sensitivity analysis of hydraulic model to morphological changes and changes in flood inundation...

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!