Long-term sediment management for sustainable hydropower

in Sayigh, A (Ed.) Comprehensive Renewable Energy. Vol. 6 (2012)

Since the second half of the 20th century and for decades to come, sedimentation in reservoirs is challenging our ability to maintain the storage capacity needed for water supply, flood mitigation and ... [more ▼]

Since the second half of the 20th century and for decades to come, sedimentation in reservoirs is challenging our ability to maintain the storage capacity needed for water supply, flood mitigation and hydroelectricity production. This complex issue needs to be accounted for from the early stages of the design of dams and reservoirs, until the definition of operation rules oriented towards sustainable management of water and sediments. With reference to the widely-used Driver, Pressure, State, Impact, Response (DPSIR) analytical framework (European Commission 2003), the present chapter reviews key figures and trends in worldwide reservoirs sedimentation, as well as analyzes contemporary sediment management strategies. Besides setting some fundamentals of reservoir sedimentation and summarizing up-to-date background information on rate and distribution of loss of storage in the world, we identify some key driving forces of soil erosion, such as land use, urban development, agriculture and deforestation. Main mechanisms of watershed erosion and sediment yield to reservoirs are also discussed, with a focus on their spatial and temporal variation. Means for quantifying the magnitude of this pressure on reservoirs sustainability are presented, including reservoir surveys, fluvial measurements and combined approaches with numerical modelling. After a review of sedimentation impacts, both locally and downstream as well as upstream of the reservoir, we describe current experience and future opportunities of response to reservoir sedimentation. Possible measures are classified into three broad categories: sediment yield reduction, sediment routing (bypass and pass-through) and sediment removal (dredging and flushing). Pro and contras are presented for the different modes of reservoir operation, as well as their expected efficiency and some specific constraints. Predicting the sedimentation pattern as well as grain sorting processes is a prerequisite for developing optimal sediment evacuation strategies, involving for instance flushing operations. Sound modelling of sediment transport and deposition in reservoirs is highlighted as highly beneficial to support sediment management in the perspective of achieving reservoir sustainability. The chapter also illustrates how spatially distributed numerical modelling may succeed in enhancing decision-making for selecting optimal sediment management measures. [less ▲]

Benefit of using flood risk analysis at the micro level for evaluating local protection measures

Conference (2012, April 23)

In numerous river basins, climate projections converge towards conditions leading to a significant increase in peak discharges both in terms of intensity and frequency. Therefore, managing flood risk will ... [more ▼]

In numerous river basins, climate projections converge towards conditions leading to a significant increase in peak discharges both in terms of intensity and frequency. Therefore, managing flood risk will remain an issue of primary importance. Besides, it is currently shifting from the search for full protection against flooding towards the management of the impacts of flooding. In this respect, the elaboration of effective flood management strategies should rely on an integrated risk-based approach, encompassing not only hydraulic criteria but also economic, social and environmental factors. In addition, the considered level of detail in the analysis should be relevant given the available data and the expected outcomes. While such risk analyses have so far been mostly undertaken at a macro- or meso-scale, the herein described analysis is performed at a micro-scale, meaning that the considered assets are the individual buildings, parcels or facilities. [less ▲]

Aménagement de la Raviège - Modèle réduit hydraulique du barrage

Report (2012)

Flow patterns and sediment deposition in rectangular shallow reservoirs

in Water & Environment Journal (2012), 26(4), 504-510

This work involves the experimental investigation of flow patterns, preferential regions of deposition and trapping efficiency in rectangular shallow reservoirs. The main flow patterns that can be ... [more ▼]

