Dissertations / Theses on the topic 'Flash floods'

Floods and Drought are some of the most catastrophic natural disasters for humanity, averaging 1 to 5 billion dollar of annually damage for flood events and 6 to 8 billion dollars respectively for drought events. To avoid this phenomena risk management science has grown in the last years and allows us to assess the risk and the possible benefits if some specific measures are implemented (e.g. mitigation / adaptation measures). A methodology for Non-Structural Measures (NSM) implementation in risk assessment has been developed for flood event management. Likewise, an uncertainty analysis has been done in order to identify the variation of the possible results in the risk assessment. An analysis has been done based on the Expected Annual Damage (EAD) to determine the optimal return period of design of a structural measure. A new indicator has been proposed based on this analysis: The Optimal Expected Annual Damage indicator (OEAD). In the present document the results of pluvial flood risk assessment are described. These results include structural and non-structural measures based on a developed methodology for Arenys de Munt basin, which belongs to the region of Catalonia in Spain. To include non-structural measures in risk assessment, mitigation coefficients where built in the methodology, and are described in the methodology. Also, steps for the optimization of their possible implementation are defined. This research shows that potential economic losses are decreasing with the construction of structural measures from approximately 6.6 M€ to 3 M€ (box culvert of €14Million), and in combination with the implementation of non-structural measures this could even decrease to 0.7 M€ if the non-structural measures are implemented (for 500 year return period event). Related potential casualties results decrease from approx. 11 casualties to 8 and even as low as 2 casualties respectively if non-structural measures are implemented (for 500 year return period). This, demonstrate that non-structural measures are a way to follow in the flood risk mitigation. For drought events, a new methodology has been developed in order to relate quantitative potential economic losses for rainfed crops with "Meteorological Drought". In the same, a method for the hazard (through the Palmer index) and vulnerability assessment was developed. The susceptibility of a particular crop due to a drought event was linked with a classification of the phenological stages according two seasons: the sowing and harvesting season. The case study was focus on the Llobregat basin, in which both, hydrometeorological and crop statistics data series were available. Results illustrate that the Llobregat basin has suffered at least 2 important periods of drought (2000/2001 and 2005/2006) during the length of the considered 16 year crop production record statistics. These periods of drought caused potential economic losses of approximately 40.13 M€ and 55.84 M€ in the geopolitical subdivision called "Comarcas" of the Llobregat basin. The related methodology, demonstrates coherence in the detection of "important" drought events, and in the quantification of individual potential losses per crop type, which shows that crops, like olives (classified in category woody crop type) are more resistant to drought than vegetables (tomato, lettuce chard etc.). Finally, in addition to the presented methodology the potential losses of crop efficiency curves are proposed, as indicators for agricultural drought risk assessment.
Les inundacions i les sequeres són alguns dels desastres més catastròfics per a la humanitat, promitjant anualment al voltant d'1 a 5 x 10³ milions de dòlars i 6 a 8 x 10³ milions de dòlars en pèrdues econòmiques respectivament. Per combatre aquests fenòmens, la ciència de la "gestió del riscs" ha anat desenvolupant al llarg dels últims anys, permetent-nos quantificar el risc i els possibles beneficis en el cas que algunes mesures siguin implementades (tals com mesures de mitigació/adaptació etc.). En el present document es presenta una metodologia per a la quantificació del risc considerant la implementació de mesures no-estructurals (NSM). Així mateix, es va desenvolupar una anàlisi d'incertesa per identificar les fonts de variància sobre els resultats en el càlcul del risc. Amb l'indicador EAD (Expected Annual Damage), es va realitzar una anàlisi per determinar el període de tornada òptima en el disseny de mesures estructurals, com a resultat del mateix, es proposa un nou indicador: l "Optimal Expected Annual Damage" (OEAD). En el present document s'exposen els resultats per a la quantificació del risc pluvial. Aquests resultats inclouen mesures estructurals i no-estructurals d'acord amb la metodologia desenvolupada per a la conca d'Arenys de Munt a Catalunya-Espanya. Per incloure en el càlcul del risc a les mesures no-estructurals, es van desenvolupar coeficients de mitigació els quals són explicats dins de la metodologia. Igualment, es defineixen els passos a seguir per a l'optimització en la implementació d'aquestes mesures. En aquesta investigació, s'il·lustra que les pèrdues potencials econòmiques disminueixen si una mesura estructural és implementada (canalització per Box Culvert de 14 M€), des de 6.6 M€ a 3 M€ (T=500 anys), i si s'implementen mesures no-estructurals en combinació amb la mesura estructural, les pèrdues potencials poden disminuir-se fins a 0.7 M€ per al període de tornada de 500 anys. Sobre les pèrdues potencials de vides humanes, la mitigació obtinguda segueix el mateix comportament que les pèrdues potencials econòmiques, disminuint des d'11 possibles víctimes a 8 amb la mesura estructural i a 2 en combinació amb la mesura no estructural. Pel fenomen de la sequera, es va desenvolupar una metodologia per relacionar la "Sequera Meteorològica" amb les pèrdues potencials econòmiques en cultius de secà. En la mateixa, es proposa un mètode per a la quantificació de la perillositat (mitjançant els índexs de Palmer) i la vulnerabilitat. La susceptibilitat d'un cultiu de secà a un esdeveniment de sequera (vulnerabilitat) es va relacionar fent una classificació d'acord a dos estats fenològics: l'època de sembra i de recol·lecció. El cas d'estudi es va enfocar a la conca del riu Llobregat-Espanya, de la qual es va disposar d'informació hidrometeorològica i de les estadístiques de producció de diferents tipus de cultius de secà. De l'anàlisi realitzada sobre la conca, es va observar que al llarg dels 16 anys d'estadístiques en els cultius, van ocórrer dos períodes importants de sequera (2000/2001 i 2005/2006). Aquests períodes de sequera van deixar respectivament pèrdues potencials econòmiques d'aproximadament 40.13 M€ i 55.84 M€ per a les comarques associades a la conca del riu Llobregat. La metodologia desenvolupada, demostra eficàcia en la detecció d'esdeveniments importants de sequera, així mateix, il·lustra una coherència en la quantificació de les pèrdues individuals en els tipus de cultiu, en les que cultius com l'oliva (classificat com a cultiu llenyós), demostra més resistència a la sequera respecte a altres cultius com l'enciam, tomàquets etc.. Finalment, com a complement a la metodologia desenvolupada, es proposa a les corbes de pèrdua d'eficiència de cultiu, com a indicadors per a la gestió del risc de sequeres en cultius de secà.

As one of the most devastating natural hazards, flash floods are responsible for major and abrupt geomorphic effects in the fluvial system as well as significant loss of life and socio-economic damages. Flash floods are characterized by strong spatio-temporal rainfall variability and therefore show variations in discharge and energy expenditure: associated geomorphic effects depend on geological controls on channel geometry and sediment characteristics, as well as on variations in flood intensity. Geomorphic effects usually take the form of erosional and depositional modification of the pre-flood channel. The central question of this thesis is to evaluate why flash floods of similar magnitudes and intensities sometimes produce dissimilar geomorphic results. The use of peak instantaneous hydraulic flow parameters such as discharge, velocity, shear stress, and stream power to quantify geomorphic changes has commonly been non-deterministic. This thesis aims at investigating how factors such as channel geometry, substrate, and flood magnitude and duration can interact and influence geomorphic effectiveness of high magnitude floods. A combined analysis of data from post-flood surveys and hydrological modelling permitted a comprehensive hydro-geomorphic investigation of seven major flash flood that occurred between 2007 and 2014 in different hydro-climatic regions in central and southern Europe. High peak discharge coupled with long flow duration ensured significant geomorphic impacts in Mediterranean basins. Values of stream power are generally consistent with observed geomorphic changes in the studied cross sections. However, bedrock channels show the highest values of energy expenditure but no visible erosion, whereas major erosion has been observed in alluvial channels. The trends in semi-alluvial channels urge the recognition of local or event-specific conditions that increase the resistance of channel bed and banks to erosion. Short flow duration caused major sedimentological effects but limited channel widening in most semi-alluvial channels. Eight rivers that were highly affected by three of the studied flash floods were selected for detailed analysis and modelling of the contiguous downstream variability in stream power. Power functions adequately interpreted the systematic downstream increase in peak discharge, whereas contrary to the usual exponential function, a quadratic function better interpreted the high downstream variability in channel gradient. The performance of the resulting empirical models for cross-sectional stream power and unit stream power were essentially influenced by channel gradient. The availability of high-resolution pre- and post-flood satellite images allowed assessment of channel changes along seven of these channels. Statistical analysis indicated that hydraulic forces alone are not adequate to interpret the rate of channel widening, which is primarily influenced by the degree of channel confinement. Together with lateral confinement, unit stream power better predicted channel widening in steep channels, whereas cumulative energy expenditure was relatively better for moderate channel reaches. The use of different erosion-resistance thresholds to quantify the geomorphological changes of riverbeds supports the conclusion that the determination of these changes is much more difficult than to determine the hydraulic variables involved.
Le piene improvvise (flash flood) sono fra i processi naturali più devastanti e sono responsabili di rilevanti e subitanei effetti morfologici, nonché della perdita di vite umane e di gravi danni economici. Le piene improvvise sono caratterizzate dalla forte variabilità spazio-temporale delle precipitazioni innescanti, cui consegue una forte variabilità delle portate e della potenza della corrente. Gli effetti geomorfologici delle piene improvvise dipendono sia dal controllo che l’assetto geologico esercita sulla geometria del canale e sulle caratteristiche del sedimento, sia dall’intensità della piena. Gli effetti geomorfologici delle piene improvvise si manifestano attraverso processi sia erosivi che deposizionali che determinano variazioni nell’assetto del canale rispetto alle condizioni antecedenti l’evento. La questione centrale di questa tesi è valutare perché piene improvvise di simile intensità producano talvolta effetti morfologici nettamente differenti. L’uso dei valori istantanei massimi di variabili di tipo idraulico, quali la portata, la velocità, lo sforzo tangenziale e la potenza della corrente, si è spesso rivelato non conclusivo nel quantificare i cambiamenti morfologici. Questa tesi mira a studiare come fattori quali la geometria del canale, il substrato, l’intensità e la durata dell’evento possano interagire e influenzare l’azione morfologia delle piene improvvise. Un’analisi combinata, basata rilievi post-evento e sulla modellazione idrologica, ha consentito di caratterizzare sette importanti eventi di piena improvvisa verificatisi fra il 2007 e il 2014 in diverse regioni dell’Europa centrale e meridionale. Nei bacini mediterranei gli elevati valori delle portate di picco, uniti alla durata relativamente lunga degli eventi, hanno determinato le condizioni favorevoli a significativi impatti geomorfologici. I valori della potenza della corrente sono generalmente coerenti con i cambiamenti morfologici osservati. Inoltre, i canali in roccia mostrano i valori di dispendio energetico più elevati ma senza erosioni apprezzabili, mente ingenti fenomeni di erosione sono stati osservati in canali alluvionali. Gli andamenti dei processi geomorfologici nei canali semi alluvionali richiedono il riconoscimento di situazioni locali che aumentano la resistenza del letto del canale e delle sponde all’erosione, o di condizioni specifiche di un particolare evento. Piene di breve durata causano talvolta abbondante trasporto solido, peraltro non associato a significativi allargamenti del canale nella maggior parte degli alvei semi-alluvionali. Otto corsi d’acqua, individuati fra quelli maggiormente interessati da tre delle piene studiate sono stati scelti per ulteriori analisi e per la modellazione della variazione longitudinale dei valori della potenza della corrente. Funzioni di potenza interpretano adeguatamente l’aumento verso valle delle portate di picco, mentre funzioni quadratiche si sono dimostrate più soddisfacenti delle relazioni esponenziali comunemente utilizzate per rappresentare la variazione longitudinale della pendenza dell’alveo. Le prestazioni dei modelli empirici per la variazione longitudinale della potenza della corrente per unità di larghezza dell’alveo (unit stream power) evidenziano il fondamentale controllo esercitato dalla pendenza dell’alveo. La disponibilità di immagini satellitari ad elevata risoluzione riprese prima e dopo gli eventi oggetto di studio ha permesso di valutare le modifiche del canale lungo sette di questi canali. Analisi statistiche hanno indicato che le sole variabili idrauliche non sono sufficienti per interpretare il tasso di allargamento del canale, che è principalmente influenzato dal grado di confinamento del canale stesso. Insieme al confinamento laterale, la potenza della corrente per unità di larghezza dell’alveo appare un valido predittore dell’allargamento in alvei ad elevata pendenza, mentre l’energia complessiva della corrente calcolata per l’intero evento fornisce prestazioni migliori nell’interpretare la variabilità dell’allargamento dell’alveo in canali a pendenza moderata. L'uso di differenti soglie di resistenza all’erosione per quantificare i cambiamenti geomorfologici degli alvei supporta la conclusione che la determinazione di tali cambiamenti è molto più difficile della determinazione delle variabili idrauliche coinvolte.

Flash floods caused by localized thunderstorms are a natural hazard of the semi -arid Southwest, and many communities have responded by installing ALERT flood forecasting systems. This study explored a rainfall- runoff modeling approach thought to be appropriate for forecasting in such watersheds. The kinematic model KINEROS was evaluated because it is a distributed model developed specifically for desert regions, and can be applied to basins without historic data. This study examined the accuracy of KINEROS under data constraints that are typical of semi -arid ALERT watersheds. The model was validated at the 150 km2, semi -arid Walnut Gulch experimental watershed. Under the conditions examined, KINEROS provided poor simulations of runoff volume and peak flow, but good simulations of time to peak. For peak flows, the standard error of estimate was nearly 100% of the observed mean. Surprisingly, when model parameters were based only on measurable watershed properties, simulated peak flows were as accurate as when parameters were calibrated on some historic data. The accuracy of KINEROS was compared to that of the SCS model. When calibrated, a distributed SCS model with a simple channel loss component was as accurate as KINEROS. Reasons for poor simulations were investigated by examining a) rainfall sampling errors, b) model sensitivity and dynamics, and c) trends in simulation accuracy. The cause of poor simulations was divided between rainfall sampling errors and other problems. It was found that when raingage densities are on the order of 1/20 km2, rainfall sampling errors preclude the consistent and reliable simulation of runoff from localized thunderstorms. Even when rainfall errors were minimized, accuracy of simulations were still poor. Good results, however, have been obtained with KINEROS on small watersheds; the problem is not KINEROS itself but its application at larger scales. The study also examined the hydrology of thunderstorm -generated floods at Walnut Gulch. The space -time dynamics of rainfall and runoff were characterized and found to be of fundamental importance. Hillslope infiltration was found to exert a dominant control on runoff, although flow hydraulics, channel losses, and initial soil moisture are also important. Watershed response was found to be nonlinear.

Effective prediction of localized flash flood regions for an approaching rainfall event requires an in-depth knowledge of the land surface and stream characteristics of the forecast area. Flash Flood Guidance (FFG) is currently formulated once or twice a day at the county level by River Forecast Centers (RFC) in the U.S. using modeling systems that contain coarse, generalized land and stream characteristics and hydrologic runoff techniques that often are not calibrated for the forecast region of a given National Weather Service (NWS) office. This research investigates the application of embedded geographic information systems (GIS) modeling techniques to generate a localized flash flood model for individual small watersheds at a five minute scale and tests the model using historical case storms to determine its accuracy in the FFG process. This model applies the Soil Conservation Service (SCS) curve number (CN) method and synthetic dimensionless unit hydrograph (UH), and Muskingum stream routing modeling technique to formulate flood characteristics and rapid update FFG for the study area of interest. The end result of this study is a GIS-based Flash Flood Forecasting system for ungaged small watersheds within a study area of the Blacksburg NWS forecast region. This system can then be used by forecasters to assess which watersheds are at higher risk for flooding, how much additional rainfall would be needed to initiate flooding, and when the streams of that region will overflow their banks. Results show that embedding these procedures into GIS is possible and utilizing the GIS interface can be helpful in FFG analysis, but uncertainty in CN and soil moisture can be problematic in effectively simulating the rainfall-runoff process at this greatly enhanced spatial and temporal scale.
Master of Science

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.
Includes bibliographical references (leaf 25).
This thesis describes an automated monitoring station designed to detect flash floods occurring in the Rio Aguan river basin, Honduras. An Atmel microcontroller polls a series of sensors in the river, logging all data for later hydrological analysis and modeling. A high-power APRS radio is used to alert a central monitoring facility of impending floods. Careful component choices and power management allows the system to run for 100 days on a single deep-cycle marine battery or practically indefinitely using a supplemental solar panel.
by Joshua A. Weaver.
M.Eng.

