1. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Multi-layered approach to 2D urban flood modelling A. S. Chen, B. Evans, S. Djordjević, D. A. Savić

2. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Improved efficiency of 2D flood modelling – Reduced-complexity models – Better numerical schemes – Parallelisation (MPI, OPEN-MP, GPU) – Adaptive meshes – Grid coarsening Motivation

3. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Building Coverage Ratio (BCR)‏ Area occupied by buildings within a grid cell Computing cell Building

4. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Conveyance Reduction Factors (CRFs) Computing cell Building Widths blocked by buildings on cell boundaries (in both x and y directions)

5. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Building alignment a bc def y x Computing cell Building

6. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Multi-layered approach

7. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Multi-layered approach

8. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Multi-layered approach

9. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Multi-layered approach

10. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 BCR & CRF values (calculated by GIS tool)

11. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Grid coarsening approaches Plain view Longitudinal elevation profile

12. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Grid coarsening approaches 1m grid resolution as the benchmark 20m grid coarsening approaches – Averaged DEM (average of ground and roof elevation) – Single layer with BCR & CRFs – Multi-layered with BCR & CRFs Error = difference from the benchmark

13. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Error map Single layer Averaged DEM Multi- layered

14. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Depth profile along the central longitudinal line

15. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Accuracy Averaged DEMSingle layerMulti-layered Overall domain RMSE (mm) 38.215.06.0 Middle section RMSE (mm) 53.020.65.7 Efficiency BenchmarkAveraged DEMSingle layerMulti-layered Grid resolution1m20m No of cells40,000100 106 Computing time [s]27,0002.41.22.7

16. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Case study

17. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 Multi-layered approach – Conclusions Keeps fine detail in coarse grid (as in fine grid) Grid-related error propagation reduced More realistic surface flow paths in coarse grid Enables much improved efficiency with little loss in accuracy Applicable to any 2D grid-based model

18. www.floodrisk.org.uk EPSRC Grant: EP/FP202511/1 a.s.chen@ex.ac.uk