Modelling of the 2005 flood event in Carlisle

1. Modelling of the 2005 flood event in Carlisle Jeff Neal1, Paul Bates1, Tim Fewtrell, Matt Horritt, Nigel Wright, Ignacio Villanuaver, Sylvia Tunstall, Hazel Faulkner, Tom Coulthard, Jorge Ramirez, Caroline Keef2, Keith Bevenand David Leedal3 1School of Geographical Sciences, University Road, University of Bristol, Bristol. BS8 1SS. 2JBA Consulting, South Barn, Broughton Hall, Skipton, N Yorkshire, BD23 3AE, UK. 3Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK.

2. Introduction • Carlisle 2005 event data • Overview • Issues • Inundation modelling • Channel hydraulics and gauges • Structural complexity • Resolution • New numerical scheme • Beyond inundation modelling • Urban futures • Probabilistic flood risk at confluences

3. 2005 event data

4. 2005 event data

5. 2005 event data

6. Channel hydraulics and gauges

7. 1D/2D model complexity

8. 1D/2D model complexity

9. Urban floodplain processes 25 m resolution 10 m resolution 5 m resolution Neal et al., 2009

10. i j • A new LISFLOOD-FP formulation • Continuity Equation • Continuity equation relating flow fluxes and change in cell depth • Momentum Equation • Flow between two cells is • calculated using: • Manning’s equation (ATS) hflow j i Representation of flow between cells in LISFLOOD-FP

11. A new LISFLOOD-FP formulation

12. A new LISFLOOD-FP formulation

13. Probabilistic flood risk mapping at confluences Q RP

14. The problem at confluences Q RP ? ? Q Q RP RP

15. The problem at confluences • Model the conditional distribution of a set of variables given that one of these variables exceeds a high threshold. Event simulation with spatial dependence

16. The problem at confluences (uncertainty) • Model the conditional distribution of a set of variables given that one of these variables exceeds a high threshold. Refit to data and run event generator may times to approximate uncertainty

17. Hydraulic modelling

18. Hydraulic modelling • LISFLOOD-FP hydraulic model (Bates et al., 2010) • 1D diffusive channel model • 2D floodplain model at 10 m resolution • Model calibrated on 2005 flood event (RMSE 0.25 m).

19. Hydraulic modelling • LISFLOOD-FP hydraulic model (Bates et al., 2010) • 1D diffusive channel model • 2D floodplain model at 10 m resolution • Model calibrated on 2005 flood event (RMSE 0.25 m). • Event simulation • 47000 events • Scaled 2005 hydrographs • Event simulation time was 0.1-2 hours • Analysis took 5 days and generated 40 GB of data

20. Run 1 flood frequency • Run 1 of the event generator using all flow data

21. Run 1 flood frequency • The maximum flood outline was a combination of multiple events. • Cannot assume same return period on all tributaries

22. Uncertainty in the 1 in 100 yr flood outline

23. Risk • MasterMap building outlines • Depth damage curve • Calculate damage from each event

24. Conclusions • Flooding at confluences is critical to the basin-wide development of flood hazard and depends on the joint spatial distribution of flows. • Assuming steady state flows over predicted flood hazard for a range of flows and event durations. • The maximum flood outline was a combination of multiple events. • Cannot assume the same return period on all tributaries • Risk assessment using the event data was demonstrated. • Expected damages increase nonlinearly. • As expected a few events caused most of the damage.