Les crues éclair (i. E. Crues soudaines provoquées par des événements pluvieux orageux) constituent sans aucun doute le risque naturel le plus destructeur en France. Malgré la menace qu'elles représentent et les nombreuses questions qu'elles suscitent, elles n'ont pas fait l'objet par le passé d'études systématiques. Dans la première partie de ce document, une méthodologie d'analyse hydrologique post-événementielle a été mise au point et testée sur cinq études de cas. Les premiers résultats obtenus sont encourageants et relativement inattendus : a) les bassins versants réagissent avec retard aux épisodes de pluies intenses, b) les volumes d'eau de pluie retenus sur les bassins et ne participant pas à la crue sont importants (de 150 à 200 millimètres dans les études de cas traitées), c) le type d'occupation des sols ne semble pas jouer un rôle déterminant sur la réponse des bassins versants. La seconde partie de la thèse est consacrée à l'analyse théorique des lois de probabilité des débits de pointes de crues. Les possibilités offertes par l'approche qualifiée de semi-déterministe, consistant à coupler un modèle mathématique de genèse aléatoire de séries pluviographiques et un modèle ``pluie-débit'' sont explorées. Il apparaît que, compte tenu des propriétés de la relation pluie-débit, les distributions des débits pointes de crues (DDPC) n'appartiennent probablement à aucun des trois types de lois des valeurs extrêmes. Elles sont asymptotiquement contrôlées par la distribution des intensités moyennes maximales des événements pluvieux, mesurées sur une durée caractéristique du bassin versant
Flash floods (i. E. Floods produced by severe thunderstorms of limited areal extent) are certainly the most destructive natural hazard in France. They also give rise to numerous questions but no systematic studies have been conducted in the past on such events. In the first part of this document, a post flood investigation methodology is proposed and tested on five case studies. The first results obtained are hopeful and reveal some original aspects of the rainfall-runoff relationship during flash floods : 1) the watershed response to the intense storm bursts is late and relatively sudden, 2) a large amount of rainwater (150 to 200 millimeters in the present case studies) is retained on the catchments and does not contribute to the flood flow, 3) no significant effects of the land use type could be identified. The second part of the document, is devoted to the theoretical analysis of the flood peak distributions. The potential of the so called derived distribution methods consisting in coupling a stochastic rainfall simulator and a mathematical ``rainfall-runoff'' model is assessed. It appears that, due to the properties of the ``rainfall-runoff'' process, flood peak distributions (FPD) belong most probably to none of the three extreme value distibution types. The asymptotic behaviour of the FPD is controlled by the maximum rainfall intensities measured over a duration characteristic of the studied watershed

La bordure septentrionale du Vénézuela (bordure méridionale de la Plaque Caraïbe) correspond pour l’essentiel à un relais de grandes failles décrochantes actives, génératrice d’une marge escarpée et accidentée (Sud de la Mer Caraïbe, Fosse et Golfe de Cariaco, Fig. 1). Cette région est donc exposée à trois sources d’aléas naturels : 1) séismes et effets directs, 2) tsunamis (liés à ces failles, à l’activité plus lointaine des Arcs Antillais, ou à des glissements sous-marins), 3) glissements et coulées aériens, parfois liés à des événements climatiques brutaux (cf. flash flood de Vargas, Décembre 1999). Le deuxième et le troisième type de phénomènes affectent directement la sédimentation soit marine (littoral et plateforme) soit lagunaire ou lacustre. Par ailleurs, les dernières variations globales (rapides) du niveau marin ont subdivisé et structuré l’empilement sédimentaire.La partie récente de ces dépôts (env. 150 000 à 200 000 ans) a fait l’objet de deux campagnes préliminaires d’imagerie sismique à haute résolution, la première consacrée à la partie orientale (Golfe de Cariaco; Audemard et al., 2007 ; Van Daele et al., 2010) et la seconde au littoral central (entre Cabo Codera et la Golfe Triste, Fig. 2). Cette seconde mission sera complétée par une nouvelle acquisition d’imagerie et la prise de carottes courtes en mer et dans les lagunes côtières. L'interprétation des sections sismiques et l'analyse sédimentologique des carottes sera utilisé pour ce travail de thèse avec un double but : 1) reconstituer l’évolution générale de la sédimentation sur la marge, et l’influence des changements environnementaux globaux, 2) connaître la distribution géographique et dans le temps (pour une période d’au moins 100 000 ans) des phénomènes catastrophiques majeurs (séismes, tsunamis, flash floods) qui se sont intercalés dans cette sédimentation. L’impact possible de la superposition de phénomènes externes et sismo-tectoniques (cf. récent séismes de Tucacas pendant un épisode pluvieux) et la localisation des zones à risques pour les tsunamis, seront modélisés et discutés
The northern border of Venezuela (southern border of the Caribbean Plate) corresponds essentially to a relay of large active strike-slip faults, generating a steep and rugged margin (South of the Caribbean Sea, Pit and Gulf of Cariaco, Fig. 1). This region is therefore exposed to three sources of natural hazards: 1) earthquakes and direct effects, 2) tsunamis (related to these faults, to the more distant activity of the West Indies bows, or to submarine landslides), 3) slips and airflows, sometimes linked to sudden climatic events (see Vargas flash flood, December 1999). The second and third types of phenomena directly affect sedimentation, whether marine (littoral and platform), lagoon or lacustrine. In addition, the latest global (fast) changes in the sea level have subdivided and structured the sedimentary stack.The recent part of these deposits (about 150 000 to 200 000 years ago) was the subject of two preliminary high resolution seismic imaging campaigns, the first devoted to the eastern part (Gulf of Cariaco, Audemard et al. 2007, Van Daele et al., 2010) and the second at the central coast (between Cabo Codera and the Sad Gulf, Fig. 2). This second mission will be complemented by a new imaging acquisition and the taking of short cores at sea and in coastal lagoons. The interpretation of the seismic sections and the sedimentological analysis of the cores will be used for this work of thesis with a double aim: 1) to reconstruct the general evolution of the sedimentation on the margin, and the influence of the global environmental changes, 2) to know the geographical distribution and over time (for a period of at least 100,000 years) major catastrophic phenomena (earthquakes, tsunamis, flash floods) that have interbedded in this sedimentation. The possible impact of the superposition of external and seismo-tectonic phenomena (see the recent Tucacas earthquakes during a rainy episode) and the location of tsunami risk areas will be modeled and discussed

This paper analyzes the anatomical response of Pinus pinaster Ait. following wounding by flash floods. A total of 14 wood samples were taken from 14 different scarred trees located on the river banks of the Arroyo Cabrera torrent (Spanish Central System). In addition, 20 increment cores were collected from undisturbed and healthy P. pinaster trees to build a local reference chronology. For the injured trees, analysis focused on growth changes in early earlywood (EE) tracheids, namely on differences in (i) lumen size; (ii) cell-wall percentage and cell-wall thickness; (iii) radial length and tangential width of tracheids; as well as (iv) in the abundance of resin ducts in earlywood (EW) and latewood (LW) following wounding. Results indicate that tissues bordering flash-flood wounds are characterized by reduced growth rates and a decrease of EE tracheid lumen area by 51%. In addition, cell-wall percentage increases by 34% in the increment rings formed after the event and significant changes are observed in the radial length and tangential width of EE tracheids. Observations on resin ducts do not yield any significant results. Based on these anatomical parameters, detecting and dating past flash-flood events in growth rings is now possible for Mediterranean species, specifically P. pinaster.

Flash floods are one of the most devastating natural hazards, claiming numerous lives and tremendous economic losses. One of the main reasons for their catastrophic potential is the limited time available for precautionary measures, such as warnings or evacuations. Early warning systems (EWSs) play a key role for emergency managers to react in a timely manner to upcoming floods and effectively mitigate the impacts. This thesis explores possibilities to enhance the methods available for flash flood early warning and thus improve the operational decision support. While a variety of existing methods aims at the prediction of the hazard component of flash floods (e.g. the peak streamflow), an increasing number of EWS developers and end-users have recognised the potential of tools that automatically translate the flash flood hazard forecasts into the expected socio-economic impacts (e.g. the population affected). These so-called impact forecasts enable more objective and rapid decisions, ultimately leading to a more effective flood response. While for fluvial floods, impact forecasts have been available for several years and over various spatial scales, the existing approaches for flash floods have been limited to a small number of prototypes focusing on individual catchments or relatively small regions. These small-scale approaches can be useful for the coordination of local emergency measures, but their potential is limited for supporting the decisions of authorities operating over larger domains (e.g. regional, national, or international civil protection mechanisms). The main goal of this thesis has been to extend the available decision support by applying the concept of flash flood impact forecasting over large spatial scales. Two methods have been developed for estimating the impacts in real time, named ReAFFIRM and ReAFFINE. The two methods take into account that emergency services operating at different spatial scales require different kinds of real-time information to make informed decisions: ReAFFIRM provides detailed impact estimates in high resolution to support regional or national authorities in the coordination of location-specific emergency measures (e.g. evacuations), whereas ReAFFINE generates order-of-magnitude impact estimates with pan-European coverage that can be useful for end-users operating across regions or countries. The application of ReAFFIRM and ReAFFINE for a number of past flood events has demonstrated their capabilities to identify flash flood impacts in real time over the different spatial scales. The developed algorithms have a moderate computational cost and require only datasets that are available throughout the EU, which facilitates the real-time implementation of the methods and their integration into the operational procedures of end-users across Europe. An additional objective of this thesis has been to explore a more integrated perspective of flood early warning. Traditionally, EWSs are designed separately for the different physical processes that lead to flooding (i.e. individual systems for fluvial, pluvial, coastal, and flash floods). This means that the end-users need to monitor a number of separate flood forecasts with potentially even contradicting outputs. Especially during events in which different flood types coincide (so-called compound floods), this can be time-consuming and confusing. The decision support could be significantly simplified by automatically integrating the forecasts of different flood types into an overall compound flood forecast. This idea has been explored through the analysis of a recent catastrophic compound flood, for which the impact estimates from ReAFFIRM have been combined with those from a system designed for fluvial floods. The combined performance of the methods has shown to be superior to the individual performances, clearly demonstrating the potential of such integrated approaches for improving the decision support.
Las avenidas torrenciales son una de las amenazas naturales más devastadoras, causando numerosas víctimas y enormes pérdidas económicas. Los sistemas de alerta temprana (SAT) juegan un papel clave para que los servicios de emergencia puedan reaccionar de manera oportuna y mitigar con eficacia los impactos. Esta tesis explora diferentes posibilidades de ampliar los métodos disponibles para la alerta temprana de avenidas torrenciales, con el objetivo de mejorar la toma de decisiones de los servicios de emergencia. Una variedad de métodos se dedica a la predicción del componente de amenaza de las avenidas repentinas (e.g. los caudales máximos instantáneos). No obstante, un número creciente de desarrolladores de SAT y usuarios finales han reconocido el potencial de herramientas que traducen automáticamente estos pronósticos de amenaza en impactos socioeconómicos (e.g. la cantidad de población afectada). Estas predicciones de impacto permiten tomar decisiones más objetivas y rápidas, que conducen a una respuesta más eficaz ante las avenidas y sus consecuencias. Los estudios realizados para la predicción del impacto de avenidas torrenciales han sido limitados a unos pocos prototipos que se enfocan en cuencas individuales o regiones relativamente pequeñas que pueden resultar útiles para la coordinación de medidas de emergencia locales, pero su potencial es limitado para apoyar las decisiones de las autoridades que actúan en dominios más amplios (e.g. autoridades de protección civil regionales, nacionales o europeas). El objetivo principal de esta tesis ha sido extender el apoyo a la toma de decisiones disponible mediante la aplicación del concepto de previsión del impacto de avenidas torrenciales en grandes escalas espaciales. Para ello, se desarrollaron dos métodos para estimar los impactos en tiempo real: ReAFFIRM y ReAFFINE. ReAFFIRM proporciona estimaciones de impacto detalladas y en alta resolución para dar apoyo a las autoridades regionales o nacionales en la coordinación de medidas de emergencia específicas (e.g. evacuaciones), mientras que ReAFFINE genera estimaciones de impacto en órdenes de magnitud con cobertura paneuropea que resultan útiles para los usuarios finales que actúan en grandes dominios espaciales. El uso de ReAFFIRM y ReAFFINE para una serie de inundaciones pasadas ha demostrado su capacidad para identificar los impactos de las avenidas torrenciales en tiempo real y en diferentes escalas espaciales. Los algoritmos desarrollados tienen un coste computacional moderado y solo requieren datos que están disponibles en toda la UE, permitiendo su implementación e integración en los procedimientos operativos de varios usuarios finales en toda Europa. Un objetivo adicional de esta tesis ha sido explorar una perspectiva más integrada de la alerta temprana de inundaciones. Tradicionalmente, los SAT son diseñados por separado para los diferentes procesos físicos que pueden resultar en inundaciones. Esto significa que los usuarios finales deben monitorear una serie de pronósticos de inundaciones por separado con resultados que podrían resultar potencialmente contradictorios, especialmente durante eventos en los que coincidan diferentes tipos de inundaciones (también llamadas inundaciones compuestas). Lo anterior puede alargar los tiempos de respuesta, generar confusión y, en última instancia, impedir una respuesta de emergencia eficaz. El apoyo a la toma de decisiones podría ser simplificada significativamente y de manera automática mediante la integración de los SAT de diferentes tipos de inundaciones en un único pronóstico que las englobe. Esta idea se explora a través de la combinación de las estimaciones de impacto de ReAFFIRM con las de un sistema diseñado para inundaciones fluviales. El rendimiento de ambos métodos combinados ha demostrado ser superior al de cada uno de manera individual, indicando el potencial de combinar el pronóstico de impacto por inundaciones
Enginyeria civil

The contribution of this thesis is twofold: 1) the development of a hydrodynamic landscape evolution model for use on high-performance computing systems and 2) assessing the sensitivity of hydrogeomorphic processes to high-resolution rainfall input data and erosional parameterisation using the model. The thesis addresses a limitation in numerical landscape evolution models regarding how spatial variation in rainfall is represented or parameterised within such models. Typically, landscape evolution models forsake a realistic representation of rainfall patterns in favour of a simpler treatment of rainfall as being spatially homogeneous across the model domain. This simplification of rainfall spatial variability is still made despite the fact that many geomorphological processes are sensitive to thresholds of sediment entrainment and transport, driven by the distribution and movement of water within the landscape. The thesis starts by exploring current limitations in rainfall representation in landscape evolution models, and assesses various precipitation data sources that could be potentially used as more realistic rainfall inputs to landscape evolution models. A numerical model of landscape evolution is developed for deployment on high-performance parallel computing systems, based on the established CAESAR-Lisflood model (Coulthard et al., 2013). The new model code is benchmarked, showing performance benefits compared with the original CAESAR-Lisflood model it is based on. The model is applied to assessing the sensitivity of flood-inundation predictions, sediment flux, and erosion distribution within river catchments to spatial variation in rainfall during extreme storm events. Two real storm events that caused localised flash flooding in the UK are used as test cases: the Boscastle storm of 2004 and the North York Moors storm of 2005. Flood extent predictions and river discharges are found to be sensitive to the use of spatially variable input rainfall data, with high-resolution rainfall data leading to larger peak flood discharges. However, the differences are less pronounced in smaller catchments. The role of sediment erosion during large floods is also assessed, but it is found to play a minor role relative to spatially variable rainfall data. In contrast, the geomorphological response of catchments to single storm events is shown to be less sensitive to the spatial heterogeneity of rainfall input and controlled more strongly by the choice of erosional process parameterisation within the model. Nonetheless, spatial variability in rainfall data is shown to increase sediment yields during flash flood simulations.

This dissertation reports the results of a field based study examining sediment transport and bed mobility in a low-ordered, ephemeral watershed. Runoff and sediment transport concentrations were sampled at the watershed outlet to determine flow discharge and sediment flux during approximately 21 flow events, from 1998 - 2007. Sediment collected in flow was measured for grain-size distribution to determine if specific grain-size fractions behave differently while in transport. The coarse sediment yield was measured for mass and grain-size distribution at the watershed outlet for two years, 2005 - 2006. Further, the arrangement and composition of the channel bed material was comprehensively mapped using terrestrial-based photogrammetry for the years, 2005 - 2006. Results show that patterns of sediment transport are complex, controlled in part by flow hydraulics but also by other phenomena. Some of the variation in sediment transport is determined by grain-size. Grain-sizes with different sources within the watershed and that transported by different transport modes were observed to follow different patterns of transport. Also, the channel bed, which serves as the source for the coarse fraction of the sediment transport, was observed to change in grain composition during periods of flow. This tendency for the bed material to evolve in time likely affected the amount and composition of the sediment grains that were entrained from it.An additional objective of this dissertation was to determine how unique the observed patterns of sediment transport were to low-order ephemeral channels. Sediment transport and yield were modeled using bed load transport formulae designed to capture the physical mechanics of transport as observed in perennial streams. Results show that contemporary transport models predict transport within the field site with similar accuracy as that in many perennial systems but not well enough to rely on their predictions for many engineering applications.

Practice-led journalism research techniques were used in this study to produce a ‘first draft of history’ recording the human experience of survivors and rescuers during the January 2011 flash flood disaster in Toowoomba and the Lockyer Valley in Queensland, Australia. The study aimed to discover what can be learnt from engaging in journalistic reporting of natural disasters. This exegesis demonstrates that journalism can be both a creative practice and a research methodology. About 120 survivors, rescuers and family members of victims participated in extended interviews about what happened to them and how they survived. Their stories are the basis for two creative outputs of the study: a radio documentary and a non-fiction book, that document how and why people died, or survived, or were rescued. Listeners and readers are taken "into the flood" where they feel anxious for those in peril, relief when people are saved, and devastated when babies, children and adults are swept away to their deaths. In undertaking reporting about the human experience of the floods, several significant elements about journalistic reportage of disasters were exposed. The first related to the vital role that the online social media played during the disaster for individuals, citizen reporters, journalists and emergency services organisations. Online social media offer reporters powerful new reporting tools for both gathering and disseminating news. The second related to the performance of journalists in covering events involving traumatic experiences. Journalists are often required to cover trauma and are often amongst the first-responders to disasters. This study found that almost all of the disaster survivors who were approached were willing to talk in detail about their traumatic experiences. A finding of this project is that journalists who interview trauma survivors can develop techniques for improving their ability to interview people who have experienced traumatic events. These include being flexible with interview timing and selecting a location; empowering interviewees to understand they don’t have to answer every question they are asked; providing emotional security for interviewees; and by being committed to accuracy. Survivors may exhibit posttraumatic stress symptoms but some exhibit and report posttraumatic growth. The willingness of a high proportion of the flood survivors to participate in the flood research made it possible to document a relatively unstudied question within the literature about journalism and trauma – when and why disaster survivors will want to speak to reporters. The study sheds light on the reasons why a group of traumatised people chose to speak about their experiences. Their reasons fell into six categories: lessons need to be learned from the disaster; a desire for the public to know what had happened; a sense of duty to make sure warning systems and disaster responses to be improved in future; personal recovery; the financial disinterest of reporters in listening to survivors; and the timing of the request for an interview. Feedback to the creative-practice component of this thesis - the book and radio documentary - shows that these issues are not purely matters of ethics. By following appropriate protocols, it is possible to produce stories that engender strong audience responses such as that the program was "amazing and deeply emotional" and "community storytelling at its most important". Participants reported that the experience of the interview process was "healing" and that the creative outcome resulted in "a very precious record of an afternoon of tragedy and triumph and the bitter-sweetness of survival".

