How restoring natural physical riverine processes can mitigate the impacts of an increasingly extreme hydrological regime.

Direct anthropogenic disturbance to watercourses through impacts such as flood embankments, canalisation, dam construction, land management and urbanisation have resulted in a dramatically altered hydrologic regime. In combination with climate change, this has resulted in a greater severity and frequency of extreme flow events (i.e. both low and high). Such 'hydromodification' usually results in extensive disconnection of the floodplain from its channel. This can significantly affect ecology and downstream flood risk (traditionally mitigated using floodwalls and other hard-engineered control techniques). However, restoring natural hydrological and geomorphic processes at the catchment scale can provide greater resilience to extreme hydrological conditions, in terms of both physical and ecological considerations. We present data from UK case studies that demonstrate the effectiveness of applying 'process-based' Natural Flood Management and restoration approaches in managing the impacts of both high and low flows. Specifically, by comparing with previous conditions, we demonstrate how river/ floodplain restoration employing channel realignment, embankment removal, gravel augmentation, large wood implementation and wetland enhancement provided: (1) a significantly enhanced physical diversity, and, (2) reduced direct impacts from high flows (i.e. damaging hydraulic forces and flooding extents) and, (3) reduced impacts on fish during low flows. Habitat modelling (incorporating micro-scale hydraulic and sedimentary conditions) quantified an order of magnitude increase in suitable biotope area for juvenile and spawning salmonids under a drought scenario (indexed by long-term Q95) between pre- and post-restoration conditions. These predictions were validated through field-based monitoring of biological response by salmonids, including high precision redd mapping and electro-fishing surveys of juvenile populations. Furthermore, habitat modelling of flood flows identified similarly improved areal extents of refugia for instream biota between pre- and post-restoration conditions.