Transnational concept for the provision of streambank erosion hazard information

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.

Sedimentological processes in the Rhone Delta subaquatic canyons (France-Switzerland)

Nicolas Le Dantec

Deltas are very sensitive environments and highly vulnerable to variations in water discharge and the amount of suspended sediment load provided by the delta-forming currents. Human activities in the watershed, such as building of dams and irrigation ditches, or river bed deviations, may affect the discharge regime and sediment input, thus affecting delta growth. Underwater currents create deeply incised canyons cutting into the delta lobes. Understanding the sedimentary processes in these subaquatic canyons is crucial to reconstruct the fluvial evolution and human impact on deltaic environments, and to carry out a geological risk assessment related to mass movements, which may affect underwater structures and civil infractructure. Recently acquired high-resolution multibeam bathymetry on the Rhone Delta in Lake Geneva (Sastre et al. 2010) revealed the complexity of the underwater morphology formed by active and inactive canyons first described by Forel (1892). In order to unravel the sedimentary processes and sedimentary evolution in these canyons, 27 sediment cores were retrieved in the distal part of each canyon and in the canyon floor/levee complex of the active canyon. Geophysical, sedimentological, geochemical and radiometric dating techniques were applied to analyse these cores. Preliminary data show that only the canyon originating at the current river mouth is active nowadays, while the others remain inactive since engineering works in the watershed occurred. However, alternating hemipelagic and turbiditic deposits in the easternmost canyons, evidence underflow processes during the last decades. Two canyons, which are located close to the Rhone river mouth, correspond to particularly interesting deeply incised crevasse channels formed when the underwater current broke through the outer bend of a meander in the proximal northern levee. In these canyons, turbidites were observed in the sediment record indicating ongoing sediment dynamics during whether extreme flood events or mass-movements due to deltaic scarp failures. The active canyon shows a classic turbiditic system with frequent spillover processes in the canyon floor/levee complex. Geotechnical measurements, a decrease in the frequency of turbidites and a fining upward sequence along the levee suggest that erosion dominates sedimentation in the canyon floor, while sedimentation dominates in the rapid levee building-up process, with sedimentation rates that exceed 2.5 cm/yr in the proximal areas. Therefore, mechanisms controlling the sedimentary evolution on the active canyon represent a complex interplay between erosion and sedimentation.

2012

Time-dependent failure analysis of large block size riprap as bank protection in mountain rivers

Mona Jafarnejad Chaghooshi

The protection of riverbanks with blocks, called riprap, is the most used method in alpine rivers to avoid uncontrolled lateral erosion. For rivers with significant bed slopes large boulders have to be used in order to withstand high flow forces. Such large boulders cannot be dumped anymore, like in the case of lowland rivers, but they have to be placed individually by machines because of their weight. Consequently the blocks are better interlocked even if a rough surface of the riprap is required. Thus a higher resistance of such individually placed or “compressed” riprap may be expected. The existing design methods have been developed for dumped riprap with relatively small block sizes. Dr. Mona Jafarnejad studied for the first time systematically the effect of compressed riprap, that means with individually placed blocks having a good interlocking, on the failure resistance. Based on a relative roughness and modified Froude number, Dr. Jafarnejad proposed an empirical relationship which can assess the limit between stable and failure conditions of the blocks and thus gives a criterion for the required minimum block size. Furthermore, and also for the first time, the time-dependant failure was analysed which is also not considered in existing design methods. Dimensionless empirical relationships between the time to failure and the bed shear stress as well as flow depth were developed. The results revealed that for a total failure of the compressed riprap a relatively high number of blocks have to be eroded. In loose dumped riprap, normally the erosion of a few blocks results in a fast failure knowing that the time to failure is very important in practice since the flood peaks have only a limited duration. Additionally Dr. Jafarnejad studied the effect of a second layer of individually placed riprap on the time-dependant failure process, which is novel. She developed also an empirical relationship, which takes into account the influence of such a second layer. For the same longitudinal channel slope and side bank slopes, the second layer stabilizes the section and postpones failure. The effect of a second layer is more significant for higher bed slopes and bank side slopes. Finally, compared to the traditional design methods, using a safety factor approach, Dr. Jafarnejad developed a probabilistic failure analysis method for riprap, which considers uncertainties of the design parameters and the future evolution of bed load transport under climate change.