L'objectif scientifique de la thèse est de progresser dans la modélisation hydrologique spatiale régionale dans le contexte de crues éclairs qui représentent l'une des catastrophes naturelles les plus destructrices dans la région Méditerranéenne. L'accent est mis sur les questions de mise à l'échelle des bassins versants et la dérivation des équations et des modèles applicables aux bassins de milieu simplifiées de grande taille pour mieux décrire l'hétérogénéité du paysage et de la complexité du processus. Telles sont les questions clés pour faciliter le modèle mis en place dans le contexte de l'ensemble du bassin versant et d'essayer son application dans les bassins non jaugés trop. Pour répondre à ces questions, une modélisation hydrologique spatiale simplifiée sur les sous-bassins versants est d'abord proposé où les paramètres sont essentiellement tirées de l'information disponible (surtout cartographique). La méthode de Kirchner (WRR, 2009) qui suppose que le débit à la sortie est la seule fonction de stockage du bassin versant, est spécifiquement étudiée dans le cadre des bassins versants Méditerranéens. L'étape suivante consiste à créer un nouveau modèle hydrologique SIMPLEFLOOD distribué sur la base de « top down » méthodologie de Kirchner dans la plateforme de modélisation JAMS. Les paramètres du modèle simple sont estimés à des endroits calibrés et une régionalisation se fait en fonction de la géologie. Le bassin versant est discrétisé en sous-bassins versants d'environ 10 km2. La dernière étape consiste à procéder à un couplage de données avec le modèle hydraulique MAGE 1D développé à IRSTEA HHLY tenir compte des effets de propagation de la rivière sur les hydrogrammes simulés. Le couplage est externe, ce qui signifie que les sorties du modèle hydrologique dans le système de modélisation de JAMS deviennent les entrées du modèle MAGE hydraulique. Les sorties sont les débits qui sont transférés dans le modèle de MAGE soit comme flux latéraux (provenant des terres adjacentes) et /ou entrées d'eau locales. L'application de la thèse est le bassin versant de l'Ardèche (2388 km ²), qui est l'un des sites pilotes français pour le programme international HyMeX (cycle hydrologique dans l'expérience de la Méditerranée, http://www.hymex.org/). La thèse proposée contribue également au projet FloodScale (multi-échelle d'observation hydrométéorologique et de modélisation pour la compréhension et simulation des crues éclairs (http://floodscale.irstea.fr/). L'application de la méthodologie Kirchner (2009) montre que les résultats de simulation des débits sont bonnes pour les bassins de granit, trouvés à être caractérisée principalement par des processus excès de ruissellement et d'écoulement sous la surface de saturation. L'hypothèse simple de système dynamique fonctionne particulièrement bien dans des conditions humides (pics et les récessions sont bien modélisés). D'autre part, la performance du modèle est moins bien représentée à l'été et les périodes de sécheresse où l'évapotranspiration est large et observations de bas-débits sont inexactes. Dans le bassin versant de l'Ardèche, les précipitations simulées correspondent bien à de stations de jaugeage observés et données de réanalyse SAFRAN pendant les périodes de non-végétation. Le modèle doit encore être amélioré pour inclure une représentation plus précise de l'évapotranspiration réelle, mais fournit un résumé satisfaisant du fonctionnement du bassin versant pendant les périodes humides et d'hiver. Le couplage du modèle hydrologique obtenue avec le modèle hydraulique MAGE 1D fournit des résultats satisfaisants mais les résultats sont si réciproques comme dans le cas du modèle hydrologique ou une équation d'onde cinématique simple pour le routage des flux existe. On peut dire que dans les situations ou débordement de la rivière est significative, le couplage serai crucial
The scientific objective of the thesis is to progress in regional spatial hydrological modeling in the context of flash floods that represent one of the most destructive natural hazards in the Mediterranean region. Emphasis is put on catchment scaling issues and derivation of simplified equations and models applicable to basins of medium to large size to best describe landscape heterogeneity and process complexity. These are the key issues in facilitating the model set up in the context of the whole catchment and trying its application in ungauged catchments too. To address these issues, a simplified spatial hydrological modeling over sub-catchments is first proposed where parameters are essentially derived from available information (cartographic utmost). For this purpose, the Kirchner (WRR, 2009) method that assumes that discharge at the outlet is only a function of catchment storage is specifically studied in the context of Mediterranean catchments. The next step is to create a new distributed hydrological model based on the data driven methodology of Kirchner within the JAMS modeling framework. The parameters of the simple model are estimated at the gauged locations and a regionalization is done according to geology. The catchment is discretized into sub-catchments of about 10 km2. The final step is to proceed with data coupling with the MAGE 1D hydraulic model developed at HHLY to consider river propagation effects on the simulated hydrographs. The coupling is external, meaning that outputs from the hydrological model in JAMS modeling system become inputs to the hydraulic model MAGE. Outputs are discharge rates in the reach network that are transferred into the MAGE model as either lateral flows (coming from adjacent land) and/or local inflows. The case study of the thesis is the Ardèche catchment (2388 km²), which is one of the French pilot sites for the HyMeX international program (Hydrological Cycle in the Mediterranean Experiment, http://www.hymex.org/). The proposed thesis also contributes to the FloodScale project (Multi-scale hydrometeorological observation and modeling for flash floods understanding and simulation, http://floodscale.irstea.fr/ ). The application of the Kirchner (2009) methodology shows that resulting discharge simulation results are good for granite catchments, found to be predominantly characterized by saturation excess runoff and sub-surface flow processes. The simple dynamical system hypothesis works especially well in wet conditions (peaks and recessions are well modeled). On the other hand, poor model performance is associated with summer and dry periods when evapotranspiration is high and operational low-flow discharge observations are inaccurate. In the Ardèche catchment, inferred precipitation rates agree well in timing and amount with observed gauging stations and SAFRAN data reanalysis during the non-vegetation periods. The model should further be improved to include a more accurate representation of actual evapotranspiration, but provides a satisfying summary of the catchment functioning during wet and winter periods. The coupling of the resulting hydrological model with the MAGE 1D hydraulic model provides satisfying results. However, the results show that the timing and magnitude of simulated discharge with coupled model is as good as by the hydrological model with a simple kinematic wave equation for flow routing. We argue that in situations when there is a significant overflow in the floodplain the interest of the coupling with the hydraulic model becomes crucial

This work describes the derivation of a set of statistics, termed spatial moments of catchment rainfall, that quantify the dependence between rainfall spatial organization, basin morphology and runoff response. These statistics describe the spatial rainfall organisation in terms of concentration and dispersion along the flow distance coordinate. These statistics were derived starting from an analytical framework, and related with the statistical moments of the flood hydrograph. From spatial moments we also created an index quantifying catchment scale storm velocity. This index measures the overall movement of the rainfall system over the catchment, reflecting the filtering effect of its morphology. We also extended spatial moments to the hillslope system, developing a framework to evaluate the relevance of hillslope and channel propagation in the flood response to spatially variable rainfall fields. Data from six flash floods occurred in Europe between 2002 and 2007 are used to evaluate the information provided by the framework. High resolution radar rainfall fields and a distributed hydrologic model are employed to examine how effective are these statistics in describing the degree of spatial rainfall organisation, which is important for runoff modelling. The size of the study catchments ranges between 36 to 2586 km2. The analysis reported here shows that spatial moments of catchment rainfall can be effectively employed to isolate and describe the features of rainfall spatial organization which have significant impact on runoff simulation. Rainfall distribution was observed to play an important role in catchments as small as 50 km2. The description timing error was further improved by the inclusion in the framework of hillslope propagation. This development allows to compare scenarios of hillslope conditions, to evaluate the sensitivity of single basins or the effect of catchment scale. The analysis of catchment scale storm velocity showed a nonlinear dependence with basin scale. The values of velocity observed were however rather moderate, in spite of the strong kinematic characteristics of individual storm elements, and did not play a relevant effect on the flood analyzed.
Questo lavoro presenta una serie di statistici, denominati "momenti spaziali di pioggia a scala di bacino", che permettono di quantificare la relazione tra l’organizzazione spaziale della pioggia, la morfologia del bacino e la forma dell’idrogramma di piena. Tali statistici descrivono la posizione e la dispersione della pioggia su un assegnato bacino idrografico. La trattazione include la derivazione di una serie di relazioni che consentono di stabilire un rapporto fra detti statistici di pioggia ed i momenti temporali dell’onda di piena. La formulazione complessiva del lavoro consente di isolare e quantificare l’effetto della variabilità spaziale della pioggia sulla struttura della risposta di piena, e di creare un indice di velocità del sistema di pioggia. Questo indice considera l'iterazione tra morfologia del bacino e spostamento della pioggia, quantificando l'influenza sull'idrogramma. I momenti spaziali stati poi ampliati alla propagazione su versante, sviluppando degli statistici per valutare l'importanza della propagazione di canale e di versante sulla risposta nella riposta di un bacino ad una pioggia distribuita. Dati relativi a sei eventi estremi di piena improvvisa verificatisi in diverse regioni Europee sono utilizzati per illustrare il significato degli statistici e le relazioni con la forma dell'idrogramma. Stime di pioggia da radar ed un modello idrologico distribuito sono utilizzati per valutare l'efficacia degli statistici nel cogliere l'organizzazione delle piogge che ha un'influenza sulla simulazione di piena. Lo studio mostra che i momenti spaziali di pioggia a scala di bacino possono essere efficaci nel cogliere questa organizzazione. Nelle piene improvvise analizzate la distribuzione della pioggia ha un effetto rilevante anche per bacini di circa 50 km2. La descrizione dell'errore temporale dell'idrogramma è ulteriormente migliorata dalla considerazione dell'effetto del versante. Questo sviluppo permette inoltre di confrontare condizioni di versante diverse, di valutare la suscettibilità di singoli bacini o l'effetto in relazione alla scala dei bacini. L'analisi della velocità di spostamento a scala di bacino mostra una relazione non lineare con le dimensioni del bacino. I valori di velocità osservati sono comunque moderati, nonostante la forte velocità di spostamento delle singole celle convettive, e non hanno avuto un ruolo rilevante nell'evento analizzato.

En cas d’alerte aux inondations, l'information en temps réel et sa diffusion à un large public sont des éléments cruciaux pour limiter les comportements à risque, détecter les premiers dégâts ou préparer les acteurs locaux à gérer la crise. Face à ces enjeux, les technologies smartphones apparaissent, deplus en plus, comme des solutions logicielles et matérielles qui pourraient compléter de façon positiveles dispositifs d’alerte institutionnels, notamment grâce à la dynamique des contenus partagés et auxnombreuses interactions dont ils sont le support. Alors que plusieurs pays (États-Unis, Norvège, Pays-Bas, Suède, Philippines) utilisent déjà de tels outils, et même si le Ministère de l'Intérieur a lancé uneapplication urgentiste (SAIP) le 8 juin 2016, la population et les services de l’État semblent beaucoupplus réticents en France. La défense de la propriété individuelle, le manque de crédit attribué auxmessages postés et les verrous technologiques sont les principaux facteurs de blocage avancés. Aprèsavoir mené une enquête dans plusieurs communes rurales des départements du Var et du Vaucluse(qui confirme la méconnaissance des applications déjà existantes alors que les besoins sont bien réelsface aux crues rapides), une nouvelle application a été développée (Al’in). Le prototype créé vise àréduire les écarts entre une approche étatique descendante (top-down), qui positionne et impose l’Étatcomme le seul lanceur d’alerte, et une initiative citoyenne (approche « Bottom-Up »), qui érigel’individu au rang de « citoyen capteur », à la fois capable de remonter ou de diffuser une alerte àtravers l’usage de son smartphone. Répondre à ce défi ne s’avère toutefois pas si simple au regard descontraintes juridiques et techniques préexistantes en France
The information given on flash floods in real time should undoubtedly answers to the citizens needs aswell as to the politic and the safety services. Face to such objective, Smartphone could positivelycomplete the official institutional systems, especially thanks to the dynamic and nature of postedmessages, and to the social interactions they support. This study questions the opportunity ofdeveloping a proactive and innovative alert system using the Smartphone technologies to face therisks due to flash flood hazards. A first study focuses on several alert systems existing actually inFrance but we quickly aim at identifying limits (legal rules, technologies and scientific problems) andchallenges usefulness and opportunities in the numeric sphere) of our objective in general manner.Finally, a local study in Var and Vaucluse permits us to see if population and local stakeholdersshould be interested (or not) by a smartphone application. The proposed solution tries to achieve amajor challenge: reducing the gap between a state approach (top-down), which positions and imposedthe state as the main official alert source, and the citizen action (bottom-up) which establishes theindividual as “Citizen sensor” (both in go up or disseminate information) through the use of issmartphone. Meeting this challenge is indispensable if we hope effectively minimizing material andhuman damages especially when high temporal acuity phenomena such as flash floods happen

Le pourtour du bassin méditerranéen subit fréquemment des épisodes de pluie diluvienne à l’origine de crues rapides pouvant provoquer de nombreuses victimes et des dégâts considérables. Afin de faire face à ce phénomène, la prévision hydrologiques, permettant au Service Central d’Hydrométéorologie et d’Appui à la Prévision des Inondations de produire des vigilances crues, tient une place centrale. Durant les dernières décennies l’efficacité des réseaux de neurones formels pour la prévision des crues rapides a été montrée sur différents bassins versant. Les travaux menés au cours de cette thèse visent à développer une méthodologie générique de mise en œuvre de réseaux de neurones, testée sur les bassins versants du Gardon d’Anduze et du Lez à Lavalette, dont le comportement hydrodynamique est particulièrement non-linéaire. Afin de limiter l’incertitude des performances en prévision en fonction de l’initialisation du modèle, nous avons, dans un premier temps, proposé un modèle d’ensemble, basé sur la médiane à chaque pas de temps des sorties d’un nombre adéquat de modèles variant uniquement par leur initialisation. D’autre part, sur le bassin du Gardon d’Anduze, afin d’améliorer les performances des réseaux de neurones récurrents par l’introduction d’informations sur l’état du bassin versant avant et pendant l’épisode de crue, différentes variables susceptibles de représenter l’état du système ont été introduites successivement afin de sélectionner celles fournissant les modèles les plus performants. Sur le bassin karstique du Lez, dont la structure est très hétérogène, nous avons appliqué la méthode KnoX permettant d’estimer les contributions au débit à l’exutoire de quatre zones aux comportements hydrologique et hydrogéologique considérés comme homogènes. Ainsi les zones les plus contributives ont été identifiées ; ceci permettra dans un second temps de rechercher les variables les mieux à même de représenter l’humidité dans ces zones. Les performances des modèles développés montrent que la méthodologie générale de conception d’un modèle pluie-débit par réseaux de neurones s’adapte de manière satisfaisante aux deux bassins cibles dont les fonctionnements hydrologiques et hydrogéologiques sont pourtant très différents. Certaines pistes de progrès restent à investiguer parmi lesquelles l’amélioration de l’information d’état est prépondérante
The Mediterranean region is frequently subjected to intense rainfalls leading to flash floods. This phenomenon can cause casualties and huge material damages. Facing to this phenomenon, hydrologic forecasting is a major tool used by Service Central d’Hydrométéorologie et d’Appui à la Prévision des Inondations to produce flood warning.During past decades, artificial neural networks showed their efficiency for flash flood forecasting on different type of watershed. The present thesis aims thus to contribute to the development of a generic methodology to design artificial neural networks, that is tested on Gardon d’Anduze and Lez at Lavalette watersheds, both displaying non-linear hydrodynamic behavior. To reduce uncertainties on forecasts, ensemble models, based on the median of forecasts calculated at each time step for an adequate number of models varying only by their initialization, have been proposed. In addition, in order to improve forecasting performances on Gardon d’Anduze, with artificial neural networks, we tried to introduce knowledge about the state of the watersheds before and during the flood. Several variables have thus been tested each one its turn, to select the one given the best performances. On the Lez karst system, that has a strongly heterogeneous structure, the KnoX method have been applicated in order to estimate the contribution to outflow from four geographical zones displaying hydrologic and hydrogeologic behavior considered as homogeneous. Thus, the most contributive zones to the discharge zones have been identified. This will help the investigation of representing humidity variables in these zones.The performances of models underlined that the general methodology of rainfall-runoff model conception could be applied on both basins, even though their hydrological and hydrogeological behavior are very different.The contribution of each zone, estimated from the KnoX methodology, improved comprehension of Lez karst system during flash floods. Selection of relevant variables representing the state of the Lez hydrosystem will be possible thanks to this new knowledge. Performances of models developed in this study underlined the difficulty to find satisfactory models, and showed the interest of the generic methodology used to design neural network adapted to the two targeted basins

Les modèles pluie-débit sont des outils essentiels pour de nombreuses applications hydrologiques, notamment la prévision des crues. L’objet de cette thèse est d’examiner les performances d’un modèle événementiel distribué, dont l’intérêt est de résumer la représentation des processus à la phase de crue, et la condition initiale à un indice de saturation du bassin facilement observable ou accessible. Ce dernier dispense de modéliser la phase inter-crue, et simplifie la paramétrisation et le calage du modèle. Le modèle étudié combine une fonction de production type SCS et une fonction de transfert type lag and route, appliquées à une discrétisation du bassin en mailles carrées régulières.Le modèle est d’abord testé sur le bassin versant du Real Collobrier. Ce bassin méditerranéen est suivi depuis plus de 50 ans par l’IRSTEA, et dispose d’une exceptionnelle densité de mesures de pluies et de débits. Cet environnement favorable permet de limiter les incertitudes sur l’estimation des pluies et d’évaluer les performances intrinsèques du modèle. Dans ces conditions, les crues sont bien reconstituées à l’aide d’un jeu de paramètres unique pour l’ensemble des épisodes testés, à l’exception de la condition initiale du modèle. Celle-ci apparaît fortement corrélée avec l’humidité du sol en début d’épisode, et peut être prédéterminée de façon satisfaisante par le débit de base ou l’indice w2 fourni par le modèle SIM de Météo-France. Les performances du modèle sont ensuite étudiées en dégradant la densité des pluviomètres, et rendent compte du niveau de performances du modèle dans les cas que l’on rencontre le plus souvent. .La variabilité spatiale des paramètres du modèle est étudiée à l’échelle de différents sous-bassins du Real Collobrier. La comparaison a permis de mettre en évidence et de corriger un effet d’échelle concernant l’un des paramètres de la fonction de transfert. Les relations entre la condition initiale du modèle et les indicateurs d’humidités des sols en début d’épisode restent bonnes à l’échelle des sous-bassins, mais peuvent être significativement différentes selon les sous-bassins. Une seule relation ne permet pas de normaliser l’initialisation du modèle sur l’ensemble des sous-bassins, à une échelle spatiale de quelques km2 ou dizaines de km2. Dans le cas de l’indice d’humidité du sol w2, une explication possible est que cet indice ne prend pas en compte suffisamment finement les propriétés des sols. Enfin, la variabilité spatiale des paramètres du modèle est étudiée à l’échelle d’un échantillon d’une quinzaine de bassins méditerranéens de quelques centaines de km2, associés à des paysages et des fonctionnements hydrologiques divers. La comparaison montre qu’à cette échelle, le lien entre les indicateurs de saturation du bassin et la condition initiale peut rester stable par type de bassin, mais varie significativement d’un type de bassin à l’autre. Des pistes sont proposées pour expliquer cette variation.En conclusion, ce modèle événementiel distribué représente un excellent compromis entre performances et facilité de mise en œuvre. Les performances sont satisfaisantes pour un bassin donné ou pour un type de bassin donné. L’analyse et l’interprétation de la variabilité spatiale des paramètres du modèle apparaît cependant complexe, et doit faire l’objet du test d’autres indicateurs de saturation des bassins, par exemple mesures in situ ou mesures satellitaires de l’humidité des sols
Rainfall-runoff models are essential tools for many hydrological applications, including flood forecasting. The purpose of this thesis was to examine the performances of a distributed event model for reproducing the Mediterranean floods. This model reduces the parametrization of the processes to the flood period, and estimates the saturation of the catchment at the beginning of the event with an external predictor, which is easily observable or available. Such predictor avoids modelling the inter-flood phase and simplifies the parametrization and the calibration of the model. The selected model combines a distributed SCS production function and a Lag and Route transfer function, applied to a discretization of the basin in a grid of regular square meshes.The model was first tested on the Real Collobrier watershed. This Mediterranean basin has been monitored by IRSTEA for more than 50 years and has an exceptional density of rainfall and flow measurements. This favourable environment made it possible to reduce the uncertainties on the rainfall input and to evaluate the actual performances of the model. In such conditions, the floods were correctly simulated by using constant parameters for all the events, but the initial condition of the event-based model. This latter was highly correlated to predictors such as the base flow or the soil water content w2 simulated by the SIM model of Meteo-France. The model was then applied by reducing the density of the rain gauges, showing loss of accuracy of the model and biases in the model parameters for lower densities, which are representative of most of the catchments.The spatial variability of the model parameters was then studied in different Real Collobrier sub-basins. The comparison made it possible to highlight and correct the scale effect concerning one of the parameters of the transfer function. The catchment saturation predictors and the initial condition of the model were still highly correlated, but the relationships differed from some sub-catchments. Finally, the spatial variability of the model parameters was studied for other larger Mediterranean catchments, of which area ranged from some tenth to hundreds of square kilometres. Once more, the model could be efficiently initialized by the base flow and the water content w2, but significant differences were found from a catchment to another. Such differences could be explained by uncertainties affecting as well the rainfall estimation as the selected predictors. However, the relationships between the initial condition of the model and the water content w2 were close together for a given type of catchment.In conclusion, this distributed event model represents an excellent compromise between performance and ease of implementation. The performances are satisfactory for a given catchment or a given type of catchment. The transposition of the model to ungauged catchment is less satisfactory, and other catchment saturation indicators need to be tested, e.g. in situ measurements or satellite measurements of soil moisture

