Littérature scientifique sur le sujet « Weir »

This thesis is devoted to the study and designs of two important proportional weirs having the discharge-head characteristics of Q α H 2/3 In the first design a geometrically simple weir in the form of a rectangular weir over a inverted V-notch (Chimney weir) is presented. This weir gives for all flows above a threshold depth a discharge proportional to H 2/3 within a maximum percentage error of ±1.5, (measured above a reference plane) within certain limits of head. Second design is concerned with the self-basing weir in which a portion of the weir above the crest acts as a base. This design is achieved by using the complementary weir profile of a Quadratic weir above the parabolic base which has the significant property of fast convergence. This weir gives discharge for all flows above the threshold depth, proportional to (head)2/3 measured above a reference plane, with increasing accuracy as head increases. Experiments with these two weirs confirm the theory by giving a constant average Coefficient of Discharge (Cd) of 0.62. The importance of these weirs as a sensitive discharge measuring device in field and laboratory is highlighted.

This thesis is devoted to the study and designs of two important proportional weirs having the discharge-head characteristics of Q α H 2/3 In the first design a geometrically simple weir in the form of a rectangular weir over a inverted V-notch (Chimney weir) is presented. This weir gives for all flows above a threshold depth a discharge proportional to H 2/3 within a maximum percentage error of ±1.5, (measured above a reference plane) within certain limits of head. Second design is concerned with the self-basing weir in which a portion of the weir above the crest acts as a base. This design is achieved by using the complementary weir profile of a Quadratic weir above the parabolic base which has the significant property of fast convergence. This weir gives discharge for all flows above the threshold depth, proportional to (head)2/3 measured above a reference plane, with increasing accuracy as head increases. Experiments with these two weirs confirm the theory by giving a constant average Coefficient of Discharge (Cd) of 0.62. The importance of these weirs as a sensitive discharge measuring device in field and laboratory is highlighted.

<p>A hydraulically undersized control structure could result in water overtopping a dam or channel banks. To increase hydraulic capacity and reduce flooding risk, nonlinear spillways are frequently replacing linear weirs. This study investigates four subjects to further knowledge for two types of nonlinear weir, the piano key and labyrinth. Weir submergence is a condition when the downstream water level of a weir exceeds the weir crest elevation, and can influence the head-discharge relationship of the structure. The effects of submergence on laboratory-scale piano key weir head-discharge relationships were evaluated experimentally and compared to published submergence data for linear and labyrinth weirs. For relatively low levels of submergence, the piano key weir requires less upstream head relative to the labyrinth weir (<6%). This increase in efficiency was reversed at higher levels. Staged labyrinth weirs feature multiple weir segments with different crest elevations, which confine base flows and/or satisfy downstream discharge requirements. Head-discharge relationships for various laboratory-scale staged labyrinth weir configurations were established. The accuracy of a head-discharge predictive technique based upon superposition and traditional labyrinth weir empirical data was evaluated, and found to be generally within ?5%. The influence of linear, labyrinth, and staged labyrinth weir head-discharge characteristics on the outflow hydrograph behavior was evaluated by numerically routing various flood discharges through a fictitious reservoir; peak outflow, maximum water surface elevation, and required detention volume data are presented for each weir alternative. A staged labyrinth weir can be an effective alternative for decreasing the peak outflow hydrograph for frequent events, while increasing discharge for higher return period storm events. Approach flow perpendicular to the labyrinth weir centerline axis may not be possible in all situations. The head-discharge characteristics of a laboratory-scale labyrinth weir were evaluated with three different approach flow angles (0?, 15?, and 45?). For approach flow angles up to 15?, no measurable loss in discharge efficiency occurred. The discharge efficiency reduced as much as 11% for the 45? approach angle case. While all data presented are specific to the weir configurations and geometries tested, these data can be beneficial to the general understanding of nonlinear weirs.

