Sensitivity analysis of hydraulic model to morphological changes and changes in flood inundation extent J.S. Wong 1, J. Freer 1, P.D. Bates 1, & D.A. Sear 2 1 University of Bristol; 2 University of Southampton Do channel changes affect flood extent?

Background and Motivation Flood inundation focus on simulation of inundation areas and flow depths influences of river geometry are neglected Morphological change increasing recognition of geomorphological impacts on flooding vital but still uncertain How do bed elevation changes influence flood extent during an extreme flood event?

Study Site - Cockermouth Background North West Cumbria, UK one major river (River Derwent) and two tributaries (Rivers Cocker & Marron) – combined catchment area of 1235km 2

Study Site - Cockermouth Why Cockermouth? extreme flood event in November 2009 significant course change deposition of debris on the floodplain

Data availability 3 datasets of observed flood extent Wrack marks 0.15m Aerial photography 1m TERRASAR-X imagery presence of pre-and post-event morphological surveyed data Number of cross- sections 234 Total length (m) MeanMaximumMinimum Width (m) Bed Elevation (m)

Model Setup 1D-2D LISFLOOD-FP inertial formulation of shallow water equations [Bates et al., 2010] 20m resolution DEM extracted from LiDAR gauged data as upstream boundary conditions free downstream boundary condition run for hrs, from 12:00 on 17 th Dec to 23:45 on 20 th Dec, 2009, across domain size of 100km 2 Monte Carlo simulations

Model Performance Channel FrictionFloodplain FrictionRMSE (m) Wrack marks – Aerial photography TERRASAR-X imagery ParameterMinimumMaximum Manning’s n (channel) Manning’s n (floodplain)

Probability Flood Map

Generation of Bed Elevation Scenarios A simplified approach initiation motion of grains at the bed, where shear stress exceeds critical shear stress maximum erosion depth is defined as focus on scouring effect, no deposition and lateral erosion ParameterMinimumMaximum Critical shear stress τ cr Grain size characteristic D

Bed Elevation Change Scenario (95% Quantile)

Reevaluation of Model Performance Channel FrictionFloodplain FrictionRMSE (m) Wrack marks ( – ) (0.0440) (0.2751) Aerial photography ( ) (0.0290) (0.4187) TERRASAR-X imagery (0.0470) (0.0200) (0.6303) ParameterMinimumMaximum Manning’s n (channel) Manning’s n (floodplain)

Differences in Flood Extent Probability

Conclusions The channel friction is insensitive on the amount of water that flows out of bank The entire valley floor is acting as a single channel unit in conveying the large flows No significant changes in flood extent before and after the bed elevation changes, possibly due to constraints of valley wall Further investigation on water depth Potential flood extent differences in response to morphological changes when given smaller flood event Potential errors in the specification of upstream gauged data

Future Work A 2D morphological model (CAESAR-LISFLOOD) will be set up to fully account for the morphological changes to flood extent and water depth Parameter space exploration of CAESAR-LISFLOOD to build up a modelling framework for identifying realistic morphological changes Application of modelling framework using Cockermouth as test site for future climate scanerios

The End Questions and comments are welcome!