Le risque hydrologique associé aux crues rapides survenant en région méditerranéenne est variable dans l'espace et le temps. Des travaux ont montré une vulnérabilité forte des personnes mobiles face aux crues touchant les bassins versants de petite taille (< 20 km²). Le risque hydrologique qui en résulte s'ajoute au risque, mieux connu, associé au débordement des grands cours d'eau. Ce constat définit les enjeux de la modélisation hydrologique régionale mise en place dans ce travail, qui a pour objectif la compréhension des processus de crue de l'échelle du petit bassin versant (1 km²) à l'échelle des bassins régionaux (> 1000 km²). Le modèle utilisé dans cette thèse est construit avec la plate-forme de modélisation LIQUID, qui permet un couplage « à la carte » de modules représentant les processus que l'on souhaite intégrer au modèle. C'est un modèle utilisé sans calibration, dans une démarche de test d'hypothèses de fonctionnement hydrologique des bassins. La zone d'étude est la région Cévennes-Vivarais. Les premières simulations effectuées montrent une sensibilité forte des résultats du modèle aux propriétés des sols (conductivité hydraulique, épaisseurs), ainsi qu'au type de condition limite employée (percolation profonde ou non). Une distinction apparaît entre les comportements en crue des bassins situés sur roche sédimentaire et des bassins schisteux situés sur les reliefs cévenols. Une version du modèle intégrant la représentation des écoulements latéraux de surface et de subsurface est également développée, et déployée sur le bassin expérimental du Cartaou (0.5 km²). Les premiers résultats soulignent l'important rôle joué par ces écoulements à petite échelle. Une méthodologie d'analyse des récessions de débit est mise en place pour l'estimation des propriétés hydrauliques et des épaisseurs des horizons de roche altérée, non-décrits par les bases de données des sols de la région. Les résultats de l'analyse suggèrent une hiérarchie dans les valeurs des paramètres, contrôlée par le type de géologie. Les horizons de roche altérée sont ensuite intégrés dans la version finale du modèle, qui est déployée à l'échelle de la région entière. Les simulations effectuées sur l'année 2008 montrent l'intérêt de la prise en compte de ces épaisseurs de roche altérée, tant pour la simulation des débits en crue que lors des périodes inter-événementielles. Les simulations mettent également en évidence des différences de comportement entre les bassins du nord de la région (Ardèche, Tarn) et ceux du sud (Cèze, Gardon, Vidourle) que l'on peut relier à la géologie
The hydrological risk associated with flash-floods in the mediteranean area is temporally and spatially variable. Recent works showed the vulnerability of mobile people during floods occurring on small catchments (area < 20 km²). The associated risk, added to the better-known risk related to the overflow of larger rivers, defines the objectives of the present thesis. This work aims at developing a regional distributed hydrological model to study the flood processes over a large range of spatial scales, from small catchments (1 km²) to large regional catchments (> 1000 km²). The model used in this thesis is built within the LIQUID hydrological modeling platform, which allows a modular coupling of the chosen hydrological processes. The model is used without calibration, with the purpose to test different hypotheses on the hydrological functioning of catchments. The studied area is the Cevennes-Vivarais region (south-east of France). The first simulations show a high sensitivity of the model results to soil properties (hydraulic conductivity, thickness), and to the bottom flux boundary condition (deep percolation). A different behavior is observed between catchments located on sedimentary rocks and catchments located in the mountain area, on metamorphic schists. A version of the model which accounts for lateral surface and sub-surface flows is developed, and tested on the Cartaou (0.5 km²) experimental catchment. Preliminary results highlight the importance of lateral flow processes in flood generation at small spatial scales. A streamflow recession analysis is performed to estimate hydraulic and thickness properties of weathered rock horizons, which are not described by regional soil databases. The results show a hierarchy in the estimated parameters, in relation with geology. The weathered rock horizons are implemented in the hydrological model, which is used at the regional scale. Simulations performed over the 2008 year bring out the better results obtained when using the weathered rock layer, for flood events simulations as well as for long-term simulations. The results also show differences between the hydrological behavior of north catchments (Ardèche, Tarn) and south catchments (Cèze, Gardon, Vidourle), which can be linked to the geology

Dans les régions méditerranéennes françaises, des épisodes pluvieux diluviens se produisent régulièrement et provoquent des crues très rapides et volumineuses que l'on appelle crues éclair. Elles font fréquemment de nombreuses victimes et peuvent, sur un seul évènement, coûter plus d'un milliard d'euros. Face à cette problématique, les pouvoirs publics mettent en place des parades parmi lesquelles la prévision hydrologique tient une place essentielle.C'est dans ce contexte que le projet BVNE (Bassin Versant Numérique Expérimental) a été initié par le SCHAPI (Service Central d'Hydrométéorologie et d'Appui à la Prévision des Inondations) dans le but d'améliorer la prévision des crues rapides. Ces travaux s'inscrivent dans le cadre de ce projet et ont trois objectifs principaux : réaliser des prévisions sur des bassins capables de ces réactions qu'ils soient correctement jaugés, mal jaugés ou non jaugés.La zone d'étude choisie, le massif des Cévennes, concentre la majorité de ces épisodes hydrométéorologiques intenses en France. Ce mémoire la présente en détails, mettant en avant ses caractéristiques les plus influentes sur l'hydrologie de surface. Au regard de la complexité de la relation entre pluie et débit dans les bassins concernés et de la difficulté éprouvée par les modèles à base physique à fournir des informations précises en mode prédictif sans prévision de pluie, l'utilisation de l'apprentissage statistique par réseaux de neurones s'est imposée dans la recherche d'une solution opérationnelle.C'est ainsi que des modèles à réseaux de neurones ont été synthétisés et appliqués à un bassin de la zone cévenole, dans des contextes bien et mal jaugés. Les bons résultats obtenus ont été le point de départ de la généralisation à 15 bassins de la zone d'étude. A cette fin, une méthode de généralisation est développée à partir du modèle élaboré sur le bassin jaugé et de corrections estimées en fonction des caractéristiques physiques des bassins. Les résultats de l'application de cette méthode sont de bonne qualité et ouvrent la porte à de nombreux axes de recherche pour l'avenir, tout en démontrant encore que l'utilisation de l'apprentissage statistique pour l'hydrologie peut constituer une solution pertinente
In the French Mediterranean regions, heavy rainfall episodes regularly occur and induce very rapid and voluminous floods called flash floods. They frequently cause fatalities and can cost more than one billion euros during only one event. In order to cope with this issue, the public authorities' implemented countermeasures in which hydrological forecasting plays an essential role.In this contexte, the French Flood Forecasting Service (called SCHAPI for Service Central d'Hydrométéorologie et d'Appui à la Prévision des Inondations) initiated the BVNE (Digital Experimental Basin, for Bassin Versant Numérique Expérimental) project in order to enhance flash flood forecasts. The present work is a part of this project and aim at three main purposes: providing flash flood forecasts on well-gauged basins, poorly gauged basins and ungauged basins.The study area chosen, the Cévennes range, concentrates the major part of these intense hydrometeorological events in France. This dissertation presents it precisely, highlighting its most hydrological-influent characteristics.With regard to the complexity of the rainfall-discharge relation in the focused basins and the difficulty experienced by the physically based models to provide precise information in forecast mode without rainfall forecasts, the use of neural networks statistical learning imposed itself in the research of operational solutions.Thus, the neural networks models were designed and applied to a basin of the Cévennes range, in the well-gauged and poorly gauged contexts. The good results obtained have been the start point of a generalization to 15 basins of the study area.For this purpose, a generalization method was developed from the model created on the gauged basin and from corrections estimated as a function of basin characteristics.The results of this method application are of good quality and open the door to numerous pats of inquiry for the future, while demonstrating again that the use of statistical learning for hydrology can be a relevant solution

Ce travail de thèse a pour ambition de comprendre les mécanismes et les processus qui régissent l’action en situation decrise, c’est-à-dire dans des conditions de gestion fortement marquées par l’urgence et la nécessité d’agir vite en situationd’incertitude. Le constat d’une vulnérabilité du réseau routier face aux crues rapides nous conduit à la question suivante :quelles sont les modalités de gestion de crise lors d’événements pluvieux violents ayant pour conséquences des fermetures, des coupures ou des submersions du réseau routier et quelles sont les conditions de l’efficacité de gestion ? Notre étude souhaite montrer comment les acteurs en charge de la gestion du réseau routier dans un département régulièrement impacté par les inondations « gèrent » ces périodes de turbulences, avec quelles ressources et avec quels outils (plans, logiciels, dispositifs techniques, fiches actions etc.). Ce que nous souhaitons montrer, ce n’est pas tant qu’il existe des décalages entre le travail prescrit et le travail réel des opérateurs en temps de crise, mais bien les modalités de ces déplacements. En ce sens, c’est la question de l’efficacité qui est au coeur de notre démarche. Au sens étymologique, l’efficacité est une « vertu », une « force » (du latin efficacitas), elle doit nous permettre dans ce travail de saisir les décalages entre se qui se rapporte d’un côté à la planification et à une gestion organisée des crises, et de l’autre ce qui se rattache à la métis, c’est-à-dire l’intelligence pratique des acteurs en charge de l’exécution des tâches planifiées. Plutôt que d’identifier des défaillances, nous souhaitons montrer ce qui semble efficace du point de vue des acteurs, ce qui est « porteur » pour l’action, ce qui fait sens (Jullien, 1996, 1992). Quels sont les mécanismes vertueux que les acteurs mettent en place afin d’assurer une certaine efficacité de gestion, et de « faire tenir » ce moment de crise caractérisé, d’après une partie de la littérature, par la perte de repères pour l’action ? Approcher les conditions de l’efficacité demande d’une part d’analyser les pratiques des acteurs (rapport à la confiance, à l’adaptation, à l’improvisation, à l’interprétation de l’environnement et des informations), d’autre part de connaître les plans d’action qui les prescrivent
This PhD dissertation aims at a better understanding of the mechanisms and processes which dictate the actions taken ina situation of crisis, that is to say, in a context very much characterized by urgency, and the necessity to act quickly inan unpredictable situation. After assessing the vulnerability of the road network in the event of a rapid rise in the waterlevel, we were led to raise the following questions : what are the various modes of crisis management in the event of violentrainstorms leading up to closings, interruptions or submersion of the road network ? What are the required conditions foran efficient management ? Our study intends to show the modus operandi of the actors in charge of the road networkmanagement in a district subject to frequent flooding to « manage » those times of trouble and the means and tools given tothem (maps, softwares, technical devices etc). What we originally aimed at was not so much emphasizing the gap betweenthe stipulated work and the effective work of operators in a time of crisis but rather focusing on the modalities of these« moves ». In a sense, matters of efficiency are at the heart of our approach. In its etymological meaning, efficiency is avirtue, a strength (from latin efficacitas) and it should enable us in this work to grasp the discrepancies between whatis linked on one side to the planning and organized managing of crises and on the other to metis, that is, the practicalintelligence of the actors in charge of the execution of the devised tasks. Instead of identifying the deficiency, we intend tohighlight what seems efficient from the actors’ point of view, what is « bearing » to the action, what makes sense (Jullien,1996, 1992). What are the virtuous mechanisms that the actors can devise in order to secure some kind of managementefficiency and to "hold together" this moment of crisis characterized, according to some writings, by the loss of a sense ofdirection in action ? This approach of the conditions of efficiency entails on the one hand the analysis of the actors’ practices(trust, adaptation, improvisation, interpretation of the surroundings and of information), and on the other, the knowledgeand understanding of the plans of action that prescribe them

D’un point de vue climatique la région méditerranéenne est propice aux évènements pluvio-orageux intenses, particulièrement en automne. Ces pluies s’abattent sur des bassins versants escarpés. La promptitude des crues ne laisse qu’un temps très court pour la prévision. L’amplitude de ces crues dépend de la grande variabilité des pluies et des caractéristiques des bassins versants. Les réseaux d'observations ne sont habituellement pas adaptés à ces petites échelles spatiales et l'intensité des événements affecte souvent la fiabilité des données quand elles existent d’où l’existence de bassin non jaugés. La régionalisation en hydrologie s’attache à la détermination de variables hydrologiques aux endroits où ces données manquent. L’objectif de cette thèse est de contribuer à poser les bases d’une méthodologie adaptée à la transposition des paramètres d'un modèle hydrologique distribué dédié aux crues rapides de bassins versants bien instrumentés à des bassins versants non jaugés, et ce sur une large zone d’étude. L’outil utilisé est le modèle hydrologique distribué MARINE [Roux et al., 2011] dont l’une des originalités est de disposer d’un modèle adjoint permettant de mener à bien des calibrations et des analyses de sensibilité spatio-temporelles qui servent à améliorer la compréhension des mécanismes de crue et à l’assimilation de données en temps réel pour la prévision. L’étude des sensibilités du modèle MARINE aborde la compréhension des processus physiques. Une large gamme de comportements hydrologiques est explorée. On met en avant quelques types de comportements des bassins versants pour la région d’étude [Garambois et al., 2012a]. Une sélection des évènements de calibration et une technique de calibration multi évènements aident à l’extraction d’un jeu de paramètres par bassin versant. Ces paramétrisations sont testées sur des évènements de validation. Une méthode de décomposition de la variance des résultats conduit aux sensibilités temporelles du modèle à ses paramètres. Cela permet de mieux appréhender la dynamique des processus physiques rapides en jeu lors de ces crues [Garambois et al., 2012c]. Les paramétrisations retenues sont transférées à l’aide de similarités hydrologiques sur des bassins versants non jaugés, à des fins de prévision opérationnelle
Climate and orography in the Mediterranean region tend to promote intense rainfalls, particularly in autumn. Storms often hit steep catchments. Flood quickness only let a very short time lapse for forecasts. Peak flow intensity depends on the great variability of rainfalls and catchment characteristics. As a matter of facts, observation networks are not adapted to these small space-time scales and event severity often affects data fiability when they exist thus the notion of ungauged catchment emerges. Regionalization in hydrology seeks to determine hydrological variables at locations where these data lack. This work contributes to pose the bases of a methodology adapted to transpose parameterizations of a flash flood dedicated distributed hydrologic model from gauged catchments to ungauged ones, and for a large study area. The MARINE distributed hydrologic model is used [Roux et al., 2011], its originality lies in the automatically differentiated adjoint model able to perform calibrations and spatial-temporal sensitivity analysis, in order to improve understanding in flash flood generating mechanisms and real time data assimilation for hydrometeorological forecasts. MARINE sensitivity analysis addresses the question of physical process understanding. A large panel of hydrologic behaviours is explored. General catchment behaviours are highlighted for the study area [Garambois et al., 2012a]. Selected flood events and a multiple events calibration technique help to extract catchment parameter sets. Those parameterizations are tested on validation events. A variance decomposition method leads to parameter temporal sensitivity analysis. It enables better understanding in catching dynamics of physical processes involved in flash floods formation [Garambois et al., 2012c]. Parameterizations are then transfered from gauged catchments with hydrologic similarity to ungauged ones with a view to develop real time flood forecasting

Les régions méditerranéennes sont particulièrement soumises au risque de crues rapides. Ces crues sont provoquées par des pluies intenses pouvant être stationnaires et qui affectent des bassins versants à la topographie marquée et aux temps de réponses courts. Ces crues peuvent être très violentes, dévastatrices voire meurtrières. Il est donc important de disposer de systèmes de prévision adaptés pour l'anticipation des pluies et des débits des cours d'eau méditerranéens. Le système couplé ISBA-TOP a été particulièrement conçu pour simuler la réponse hydrologique de bassins versants à dynamique rapide. Il s'agit d'un couplage entre le modèle de surface ISBA et une version du modèle hydrologique TOPMODEL adaptée aux crues rapides. Une première étape du travail de thèse a été d'évaluer l'apport d'une meilleure représentation des processus hydrologiques dans le sol pour la simulation des débits avec ISBA-TOP. Une version d'ISBA-TOP basée sur la version diffusive d'ISBA (ISBA-DF qui découpe verticalement les colonnes de sol en de multiples couches) a donc été comparée à la version originale d'ISBA-TOP qui utilisait l'approche ISBA-3L où 3 couches de sol sont représentées. Sur un échantillon d'une vingtaine de cas d'étude, on montre une légère amélioration des simulations de débits avec la nouvelle version d'ISBA-TOP. Cette nouvelle version permet de s'affranchir du calage de deux paramètres gérant la répartition verticale de l'eau dans les colonnes de sol qu'il était nécessaire de réaliser pour chacun des bassins étudiés. La simulation de petits bassins versants non jaugés, pour lesquels on ne dispose pas d'observations pour réaliser le calage des paramètres, peut donc être envisagée. Il n'en reste pas moins que la modélisation hydrologique avec ISBA-TOP (comme avec tout autre modèle) reste entachée d'incertitude. La seconde partie de cette thèse vise à identifier ces incertitudes et à les quantifier. Dans le cas des crues rapides, les données de précipitation utilisées en entrée du modèle hydrologique constituent la source majeure d'incertitude mais elle n'est pas la seule. La connaissance des conditions initiales d'humidité des sols est également incertaine et le modèle hydrologique, lui-même, est entaché d'incertitude de par la `` paramétrisation '' des processus physiques et de par sa structure. Afin d'explorer ces deux sources d'incertitude et de les hiérarchiser, une étude de sensibilité du modèle ISBA-TOP à ses paramètres et à ses conditions initiales d'humidité des sols a été menée. Cette étude de sensibilité a d'abord été conduite pour un cas idéalisé, d'interprétation plus facile, avant de confirmer et d'approfondir les résultats sur des cas réels. Les simulations de débit avec ISBA-TOP se sont avérées particulièrement sensibles à trois des paramètres hydrodynamiques : le contenu en eau à saturation, la conductivité hydraulique à saturation et le taux de décroissance de la transmissivité avec la profondeur. Cette sensibilité, toutefois, est très dépendante des conditions initiales d'humidité des sols. Sur la base des résultats de l'étude de sensibilité, une méthode de perturbation des paramètres du modèle hydrologique a été conçue afin de tenir compte des erreurs de modélisation. Elle est complétée par une méthode de perturbations des conditions initiales en humidité des sols afin de constituer un système de simulation hydrologique d'ensemble. Evalué sur six cas réels, il a été montré que cet ensemble a un apport pour la simulation des débits par rapport à la version déterministe du modèle
Intense precipitation events in the Mediterranean often lead to devastating flash floods. The affected watersheds are characterized by steep slopes and a short response time. Flash floods can be violent and destructive. Dedicated meteorological and hydrological systems are thus necessary to anticipate those flash floods. The ISBA -TOP coupled system was developed to simulate the hydrological reaction of fast responding rivers. It is a coupling between the ISBA surface scheme and a version of the TOPMODEL hydrological model dedicated to mediterranean flash floods simulation. A first part of the thesis aims at assessing the benefit of a better representation of hydrological processes within the soil for discharge simulation with ISBA-TOP. Moreover, it would allow the use of ISBA-TOP for any watershed even ungauged. A version of ISBA-TOP based on ISBA-DF (the diffusive version of ISBA that discretizes soil columns in multiple layers) has been compared to the original ISBA-TOP that relies on ISBA-3L (with only 3 soil layers). On 18 study cases, better discharge simulations are obtained basically using the new ISBA-TOP version. So this improved representation of hydrological processes in the soil allows to improve discharge simulations and to be confident into the results obtained for small ungauged catchments. The second part of this PhD work concerns the uncertainty that affects ISBA-TOP simulations. For flash floods, rainfall data used to drive an hydrological model are the major source of uncertainty. But initial soil moisture knowledge is also uncertain and the hydrological model himself is affected by uncertainty. The sensitivity of ISBA-TOP model to its parameters and initial soil moisture is investigated to document those two sources of uncertainty. First an academic case is used to verify some preliminary hypotheses and then real cases are studied to properly consider the different data heterogeneities. Discharge simulations with ISBA-TOP are sensitive to three hydro dynamical parameters : the saturated soil water content, the saturated hydraulic conductivity and the rate of decrease of the transmissivity with depth. This sensitivity is found very dependant on initial soil moisture conditions. Perturbation methods varying the 3 parameters that have the highest impact on discharge simulations as well as initial soil moisture allow to design an ensemble prediction system. This ensemble has been assessed for 6 real cases. Using this ensemble-based approach for discharge simulation lead to better results than using the deterministic version. The skill of the ensemble with 30 to 50 members is close to the skill of this ensemble with 100 members whereas it outerperfoms an ensemble with 10 members. The last part of the thesis is the conception of a complete hydrometeorological ensemble prediction system (HEPS). The hydrological ensemble defined in the second part of the thesis is used to take the uncertainty that affects the hydrological modelling and initials soil moisture into account. The uncertainty that affects precipitation fields is sampled using the AROME ensemble prediction system (AROME-EPS). The skill of this complete chain is better than an HEPS based on AROME-EPS and the deterministic version of ISBA-TOP. But both HEPS exhibit a low bias for discharge simulation on the study cases sample. This bias can come from a low bias that is found for the AROME-EPS rainfall forecasts. A simple bias correction applied on rainfall forecasts improves the bias on discharge forecasts

