2. © NERC All rights reserved Groundwater flooding FLOOD1 – Interreg IIIA project (BGS, BGRM, Brighton Uni.) Aim to understand processes leading to groundwater flooding in Chalk and produce predictive models for Somme and Brighton Somme valley at Abbeville, April 2001 (Ministère de l’Ecologie et du Développement Durable) Flooding of the Patcham area, Brighton November-December 2000 ( John Ellis, EA) www.flood1.info

3. © NERC All rights reserved Outline What specific questions are we trying to answer using models? How do we represent the processes of interest? With current commonly applied techniques, what can we do well, and where is there room for improvement?

4. © NERC All rights reserved Groundwater flooding Definition: ”A groundwater flood event results from a rise in groundwater level sufficient for the water table to intersect the ground surface and inundate low lying areas” Flooding from other sources (HA4a), 2006, jba Consulting

5. © NERC All rights reserved What do we want models to do? Explain – why and how Predict – where and when Constraint – need a methodology that can be applied in other regions

6. © NERC All rights reserved Processes of interest Response of water table to rainfall Effect of “micro” geology on water table

7. © NERC All rights reserved What have we tried Using DTM and water table contours Distributed recharge model Regional groundwater model with simplified geology Unsaturated zone model

8. © NERC All rights reserved What have we tried Using DTM and water table contours Distributed recharge model Regional groundwater model with simplified geology Unsaturated zone model

9. © NERC All rights reserved

11. What have we tried Using DTM and water table contours Distributed recharge model Regional groundwater model with simplified geology Unsaturated zone model

12. © NERC All rights reserved Average recharge (1990-2003)

13. © NERC All rights reserved

15. What have we tried Using DTM and water table contours Distributed recharge model Regional groundwater model with simplified geology Unsaturated zone model

16. © NERC All rights reserved Average groundwater levels

17. © NERC All rights reserved Transmissivity distribution

18. © NERC All rights reserved Modelled water levels

19. © NERC All rights reserved Difference between model and observed water levels

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25. What have we tried Using DTM and water table contours Distributed recharge model Regional groundwater model with simplified geology Unsaturated zone model Data

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27. What have we tried Using DTM and water table contours Distributed recharge model Regional groundwater model with simplified geology Unsaturated zone model Data Boundary conditions

28. © NERC All rights reserved The way forward for prediction Analysis of hydrographs

29. © NERC All rights reserved The way forward for prediction Analysis of hydrographs Interested in GWL maxima Water level related to rainfall Want to predict how high water level may rise during next recharge season

30. © NERC All rights reserved Predicting groundwater floods 1. Calibrate and validate model using historic GWL data Multiple linear regression model where annual maximum groundwater level (m OD) is a function of previous annual minimum and winter rainfall (mm)

31. © NERC All rights reserved Predicting groundwater floods 1. Calibrate and validate model using historic GWL data Multiple linear regression model where annual maximum groundwater level (m OD) is a function of previous annual minimum and winter rainfall (mm)

32. © NERC All rights reserved 2. Predictive model Use Monte Carlo simulation to predict a range of annual maximum groundwater levels, based on a range of possible winter rainfall scenarios, up to nine months ahead 1. Calibrate and validate model using historic GWL data Multiple linear regression model where annual maximum groundwater level (m OD) is a function of previous annual minimum and winter rainfall (mm) Predicting groundwater floods

33. © NERC All rights reserved 1. Calibrate and validate model using historic GWL data Multiple linear regression model where annual maximum groundwater level (m OD) is a function of previous annual minimum and winter rainfall (mm) Predicting groundwater floods 2. Predictive model Use Monte Carlo simulation to predict a range of annual maximum groundwater levels, based on a range of possible winter rainfall scenarios, up to nine months ahead

34. © NERC All rights reserved