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