In our region, heavy rains may occur virtually everywhere. Nowadays there are instruments to predict these events in sufficient advance, but without precise localisation, which is a problem. Present instruments for searching endangered watersheds are focused on operative evaluation of meteorological situation and actual precipitation forecast processing (nowcasting). The thesis brings quite different approach. Potentially endangered areas are detected with evaluation of long-term statistical variables (N-year discharges and rain characteristics) and properties of specific watershed. The whole issue is handled out of situation of actual danger, this attitude is so called off-line solution. The thesis describes a model based on selected artificial intelligence methods. The model forms the core of final map application. The use of model and final application is supposed to be used in area of preventive flood protection, and related investment decision-making. The model focuses on heavy rains and flash floods.

A major area targeted for hydrometeorological forecast service improvements is in flash flood forecasting. Verification data show that general public service products of flash flood forecasts do not provide enough lead time in order for the public to make effective response. Sophisticated users of flash flood forecasts could use forecast probabilities of flash flooding in order to make decisions in preparation for the predicted event. To this end, a systematic probabilistic approach to flash flood forecasting is presented. The work first describes a deterministic system which serves as a conceptual basis for the probability system. The approach uses accumulated rainfall plus potential rainfall over a specified area and time period, and assesses this amount against the water holding capacity of the affected basin. These parameters are modeled as random variables in the probabilistic approach. The effects of uncertain measurements of rainfall and forecasts of precipitation from multiple information sources within a time period and moving forward in time are resolved through the use of Bayes' Theorem. The effect of uncertain inflows and outflows of atmospheric moisture on the states of the system, the transformation of variables, is resolved by use of convolution. Requirements for probability distributions to satisfy Bayes' Theorem are discussed in terms of the types and physical basis of meteorological data needed. The feasibility of obtaining the data is evaluated. Two alternatives for calculating the soil moisture deficit are presented--one, an online automatic rainfall/runoff model, the other an approximation. Using the soil moisture approximation, a software program was developed to test the probabilistic approach. A storm event was simulated and compared against an actual flash flood event. Results of the simulation improved forecast lead time by 3-5 hours over the actual forecasts issued at the time of the event.

En Europe, les crues éclair affectent principalement les régions méditerranéennes et montagneuses. Les bassins concernés sont le plus souvent de petite taille. Anticiper ce type d'événements demeure un exercice ardu parsemé de multiples difficultés. Cette thèse s'inscrit dans le cadre d'un projet commun à IRSTEA et Météo-France (projet RHYTMME) et ses deux objectifs sont :l'adaptation de modèle hydrologique de la méthode AIGA aux régions montagneuses, en tenant compte des caractéristiques de cet environnement ;l'évaluation du nouveau modèle sur des bassins de petite taille, qui sont réellement non-jaugés.Pour atteindre le premier objectif, le modèle est complexifié (prise en compte de la neige), testé sur 118 bassins versants jaugés et régionalisé. Quant au second but, des relevés de dégâts, issus de la base de données des services de Restauration des Terrains en Montagne (RTM), sont utilisés afin d'évaluer le modèle sur 123 très petits bassins dans des conditions aussi proches que possible de la réalité du terrain.Pour réaliser ce second objectif, une méthode d'évaluation est développée, fondée sur des statistiques de contingence, illustrant la concomitance entre dégâts observés et dépassement de seuil des débits simulés. Il est proposé un graphe renseignant le taux de détection (POD) et le taux de succès (SR) pour différents seuils simultanément à l'instar des courbes de ROC. Le principal résultat est qu'une évaluation sur bassins jaugés est une première étape nécessaire mais pas suffisante. En effet, les travaux illustrent le fait que les meilleurs modèles sur les bassins jaugés ne le sont pas forcément sur les petits bassins non-jaugés, sujets aux crues éclair.

A flash flood is a flood that follows the causative storm event in a short period of time. The term “flash” reflects a rapid response, with water levels in the drainage network reaching a crest within minutes to a few hours after the onset of the rain event, leaving extremely short time for warning [Creutin and Borga, 2003; Borga et al., 2008]. Flash floods are localized phenomena that occur in watersheds of few hundred kilometres or less, with response times of a few hours or less [Creutin and Borga, 2003; O’Connor and Costa, 2004]. Such basins respond rapidly to intense rainfall because of steep slopes and impermeable surfaces, saturated soils, or because of human (i.e., urbanization) or fire-induced alterations to the natural drainage. Causative events are generally excessive storms, but can also be the sudden release of water impounded by a natural jam (i.e., formed by ice or rock, mud, and wood debris) or human-made dam or levee. This thesis focuses on flash flood events associated with heavy rainfall. Europe experienced several catastrophic flash floods in the last decades. Data concerning a number of these floods occurred during the last 15 years have been reported in Marchi et al. (2010). Examination of these data and references therein shows that: Flash floods occur in any of the hydroclimatic regions of Europe, even though three regions appear to be characterized by high flash flood potential: Mediterranean, Alpine Mediterranean, and Inland Continental Europe; Heavy rainfall accumulation is a necessary but not sufficient condition for flash floods, since hydrology critically controls flash-flood-triggering. Without hydrological analysis, it is impossible to evaluate the flood potential of storms, particularly in the fringe of the flood/no flood threshold; Flash flood hazard is related to both stream response (flood) and landscape response (landslide and erosion). The intense erosion and solid transport associated with these extreme events add to the hazard and strongly influence the quality of soils, waters and ecosystems. The twofold consequence of the above observations is that forecasting of flash-floods: Depends critically on meso-scale storm forecasting, with a specific attention to the processes leading to slow movement of the precipitation system; Necessitates real time hydrological modelling, with a specific attention to the runoff generation processes over a wide range of scales. Although they are seldom all deployed at the same time, the technical requirements for a hydrometeorological flash flood forecasting system include: A numerical weather prediction (NWP) model, capable to provide short-range Quantitative Precipitation Forecasts (QPF); A remote sensing based (radar, satellites) precipitation detection system, for storm monitoring and for the possible initialization and conditioning of the NWP model, and A hydrological-hydraulic forecasting model, capable to forecast the stream response from the rain input. These requirements are similar to those of more common riverine flood forecasting systems. However, some features characterise flash flood forecasting with respect to riverine flood forecasting and point out to their larger uncertainty. These are: The short lead time, which implies both the integration of meteorological and hydrologic forecast, and the difficulties of using data assimilation procedures based on real time observed discharges to reduce uncertainty in hydrologic predictions; The need to provide local forecasts, which means that, on one hand, the rainfall must be monitored and forecasted on a wide range of space/time scales, and, on the other hand, every tributary of a monitored basin can be considered as a potential target for flood warning. Estimation of extreme rainfall rates by weather radar at the appropriate time and space scales is the cornerstone of flash flood analysis and forecasting. A large body of research work has greatly improved in the last two decades radar technology and algorithms for rain quantification. This work has shown that well maintained conventional radar systems can estimate rainfall at ground level provided that a number of precautions are taken, and in particular: The siting of the instrument and its scanning protocol must be carefully selected and analysed; The quality of the instrument must be routinely checked; The signal processing must take into account the physics of the instrument as well as the properties of the atmospheric and ground targets. A downstream control of the radar rainfall processing can rely on rain-gauge measurements at ground level using a variety of methods. When these precautions are taken, different studies have shown that radar-based rainfall estimates are reliable and may be used as input in rainfall-runoff models for flood modelling and forecasting [Borga et al., 2000; Delrieu et al., 2005; Borga et al., 2002]. These very positive results must not be hiding some weaknesses: Most of these results never had the opportunity to be coherently validated over a significant number of flash floods events. The use of specific experiments or of limited operational radar data sets is insufficient to test complex combinations of algorithms, especially if high rain intensities are of interest. Very few results have been translated into operational hydrologic applications. This thesis aims to investigate the use of weather radar for the purpose of understanding the hydrometeorological mechanisms leading to flash floods, and then for flash flood forecasting. The outline of the thesis work is as follows. Chapter 1 provides a literature review of the rainfall estimation by weather radar for flash flood-generating storms. Chapter 2 describes a number of procedures for the rainfall estimation at the ground during flash flood events in mountainous catchments. A metric for the analysis of the rainfall field spatial patterns is proposed in Chapter 3, in the context of the analysis of a number of Romenian flash floods. This metric is used for the analysis of two flash flood events, respectively occurred in 2003 in the Eastern Italian Alps (Chapter 4) and in Western Slovenia (Chapter 5). Major conclusions from the work are reported in Chapter 6.
Una piena improvvisa è una piena che segue l’evento precipitativo che la ha causata entro un breve periodo di tempo. Il termine “improvvisa o flash” riflette una risposta rapida, con il picco di piena che si verifica nella rete di drenaggio nel volgere di alcuni minuti fino a poche ore dopo l’inizio dell’evento di pioggia. Questo fatto lascia intendere quanto poco tempo ci sia per l’allerta [Creutin and Borga, 2003; Borga et al., 2008]. Questo tipo di bacini rispondo rapidamente ad una precipitazione intensa a causa di pendii ripidi e superfici impermeabili, terreni saturi, o a per fattori determinati dall’uomo (vedi per esempio l’urbanizzazione) o a causa di alterazioni del drenaggio naturale del terreno dovuto ad incendi. Gli eventi scatenanti le piene improvvise sono generalmente precipitazioni che portano all’eccesso di drenaggio, ma questo tipo di piene possono anche essere scatenate dal rilascio improvviso di acqua trattenuta da impedimenti naturali (per esempio formati da ghiaccio e roccia, fango e detriti di legno) o di tipo artificiale come dighe e argini. Questa tesi si concentra su eventi di piena improvvisa associati a precipitazioni intense. L’Europa ha conosciuto diverse inondazioni catastrofiche negli ultimi decenni. I dati relativi un certo numero di queste inondazioni che si sono verificate nel corso degli ultimi 15 anni sono riportati da Marchi et al. (2010). Dall’analisi di questi dati e di queste fonti risulta che: Una piena improvvisa si può verificare in qualsivoglia regione idroclimatica dell’Europa, anche se tre regioni sembrano essere caratterizzate da una grande incidenza di di piene improvvise: l’area Mediterranea, quella Alpino-Mediterranea, e quella Continentale; Una gran quantità di pioggia accumulata è una condizione necessaria ma non sufficiente al verificarsi di una piena improvvisa, dal momento che l’idrologia controlla in modo decisivo l’innesco della piena improvvisa. Senza un’analisi di tipo idrologico, risulta impossibile valutare la probabilità che una data precipitazione scateni una piena, in praticolare in termini di una soglia oltre la quale si verifica la piena; La pericolosià delle piene improvvise è collegata sia alla risposta del fiume (la piena) che alla risposta del terreno (fenomeni di tipo franoso ed erosivo). L’intensa erosione ed il trasporto solido associati a questi fenomeni estremi si aggiungono alla pericolosità ed influenzano in modo significativo la qualità dei terreni, delle acque e degli ecosistemi. La duplice conseguenza delle osservazioni appena fatte è che la previsione di piene improvvise: Dipende in modo determinante dalle previsioni delle precipitazioni che si sviluppano alla meso-scala, con una attenzione specifica ai processi che frenano la circolazione del sistema di precipitazione; Richiedone modelli idrologici che lavorino in tempo reale, con una particolare attenzione ai processi du generazione del deflusso a vasta scala. Anche se raramente sono tutti utilizzati contemporaneamente, i requisiti tecnici per un sistema di previsione idrometeorologica per le piene improvvise comprendono: Un modello numerico di previsione (NWP2), in grado di fornire previsioni quantitative di pioggia a corto raggio (QPF3); Un sistema di rilevamento in remoto per la pioggia (radar, satellite), per il monitoriraggio dei fenomeni temporaleschi e la possibilie inizializzazione e condizionamento del modello NWP, e Un modello di previsione idrologico-idraulico, in grado di prevedere la risposta del corso d’acqua all’input pioggia. Tali requisiti sono simili a quelli più comuni utilizzati per la previsione delle alluvioni dei sistemi fluviali. Tuttavia, alcuni elementi caratterizzano la previsione delle piene improvvise rispetto alla previsione delle alluvioni e ne sottolineano la grande incertezza. Questi sono: Il breve periodo durante il quale questi processi si sviluppano, che implica sia l’integrazione di un sistema di previsione di tipo meteorologico e idrologico, che la difficoltà nell’utilizzo di procedure di assimilazione di dati basate sull’osservazione in tempo reale delle portate al fine di ridurre l’incertezza nelle previsioni idrologiche; La necessità di fornire previsioni a scala locale, il che significa da una parte che la pioggia deve essere monitorata e prevista su una vasta scala spazio-temporale, all’altra che ciascun tributario del bacino monitorato può essere considerato come un bersaglio potenziale per un allarme di piena. La stima di fenomeni precipitativi estremi tramite l’utilizzo del radar meteorologico alla appropriata scala spazio-temporale è una pietra miliare dell’analisi e della previsione delle piene improvvise. Una grande branca della ricerca in questo campo ha favorito un notevolmente migliorato, negli ultimi due decenni, delle tecnologie radar e degli algoritmi per la stima di pioggia. Questo lavoro ha dimostrato che anche utilizzando sistemi radar convenzionali si possono ottenere stime di precipitaziona a livello del suolo, a condizione che vengono adottate una serie di precauzioni, in particolare: L’ubicazione dello strumento e del suo protocollo di scansione devono essere attentamente selezionati ed analizzati; La qualità dello strumento deve essere sottoposta a controlli ordinari; L’elaborazione del segnale deve tener conto della fisica dello strumento così come delle proprietà atmosferiche e dei bersagli di terra. Un controllo a valle del trattamento delle precipitazioni radar può essere fatto tramite misurazioni da pluviometro a livello del suolo utilizzando una varietà di metodi. Quando si sono prese queste precauzioni, diversi studi hanno dimostrato che le stime di precipitazione basate su radar meteorologico sono affidabili e possono essere utilizzate come input di modelli afflussodeflusso per la modellazione e la previsione delle piene [Borga et al., 2000; Delrieu et al., 2005; Borga et al., 2002]. A fronte di questi risultati molto positivi non devono però essere nascosti alcuni punti deboli: La maggior parte di questi risultati non hanno mai la possibilità di essere coerentemente convalidati su un numero significativo di eventi di piena improvvisa. L’utilizzo di esperimenti specifici o di una banca dati limitata di dati radar è insufficiente a testare la combinazione complessa degli algoritmi utilizzati, specialmente se si è interessati ad intensità di pioggia elevata. Un numero molto limitato di risultati positivi è stato tradotto in applicazioni idrologiche operative. Questa tesi si propone di esaminare l’uso del radar meteorologico ai fini della comprensione dei meccanismi idrometeorologici che portano alla formazione di piene improvvise, e quindi alla loro previsione. L’organizzazione del lavoro di tesi è la seguente. Il Capitolo 1 fornisce una revisione della letteratura sul tema della stima di precipitazione tramite radar meteorologico per le precipitazioni che causano la formazione di piene improvvise. Il Capitolo 2 descrive una serie di procedure per la stima delle precipitazioni al suolo durante gli eventi di piena improvvisa in bacini montani. Una metrica per l’analisi spaziale del campo di pioggia viene proposta nel Capitolo 3, nel contesto dell’analisi di una serie di piene improvvise verificatesi in Romania. Questa metrica è utilizzata per l’analisi di due eventi di piena, accaduti rispettivamente nel 2003 nelle Alpi Italiane friulane e nella parte ovest della Slovenia (Capitolo 5). Le conclusioni principali del lavoro di tesi sono riportate nel Capitolo 6.

