Geomorphic response of a mountain gravel-bed river to an extreme flood in Aberdeenshire, Scotland

This study uses the 2015 'Storm Frank' flood on the River Dee, Aberdeenshire, to assess the impact of extreme events on river dynamics. The Storm Frank flood (> 200 year recurrence interval) caused significant local morphological change that was concentrated in the middle portion of the 140 km long river and overall net degradation that primarily occurred through lateral adjustment processes. Although the flood did not cause widespread change in channel planform, morphological change at the reach scale (<1 km) was significant. Bank scour resulted in channel expansion and lateral migration as well as widespread aggradation on existing gravel beds. The HEC–RAS and CAESAR–Lisflood models were used to determine the impact of morphological changes from the Storm Frank flood on a series of future hypothetical floods. The results show that inundation is highly influenced by the degree of morphological change for moderate floods, but not for high magnitude ones. In-channel scour and bank erosion can lead to an increase in channel capacity, thereby decreasing inundation. Conversely, where conveyance capacity is decreased by aggradation, flood risk inherently increases. The impact of these changes was great for a 5-year return period flood, but minimal for a magnitude flood comparable to Storm Frank's. Our modelling results also reveal the inundation model is sensitive to the grain size and channel bed roughness input parameters, as these parameters impact flow discharge and flood hydraulics. Accurate determination of sediment parameters and degree of morphological change is therefore critical in flooding modelling and flood hazard management.