<p> Streambank Erosion Hazard Mapping on the Venoge River, Switzerland John Beck 31 August 2005</p><p>Streambank erosion due to high river flows often causes significant damage and financial loss. Many times these losses are greater than those occurred from the flood waters (OFEG, OFAT et al. 1997). Large damages often result from bank protection failures. Even if the resulting erosion behind the failed protection is insignificant, the cost of replacing the bank protection is not. Since many European rivers are not allowed to meander (especially in urban areas), it has been forgotten how much erosion can occur when a bank protection failure does occur. These problems point to the necessity of being able to model streambank erosion. Streambank erosion modelling will allow a prediction of possible erosion for natural banks or for protected banks that have failed. Within the SAFER project, partners have been confronted by the streambank erosion problem. In Switzerland, the Federal Office of Water and Geology has ordered the Swiss cantons to include streambank erosion hazard maps in the natural hazards maps that are to be drawn up throughout the country (OFEG, OFAT et al. 1997). For the moment, streambank erosion hazard mapping is largely being done by fluvial geomorphologists who through fieldwork estimate the streambank erosion danger. There is a need in Switzerland to investigate complementary ways to produce the streambank erosion hazard map. In the Neckar catchment, there is a need for a preliminary streambank erosion map to diagnose areas for further attention. The Enrick River in Scotland poses serious problems of farmland erosion that need to be investigated. The River Dodder in the Dublin area has also caused erosion problems in the urban area. These problems pushed the SAFER project to include the development of streambank erosion modelling techniques (for detailed local analysis and regional analysis) in the hazard mapping part of the project. Model development is being conducted by the HYDRAM laboratory of the Ecole Polytechnique Fédérale de Lausanne, Switzerland, with data from the Venoge River in Switzerland. The Lower Venoge River has four alluvial zones which are mostly free to meander. Erosion has been monitored event after event during nearly two years in two of these alluvial zones. Stage recorders in these two detailed monitoring zones give an idea of the water depth during flooding. Particle size distributions for the bank and bed sediment have been determined, as well as the plasticity and liquid limits for the bank material. The work done on the banks allows an estimation of the cohesion, friction angle, and critical shear stress necessary for the model. Besides sediment data, the model needs very detailed topographic data. Detailed surveying of the Venoge has been done and a program to interpolate cross- sections in bends is under development. Discharge input into the model has been insured by the hydrologic model, HydroRoute, which is a semi-distributed, continuous, conceptual model capable of taking into account snowfall and snowmelt. For detailed erosion modelling, the model CCHE1D of the National Center for Computational Hydroscience and Engineering, University of Mississippi, USA is being used. This model is a 1D dynamic/diffusive wave hydraulic model that includes non-equilibrium sediment transport and bank erosion/mass failure. Model adaptations are being tested to improve shear stress estimations in bends and improve bank erosion estimations. Preliminary runs for the Venoge River show realistic failure widths. Bank erosion parameters will be calibrated against observed erosion. A simulation with the hydrologic data that spans from 1983-2005 will be done. Frequency analysis of bank failures will then allow a prediction of the frequency of a given erosion failure. These failure widths and their probability will then be mapped in the streambank erosion hazard map. Regional erosion modelling will try to take advantage of regional hydraulic modelling results and lidar topographic data, two data sets becoming more and more available to state water offices. The regional modelling technique will highlight areas with high banks (visible with the lidar developed Digital Terrain Model) and/or high shear stresses (developed from regional hydraulic modelling results). This modelling technique will help prioritize the order in which streambank erosion hazard mapping could be conducted.</p><p>Reference: OFEG, OFAT, et al. (1997). Danger naturels : recommandation : Prise en compte des dangers dus aux crues dans le cadre des activités de l'aménagement du territoire. Bienne.</p>