ABSTRACT: “Flash flood analysis and modelling in mountain regions” Flash flood are rare and localized phenomena, triggered by meteorological event with a pronounced spatial variability, with a precipitation gradient, at event scale, up to 20-50 mm/km. The consequences of these features is that the scientific and operational communities working on flash flood analysis have to deal with an impressive lack of data. Even a dense raingauge network may not be able to represent spatial variability of rainfall patterns associated with convective storms that trigger flash floods. Radar rainfall estimations, when correctly elaborated, are able to represent spatial patterns, but quantitative precipitation volume estimations need to be validated. In addition, concerning discharge data, the majority of the upstream and larger catchments affected by flash floods are not gauged and stream gauges, where present, are often damaged, so that peak discharge distribution along main and secondary river network is even less known than precipitation fields. This study aims at covering the gap between needed and available data for flash flood event analysis, combining different methodologies. An Intense Post Event Campaign (IPEC) may be very useful to collect peak discharge estimations and time sequence of the flood in ungauged sections. Simplified hydrological model, based on rough runoff excess computation and set velocity propagation, can be used to cross validate quantitative distributed precipitation data from weather radar and peak discharge estimations collected during an IPEC. More complex and detailed model may help to improve the knowledge about flash flood associated phenomena, like debris flow. Another objective of this thesis is to investigate the role of rainfall spatial variability in flash flood triggering. First a standard procedure to describe the variability catchment scale is needed. It will be so possible to study the relationship between rainfall input distribution and flood propagation dynamics. Then a simplified hydrological model is used to investigate the role of spatial variability in precipitation patterns: systematic studies are carried to describe the accuracy of rainfall volumes at basin scale and the effect of spatial variability within the basin. Often the studies about flash flood dynamics are slow down or stopped because no measured data are directly to hand, or, if so, because they are not considered sufficiently accurate. This work shows the possibility to combine together data with an assured degree of uncertainty, the only available or collectable existing data, and processed them with simple statistical and hydrological tools to obtain a more precise knowledge about past flash floods. The remainder of this dissertation is organised as follows. Chapter 1 “Introduction”. The work starts with the aim to define what a “flash flood” is, underlying the importance to characterize such event according spatial and temporal proprieties. From this definition it follows that a generic flood can be classified according its own spatial and temporal proprieties and located in a specific point of a segment delimitated by the two ideal cases of “flash flood” and “flood at large scale”. Chapter 2 “Literature review”. Spatial and temporal characterization lead to describe typical features of flash flood in different climates. Meteorological conditions able to trigger this kind of events are described and analyzed, with particular care about convective cells system organized in mesoscale structures. Finally some literature examples are reported to show different possible approach and to underline usual uncertainty when dealing with flash flood. Chapter 3 “Materials an methods”. This chapter summarizes and describes the tools used in this thesis to carry on flash flood analysis. 3.1 Weather radar data are used to describe rainfall spatial distribution and obtain quantitative estimations of rainfall patterns. Data acquiring and processing are described and most common errors are summarized along with most common procedures and algorithms to avoid and correct them. It is finally shown how merging radar and conventional raingauge network information can provide a more exhaustive description of rainfall fields, with quantitative estimation. This data processing is very useful for further characterization and analysis of past flash flood events. 3.2 Post event surveys are presented as an essential tool to collect the richest possible documentation. Measure campaigns are valorised to obtain qualitative and quantitative description of past floods. The goal is to complete the spatial and temporal precipitation knowledge and dynamic description, focusing on discharge estimation along hydrological network in term of peak values and timing. 3.3 Hydrological models can be routed for a better comprehension of flood dynamics at event scale. Two hydrological models, then used for flash flood analysis, are described in detail. The first one is applied at large event scale and starts from a distributed precipitation input. Hortonian runoff generation is applied punctually and superficial flood propagation is computed basing on fixed hillslope and channel velocity. The second model is built to be applied at very small catchment scale and simulate infiltration and transport processes for surface and subsurface flow through uniform hypothesis equations. Chapter 4 “Analysis of past flash flood events”. Some specific post flood analysis are collected in three section. 4.1 Five flash flood events occurred in Romania are analysed with HYDRATE European project contribution. This study shows that even if the conventional hydrometeorological data are poor, weather radar information and hydrological modelling can help in understanding specific past flood dynamics. 4.2 HYDRATE project was also involved in the analysis of a flash flood occurred in Slovenia in September 2007, including radar processing and post event surveys. It is shown how this approach, characterize by time and cost significant efforts, is a precious tool to collect data and information for a detailed description that would be not possible through traditional hydrometeorological network. 4.3 A detailed model is used to describe surface and subsurface flow dynamics during the debris flow occurred in two small subcatchments in Fella river valley (North Est of Italy), hit by a flash flood on August 29, 2003. The study mainly consists on liquid and solid mass balance during the different phases of the event. Chapter 5 “Spatial variability in flash flood events”. An analysis on rainfall spatial distribution is carried with the same tools on two different basin interested by flash flood event. The studies includes a fist detailed analysis on rainfall spatial variability within selected subcatchments at different scales: spatial variability is described through time distance calculated in base of hydrological network. Then a simplified hydrological model is used to investigate spatial aggregation effects on mean areal rainfall and peak discharge value at subcatchment scale. 5.3 For Fella river basin (in Friuli Venezia Giulia region), ten subcatchments from 10.5 and 623km² are choosen. 5.4 For Cervo River (Piomente region, North West Italy) the study is applied to three flood events characterized by different rainfall spatial variability, and focused on four subcatchments (from 75 to 983km²). Chapter 6 “Conclusions”. Are here reported and summarized the main observations coming from the specific studies describe in the two previous chapters as long as recommendation for future research.
RIASSUNTO: “Analisi e modellazione di piene improvvise in zone montane” Le piene improvvise sono fenomeni rari e localizzati, causati da eventi meteorologici caratterizzati da una spiccata variabilità spaziale, con gradienti di precipitazione che possono raggiungere, a scala di evento, i 20-50 mm/km. La conseguenza di ciò è che la comunità scientifica e gli enti operativi interessati nell’analisi dei fenomeni di piena si relazionano quotidianamente con una carenza di dati. Anche una fitta rete di pluviometri non è in grado di rappresentare la variabilità spaziale dei campi di precipitazione associati a fenomeni convettivi che innescano piene improvvise. Le stime di precipitazione ottenute attraverso il radar meteorologico, opportunatamente elaborate, sono in grado di rappresentare i pattern spaziali, ma i valori di volumi di pioggia necessitano di essere validati. Inoltre, per quanto riguarda i dati di portata, la maggior parte dei bacini colpiti da piene improvvise non sono strumentati e gli strumenti, dove presenti, risultano spesso danneggiati, cosicché la conoscenza della distribuzione delle portate al picco, lungo la rete idrologica principale e secondaria, è persino più approssimativa di quella della distribuzione spaziale della precipitazione. Questo studio si prefigge di colmare la distanza tra i dati disponibili e quelli richiesti per un’analisi a scala di evento con riferimento a fenomeni di piena improvvisa. Un’approfondita campagna di rilievi post evento (in inglese Intense Post Event Campaign, IPEC) può risultare estremamente utile per raccogliere le stime di portate al picco e la sequenza cronologica dello svilupparsi della piena in sezioni non monitorate. Modelli idrologici semplificati, dotati di metodi elementari per la separazione dei deflussi e predeterminate velocità di propagazione, possono essere utilizzati per una validazione incrociata tra una descrizione quantitativa della distribuzione di precipitazione ottenuta attraverso il radar meteorologico e le stime di portate al picco raccolte durante un IPEC. Modelli più complessi e dettagliati possono migliorare il livello di conoscenza riguardo fenomeni associati alle piene improvvise, come le colate detritiche. Un altro obiettivo di questa tesi è quello di investigare il ruolo della variabilità spaziale della precipitazione nei fenomeni di piena improvvisa. In primo luogo è necessario impostare una procedura che permetta di caratterizzare tale variabilità all’interno di un particolare bacino idrografico, mettendo in relazione la distribuzione degli apporti meteorici con le modalità di propagazione della piena. In secondo luogo si vuole indagare, attraverso l’applicazione di modelli idrologici semplificati, il ruolo della risoluzione spaziale della precipitazione. A questo fine è necessario separare due aspetti: l’accuratezza della stima dei volumi piovuti a scala di bacino e l’influenza della variabilità spaziale all’interno del bacino stesso. Spesso gli studi che si concentrano sulle dinamiche delle piene improvvise sono rallentati o resi impossibili per il fatto che nessun dato misurato risulta utilizzabile così come disponibile, oppure perchè i dati di partenza non sono ritenuti sufficientemente accurati. Questo lavoro si prefigge di mostrare come sia possibile, partendo dai soli dati esistenti, disponibili o recuperabili, caratterizzati da un certo grado di incertezza, passare attraverso un’elaborazione tramite semplici strumenti statistici e idrologici al fine di ottenere una conoscenza più precisa riguardo passati eventi di piena improvvisa. Si riporta una breve descrizione del contenuto dei capitoli della tesi, che sarà elaborata in lingua inglese. Capitolo 1 “Introduction”. Introduzione alla tematica che comprende una definizione del termine “piena improvvisa”, convenendo sulla necessità di caratterizzare tali eventi in termini di proprietà spazio-temporali. Si nota che, a partire da questa definizione, è possibile classificare una generica piena in un punto di un segmento ai cui estremi ci sono i casi ideali di “piena improvvisa” e “piena a larga scala”. Capitolo 2 “Literature review”. Partendo dalla caratterizzazione spazio temporale si descrivono le caratteristiche tipiche delle piene improvvise nei diversi tipi di clima, si individuano le condizioni meteorologiche in grado di innescare tali fenomeni, quali le celle convettive organizzate in strutture di mesoscala. Si riportano, infine, alcuni esempi di studi in letteratura che mostrano diverse tipologie di approcci e che sono indicativi dell’incertezza in cui si è soliti lavorare quando si approfondiscono questi temi. Capitolo 3 “Materials an methods”. In questo capitolo vengono presentati i principali strumenti comuni a tutte le analisi di fenomeni di piena improvvisa presentati in questa tesi. 3.1 L’utilizzo del radar meteorologico per studiare, dal punto di vista quantitativo, la distribuzione spaziale della precipitazione. Vengono approfondite la modalità di acquisizione del dato, sottolineando le possibili fonti di errore ed i metodi più comuni per ovviare a questi inconvenienti. Viene anche mostrato come l’utilizzo combinato di radar e tradizionali pluviometri renda più completa la caratterizzazione della precipitazione ai fini di un analisi di una piena improvvisa. 3.2 Le indagini post evento, necessarie per raccogliere la maggior documentazione possibile, sono valorizzate al fine di una ricostruzione, anche qualitativa, delle dinamiche caratteristiche di una specifica piena. Queste, attraverso diverse metodologie, devono aiutare a descrivere la struttura spazio temporale della precipitazione e la stima di portata, distribuita lungo la rete idrica, in termini di valore al picco e di tempistica 3.3 L’uso della modellistica idrologica applicata ad una miglior comprensione delle dinamiche a scala di evento. In particolare vengono descritti i due modelli idrologici utilizzati. Il primo, da applicare a larga scala, parte da un input di precipitazione spazialmente distribuito e, attraverso un meccanismo hortoniano di separazione dei deflussi applicato puntualmente, propaga la piena in base a fissate velocità di versante e di canale. Il secondo, da applicare a bacini di piccolissima dimensione, simula i processi di trasporto superficiale e sottosuperficiale integrando le note equazioni di moto uniforme. Capitolo 4 “Analysis of past flash flood events”. Vengono qui presentate alcune analisi di eventi, distinte in tre sezioni. 4.1 Analisi di cinque eventi di piena improvvisa avvenuti in Romania nell’ambito del progetto europeo HYDRATE. Da questo studio risulta che, pur in presenza di scarsi dati provenienti dalle tradizionali fonti di monitoraggio idro-meteorologico, l’informazione proveniente da radar meteorologico e la modellistica idrologica possono aiutare nella ricostruzione delle dinamiche dell’evento preso in considerazione. 4.2 Analisi di una piena improvvisa avvenuta in Slovenia nel settembre 2007 per la quale, attraverso il progetto HYDRATE si è condotta un indagine post evento. La ricchezza di questo approccio, pur dispendioso in termini di tempo, mostra un possibile percorso per recuperare le maggior informazioni possibili per eventi di piena che non sono ricostruibili solo attraverso le normali reti di monitoraggio idrometeorologico. 4.3 Analisi attraverso un modello dettagliato di deflusso superficiale e sottosuperficiale della colata detritica avvenuta in due piccoli sottobacini nella valle del fiume Fella, colpita da una piena improvvisa il 29 agosto 2003. Lo studio consiste essenzialmente nel bilancio di massa liquido e solido durante le diverse fasi dell’evento. Capitolo 5 “Spatial variability in flash flood events”. Questa analisi sulla distribuzione spaziale della precipitazione è stata condotta con le medesime metodologie in due diversi bacini. Gli studi comprendono un primo approfondimento della variabilità spaziale della precipitazione all’interno di sottobacini di diversa estensione: la variabilità è descritta in funzione del reticolo idrografico del bacino preso in considerazione. Successivamente, attraverso un modello idrologico semplificato, si è valutata l’influenza della variabilità spaziale della precipitazione analizzando gli effetti dell’aggregazione spaziale in termini di precipitazione media su bacino e di portata al picco simulata. 5.3 Per l’analisi nel bacino del fiume Fella (FVG), colpito da una piena improvvisa il 29 agosto 2003, si sono scelti dieci sottobacini di dimensione variabile tra i 10.5 e i 623km². 5.4 Nel caso del fiume Cervo (Piemonte) lo studio ha riguardato tre eventi di piena con diversa variabilità spaziale della precipitazione e si è concentrato su quattro sottobacini (tra i 75 e i 983km²). Capitolo 6 “Conclusions”. Vengono riassunte le principali osservazioni ricavate dalle analisi descritte nei due capitoli precedenti e indicazioni per possibili future linee di ricerca.

Les régions méditerranéennes sont régulièrement exposées aux événements fortement précipitants et aux crues rapides. En situation de crise, disposer d'une prévisionhydrométéorologique sur quelques heures d'échéance peut s'avérer crucial, notamment pour l'intervention des services de secours. La prévision des conséquences hydrologiques des épisodes méditerranéens de pluie intense aux échéances de la prévision immédiate, jusqu'à 6h d'échéance, est le sujet de cette thèse. Deux aspects ont été étudiés : la modélisation des débits et du ruissellement à fine échelle, et l'utilisation de prévisions immédiates de pluie, notamment des nouveaux systèmes de prévision de Météo-France, pour anticiper les crues. L'apport d'une représentation plus fine de l'occupation et de la texture des sols pour la simulation des débits des cours d'eau et du ruissellement sur des zones urbaines et péri-urbaines a d'abord été examiné. Une étude de sensibilité a été menée sur l'impact des descripteurs de sol et de la résolution spatiale (1km et 300m) avec le modèle hydrologique ISBA-TOP sur 12 cas de crues passées, incluant celui des inondations meurtrières dans la région de Cannes en 2015. Une analyse approfondie de ce cas a été conduite en exploitant les estimations de débits obtenues dans le cadre des enquêtes post-évènement du programme HyMeX et en explorant le potentiel dedonnées d'impacts pour évaluer le ruissellement simulé. La résolution spatiale du modèle s'estmontrée avoir le plus d'impact, suivie de la description de la texture du sol et enfin de ladescription de l'occupation du sol. Le potentiel des prévisions immédiates de pluie pour laprévision des crues méditerranéennes jusqu'à 6h d'échéance a ensuite été étudié. En particulier,les prévisions de pluie issues d'une version dédiée à la prévision immédiate du modèle deprévision numérique du temps AROME (AROME-PI), et d'une méthode de fusion entreextrapolation des lames d'eau radar et prévision numérique du temps (PIAF) ont été examinées.L'évaluation a été conduite en considérant les temps de mise à disposition réelle des prévisions,qui est un paramètre important à prendre en compte pour la prévision jusqu'à quelques heuresd'échéance. Ces pluies ont été évaluées d'un point de vue hydrologique, en comparant les lamesd'eau observées et prévues sur des bassins versants affectés par des crues passées, puis àl'échelle régionale sur un grand domaine Sud-Est. Les résultats sont globalement les mêmes pourles deux évaluations. PIAF est de très bonne qualité jusqu'à 1h de prévision, mais cette qualité sedégrade très rapidement, pour atteindre une qualité comparable voire inférieure à AROME-PI audelà de 1h15/1h30 de prévision. Entre 2h et 3h d'échéance, AROME-PI est de qualité supérieure ou équivalente à PIAF. Des ensembles formés par les prévisions successives de AROME-PI et dePIAF respectivement, ont aussi été étudiés. La sensibilité des ensembles à leur taille et à l'ajout d'une tolérance temporelle sur la prévision de chacun des membres a été examinée. Les résultats indiquent que plus un ensemble contient de membres, plus il est performant. Il en est de même pour les ensembles avec une tolérance temporelle de 15 ou de 30min. Une évaluation des débits aux exutoires simulés avec ISBA-TOP et MARINE forcés par les prévisions immédiates de pluie AROME-PI et PIAF, utilisées seules ou en tant qu'ensembles, a aussi été effectuée sur deux cas de crues exceptionnelles (l'Aude en 2018 et Cannes en 2015). Pour les meilleurs scenarii basés sur les prévisions AROME-PI, l'anticipation de la bonne intensité des pics de crue et de la décrue peut atteindre jusqu'à 5h, et un peu plus pour la montée. Pour ceux basés sur les prévisions PIAF, l'anticipation varie entre 20min et 4h, selon le phénomène, le bassin et le modèle hydrologique étudié
The Mediterranean regions are regularly exposed to heavy precipitating events and flash floods. Hydrometeorological forecasts up to a few hours are crucial for planning the intervention of emergency services in these situations. The prediction of the hydrological consequences of Mediterranean events of intense rainfall at the nowcasting ranges (few minutes to 6h) is the topic of this Ph. D. thesis. Two areas were studied: modelling of river flows and runoff at a fine scale, and the use of rainfall nowcasting, and particularly those from Météo-France new forecasting systems, to anticipate floods. The sensitivity to a more detailed representation of land use and texture in ISBA-TOP for simulating river flows and runoff over urban and peri-urban areas was first studied. The influence of terrain descriptors and spatial resolution (1km and 300m) has been analyzed for 12 flood events, including the major flood event in 2015 over the Cannes region. A more detailed analysis of this case was conducted using streamflow estimates at fine scale obtained from the HyMeX post-event survey and exploring the potential of impact data to evaluate simulated runoff. The results reveal that the spatial resolution has the largest impact on the hydrological simulations, larger than soil texture and land cover. Then, the potential of rainfall nowcasting for forecasting Mediterranean flash floods up to 6h was studied. The rainfall forecasts from the nowcasting suite based on the numerical weather prediction system AROME (AROME–PI), and from the nowcasting system blending numerical weather prediction and extrapolation of radar estimation (PIAF) were examined. The availability times of forecasts, based on the operational Météo-France suites, are taken into account when performing the evaluation. The evaluation of rainfall has adopted a hydrological point of view, by comparing observed and forecast rainfall over watersheds affected by past floods. A more classical evaluation comparing rainfall observation and forecast at the same location over Southeastern France has been also carried out. The results generally led to the same conclusions for both evaluations. The performance of PIAF is very good over the first hour of forecasting, but it deteriorates very quickly, to reach about the same or even a lower skill than AROME-PI beyond about 1h15/1h30 of forecasting. Between 2 and 3 hours of forecasting, AROME-PI performs better or at the same level as PIAF. Time-lagged ensembles based on AROME-PI and on PIAF forecasts respectively, were also studied. The sensitivity of the ensembles to their size and to the addition of a time tolerance on the forecast for each member was examined. The results indicate that the more members an ensemble has, the better it performs. The same applies to the ensembles with a time tolerance of 15 or 30 minutes. An assessment of river discharges simulated with ISBA-TOP and MARINE forced by AROME-PI and PIAF rainfall forecasting, used alone or as an ensemble, was also confducted on two exceptional past flash flood events (Aude in 2018 and Cannes in 2015). For the best scenarios based on AROME-PI, the anticipation of the flood peak intensity and of the instant of recession can reach up to 5h, and a little more for the first increase of flow. For those based on PIAF, the anticipation varies between 20 minutes and 4h, depending on the phenomenon, the watershed and the hydrological model studied

