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

J.S. Wong1, J. Freer1, P.D. Bates1, & D.A. Sear2

1University of Bristol; 2University 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 1235km2

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) 28567.04Mean Maximum Minimum

Width (m) 34.97 195.83 18.51Bed Elevation (m) 36.93 67.44 4.20

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

Model PerformanceChannel Friction Floodplain Friction RMSE (m)

Wrack marks 0.0220 – 0.0420 0.0440 0.2751

Aerial photography 0.0380 - 0.1350 0.0290 0.4187

TERRASAR-X imagery 0.0470 0.0200 0.6303

Parameter Minimum MaximumManning’s n (channel)

0.02 0.14

Manning’s n (floodplain)

0.02 0.14

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

Parameter Minimum MaximumCritical shear stress τcr 0.047 0.083Grain size characteristic D50

0.042 0.082

Bed Elevation Change Scenario (95% Quantile)

Reevaluation of Model PerformanceChannel Friction Floodplain Friction RMSE (m)

Wrack marks 0.0660 (0.0220 – 0.0420)

0.0490 (0.0440)

0.3147 (0.2751)

Aerial photography 0.1240(0.0380 - 0.1350)

0.0340 (0.0290)

0.4179 (0.4187)

TERRASAR-X imagery 0.0470(0.0470)

0.0200(0.0200)

0.6065 (0.6303)

Parameter Minimum MaximumManning’s n (channel)

0.02 0.14

Manning’s n (floodplain)

0.02 0.14

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 EndQuestions and comments are welcome!