Academic literature on the topic 'Fluvial envelope'

AbstractThe morphological evolution of a carbonate fault line scarp from southern Italy, generated by transpressional faulting and evolved by slope replacement, has been reconstructed. 14C dating of faulted slope deposits (ages included between 18 ka and ∼8 ka BP) have been performed to constrain the Late Pleistocene — Holocene evolution of that scarp. Long-to short-term denudation rates have been also evaluated for the understanding of the mountain front origin. The slope shows well-defined triangular facets combined with the presence of N-S-striking mountainward-dipping fault planes. The envelope of the slope foot appears slightly curved in a planimetric view and shows an E-W-trending offset in its southern part, making such a feature quite different from the recurrent rectilinear fault scarps, often related to normal faulting. Morphostructural analysis showed that: i) the oldest displacement was generated by a fault with a reverse component of movement; ii) the slope represents an inherited feature, only recently exhumed, and developed starting from a high-angle curved surface; iii) the upper Pleistocene — Holocene extensional faulting has only affected the slope foot and associated waste deposits, causing a series of collateral morphological effects, as fluvial cut of preexisting valleys and the genesis of conspicuous mass movements.

AbstractOne of the principal Quaternary deposits in Central Southern England are extensive spreads of sandy fine to coarse, mainly flint gravels: those at the higher elevations being referred to as “Plateau” Gravels to distinguish them from the “Valley” Gravels. The most widely accepted view of the origin of the Plateau Gravels is that they represent fluvial terraces deposited under periglacial conditions. Although they have provided excellent conditions for foundations, their actual in-situ density and shear strength characteristics were virtually unknown. To remedy this deficiency, a series of in-situ density tests using the sand replacement technique and direct shear tests using 300 mm square samples have been carried out.Samples of sandy gravel from Highcliffe were compacted by hand tamping to minimise particle breakdown and tested in a saturated condition with a dry density approximately 97% of the in-situ value. The results show a curved failure envelope which in the usual form of power law notation gives a best fit result of:- τ = 1.475 σn ′ 0-955 Over the range of normal Stresses from 80 to 370 kPa used in the tests, this corresponds to secant φ′ values from 51°. to 48°. Tests carried out on dry samples gave φ′ values up to 4° higher. Tests on other samples with different grain size distributions show that the shearing resistance increases with the percentage gravel (as opposed to sand) content. These results have been combined with published work on other well graded, mainly flint gravels to give a preliminary quantitative evaluation of the influence of percentage gravel content on shearing resistance.

AbstractThe reported occurrence of Albian- and Cenomanian-aged braided fluvio-deltaic channels in the Orange Basin, South Africa, opens a window of exploration activities to characterize these channels as they are renowned to form some of the world’s giant oil field. In this study, a seismic acoustic impedance inversion and seismic attributes (instantaneous frequency and iso-frequency) analysis is used to investigate potential Albian and Cenomanian fluvio-deltaic channels in offshore, northern Orange Basin. Reservoirs were mapped using a well and 3D seismic volume (8-bit) after initial dip-steering coherency filtering had been performed on the seismic volume to remove incoherent noise and improve data resolution. Model-based acoustic impedance inversion was applied on the seismic volume to delineate fluvio-deltaic channels in addition to using the RMS (root mean square) amplitude attribute. Iso-frequency using the cosine correlative transform (CCT) method was equally applied to delineate these channels. Instantaneous frequency attribute was analyzed for potential hydrocarbon-charged sediments. This was achieved by utilizing thirty-three seismic traces as an input in the Hilbert transform window, after which trace envelope and instantaneous phase were transformed into instantaneous frequency. Acoustic impedance inversion results reveal the presence of two channels within the Cenomanian sequence, which shows high porosity (∼40%) along its geometry. The CCT method shows that the 8 Hz frequency window resolved the presence of a channel within the Albian sequence. A meandering channel within the Albian sequence was equally delineated by the RMS, while the application of instantaneous frequency (IF) attribute indicates the presence of hydrocarbon-charged sediments of Cenomanian age in proximity to a listric normal fault because of the attenuation of frequency observed close to the fault. This study demonstrates a case study of the application of seismic impedance inversion and seismic attributes for the delineation of potential reservoirs and hydrocarbon-charged sediments in a basin.

A database of 3023 river basins in Africa was assembled to investigate the relative effects of uplift and erosion on landscape development. The volume of rock eroded from each basin was calculated by integrating the difference between a topographic summit envelope fit across drainage divides and interfluves, and present-day topography over basin area. Africa is an excellent natural laboratory for this procedure because drainage patterns reflect the Neogene development of topographic basins and swells, themselves surficial manifestations of sub-lithospheric mantle convection. As a result, the loci of major offshore deltas and drainage divides have remained largely static, while epeirogenic (vertical) surface motions are more important than shortening. Eroded rock volume is presented as a proxy for fluvial incision and correlates strongly with long-wavelength gravity anomalies across Africa, but not with mean precipitation, which was calculated by merging satellite estimates with rain gauge data. This finding implies that spatial variations in epeirogenic uplift govern landscape evolution across the continent. Other variables that alter drainage basin geometry and the magnitude of eroded rock volumes, such as varying climate, bedrock erodibility, and drainage capture, are likely subordinate to these variations. First-order estimates of eroded rock volume onshore are potentially the most accurate indicator of offshore sedimentation because they implicitly include information pertaining to basin area and relief, which together control sediment load.

