Sediment flushing downstream dams: a study on the clogging by fine sediments

Water reservoirs used to produce electricity have an impact on the environment and durability by : • stopping the natural sediment flux • changing the flow regime downstream of the reservoirs • storing (fine) sediments that reduce the storage volume Those 3 issues have been leading to the development of strategies in order to improve the equilibrium of the ecosystem downstream such structures, with solution such as sediment flushing and simple water flushing reproducing flood events. Those operations produce excessive sediment inflow on certain river sections, which can lead to the clogging of the gravel bed. River construction work, soil erosion, emergency actions and natural river bank erosion can also bring similar problematic. For example, a dramatic event happened in 2013 on the Spöl River in Eastern Switzerland. Due to some operations on a Punt dal Gall dam, important amountsof fine sediments were flushed downstream the reservoir and resulted in significant damages to the river ecosystem1. A strong clogging of the river bed was noted. A clean water flushing a few months later led to a cleaning of the clogged areas.

Transnational concept for the provision of streambank erosion hazard information

John Beck

Streambank erosion is an aspect of flood hazards that has been largely neglected to date due to its complexity. The INTERREG IIIb NWE project « Strategies and Actions for Flood Risk Management » (SAFER) has recognized its importance by including this aspect in the flood hazard component of the project. SAFER promotes integrated flood management by developing strategies for flood hazard mapping, flood partnerships and flood emergency management. These strategies are transnational due to the cooperation of partners from different geographical and administrative regions in their development. A transnational strategy for streambank erosion hazard mapping is developed. The demand for streambank erosion hazard information is listed so as to better develop the methodology with the end-user in mind. The methodology involves a regional model useful for prioritizing regions for more detailed analysis. The regional model consists of confronting calculated river shear stresses (via modelling or simplified formulas) with areas of high vulnerability. River reaches with the greatest conflicts can then be analyzed further. Geomorphological mapping is necessary before any further modelling can be done. It will permit an understanding of the sediment movements in the river catchment. The next step is to do detailed streambank erosion modelling with a geofluvial model that combines hydrodynamic, sediment transport and geotechnical modelling. The model CCHE1D of the National Center for Computational Hydroscience and Engineering of the University of Mississippi is adapted in the SAFER project for the detailed modelling. The adaptation is very significant because it provides a 1D model that can be applied to a meandering river reach of tens of kilometres. Data needs for regional and detailed modelling procedures are listed. The streambank erosion hazard mapping methodology was developed on the Venoge River in Switzerland and applied to the River Enrick in Scotland and the Lauter River in Germany. Streambank erosion monitoring on the Venoge River allowed for data collection techniques to be considered with respect to streambank erosion modelling. It also provided data for testing the adapted CCHE1D model. Streambank erosion danger maps were produced in accordance with Swiss criteria that classify the danger level according to the combination of the event frequency and magnitude. Shear stress maps as necessary in the regional modelling were produced for the Lauter River. These maps were rapidly produced from the channel geometry and hydraulic information from previous flood inundation studies. Observed erosion areas coordinate well with high shear stress areas. Shear stress and erosion maps were also produced for the River Enrick. This river permitted the testing of the CCHE1D model for streambank erosion hazard mapping in the case of a steep river with a high sediment load. The model permitted the calculation and mapping of the shear stresses and erosion for the historical period of 1993-2004 and for design flows. The River Enrick modelling showed the necessity of further research in modelling and methodological procedures for braiding areas. Application of the SAFER streambank erosion methodology to these three varied river settings permitted the development of a transnational strategy.

2007

Experimental assessment of riverbed sediment reinforcement by vegetation roots

Paolo Perona

Vegetation roots are known to increase soil stability on hillslopes, crops and river banks, thus reducing slide hazards, surface erosion and general processes affecting lateral channel migration. Many studies have tackled this classical engineering problem from either experimental or modelling viewpoints, in general describing the role of roots entanglement of soil particles as an additional cohesion, though non-physical. On river bedforms, the alluvial material on which pioneering plants grow is often non-cohesive. Moreover, both local and non-local erosion processes drive the mechanisms of vegetation uprooting. While the first are influenced by the ratio between non-porous obstacles diameter vs sediment grain size, the second have rather a morphodynamic origin, and are responsible for the onset and migration of alluvial bedforms. In both cases, ablation of particles does not occur as the failure of a volume along preferential sliding planes, but rather gradually when local hydrodynamic bed shear stress conditions exceed some critical value. Hence, classic soil stability models are inadequate to assess the role of roots on stabilizing riverbed sediment, and experimental methods are also not well established. This study proposes an analysis of the data available from the experimental setup of (Pasquale et al. 2011, Pasquale et al. in press), in order to inquire the effect of root reinforcement of river bedforms alluvial material. We compared the amount of sediment scouring recorded inside square plots (4 m(2)) with growing Salix cuttings with that occurring around them and caused by pure morphodynamic erosion, i.e. under the absence of obstacles inducing either deposition or erosion. We show that for those cases where the aforesaid assumptions hold, then bedload transport models can be used in an inverse way in order to explain the reduced erosion in the presence of roots as an increased critical shear stress condition, which might be more useful to morphodynamic modelling purposes. We discuss the accuracy of the proposed methodology in relation to data availability, and propose improvements to design more effective laboratory experiments.

Crc Press-Taylor & Francis Group2014

Physical modelling of artificial river replenishment techniques to restore morphological conditions downstream of dams

Elena Battisacco, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

First applied in the 1980’s, the artificial replenishment of sediments is one technique proposed to solve the problem of sediment deficit downstream dams. The present experimental research aims at improving the technique of river replenishment and at providing an engineering answer and framework for its application. A systematic series of laboratory tests are run to understand the hydrodynamics of the river flow when the replenishment technique is applied. Erodible volumes, with different lengths (occupancy of the replenishment volume is 1/3 of the channel width) and submergence conditions (100% and 130% submergence of the replenished volume), reproducing sediment replenishments volumes, are placed along a channel bank. Different geometrical combinations (single volume, double aligned volumes, double aligned half-volumes, double alternated half-volumes) of erodible sediment volumes are tested. The influence of discharge, the distance travelled by the eroded sediments and the time evolution of the erosion process are presented and discussed.