Thesis (MEng)--Stellenbosch University, 2015.<br>ENGLISH ABSTRACT: General objective of the study This thesis aims to evaluate the hydraulics of an inflatable weir in its fully inflated position to the almost fully deflated position using different diameter circular weirs with varying discharges, by considering the change in the weir radius and the dynamic pressures on the weir. In the evaluation, three cylindrical weirs were installed in a 2m wide flume and tested over various discharges. Methodology The three weirs, one with a 300mm diameter, another with a 250mm diameter, and the last one with a 100mm diameter, were used to determine the effects of over flow water on the weir as seen in the different stages of the normal operation of an inflatable weir. Simulation involved measurement of the upstream and downstream water levels with the weir height involved at stable over flow conditions. Measurement of pressure variations was done on the weir faces with different water inflow rates to the test flume with three pressure sensors installed on each weir at 0°, 11.25° and 22.5° from the crest to the downstream. Additionally a single 0.15m radius weir was tested for pressures 67.5°, 78.75° and 90° from weir crest. Water level variation on the downstream of the weir was created by means of a variable tail gate to observe its effects. Results of the investigation The effects of upstream arches, stage, radius of curvature, discharge, pressure, energy losses over the weir and the downstream hydraulic jump were investigated in the inflation and deflation of the inflatable weir. The findings were as follows: ►Based on literature by Chanson and Montes (1998), Shabanlou et al. (2013), Schmockeret al. (2011) and Bahzad et al. (2010), upstream arches have insignificant influence onthe performance of the inflatable weir. There is rather reduced afflux due to the shape ofthe upstream of the weir from the Bernoulli’s equation. This shape of the upstream of theweir also contributes to the transport of sediments Gebhardt et al. (2012). ►Investigation of the discharge coefficient and factors influencing showed that: oAs the weir radius is reduced during the deflation, the unit discharge over each weirincreased with increase in head above the crest. oDischarge coefficient of the inflatable weir increases with the increase in head aboveweir crest, and the discharge coefficient is inversely proportional to the radius ofcurvature of the weir. ►Investigation of pressures on the downstream face of the weir models showed that: oThe negative (suction) pressure acting on the downstream face of the weir becomesincreasingly negative with increase in H/R values. oPoint of separation of nappe was seen with pulsations of pressure of the recordpressure. Generally, energy dissipation over the weir decreases with the decrease in the weir radius and the jump is more stable with the smallest circular weir and can be more accurately determined in the case of a small weir. Conclusions and Recommendations The inflatable weir has a high discharge at its fully inflated position. Its hydraulic performance is largely influenced by inflow head and is inversely proportional to the radius of curvature. Nappe pulsation as seen in the nappe vibrations can cause the vibration of weir. Future research on inflatable weirs should aim to monitor the negative pressure on measuring pressures further down the face of the weir because larger negative pressures are expected to develop after 90˚ as with this study.

A hydraulically undersized control structure could result in water overtopping a dam or channel banks. To increase hydraulic capacity and reduce flooding risk, nonlinear spillways are frequently replacing linear weirs. This study investigates four subjects to further knowledge for two types of nonlinear weir, the piano key and labyrinth. Weir submergence is a condition when the downstream water level of a weir exceeds the weir crest elevation, and can influence the head-discharge relationship of the structure. The effects of submergence on laboratory-scale piano key weir head-discharge relationships were evaluated experimentally and compared to published submergence data for linear and labyrinth weirs. For relatively low levels of submergence, the piano key weir requires less upstream head relative to the labyrinth weir (<6%). This increase in efficiency was reversed at higher levels. Staged labyrinth weirs feature multiple weir segments with different crest elevations, which confine base flows and/or satisfy downstream discharge requirements. Head-discharge relationships for various laboratory-scale staged labyrinth weir configurations were established. The accuracy of a head-discharge predictive technique based upon superposition and traditional labyrinth weir empirical data was evaluated, and found to be generally within ±5%. The influence of linear, labyrinth, and staged labyrinth weir head-discharge characteristics on the outflow hydrograph behavior was evaluated by numerically routing various flood discharges through a fictitious reservoir; peak outflow, maximum water surface elevation, and required detention volume data are presented for each weir alternative. A staged labyrinth weir can be an effective alternative for decreasing the peak outflow hydrograph for frequent events, while increasing discharge for higher return period storm events. Approach flow perpendicular to the labyrinth weir centerline axis may not be possible in all situations. The head-discharge characteristics of a laboratory-scale labyrinth weir were evaluated with three different approach flow angles (0°, 15°, and 45°). For approach flow angles up to 15°, no measurable loss in discharge efficiency occurred. The discharge efficiency reduced as much as 11% for the 45° approach angle case. While all data presented are specific to the weir configurations and geometries tested, these data can be beneficial to the general understanding of nonlinear weirs.

