Bank and bed stabilization using riprap: geomorphological impacts on embeddedness and implications for fish habitat

English Abstract:

Bank and bed stabilization, through riprapping in most cases, is a common and necessary measure to protect infrastructures such as bridges and roads from fluvial erosion. However, banks artificialization may trigger hydrological, biological and geomorphological changes. In particular, in a lowland context, the coarsening of the substrate is a drastic modification from the pristine river state, but it can be mitigated by the embeddedness of riprap with finer sediments. The research objectives of this study are 1) to identify factors that may improve the substrate through embeddedness at riprapped reaches ; 2) to provide an insight on the relevance of artificially adding fine sediment into riprap to increase embeddedness ; 3) to better assess the role of embeddedness on low-flow water stage essential for free passage of fish . Visual estimation of the embeddedness at 42 sites which were recently stabilized with riprap was conducted in the region of Montérégie-Est, Quebec, Canada. Results from a combination of field topography measurements and Lidar data reveal that embeddedness depends on local topography. No temporal trend in embeddedness was observed, indicating that sediment infilling happens quickly under favourable conditions, or does not occur at all depending on conditions. It is consequently argued that artificially adding fine sediment to promote naturalization of the substrate is a worthless operation. In addition, observations show that riprapping is often linked with bed rising which can have a negative effect on low-flow stage, potentially impacting free passage of fish during the summer. A conceptual model is presented displaying interactions between groundwater base flow, surface water stage, artificial bed heightening and embeddedness.