Sensitivity analysis of hydraulic model to morphological changes and changes in flood inundation extent

1. Sensitivity analysis of hydraulic model to morphological changes and changes in flood inundation extent Do channel changes affect flood extent? J.S. Wong1, J. Freer1, P.D. Bates1, & D.A. Sear2 1University of Bristol; 2University of Southampton

2. 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?

3. Study Site - Cockermouth • Background • North West Cumbria, UK • one major river (River Derwent) and two tributaries (Rivers Cocker & Marron) – combined catchment area of 1235km2

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

5. 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

6. 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 167.75hrs, from 12:00 on 17th Dec to 23:45 on 20th Dec, 2009, across domain size of 100km2 • Monte Carlo simulations

7. Model Performance

8. Probability Flood Map

9. 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

10. Bed Elevation Change Scenario (95% Quantile)

11. Reevaluation of Model Performance

12. Differences in Flood Extent Probability

13. 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

14. 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

15. The End Questions and comments are welcome!