Cette étude évalue la performance d'un modèle hydrologique à base physique, distribué, appelé MARINE, pour la simulation du transport de sédiments en suspension pendant les crues soudaines à l'échelle du bassin versant. Les crues soudaines et intenses peuvent induire une érosion du sol et un transport de sédiments importants, entraînant une perte de sol durable. MARINE permet de prendre en compte la variabilité spatiale des caractéristiques du bassin versant et des précipitations. L'étude est menée sur deux petits bassins versants du sud de la France, la Claduègne en Ardèche (42,3 km2) et Auradé dans le Gers (3,28 km2). MARINE utilise une équation d'advection par l'écoulement moyen pour simuler l'évolution spatio-temporelle de la concentration en sédiments non cohésifs, en prenant en compte des termes sources représentant l'érosion par impact des précipitations et l'érosion par cisaillement.L'étude examine l'influence de divers paramètres sur la simulation de la concentration pendant les crues rapides dans ces bassins versants, y compris : (i) le diamètre médian des particules de sédiments, (ii) la cote de l'interface entre la charge de lit et la charge en suspension, (iii) le coefficient de sensibilité du sol à l'érosion par cisaillement, (iv) le coefficient de sensibilité du sol à l'érosion par impact des gouttes de pluie. Les résultats des analyses de sensibilité appliquées à des crues éclair sélectionnées mettent en évidence la sensibilité du modèle à deux paramètres clés pour les deux bassins versants : le coefficient de sensibilité du sol à l'érosion par cisaillement et le diamètre médian des particules de sédiments. Grâce à l'analyse de sensibilité des paramètres du modèle, les simulations permettent de classer les inondations en deux catégories : les crues dominées par l'érosion par impact des gouttes de pluie et celles dominées par l'érosion par cisaillement. En outre, l'analyse des résultats souligne la nécessité de prendre en compte la variabilité spatiale de la sensibilité du sol à l'érosion en identifiant notamment l'emplacement des sources potentielles de sédiments.Pour évaluer les performances du modèle en matière d'estimation de l'érosion, les résultats sur le bassin versant de la Claduègne sont comparés à deux modèles empiriques largement utilisés, dérivés de l'équation universelle de perte de sol (USLE), à savoir RUSLE et MUSLE. Pour comparer les trois modèles avec MARINE comme référence, une analyse de sensibilité est menée sur le facteur d'occupation du sol intervenant dans MUSLE et RUSLE. Bien que l'érosion simulée par les trois méthodes soit généralement comparable, des variations apparaissent pour les événements dominés par l'érosion par impact des gouttes de pluie, ce qui suggère la nécessité de poursuivre les recherches pour améliorer la modélisation de cette érosion dans le cadre du modèle MARINE.Les simulations du modèle MARINE offrent également la possibilité de générer des cartes d'aléas érosion/déposition qui, combinées à une carte de vulnérabilité, peuvent être utiles aux décideurs et planificateurs environnementaux pour identifier les zones exposées aux risques d'érosion et de dépôt
This study evaluates the performance of a physically-based, distributed hydrological model, called MARINE, for simulating suspended sediment transport during flash floods at the catchment scale. Intense flash floods can induce significant soil erosion and sediment transport, leading to long-lasting soil loss. MARINE takes into account the spatial variability of catchment characteristics and precipitation. The study was carried out on two small catchments in southern France, La Claduègne in Ardèche (42.3 km²) and Auradé in Gers (3.28 km²). MARINE uses a mean-flow advection equation to simulate the spatio-temporal evolution of non-cohesive suspended sediment concentration, taking into account source terms representing raindrop erosion and shear stress erosion.The study examines the impact of various parameters on the simulation of suspended sediment concentration during flash floods in these catchments, including (i) the median diameter of sediment particles, (ii) the location of the interface between bed load and suspended load, (iii) the soil sensitivity coefficient to shear stress erosion, (iv) the soil sensitivity coefficient to raindrop erosion. The results of the sensitivity analyses applied to selected flash flood events highlight the model's sensitivity to two key parameters for both catchments: the soil sensitivity coefficient to shear stress erosion and the median sediment particle diameter. Thanks to the sensitivity analysis of the model parameters, the simulations led to the classification of floods into two categories: those dominated by raindrop erosion and those dominated by shear stress erosion. Analysis of the results also highlights the need to take into account the spatial variability of soil sensitivity to erosion, in particular by identifying the location of potential sources of sediment.To assess the model's performance in estimating erosion, the results for the Claduègne catchment are compared with two widely used empirical models derived from the Universal Soil Loss Equation (USLE), namely RUSLE and MUSLE. To compare the three models with MARINE as a reference, a sensitivity analysis is carried out on the land-use factor involved in MUSLE and RUSLE. Although the erosion simulated by the three methods is generally comparable, variations appear for events dominated by raindrop erosion, suggesting the need for further research to improve raindrop erosion modeling within the MARINE model.MARINE model simulations also offer the possibility of generating erosion/deposition hazard maps which, combined with a vulnerability map, can be useful to environmental decision-makers and planners in identifying areas at risk from erosion and deposition

Ce travail de thèse concerne la caractérisation de la structure spatio-temporelle des fortes précipitations dans la région Cévennes-Vivarais. La région est soumise à des événements de pluie catastrophiques dont la magnitude gouverne les conséquences à différentes échelles de temps et d'espace. La détermination de la probabilité d'occurrence des orages est problématique à cause du caractère extrême des ces événements, de leur dimension spatio-temporelle et du manque de données pluviométriques aux échelles d'intérêt. Nous proposons d'adopter des approches d'invariance d'échelles afin d'estimer la fréquence d'occurrence de ces événements. Ces approches permettent d'extrapoler la distribution de la pluie à haute résolution à partir de données d'intensité pluvieuse à plus faible résolution. La paramétrisation de ces modèles étant fortement dépendante de l'incertitude de la mesure, nous avons d'abord caractérisé l'erreur commise dans la mesure de la pluie par un réseau de pluviomètres à augets. Nous avons ensuite exploré le comportement des pluies extrêmes dans la région d'étude, identifiant les gammes d'invariance d'échelles des extrêmes. Dans cette gamme d'échelles, nous présentons un modèle régional Intensité-Durée-Fréquence qui prend en considération l'hétérogénéité spatiale des extrêmes dans la région. Étant donné que le réseau pluviométrique ne permet pas de détecter les propriétés d'invariance d'échelle spatiale des champs de pluie, nous avons adopté une méthode semi-empirique pour modéliser des intensités de pluie intégrés sur des surfaces données (pluie surfacique) sur la base du concept de la mise en échelle dynamique (" dynamic scaling "). Cette modélisation permet la construction d'un modèle régional Intensité-Durée-Fréquence-Surface. Enfin, nous avons appliqué ce modèle à la construction des diagrammes de sévérité pour trois événements marquants en région Cévennes-Vivarais, afin d'identifier les échelles spatio-temporelles critiques pour chaque événement. Grâce aux diagrammes de sévérité, nous avons pu évaluer, pour ces mêmes événements, la performance d'un modèle météorologique de méso-échelle.

An empirical method is developed for constructing likelihood functions required in a Bayesian probabilistic flash flood forecasting model using data on objective quantitative precipitation forecasts and their verification. Likelihoods based on categorical and probabilistic forecast information for several forecast periods, seasons, and locations are shown and compared. Data record length, forecast information type and magnitude, grid area, and discretized interval size are shown to affect probabilistic differentiation of amounts of potential rainfall. Use of these likelihoods in Bayes' Theorem to update prior probability distributions of potential rainfall, based on preliminary data, to posterior probability distributions, reflecting the latest forecast information, demonstrates that an abbreviated version of the flash flood forecasting methodology is currently practicable. For this application, likelihoods based on the categorical forecast are indicated. Apart from flash flood forecasting, it is shown that likelihoods can provide detailed insight into the value of information contained in particular forecast products.

Improving the capacity to make predictions in ungauged basins is one of most difficult challenge for the scientific community (see for example the current initiative Prediction Ungaged Basins (PUB) launched by the International Association of Hydrological Sciences, IAHS). Whatever hydrological models are used, in view of the tremendous spatio-temporal heterogeneity of climatic and landscape properties, extrapolation of information, or knowledge, from gauged to ungauged basins remains fraught with considerable difficulties and uncertainties, especially in the light of the generally poor understanding of where water goes when it rains, what flow path it takes to the stream, and the age of the water that emerges in the channel. The PUB problem is the key concept of this thesis and it is analysed from several point of view. Methodologies able to observe, model and predict the hydrological response at the regional scale are proposed.

Flash flood is among the most hazardous natural disasters, and it can cause severe damages to the environment and human life. Flash floods are mainly caused by intense rainfall and due to their rapid onset (within six hours of rainfall), very limited opportunity can be left for effective response. Understanding the socio-economic characteristics involving natural hazards potential, vulnerability, and resilience is necessary to address the damages to economy and casualties from extreme natural hazards. The vulnerability to flash floods is dependent on both biophysical and socio-economic factors. This study provides a comprehensive assessment of socio-economic vulnerability to flash flood alongside a novel framework for flash flood early warning system. A socio-economic vulnerability index was developed for each state and county in the Contiguous United States (CONUS). For this purpose, extensive ensembles of social and economic variables from US Census and the Bureau of Economic Analysis were assessed. The coincidence of socio-economic vulnerability and flash flood events were investigated to diagnose the critical and non-critical regions. In addition, a data-analytic approach is developed to assess the interaction between flash flood characteristics and the hydroclimatic variables, which is then applied as the foundation of the flash flood warning system. A novel framework based on the D-vine copula quantile regression algorithm is developed to detect the most significant hydroclimatic variables that describe the flash flood magnitude and duration as response variables and estimate the conditional quantiles of the flash flood characteristics. This study can help mitigate flash flood risks and improve recovery planning, and it can be useful for reducing flash flood impacts on vulnerable regions and population.

Flash flooding in the semi-arid United States poses a significant danger to life and property. One effective way to mitigate flood risk is by implementing a rainfall-runoff model in a real-time forecast and warning system. This study investigated the feasibility of using the mechanistic, distributed semi-arid rainfall-runoff model KINEROS2 driven by high resolution radar rainfall input estimates obtained from the NEXRAD WSR-88D DHR reflectivity measurements in such a system. The original procedural paradigm-based KINEROS2 Fortran 77 code with space-time looping was recoded into an object-oriented Fortran 90 code with time-space looping for this purpose. The recoded form is now applicable to large basins, is easily future-extensible, and individual modules can be incorporated into other models.Sources of operational uncertainty in the above system were investigated for their influence over several events within a sub-basin of the USDA-ARS Walnut Gulch Experimental Watershed. Uncertainties considered were in the rainfall estimates, the model parameters, and the initial conditions. The variance-based Sobol' method of global sensitivity analysis conditioned on the observed streamflow showed that the uncertainty in the modeled response was heavily dominated by the operational variability of biases in the radar rainfall depth estimates. Sensitivities to KINEROS2 parameters indicates the need for improved representation of semi-arid hillslope hydrology in small basins, while pointing to specific influential, but poorly identified model parameters towards which field investigations should be directed. The significant influence of initial hillslope soil moisture showed the requirement of a sophisticated inter-storm model component for a continuous forecasting model.A synthetic study data was used to further explore the phenomena seen in the above real data study, of behavioral modifier set inconsistency across all events and of irreducibility in the spatial modifier ranges. The former was found to be attributable to wide uncertainty ranges in the sources of uncertainty, and the latter to the high distributed model non-linearity with associated interactions. These contribute towards a high predictive uncertainty in operational forecasting.Overall, the GLUE-based predictive uncertainty method with behavioral classification and accommodation of wide operational source uncertainty ranges is recommended as a simple and effective setup for operational flash flood forecasting.

En Europe, les crues éclair affectent principalement les régions méditerranéennes et montagneuses. Les bassins concernés sont le plus souvent de petite taille. Anticiper ce type d'événements demeure un exercice ardu parsemé de multiples difficultés. Cette thèse s'inscrit dans le cadre d'un projet commun à IRSTEA et Météo-France (projet RHYTMME) et ses deux objectifs sont :l'adaptation de modèle hydrologique de la méthode AIGA aux régions montagneuses, en tenant compte des caractéristiques de cet environnement ;l'évaluation du nouveau modèle sur des bassins de petite taille, qui sont réellement non-jaugés.Pour atteindre le premier objectif, le modèle est complexifié (prise en compte de la neige), testé sur 118 bassins versants jaugés et régionalisé. Quant au second but, des relevés de dégâts, issus de la base de données des services de Restauration des Terrains en Montagne (RTM), sont utilisés afin d'évaluer le modèle sur 123 très petits bassins dans des conditions aussi proches que possible de la réalité du terrain.Pour réaliser ce second objectif, une méthode d'évaluation est développée, fondée sur des statistiques de contingence, illustrant la concomitance entre dégâts observés et dépassement de seuil des débits simulés. Il est proposé un graphe renseignant le taux de détection (POD) et le taux de succès (SR) pour différents seuils simultanément à l'instar des courbes de ROC. Le principal résultat est qu'une évaluation sur bassins jaugés est une première étape nécessaire mais pas suffisante. En effet, les travaux illustrent le fait que les meilleurs modèles sur les bassins jaugés ne le sont pas forcément sur les petits bassins non-jaugés, sujets aux crues éclair
In Europe, flash floods primarily occur in the Mediterranean and mountainous areas. The concerned basins are often small and ungauged with a short lag time. Anticipating such events is a tricky task with many difficulties. This thesis is part of a common project between Irstea and Météo-France (RHYTMME project) with two objectives : the adaptation of the hydrological model of AIGA method to the mountainous areas, taking into account the environment characteristics the evaluation of the new model on small basins, which are actually ungauged. To achieve the first objective, the model is complicated by the integration of a snow modelling on 118 gauged basins. Then the model is regionalized. The second objective is to use flood reports from the mountain area restoration services database (services de Restauration des Terrains en Montagne (RTM)). The model is evaluated with these reports on 123 very small basins under conditions as close to as possible the reality. To overcome the second objective, an evaluation method is developed, based on contingency statistics, illustrating the coincidence between observed damages and threshold crossing by simulated flows. A graphic with the probability of detection (POD) according to the success rate (SR) is introduced for different thresholds. Thereby, a multi-threshold approach is used to compare hydrological models like ROC-curves. The main result, highlighted by this thesis is an evaluation on gauged basins is a necessary first step but not sufficient. Indeed, the works illustrate that the decided compromises on the large gauged basins don?t lead automatically to the best performances on the small basins occurring flash floods

The dangerous hazard posed by flash flooding to upland communities is likely to increase due to climate change. The flood risk management policy approach has become predominant since the 1990s, with an emphasis on the public awareness of, and responses to, flood risks; however, the unpredictable nature of upland flash flooding means that responses to such hazards are uncertain. This thesis uses an integrated analysis of social and physical science datasets to study responses by local residents and the Environment Agency to flash flooding, using a case study of a major upland flood in North Yorkshire. Responses to flash flooding within upland communities were found to be mostly present as changes to individual behaviour and awareness. However, physical, damage reducing modifications were limited. Flash flood hazard perception was found to be linked to knowledge and experience of local flooding. Major flash flood events occurring in areas which have not experienced other recent floods are unlikely to increase perceptions or provoke responses. Although local awareness of changing weather patterns was found, supporting analyses of rainfall records, local flood risks were frequently framed in the context of river management, rather than climate change. The implementation of policy changes and responses to flash flooding by the Environment Agency will prove difficult at the local level, due to the nature of attitudes and perceptions encountered at the local level, including important differences in the perception of the flash flood hazard between local residents and representatives from nationwide organisations. Encouraging property-level modifications following flash floods, in accordance with national policies, is very difficult. In order to increase local perceptions of the flash flood hazard, the use of participatory work, focusing on long-term awareness raising and the sharing of locally held flood knowledge may be beneficial, alongside the support of existing resilience in upland communities.

This study was conducted at the mountainous catchment part of Batinah Region of the Sultanate of Oman called Al-Awabi watershed which is about 260km2 in area with about 40 Km long Wadi main channel. The study paper presents a proposed modeling approach and possible scenario analysis which uses 1D - hydraulic modeling for flood routing analysis; and the main tasks of this study work are (1) Model setup for Al-Awabi watershed area, (2) Sensitivity Analysis, and (3) Scenario Analysis on impacts of rainfall characteristics and transmission losses. The model was set for the lower 24 Km long of Al-Awabi main channel (Figure 13). Channel cross-sections were the main input to the 1D-Hydraulic Model used for the analysis of flash flood routing of the Al-Awabi watershed. As field measurements of the Wadi channel cross-sections are labor intensive and expensive activities, availability of measured channel cross-sections is barely found in this study area region of Batinah, Oman; thereby making it difficult to simulate the flood water level and discharge using MIKE 11 HD. Hence, a methodology for extracting the channel cross-sections from ASTER DEM (27mX27m) and Google Earth map were used in this study area. The performance of the model setup was assessed so as to simulate the flash flood routing analysis at different cross-sections of the modeled reach. And from this study, although there were major gap and problems in data as well as in the prevailing topography, slope and other Hydro Dynamic parameters, it was concluded that the 1D-Hydraulic Modelling utilized for flood routing analysis work can be applied for the Al-Awabi watershed. And from the simulated model results, it was observed that the model was sensitive to the type of Boundary Condition chosen and taken, channel cross sections and its roughness coefficient utilized throughout the model reach.