This work involves the experimental investigation of flow patterns, preferential regions of deposition and trapping efficiency in rectangular shallow reservoirs. The main flow patterns that can be encountered in rectangular shallow reservoirs are described: symmetrical flows without any reattachment point (S0), asymmetrical flows with one reattachment point (A1), and asymmetrical flows with two reattachment points (A2). The influence of geometrical and hydraulic parameters on reattachment lengths is intensively investigated. A shape parameter is introduced to classify symmetrical and asymmetrical flows. For each flow pattern, the preferential regions of deposition are studied. To conclude, a number of practical recommendations are given. Reservoirs with a shape parameter lower than 6.2 limit sediment deposition. Reservoirs with a shape parameter greater than 6.8 are favourable for sediment deposition. Finally, perspectives for maximizing and minimizing deposition are given, respectively by exploiting the great trapping potential of the flow pattern A1 and the poor trapping potential of the flow pattern S0. [less ▲]

The Taoussa project (Mali): an example of effective composite modeling

in 4th IAHR International Symposium on Hydraulic Structures (2012, February)

Physical modeling and numerical modeling are two efficient analysis approaches in hydraulic engineering. The interactive application of both methods is obviously the more effective response to most flow ... [more ▼]

Physical modeling and numerical modeling are two efficient analysis approaches in hydraulic engineering. The interactive application of both methods is obviously the more effective response to most flow problems analyses. This paper presents the results of a successful application of such a composite numerical – physical study carried out by the Research Group Hydraulics in Environmental and Civil Engineering (former HACH) of the University of Liège on behalf of Coyne et Bellier (Tractebel Engineering). It concerned, at the stage of detailed draft, the hydraulic study of the Taoussa Project on the Niger River in Mali. The studies, performed in less than 6 months, focused on the flow characteristics at the scale of the reservoir and the river, using the numerical approach, as well as on hydrodynamic details in the spillway using a large scale factor physical model with boundary conditions defined on the basis of numerical modeling. [less ▲]

Influence of the relative alveoli widths on Piano Key Weirs efficiency for varied weir heights

in Proceedings of Int. Symp. on Dams for a changing world (2012)

In the scope of dam rehabilitation to manage floods increase or to increase water storage, the Piano Key Weir is a good solution for concrete dams. Indeed, its labyrinth shape allows discharge capacities ... [more ▼]

In the scope of dam rehabilitation to manage floods increase or to increase water storage, the Piano Key Weir is a good solution for concrete dams. Indeed, its labyrinth shape allows discharge capacities until 4 times more important than traditional ogee-crested weirs and 10 percent more important than labyrinth weirs of same horizontal geometry. Furthermore, the use of up- and downstream overhangs reduces drastically its footprint and enables its use directly on dams crest. The efficiency of Piano Key Weirs is now well demonstrated through various experimental studies. Even if parametrical studies are currently undertaken, the definition of the optimal shape to give to the structure is still missing due to the lack of knowledge in the influence of the large set of geometrical parameters. This paper presents the results of a combine experimental and numerical study about the influence of the relative alveoli widths on the Piano Key Weir discharge capacity. 14 models have been tested providing results for a large range of the inlet to outlet widths ratio and considering variation of the weir height in accordance with former studies. The results of the experiments enable to define optimal values for a common variation of these two parameters. [less ▲]

Composite modeling to enhance hydraulic structures studies

in Proceedings of SimHydro 2012 (2012)

Physical modeling and numerical modeling are two efficient analysis approaches in hydraulic engineering. The interactive application of both methods is obviously the more effective response to most flow ... [more ▼]

Physical modeling and numerical modeling are two efficient analysis approaches in hydraulic engineering. The interactive application of both methods is obviously the more effective response to most flow problems analyses. Indeed, it enables combining the inherent advantages of both approaches, which are complementary, while being beneficial to the delays as well as the quality of the analysis. The paper presents the way composite modeling is applied for years at the HECE - Laboratory of Engineering Hydraulics (University of Liege) to enhance hydraulic structures studies. Besides numerical model validation for which experimental benchmarks constitute the first reliable data source, simultaneous application of both modeling approaches may be envisaged in three different ways. Composite modeling may be used to increase the scale factor of physical models by reducing the layout of the real structure to be modeled, to provide a better answer to specific problems than a single approach study or to maximize the efficiency of experimental tests by reducing the range of variation of the unknown parameters to be tested. For each of these three issues depicted in the paper, several examples show how the combined use of efficient numerical solvers together with physical scale models enables to increase the overall quality and scope of the analyses while decreasing the delays and possibly the costs. [less ▲]

