ABSTRACT
Design and analysis of flood defence schemes is normally based on a single https://www.w3.org/1998/Math/MathML"> 1 : N https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq14662.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> year extreme flow event using a ‘snapshot’ survey of channel morphology. This assumes that channel capacity is identical for all https://www.w3.org/1998/Math/MathML"> 1 : N https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq14663.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> year events, thus failing to account for morphodynamic change between events. The aim of this paper is to use the 1D sediment transport model HEC-RAS to investigate how morphological change occurring during a sequence of flood events influences river morphology; and the extent to which it may compromise flood defences. The HEC-RAS model is widely used in practical engineering analysis and understanding its performance is essential to appropriate application in design and flood risk analysis. Using the gravel-bed American River, California, USA as a case study, four synthetic sequences of flood events were generated from combinations of estimated 5,10,25,50 and 100 year extreme flow events. For each flood sequence, changes to the channel invert (lowest elevation) profile and median diameter of the bed sediment were analysed and compared to investigate reach scale and system wide changes. Data show generally higher channel erosion at reach scale locations when high flow events are clustered at the start of a flow sequence. Conversely, multiple small ( https://www.w3.org/1998/Math/MathML"> ≤ 10 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq14664.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> year return period) events at the start of a flow sequence mitigate erosion during subsequent high flow conditions. Local adjustments in bed level (up to 0.65 m, where the average flow depth is 5.4 m ) were predicted, which are significant in terms of flood defence performance and subsequent flood risk analysis. System-wide morphological change appears relatively insensitive to flood sequence.