Au XXIe siècle, la prévision de l'aléa hydrométéorologique et des impacts associés aux crues rapides demeurent un défi pour les prévisionnistes et les services de secours. Les mesures structurelles et / ou les avancées des systèmes de prévision hydrologique ne garantissent pas, à elles seules, la réduction des décès lors de ces phénomènes d'inondation rapide. La littérature souligne la nécessité d'intégrer d'autres facteurs, liés aux processus de vulnérabilité sociaux et comportementaux, afin de mieux prendre en compte les risques encourus par les populations lors de ces épisodes extrêmes. Cette dissertation conduit une analyse théorique couplés à ceux de une analyse des accidents historiques mortels afin d'expliquer les interactions qui existent entre les processus hydrométéorologiques et sociaux responsables de l'apparition de vulnérabilités humaines lors de crues rapides aux États-Unis. Des données d'enquêtes liées aux crues rapides sont examinées afin d'élaborer un système de classification des circonstances du décès (en voiture, à l'extérieur, à proximité d'un cours d'eau, dans un camping, dans un bâtiment ou en mobile-home). L'objectif est d'établir un lien entre la conception des vulnérabilités et l'estimation des pertes humaines liées à ces catastrophes naturelles. "Random forest" est utilisé et est basé sur un arbre de décision, qui permet d'évaluer la probabilité d'occurrence de décès pour une circonstance donnée en fonction d'indicateurs spatio-temporels. Un système de prévision des décès liés à l'usage de la voiture lors des crues rapides, circonstance la plus répandue, est donc proposé en s'appuyant sur les indicateurs initialement identifiés lors de l'étude théorique. Les résultats confirment que la vulnérabilité humaine et le risque associé varient de façon dynamique et infra journalière, et en fonction de la résonance spatio-temporelle entre la dynamique sociale et la dynamique d'exposition aux dangers. Par exemple, on constate que les jeunes et les personnes d'âge moyen sont plus susceptibles de se retrouver pris au piège des crues rapides particulièrement soudaines(par exemple, une durée de près de 5 heures) pendant les horaires de travail ou de loisirs en extérieur. Les personnes âgées sont quant à elles plus susceptibles de périr à l'intérieur des bâtiments, lors d'inondations plus longues, et surtout pendant la nuit lorsque les opérations de sauvetage et / ou d'évacuation sont rendues difficiles. Ces résultats mettent en évidence l'importance d'examiner la situation d'exposition aux risques en tenant compte de la vulnérabilité dynamique, plutôt que de se concentrer sur les conceptualisations génériques et statiques. Ce concept de vulnérabilité dynamique est l'objectif de modélisation développée dans cette thèse pour des vulnérabilités liés aux véhicules. À partir de l'étude de cas sur les crues rapides survenues en mai 2015, et en analysant principalement les états du Texas et de l'Oklahoma, principaux états infectés par ces évènements,le modèle montre des résultats prometteurs en termes d'identification spatio-temporelle des circonstances dangereuses. Cependant, des seuils critiques pour la prédiction des incidents liés aux véhicules doivent être étudiés plus en profondeur en intégrant des sensibilités locales non encore résolues par le modèle. Le modèle établi peut être appliqué, à une résolution journalière ou horaire, pour chaque comté du continent américain. Nous envisageons cette approche comme une première étape afin de fournir un système de prévision des crues rapides et des risques associés sur le continent américain. Il est important que la communauté scientifique spécialisée dans l'étude des crues éclairs récoltent des données à plus haute résolution lorsque ces épisodes entrainement des risques mortels, et ce afin d'appuyer la modélisation des complexités temporelles et spatiales associées aux pertes humaines causées par les futures inondations soudaines
In the 21st century the prediction of and subsequent response to impacts due to sudden onset and localized flash flooding events remain a challenge for forecasters and emergency managers. Structural measures and/or advances in hydrological forecasting systems alone do not guarantee reduction of fatalities during short-fuse flood events. The literature highlights the need for the integration of additional factors related to social and behavioral vulnerability processes to better capture risk of people during flash floods. This dissertation conducts a theoretical analysis as well as an analysis of flash flood-specific historic fatalities to explain complex and dynamic interactions between hydrometeorological, spatial and social processes responsible for the occurrence of human life-threatening situations during the "event" phase of flash floods in the United States (U.S.). Individual-by-individual fatality records are examined in order to develop a classification system of circumstances (i.e., vehicle-related, outside/close to streams, campsite, permanent buildings, and mobile homes). The ultimate goal is to link human vulnerability conceptualizations with realistic forecasts of prominent human losses from flash flood hazards. Random forest, a well-known decision-tree based ensemble machine learning algorithm for classification is adopted to assess the likelihood of fatality occurrence for a given circumstance as a function of representative indicators at the county-level and daily or hourly time steps. Starting from the most prevalent circumstance of fatalities raised from both the literature review and the impact-based analysis, flash flood events with lethal vehicle-related accidents are the subject to predict. The findings confirm that human vulnerability and the subsequent risk to flash flooding, vary dynamically depending on the space-time resonance between that social and hazard dynamics. For example, it is found that younger and middle-aged people are more probable to get trapped from very fast flash floods (e.g., duration close to 5 hours) while participating in daytime outdoor activities (e.g., vehicle-related, recreational). In contrary, older people are more likely to perish from longer flooding inside buildings, and especially in twilight and darkness hours when rescue and/or evacuation operations are hindered. This reasoning places the importance of situational examination of dynamic vulnerability over generic and static conceptualizations, and guides the development of flash flood-specific modeling of vehicle-related human risk in this thesis. Based on the case study of May 2015 flash floods with a focus in Texas and Oklahoma, the model shows promising results in terms of identifying dangerous circumstances in space and time. Though, critical thresholds for the prediction of vehicle-related incidents need to be further investigated integrating local sensitivities, not yet captured by the model. The developed model can be applied on a daily or hourly basis for every U.S. county. We vision this approach as a first effort to provide a prediction system to support emergency preparedness and response to flash flood disasters over the conterminous U.S. It is recommended that the flash flood disaster science community and practitioners conduct data collection with more details for the life-threatening scene, and at finer resolutions to support modeling of local temporal and spatial complexities associated with human losses from flash flooding in the future

The height of an overflow dam must be designed low enough to prevent the reservoir water level from exceeding a flood plain during flooding conditions. Because of this constraint, much of the available water storage area is wasted and the available pressure head for power generation will be less than maximum during normal conditions. Crest control gates alleviate this problem by providing a variable spillway height. The Flash Gate Board is a passive automatic crest control gate. Its purpose is to regulate flood water while providing increased water pressure for power generation or for additional water storage for a municipality. The governing equations for the Flash Gate Board system are derived and used to formulate models of the system. Computer simulations are used to examine the system response in a variety of operating conditions. The results of these simulations are presented and discussed. The results include an investigation which developed an optimum gate height to maximize the potential of the Flash Gate Board. An experimental model was developed to verify analytical results and to provide additional insight. Conclusions from the study, recommendations for future work, and modifications for a trouble-free design are discussed.
Master of Science

Dans les pays du Sud comme ailleurs, la gravité croissante des conséquences des catastrophes naturelles traduit le rôle essentiel de l’augmentation de la vulnérabilité des populations et des biens, liée aux inégalités, à la croissance démographique et à l’insuffisance de planification. Au Brésil, des réponses législatives et opérationnelles spécifiques ont suivi les désastres survenus entre 2008 et 2011 dans plusieurs régions. Dans la préparation de tels dispositifs, l’apport des géographes devrait être essentiel, d’où cette thèse fondée sur une approche géographique de la question dans la région du Cariri (Nordeste). Elle comporte aussi une comparaison destinée tirer les enseignements de l’expérience française en matière de traitement et de prévention des risques naturels, ainsi que les possibilités de les transposer dans le cadre brésilien. L’analyse des facteurs géographiques locaux conditionnant les types d’aléas et d’enjeux est suivie de celle des informations obtenues sur les inondations contemporaines de la recherche et antérieures. Elle comporte aussi des données originales sur les mouvements de terrain qui l’affectent. La perception et la gestion des risques sont abordées à travers une enquête menée auprès des populations riveraines des cours d’eau dangereux. Une partie des données est transcrite sous forme de cartes dans un essai de zonage des aléas, des enjeux et des risques à l’échelle locale, destiné à fournir aux décideurs des éléments utiles à l’élaboration d’une planification urbaine comportant la gestion et la prévention des risques naturels. Les résultats sont mis en perspective grâce à une revue des catastrophes et des risques concernant l’ensemble du Brésil, et des mesures prises à l’échelle fédérale et à des niveaux plus locaux. Elle sert aussi de substrat pour la brève étude comparative qui porte sur le traitement des catastrophes naturelles en France (Draguignan). Si les besoins en matière de délimitation des aires de risque sont les mêmes dans les deux pays, les conséquences qui en sont tirées sont différentes puisque, jusqu’à présent, on ne retrouve pas dans la région étudiée au Brésil l’équivalent du zonage réglementaire des PPRI français. Pourtant, si des enseignements peuvent être transposables d’un pays à l’autre, c’est surtout dans ce domaine réglementaire. Les orientations futures qui peuvent se dégager de ce travail résident surtout dans les améliorations de connaissances que l’on peut attendre de collaborations interdisciplinaires étendues et mieux ordonnées
In the Southern countries as elsewhere, the increasing seriousness of the consequences of natural disasters illustrates the essential part of an increasing vulnerability of populations, property and infrastructures, linked with demographic growth, poverty, social disparities and insufficient planning. In Brazil, legislative and operational responses followed the disasters which had occurred between 2008 and 2011 in several regions. Geographers should play an essential part in the preparation of such measures. This is the scope of this thesis, which is based upon a geographical approach of the question in the Cariri region of Northeast Brazil. This work also includes a comparative approach, in order to analyze lessons from the French experience in the domain of risk management and prevention, and to study the possibilities of transposing them in the Brazilian context. The analysis of local geographical factors (types of hazards, population and property at risk) is followed by that of information obtained about floods which occurred in the study area during the time of our research work and before. It also includes original data on mass movements at various scales which were identified from field work. Risk perception and management are investigated by the means of a questionnaire submitted to residents neighbouring rivers identified as dangerous. Part of these data are transcribed in the form of maps, in an attempt to draw a zonation of hazards, population and property at risk at local scale, in order to provide useful elements for an urban planning including risk management and prevention. This work is put in perspective through a review of disasters and risks at national scale, and of the measures that were taken at federal and more local scales. This review is used as a basis for a comparative study which bears on the management of natural disasters in France (Draguignan). Although the needs in delimitation of areas at risk are the same in both countries, the consequences which are drawn are different since, until now, there is no equivalent of the regulatory zoning of the French « PPRI », at least in the study area. However, if lessons can be transposed from one country to the other, this is mainly in this regulatory domain. Future orientations that can be retained from this work are mainly found in improvements that can be expected from wider and better organized interdisciplinary collaborations

The frequent occurrence of natural disasters; especially flash floods, are resulting the significant threats to many countries around the world. The truth that cannot be ignored, is that the effects of flash floods on the developing countries’ societies and economies are massive, compared with developed countries. Petra region; which is located in Jordan, is exposed to flash flood risks, which led to losses in lives, public and private properties. While the frequencies and impacts of flash floods might not be controlled easily, the need for more effective early warning systems has become extremely important. United Nations International Strategy for Disaster Reduction (UNISDR) and many researchers assumed that if an effective tsunami early warning system had been in place in the Indian Ocean region on 26 December 2004, thousands of lives would have been saved. Accordingly, Petra Region’s communities have experienced the impacts of flash floods in recent years due to absence of early warning systems and the lack of knowledge among communities about flash flood risks. These problems provide the context and demonstrate the significance of this study. Research aims to develop a responsive Flash Flood Early Warning System Guidance (FFEWSG) to enhance resilience in Petra Region. This research takes the social constructivism (interpretivism) stance in the continuum of philosophy and adopts a case study research strategy with qualitative method of research techniques. The research data collection was conducted in three phases. During the first phase, pilot semi structured interviews were conducted among people in Petra Region while the second phase focused on collecting the data from disaster affected communities, and disaster experts using semi-structured interviews. The third phase gathered information from Petra Development and Tourism Region Authority (PDTRA) documents. Data was analysed using content analysis. The research investigated flash floods in developed and developing countries; reviewed previous reports of flash flood events in Petra Region and how they affect the study area; and current early warning systems related to flooding. The research recommended a flash flood early warning system that could empower the local governmental institutions to mitigate flash flood impacts and enhance the resilience in Petra Region. It is expected that the research will add significant empirical evidence on the elements of the guidance within early warning system for flash flood, and will provide a useful tool in Petra Region for stakeholders, particularly for the government or the implementing agencies, helping to ensure the success of reducing the flash flood risks by the development of FFEWSG.

This thesis presents a novel analytical solution strategy for the zero-inertia (ZI) equations of free surface flow. These equations are utilized herein for routing flood flow in open channels and for simulating excess rainfall runoff on overland planes. The novel solution approach is shown to be both accurate and robust, especially under the complicated and intricate conditions of infiltrating flow on initially dry river beds or soils, e.g., as present in arid and semiarid areas. This is underlain by comparing modeling results of the novel analytical procedure with those of validated numerical solutions. Furthermore, it is shown that the analytical ZI model can deliver a process-oriented portrayal of runoff concentration in the flood-generating parts of the catchment. Subsequently, the novel analytical ZI model is applied for a real-world water management problem in the Sultanate of Oman, Arabian Peninsula. Within an integrated flash flood routing model—which is also presented in this thesis—the novel analytical routing approach helps in accurately matching the dynamics of advancing and infiltrating ephemeral river flow, established as a consequence of release from a groundwater recharge dam. The integrated modeling system houses the aforementioned analytical downstream model and tailor-made, state-of-the-art modeling components to portray the upstream flow processes, dam operation (including evaporation), and spillway release flow. The proposed modeling system can aid in rendering a realistic image of transient transmission losses and dependent flow dynamics. This is of extremely high importance for water resources assessment, as well as for optimizing recharge dam operation strategies in order to maximize downstream transmission losses and, thus, groundwater recharge
Diese Dissertation präsentiert einen neuartigen analytischen Lösungsansatz für das beschleunigungsfreie Wellenmodell (bzw. „Zero-Inertia-Modell“, „ZI-Modell“, oder „diffusives Wellenmodell“). Im Rahmen der Arbeit wird das hergeleitete hydrodynamische Modell sowohl zur Simulation von Freispiegelabflüssen in nichtprismatischen und durchlässigen Gerinnen, als auch für die Beschreibung von auf der Landoberfläche abfließendem Infiltrationsüberschuss eingesetzt. Es wird gezeigt, dass der neuartige analytische Ansatz — im Hinblick auf Massenerhaltung und die exakte Abbildung der Abflussdynamik — akkurate Ergebnisse liefert und gleichzeitig unter komplexen und verwickelten Prozessbedingungen anwendbar ist. So belegt eine vergleichende Analyse mit validierten numerischen Lösungsansätzen die Robustheit des analytischen ZI-Modells. Insbesondere die im Sinne der numerischen Mathematik stabile und genaue Modellierung der gekoppelten Abfluss- und Infiltrationsvorgänge in anfänglich trockenen Gerinnen ist dabei ein Novum. Weiterhin wird die Eignung und Anwendbarkeit des neuartigen Modellansatzes zur Beschreibung der Abflusskonzentrationsprozesse gezeigt. Der neuartige Lösungsansatz wird im Folgenden für ein reales Wassermanagementproblem im Sultanat Oman, Arabische Halbinsel eingesetzt. Als Bestandteil eines integrierten Modellsystems, welches ebenfalls im Rahmen der Dissertation vorgestellt wird, dient das analytische ZI-Modell zur Simulation von infiltrierendem Wadiabfluss, welcher unterstrom von Grundwasseranreicherungsdämmen starke Verluste von Masse und Impuls erfährt. Zusammen mit maßgeschneiderten und dem Stand der Technik entsprechenden Komponenten für die Betriebssimulation des Anreicherungsdammes (inklusive Verdunstung von der freien Seefläche) sowie für die Abbildung der oberstromigen hydrodynamischen Prozesse (ebenfalls inklusive Infiltration) wird der neuartige analytische Ansatz in einem Modellsystem zusammengefasst. Das Modellsystem ist in der Lage ein realistisches Bild der raumzeitlichen Dynamik des Abflusses sowie der Grundwasserneubildung aus infiltrierendem Wadiabfluss zu liefern. Damit stellt das Modellsystem ein wertvolles Werkzeug sowohl zur Wasserdargebotsermittlung, als auch für die Optimierung des Betriebes von Grundwasseranreicherungsdämmen dar

This study presents an integrated approach to estimating the risk of flash floods in arid wadis. Flash floods impact seriously on the infrastructure and development of Egyptian desert towns, such as Marsa Alam, and their surrounding areas. The study developed a method using the example of the wadi El-Alam, which has an area of approximately 407 km2. The method has three stages. First, the construction of a Digital Elevation Model from which morphometric properties of the brain were calculated. In particular, representing the flat areas and the sharp junctions between badland slopes and the adjacent wadi system (which generally characterised arid landscapes), were among the major issues facing this study. Second, the derivation of land cover from satellite remote sensing data which were rigorously pre-processed with the aid of aerial photographs and fieldwork measurements. Third, the application of a hydrological model, incorporating the basin morphometry and land cover data, to transform rainfall into runoff and route the floodwater through the sub-catchments of the wadi. The hydrological model system (HMS) was run to simulate discharge at the main wadi outlet (where the town of Marsa Alam is located) and at each sub-basin intersecting the Idfu-Alam highway that runs through the wadi. From these results, sites vulnerable to flash flood and their risk classes were identified. The sites judged to have the highest flood hazard along the highway were exactly the sites damaged in the 1991 flash flood. Thus, as an outcome of this study, the first digital database that includes information on land cover, geomorphology and hydrology of wadi El-Alam was developed. This database can be used to highlight regions vulnerable to flood damage.

Various models have been applied in the analysis of hydrologic conditions in different watershed areas. The whole spectrum of models available as tools for natural resource managers is because the use of modeling techniques are limited for specific areas and/or purposes. Constraints and limitations have to be realized in order to properly select a model that answers the needs of a certain locality, for a stated goal. It is the purpose of this paper to review existing hydrologic models, and in the process, select the most promising for application in the southwestern part of Saudi Arabia for the purpose of designing flood control and warning systems. The six models under investigation are the SCS Method, SCS TR-20, Stanford, USDA HL-74, HEC-1 and ANSWERS. Based on the scope and limitations of each model, as well as certain restrictions found within the area of study, it became evident that the SCS models are the most appropriate. This became more apparent having considered the type of data input the models require that can be provided for in the study area, as well as the simplicity of the models and scale of application.

The North American Monsoon System (NAMS) is a significant weather and climate phenomenon that brings critical rainfall to the southwestern United States and northwestern Mexico. As a result of the North American Monsoon Experiment, and research efforts surrounding the field campaign, the understanding of the NAMS has increased considerably over the last 15 years. In addition questions concerning potential flash flood causing mechanisms of the NAMS have not been thoroughly investigated. This dissertation is comprised of two papers that collectively address the aspects of the literary understanding of the NAMS as we know it today and conduct an investigation into the complex interactions between various weather systems that may influence the NAMS. In the first paper, a review of the major research of the NAMS literature since the last comprehensive review 15 years ago is conducted. The results of his review are assessed for where our understanding has been improved and where future research needs to be guided for purposes of the second paper. Based upon the results from the literature review, the second paper focuses on identification of inverted troughs and gulf surges based upon lower- and mid-level atmospheric parameters for purposes of assessing the impacts on National Weather Service Storm Report flash flood dates. This research contributes to the synthesis of the current knowledge of the NAMS in general and to the specific regional impacts that do occur during periods of heavy precipitation over the NAMS region for purposes of improving meteorological predictability of flash flooding. The results can (1) gauge our understanding of the NAMS literature to date and (2) improve meteorological forecasts through the recognition of synoptic and sub-synoptic patterns related to the NAMS that are most likely to cause flash floods.