Influence of weir height and keys slope on PKW discharge

in Proceedings of 4th IAHR International Symposium on Hydraulic Structures (2012)

Piano Key Weir is a cost effective solution for rehabilitation as well as for new dam projects with a high level of constraints (limited space, high specific flood discharge, small reservoir level ... [more ▼]

Piano Key Weir is a cost effective solution for rehabilitation as well as for new dam projects with a high level of constraints (limited space, high specific flood discharge, small reservoir level variation). While the higher efficiency of the Piano Key Weir compared to standard linear weirs has already been demonstrated, its optimal geometry is still poorly defined. Even if former studies highlighted the main influence of the weir height on its discharge capacity, the distinction between weir height effect and keys slope one is not so straight forward. In order to distinguish the influence of these two parameters, the use of parapet walls has been tested to increase weir height keeping bottom slope constant, and to decrease keys slope keeping the weir height constant. The experimental results presented in this paper enable to distinguish the relative influences of the keys bottom slope and of the weir height on the Piano Key Weir release capacity. Comparisons with former experimental results as well as design guidelines are also provided. [less ▲]

Composite modeling to enhance hydraulic structures studies

in Houille Blanche (2012), 6

Physical modeling and numerical modeling are two efficient analysis approaches in hydraulic engineering. The interactive application of both methods is obviously the more effective response to most flow ... [more ▼]

Physical modeling and numerical modeling are two efficient analysis approaches in hydraulic engineering. The interactive application of both methods is obviously the more effective response to most flow problems analyses. Indeed, it enables combining the inherent advantages of both approaches, which are complementary, while being beneficial to the delays as well as the quality of the analysis. The paper presents the way composite modeling is applied for years at the HECE - Laboratory of Engineering Hydraulics (University of Liege) to enhance hydraulic structures studies. Besides numerical model validation for which experimental benchmarks constitute the first reliable data source, simultaneous application of both modeling approaches may be envisaged in three different ways. Composite modeling may be used to increase the scale factor of physical models by reducing the layout of the real structure to be modeled, to provide a better answer to specific problems than a single approach study or to maximize the efficiency of experimental tests by reducing the range of variation of the unknown parameters to be tested. For each of these three issues depicted in the paper, several examples show how the combined use of efficient numerical solvers together with physical scale models enables to increase the overall quality and scope of the analyses while decreasing the delays and possibly the costs. [less ▲]

Modelling sediment transport over partially non-erodible bottoms

in International Journal for Numerical Methods in Fluids (2012), 70(2), 186-199

In depth- and cross section-averaged morphodynamic models, based on explicit time integration, it may happen that the computed bed level becomes lower than the top level of a non-erodible layer (e.g ... [more ▼]

In depth- and cross section-averaged morphodynamic models, based on explicit time integration, it may happen that the computed bed level becomes lower than the top level of a non-erodible layer (e.g. concrete, bedrock or armoured layer). This is a standard pitfall which has been addressed in different ways. In this paper, we present an original approach for avoiding computation of non-physical bed levels, using an iterative procedure to correct the outward sediment fluxes. The procedure is shown to be computationally efficient while it achieves a high accuracy in terms of mass conservation. We compare our original approach with the existing Struiksma’s method and with a reformulation of the problem in terms of mathematical optimization of a linear or non-linear objective function under linear constraints. The new procedure has been incorporated into an existing finite volume morphodynamic model. It has been validated with several 1D benchmarks leading to configurations with sediment transport over non-erodible bottom. The computation time has been verified not to increase by more than 15% compared to runs without non-erodible bottom. [less ▲]