2023 CSDMS meeting-114


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River morphodynamics and flooding on the Nooksack River, Washington State: an investigation of drivers and feedbacks

Shelby Ahrendt, (she/her),University of Washington, Civil & Environmental Engineering Dept. Seattle Washington, United States. sahrendt@uw.edu
Alex Horner-Devine, University of Washington, Civil & Environmental Engineering Dept. Seattle Washington, United States.
Jacob Morgan, Paradigm Environmental Nashville, TN Tennessee, United States.
Wuming Ni, Zhejiang University, Department of Civil Engineering Hangzhou , China.
Guillaume Mauger, University of Washington, Climate Impacts Group Seattle Washington, United States.
Erkan Istanbulluoglu, University of Washington, Civil & Environmental Engineering Dept. Seattle Washington, United States.

River morphodynamics can affect overbank flooding if changes to cross-sectional area or roughness reduce the flow conveyance capacity of the channel. Correspondingly, extreme floods can also cause drastic adjustments to river morphology on relatively short timescales. These co-occurring processes raise the question: how do flood dynamics and river morphology co-evolve? The difficulty of conducting rapid field measurements of river geometry during peak flows has limited existing research on channel adjustment and recovery during a flood hydrograph. Here, we leverage a one-month Delft3D simulation to investigate flood hydraulics and river morphodynamics in a November 2021 flood event in the Nooksack River, western Washington State (WA). This flood devastated river-adjacent WA communities along the Lower Nooksack. Flood waters additionally overtopped the levees near Everson, WA and traveled over 25 km north towards the Fraser River, causing extensive and costly damages across the U.S.-Canada border. To understand the feedbacks between river morphodynamics and floods, we analyze streamwise changes in flood hydraulics (flow velocity & shear stress) and morphodynamics (bed elevation change). Within the Everson overflow region, we find that spatial gradients in flow velocity shift between in-bank- and peak-flow conditions. Bed elevation changes are commonly located where hydrodynamic gradients intensify during peakflows. Importantly, at the Everson overflow location, bed deposition co-occurs with a drastic along-channel velocity decrease and overbank flow, suggesting that the hydraulics of overbank flooding can contribute to local morphodynamic adjustment. We additionally investigate how channel adjustments during the first November 2021 flood peak affect flooding during a secondary flood peak occurring two weeks later. These results are of particular interest to the Nooksack River floodplain managers who are eager for insights on the contribution of channel morphodynamics to flooding and who are actively investigating methods to alleviate overtopping including setback levees and dredging.