Compound flooding is a physical phenomenon that has become more destructive in recent years. Moreover, compound flooding is a broad term that envelops many different physical processes that can range from preconditioned, to multivariate, to temporally compounding, or spatially compounding. This research aims to analyze a specific case of compound flooding related to tropical cyclones where the compounding effect is on coastal flooding due to a combination of storm surge and river discharge. In recent years, such compound flood events have increased in frequency and magnitude, due to a number of factors such as sea-level rise from warming oceans. Therefore, the ability to model such events is of increasing urgency. At present, there is no holistic, integrated modeling system capable of simulating or forecasting compound flooding on a large regional or global scale, leading to the need to couple various existing models. More specifically, two more challenges in such a modeling effort are determining the primary model and accounting for the effect of adjacent watersheds that discharge to the same receiving water body in amplifying the impact of compound flooding from riverine discharge with storm surge when the scale of the model includes an entire coastal line. In this study, we investigated the possibility of using the Advanced Circulation (ADCIRC) model as the primary model to simulate the compounding effects of fluvial flooding and storm surge via loose one-way coupling with gage data through internal time-dependent flux boundary conditions. The performance of the ADCIRC model was compared with the Hydrologic Engineering Center- River Analysis System (HEC-RAS) model both at the watershed and global scales. Furthermore, the importance of including riverine discharges and the interactions among adjacent watersheds were quantified. Results showed that the ADCIRC model could reliably be used to model compound flooding on both a watershed scale and a regional scale. Moreover, accounting for the interaction of river discharge from multiple watersheds is critical in accurately predicting flood patterns when high amounts of riverine flow occur in conjunction with storm surge. Particularly, with storms such as Hurricane Harvey (2017), where river flows were near record levels, inundation patterns and water surface elevations were highly dependent on the incorporation of the discharge input from multiple watersheds. Such an effect caused extra and longer inundations in some areas during Hurricane Harvey. Comparisons with real gauge data show that adding internal flow boundary conditions into ADCIRC to account for river discharge from multiple watersheds significantly improves accuracy in predictions of water surface elevations during coastal flooding events.

Changes in external forcing having traditionally been the main area of interest in trying to understand paleo-depositional environments in sedimentary systems; however, autogenic variability has been rising in importance, while autogenic behavior has been thought of as a “noise” generator. Recently, autogenic variability has been rising in attention because decoupling allogenic signatures (externally driven) from the stratigraphic record requires robust understanding of autogenic variations (internally generated). This study aims to quantitatively measure autogenic processes under a range of flux conditions and to show that autogenic processes generate distinct signatures rather than random noise. We present data from a matrix of nine different tank experiments in order to systematically evaluate the effects of sediment flux and water discharge variations on the autogenic timescale of fluvial sediment storage and release processes and the implications of this data to the stratigraphic record. The sediment flux tow ater discharge ratio and the absolute values of these two discharges control the autogenic timescale. Variations in sediment supply yield two competing effects on the autogenic timescale. The primary sediment flux control causes a reduction in the autogenic timescale as an increase in sediment supply yields an increased rate of filling the “fluvial envelope” (the space between the maximum and minimum fluvial slopes obtained during storage and release events). In contrast, the secondary sediment flux control increases the size of the fluvial envelope and works against the primary sediment flux control. Increasing the water discharge increases the autogenic timescale by widening the fluvial envelope during the organization of the fluvial system and more importantly, diminishes the functionality of the secondary sediment control. A competition exists between these factors, causing a non-linear range of autogenic timescales for a given sediment flux to water discharge ratio. In the nine experiments here, as the ratio decreases, the secondary effects of variations in sediment supply are suppressed by the relatively high water discharge, and the timescale is more predictable using the primary sediment control. As the ratio increases, the secondary effects from sediment supply are enhanced by a poorly organized fluvial system, and the timescale converges to a narrow range. This suggests significant implications for autogenic sediment delivery and stratigraphic development in a wide range of discharge conditions in field cases.<br>text

The Mesozoic Hartford Basin, a fault-bounded half-graben in New England, is composed of four sedimentologic units displaying lacustrine, playa, and alluvial conditions separated by three tholeiitic basalt flows. Limited outcrop, however, has restricted analyses across the basin. The Jurassic East Berlin Formation, in particular, crops out only in the southern and northern extents of the basin, exposing the upper 100-118-m of deposits. As a result, a new core analysis across a 600-m-transect of East Berlin rocks has been completed in the central region of the basin, exposing the entire 195-m thickness of the formation for the first time. Cores expose eight 3-m-thick lacustrine mudrock units, the upper six of which are correlative to lake deposits identified in the southern and northern extents of the basin. Additionally, thin chicken-wire evaporites demarcate the lowermost, previously unexposed, lacustrine unit, 7-m beneath a 15-cm-thick tufa horizon. Thin playa deposits and thick sheetflood and Vertisol packages separate these lake sequences over 5-30-m of vertical distance.To supplement these sedimentologic data, and better understand lake geochemistry of the basin during East Berlin time, new biomarker analyses have been applied to each of the eight lacustrine mudrock units for the first time. Biomarker data are useful for determining the lake-basin type, a paleolake classification system derived by Bohacs, Carroll, and others to describe predictable physical and geochemical evolution within rift basins from fluvial facies to over-filled, balance-filled, and under-filled lacustrine facies; subsequently, balance-filled lacustrine facies grade to a terminal fluvial facies during changes in accommodation space through time. While fluvial facies envelope lake deposits within the Hartford Basin, identifying the lake types within the East Berlin has been problematic because of limited exposures. These new sedimentologic and biomarker analyses, however, suggest balance-filled lacustrine conditions at the base of the East Berlin that grade into under-filled conditions upsection. These new biomarker data finally provide definitive evidence for changing lake types during East Berlin time.