Embankment lowering and natural self-recovery improves river-floodplain hydro-geomorphic connectivity of a gravel bed river

Abstract

Floodplain reconnection can potentially improve the hydrogeomorphology of river corridors and attenuate flood peaks. However, empirical evidence of its effectiveness – needed to inform future restoration – is limited. A 70 m long flood embankment was lowered on the upper River Dee, a medium sized gravel bed river in north-east Scotland to reconnect a backwater and floodplain. Comparison of two years pre- and three years of post-restoration hydro-geomorphic monitoring, shows the lowering and subsequent adjustment, assisted by several floods including a <1% annual exceedance probability event, have altered the morphology and hydrological dynamics. Channel aggradation of up to 1 m occurred. Erosion of the bank by up to 0.41 m in depth and deposition of gravel or sand within the backwater and the floodplain were the dominant geomorphic responses elsewhere. The channel adjustment, unexpected in the restoration design, improved hydrological connectivity with the floodplain; annual median water table levels were on average 0.037–0.089 m higher post-restoration although the correlation between river discharge and floodplain water levels did not change. Based on 1D modelling, the threshold river discharge for backwater connection decreased by 55% from 93 m3 s−1 (discharge exceedance percentile: Q0.4) to 42 m3 s−1 (Q8). Thus, overspill frequency increased. For a given peak discharge, floodplain water levels were higher on average by 0.25 m with a statistically significant difference (Mann Whitney U test: p < 0.05), due to improved hydrological connectivity. The monitoring demonstrates that localised but significant improvement of channel to floodplain hydro-geomorphic connectivity can result from targeted embankment lowering. Such actions could enhance future catchment resilience by improving water storage and biogeochemical processing.