Sediment continuity through the upland sediment cascade: geomorphic response of an upland river to an extreme flood event

Abstract

Hillslope erosion and accelerated lake sedimentation are often reported as the source and main stores of sediment in the upland sediment cascade during extreme flood events. While upland valley floodplain systems in the transfer zone have the potential to influence sediment continuity during extreme events, their geomorphic response is rarely quantified. This paper quantifies the sediment continuity through a regulated upland valley fluvial system (St John's Beck, Cumbria, UK) in response to the extreme Storm Desmond (4–6 December 2015) flood event. A sediment budget framework is used to quantify geomorphic response and evaluate sediment transport during the event. Field measurements show 6500 ± 710 t of sediment was eroded or scoured from the river floodplains, banks and bed during the event, with 6300 ± 570 t of sediment deposited in the channel or on the surrounding floodplains. <6% of sediment eroded during the flood event was transported out of the 8 km channel. Floodplain sediment storage was seen to be restricted to areas of overbank flow where the channel was unconfined. Results indicate that, rather than upland floodplain valleys functioning as effective transfer reaches, they instead comprise significant storage zones that capture coarse flood sediments and disrupt sediment continuity downstream.