High Resolution UAV Image Processing to Monitor Urban Coarse Sediment: Implications for Flood Management

Reactive management of watercourses following flooding has concentrated on removal of gravel and clearing of vegetation, both of which are perceived to influence local flooding. Drone-based digital elevation models and orthophotography were employed to critically assess the location, type and magnitude of sediment mobilization on two watercourses in the English Lake District (River Kent at Kendal, and River Greta at Keswick), following several geomorphologically effective floods between 2018 and 2020. The detailed 0.02 m resolution imagery was interrogated to quantify individual clast scale movement across the range of grain sizes present. It would appear that both sites have become armored over time with large cobbles remaining generally immobile during flood events, while smaller gravels are being supplied and transported through the study reaches until the falling limb when this material is prone to stalling in over-wide reaches. This flood mobility suggests that material of this caliber, although present in the reaches, is soon mobilized and moved downstream in a flood and does not act to increase local flood risk unless stabilized by vegetation.