EPFL - LCH2016

Streambank erosion hazard mapping

John Beck

Streambank erosion hazard mapping has received much less attention than flood inundation mapping in the past due to the complexity of the task as well as bank protection works that have reduced bank erosion and unfortunately, the ecological functions of our watercourses at the same time. Damages due to streambank erosion in some flooding contexts are greater than the flood water damages (Loat and Petrasheck, 1997). For these reasons, streambank erosion hazard mapping should be an integral part of flood hazard mapping and methods must be developed to accomplish it. This research proposes a methodology for mapping streambank erosion hazards based on the directives of the Swiss Federal Office for Water and Geology (now within the Swiss Federal Office for the Environment). It permits the calculation of bank failure widths and their probability as opposed to future channel migration paths. This research also investigates the input data necessary for streambank erosion hazard modeling. Geomorphological mapping must be the first step to streambank erosion hazard mapping as it permits the identification of the sediment movements in the catchment. After this step, modeling of streambank erosion can be undertaken. Geofluvial models that combine hydraulic sediment transport and geotechnical modeling are well suited for streambank erosion modeling. The model CCHE1D is such a model and was adapted for the calculation of the streambank erosion hazard on an 8 kilometer reach of the Lower Venoge River, Switzerland. CCHE1D performs one dimensional hydraulic calculations. A shear stress correction function based on channel curvature distributes mean boundary shear stress appropriately to outer and inner bank toes and the phase lag of maximum toe shear stress compared to the apex of the bend curvature is ensured by a convolution of upstream shear stresses. Tension cracking was added to the slab failure algorithm due to its significant effect on bank failure widths. After a bank failure, the cross section shape does not change which allows the flow conditions to remain the same and in turn allows the probability of failure for the modeled bank profile to be evaluated. To gain a better understanding of streambank erosion on the Lower Venoge River, detailed erosion and flow depth monitoring were done on two 1 kilometer river reaches from November 2003 through September 2005. These measurements showed the mass failures to be mainly soil falls and cantilever failures. Measured bank erosion was linearly related to the product of maximum discharge and flood volume. Bank and bed sediment data were also collected for the study reach. Eighty-two cross sections were surveyed in 2004 in the 8 kilometer study reach. A new cross section tool was developed to properly reproduce scour holes in bends. It calculates transverse position and distance, graphs the cross section to allow identification of bank definition points, linearly interpolates, calibrates bed topography parameters based on surveyed cross sections, and interpolates with respect to channel curvature. Hydrological modeling allowed for the generation of input hydrographs for the period January 1979 - February 2005. This information was combined with historically based low probability floods to construct three 300 year discharge series. Flow and erosion measurements allowed for the calibration of roughness and critical shear stress parameters, respectively, in the CCHE1D model. Where detailed erosion measurements were not available, past channel migration served as a guide for estimating the critical shear stress. Calibrated critical shear stress was poorly correlated with measured bank properties indicating the necessity of measuring critical shear stress. A regression equation involving the percentage of fine sand – large silt and the fraction of non-vegetated bank explained 75% of the variation. The three 300 year discharge series were simulated with CCHE1D. Bank failures series were output for each of the banks of the 1149 computational nodes in the study reach. Empirical frequency was used to determine the bank failure width of a given probability. These bank failure widths and their probabilities were used to calculate a streambank erosion danger. To further qualify this danger, it was mapped with a bar proportional to the mean annual simulated erosion rate. The extreme failure width for the entire reach was determined by multiplying the maximum simulated failure width by a safety factor. The erosion hazard in straight reaches is too high showing that the shear stress reduction due to the highly vegetated banks needs to be taken into account better in the shear stress correction function. This research has demonstrated the feasibility of streambank erosion hazard mapping, although the quantity of input data necessary is prohibitive. Data acquisition methods must be researched and improved to reduce costs, and research must be continued to improve understanding of bank failure processes to be included in geofluvial models.

EPFL2006