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The prediction of river morphology evolution is very complicated, especially in the case of mountain and Piedmont rivers with complex morphologies, steep slopes, and heterogeneous grain sizes. The Lac des Gaves (LDG) reach, located within the Gave de Pau River in the Hautes-Pyrénées department, France, has precisely the complex morphological characteristics mentioned above. This reach has gone through severe sediment extractions for over 50 years, leading to the construction of two weirs for riverbed stabilisation. Two large floods resulted in changes in the LDG's hydromorphological characteristics as it went from a single channel river section to a braided river reach. In this study, a 2D hydromorphological model is developed with the TELEMAC-MASCARET system to reproduce the evolution of the channel following a flood that occurred in 2018. The model's validity is assessed by comparing the simulated topographic evolution to the observed one. The results reveal the challenge to choose well-fitted sediment transport equations and friction laws that would make it possible to reproduce such complex morphology. Although the exact localisation of the multiple channels forming the braided nature of the LDG was challenging to reproduce, our model was able to provide reliable volumetric predictions as it reproduces the filling of the LDG correctly. The influence of the two weirs on the river's current and future morphology is also studied. The aim is to provide decision-makers with more reliable predictions to design suitable restoration measures for the LDG reach. [ABSTRACT FROM AUTHOR]

Water control structures, used to regulate water levels and flow exchange in coastal marshes, act as barriers during fish migration between the ocean and brackish or freshwater ecosystems. Usual fish pass solutions may be unsuitable for obstacles subject to significant water level variations such as tidal range. This study proposes new solutions that were developed, implemented and evaluated on a marsh controlled by a series of hydraulic structures. These solutions were based on soft physical modifications (passive management) of the control gates, and on adaptations of their operation rules (active management). To evaluate the impacts of these adaptations, a hydraulic model of the marsh was built. It solves the one-dimensional Saint-Venant equations and appropriate gate equations. The model was used to identify management rules of control structures in a way to improve fish migration without significantly affecting the initial hydraulic management of the marsh (i.e. targeted seasonal water levels). Fish passability of upstream structures could be improved by managing downstream ones. It was concluded that the combination of active and passive management of water control structures could largely increase the passability of these obstacles during glass eel migration, while limiting seawater intrusion in the marsh and maintaining water levels into a range compatible with marsh management needs. [ABSTRACT FROM AUTHOR]

Hydropower plants can profoundly modify the natural morphology and hydrology of rivers and can alter the functionality of habitats for fish living and reproduction. In particular, hydropeaking leads to rapid and frequent changes of the hydrodynamic conditions and it is crucial to ensure aquatic habitat quality is maintained as much as possible during these periods. One present method to determine mitigation measure (minimum and maximum flow, rate of change) is to perform hydraulic simulations in 1D or 2D in the range of flow variation and to evaluate habitat quality for fish with the microhabitat method. The hydraulic model calibration has to be conducted carefully since the model has to reproduce precisely the hydraulic conditions from low to high flow rates (up to several times the mean flow of the rivers). Within this range, the friction coefficient can evolve greatly because at low flows the size of roughness elements become comparable to the water depth.. This phenomenon is observed by performing the modelling and the calibration at 2 stations on a river in French Pyrenees with different cross section shapes, one with progressive overflowing of some banks with large blocks and the other one without such phenomena. Thanks to field measurements of water levels at low and high discharge, the calibration process has shown that the friction coefficient can be multiplied by 2 as a function of the discharge. The paper proposes a methodology to evaluate the most appropriate tool. As water depth is concerned, the 2D simulations (TELEMAC 2D) provide similar results to those obtained with 1D (HEC-RAS) because flow remains unidirectional. Then a sensitivity analysis is carried out to estimate the uncertainty on the fish habitat outputs for a fish species (brown trout in the present study) resulting from several widely used friction laws. These friction laws can lead to different conclusions about habitat suitability depending on the calibrated coefficient. Finally, to perform relevant habitat modelling, it is necessary to measure water levels at several discharges and to describe accurately the spatial variability of roughness height.

Vegetation is a common feature in natural coastal and riverine waters, interacting with both water flow and sediment transport. However, the physical processes governing these interactions are still poorly understood, which makes it difficult to predict sediment transport and associated morphodynamics in a vegetated environment. In this context, an experimental study was conducted in laboratory with a movable bed trapped in artificial vegetation. The experimental flume is a rectangular open channel 5.75 m long and 0.29 m wide. For flow measurements, the channel is equipped with a fast camera and ADV probe. This work focuses on identifying the vegetation effects on flows and sediment transport. In fact, it was shown that the vegetation presence in a watercourse promotes deposition and sediment accumulation. This is explained by a reduction of the bed shear stress, since the friction occurs mainly by the drag force effect exerted by the vegetation. It was shown too that the vegetation reduced the bedload transport. Thanks to the partitioning of shear stress, it was possible to predict the bedload transport using standard formulas with a reasonable accuracy.

The contraction coefficient under sluice gates on flat beds is studied for both free flow and submerged conditions based on the principle of momentum conservation, relying on an analytical determination of the pressure force exerted on the upstream face of the gate together with the energy equation. The contraction coefficient varies with the relative gate opening and the relative submergence, especially at large gate openings. The contraction coefficient may be similar in submerged flow and free flow at small openings but not at large openings, as shown by some experimental results. An application to discharge measurement is also presented. DOI: 10.1061/(ASCE)HY. 1943-7900.0000122 CE Database subject headings: Gates; Contraction: Coefficients; Hydraulic structures; Potential flow; Submerged flow; Free flow; Discharge measurement.

An analytical model is developed to calculate the stage-discharge relationship for emergent and submerged rock-ramp fish passes. A previous model has been modified and simplified to be adapted to a larger range of block arrangement. For submerged flows, a two-layer model developed for aquatic canopies is used. A turbulent length scale is proposed to close the turbulence model thanks to a large quantity of data for fully rough flows from the literature and experiments. This length scale depends only on the characteristic lengths of arrangements of obstacles. Then the coefficients of the logarithmic law above the canopy can also be deduced from the model. As a consequence, the total discharge through the fish pass is computed by integrating the vertical velocity profiles. A good fit is found between the model and commonly observed values for fish pass or a vegetated canopy. The discharge of the fish pass is then accurately estimated for a large range of hydraulic conditions, which could be useful for estimating fish passability through the structure.

A numerical procedure is proposed to estimate the velocity distribution in open channel flow, for engineering applications. A method based on the mixing length model and sub-division of the wetted surface, is modified to easily integrate a lateral distribution of roughness. Several friction models for vegetated or rough flows can be added, two of which have been tested. The numerical procedure was developed with a commercial software (Matlab) to ensure fast computation and possible coupling with 1D hydraulic models. The validation of the computation has been done with rectangular and compound channels with spatially distributed roughness. The usefulness of the method for ecohydraulics is demonstrated with two applications: box culvert design for fish passes and 1D modelling of mountain river flows for which experimental results from a laboratory scale-model is compared to predictions. The interest of a friction law calibrated with a real grain roughness size is put forward. [ABSTRACT FROM AUTHOR]