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SESSION 2017 FLOODPLAIN REPRESENTATION AND ACCURACY OF PROCESSES 1D/2D SUPERVISORS James CELRAY Leslie SALVAN TEAM MEMBERS Tessie BARJAT Anaïs MAYAU Tamsin JONES Aymeric FRANCOIS Laura TALBA Konlavach MENGSUWAN Andres GONZALEZ INIGUEZ Miquel SARRIAS MONTON Oussama MOKHTARI Chenyu LIU 2 INTRODUCTION Floodplain: • Area of low-lying ground adjacent to a river subject to flooding • Need to understand how water flows in these areas Accuracy of processes: • Depends upon the model used • Ability to model ‘real processes’ occurring in the floodplain • Need to produce a 1D and 2D model – compare • 3 November 2011 event chosen 3 INTRODUCTION Aim: Compare 1D and 2D model processes for the floodplain – find the limitations of processes Objectives: represented. Vs. 1. Discuss processes of 1D and 2D models 2. Produce an output for HEC-RAS (1D). 3. Produce an output for Telemac (2D). The var river 4. Compare the representation of the floodplain for each model 4 1D VS 2D MODELS 2D MODELS 1D MODELS Cross-sections taken perpendicular to river flow (x) Uniform water level across the cross-section Uniform velocity across the cross-section Better for low gradients Velocity is not considered to be uniform Mathematical governing equations Terrain represented as a continuous surface (x,y) Better for steep gradients Water depth is not considered to be uniform 5 DOMAIN DEFINITION - HECRAS ● Lower Reach of the Var River Basin → Covering the Floodplain ● Upstream Boundary → La Manda Bridge (observed discharge data) ● Downstream boundary → Mediterranean Sea 6 TOPOGRAPHY REPRESENTATION Original DEM DEM after bridges extraction 7 TOPOGRAPHY REPRESENTATION WEIRS 8 CHOICE OF MODEL - HECRAS Way to Mike 11solve case problem of uniform water level by the implementation of levees Meters Meters 9 HEC-RAS (MODEL SET-UP) Topography 5m resolution DEM ArcGIS +HEC-GeoRAS Cross sections 10 HEC-RAS (MODEL SET-UP) Roughness coefficient RIVER BED n (s/m1/3) Dense vegetation 0.080 Some vegetation 0.050 No vegetation 0.033 FLOOD PLAIN n (s/m1/3) Agricultural areas 0.050 Industrial areas 0.100 Urban areas 0.150 11 HEC-RAS (MODEL SET-UP) Hydrograph The hydrograph used as upstream boundary condition was observed on La Manda bridge on November 2011. 12 HEC-RAS (MODEL SET-UP) Boundary Conditions ● Upstream: Flow Hydrograph (La Manda bridge, 2011) ● Downstream: Rating curve obtained from Manning equation, assuming a rectangular cross-section. Initial Conditions ● Initial distributed flow (20m3/s) 13 HEC-RAS (MODEL SET-UP) Computation aspects 1D Courant–Friedrichs–Lewy condition: ● 17 days computed ● 30s computational time step ● 30min output interval ∆x ≈ 100m u ≈ 2m/s Cmax = 1 ∆t < 50s about 5 min to compute 14 HEC-RAS (FLOODPLAIN REPRESENTATION) 10 meters between cross sections 100 meters between cross sections 15 HEC-RAS (RESULT) 16 TELEMAC (MODEL SET-UP) MANNING ● Mesh characteristics: DISTRIBUTION ○ 25 m size ○ part of the sea 0.05 0.08 ○ buildings ○ try to remove bridges elevation ● Simulation parameters & equations: ○ 2 boundary conditions ○ Numerical scheme SaintVenant FE ○ Manning’s friction law 0.1 0.15 0.033 BUILDINGS DISTRIBUTION 17 TELEMAC (RESULT) • Many observed problems for: false flux observed bridges impact the flood map mesh size too big time step too high faced turbulence TURBULENCE WATER DEPTH & VELOCITY AT THE BRIDGE 18 TELEMAC (RESULT) IMAGE WATER DEPTHS CALCULATED AIRPORT ZOOM 19 TELEMAC (RESULT) IMAGE VELOCITY DISTRIBUTION AIRPORT ZOOM 20 CONCLUSION 1D: ● Quicker process time ● Limitations - uniform velocity ● Accuracy - increase number of cross-sections (slower). Depends on modellers experience 2D: ● Accounts for variability in the river ● Limitations - computation time ● Accuracy - finer mesh Depends on the purpose - flood forecasting, scheme design QUESTIONS? Thank you to all lecturers especially our supervisors James and Leslie for their advice and experience We are grateful for the opportunity to take part in HydroEurope 21