A piano key (PK) weir is a type of nonlinear (labyrinth-type) weir developed specifically for free-surface flow control structures with relatively small spillway footprints. Currently, no generally accepted standard PK weir design procedure is available. This is due, in part, to the large number of geometric parameters and a limited understanding of their effects on discharge efficiency (discharge efficiency is quantified by the discharge coefficient of the standard weir equation). However, Hydrocoop, a non-profit French dam spillways association, has recommended a PK weir design and a head-discharge relationship specific to that geometry. To develop a better understanding of the effects of PK weir geometry on discharge efficiency, 13 laboratory-scale, 4-cycle PK and rectangular labyrinth weir configurations were tested. As a result, the influence of the following PK weir geometries and/or modifications on discharge efficiency were partially isolated: the inlet-to-outlet key width ratio, upstream, and downstream apex overhangs; sloped floors; raising the crest elevation via a parapet wall; fillets underneath the upstream overhangs; and the crest type. The physical model test matrix also included a PK weir configuration consistent with the Hydrocoop-recommended design. From the experimental results, the appropriateness of the Hydrocoop-recommended head-discharge relationship was evaluated, along with the discharge coefficient behavior associated with the standard weir equation. Finally, trapezoidal labyrinth weirs were compared to PK weirs to make a relative comparison of nonlinear weir discharge efficiency; comparisons were made considering crest length and structure footprint.

If humanity is to make the best of this planet then it is crucial that we develop the capacity to implement the most effective environmental management practices. Essential is a holistic approach to management, as is advocated by integrated catchment management (ICM), which proposes that catchment management issues will be best dealt with when interventions are planned together at the catchment scale and all stakeholder interests are given consideration during decision making. The issue of weir modification is a good example of a problem that would benefit from these principles. Many stakeholder interests are affected by weir modification, and if effective and fair weir modification decisions are to be made, all must be used to evaluate alternative weir modification options. So that decision makers can make the most of the synergies and avoid the conflicts that can occur between interventions, they need to know how multiple weir modifications interact. To do this decision makers must be able to manage and utilise a large amount of information and use it to help them make effective decisions. The objective of the research presented in this thesis is to develop an approach to the management of weirs in the Don Catchment that is holistic both a spatial sense and in terms of the assessment of alternative management options. An evaluatory framework for weir modifications is formulated by adapting published typologies of river ecosystem services (ESs). The prediction of how catchment interventions affect sociocultural ESs is recognized as a particularly challenging to the application of this framework because their qualitative and subjective nature makes them hard to predict. Bayesian Networks (BNs) are identified as a potential solution as they use probabilities to describe the relationships between variables. A BN was built to predict how weir modification affected weir danger and weir fun for canoeists by utilising the knowledge of canoeing groups. It is concluded that despite a number of caveats, BNs offer a potentially important method for allowing sociocultural ESs to be predicted in decision making processes. The consideration ofthe complex interdependencies multiple weir modifications can have is recognised as another of the challenges facing weir management decision making. A spatially explicit modelling approach is developed that can account for the interactive effect multiple weir modifications have on river connectivity for several river species in the Don Catchment. Expert judgement and hydrological modelling are used to discriminate between different levels of habitat quality for European eel (Anguilla anguilla) and Atlantic salmon (Safrna safar). Several strategies to increase connectivity in the Don Catchment were explored. It was found that each had its own set of winners and losers, indicating trade-offs between species need to be considered when planning connectivity enhancements. The modelling approach shows the interdependent effects of weir modifications are vet: important in determining habitat accessibility, particularly the cumulative effect of multiple fish passes.