1. © NERC All rights reserved The use of a gw model to improve the conceptual understanding of gw flow in the Braunton Burrows dune systems – the known unknowns Corinna Abesser Andrew Hughes British Geological Survey

2. © NERC All rights reserved Braunton Burrows 5km 2km Contains Ordnance Survey data © Crown copyright and database rights 2010. Ordnance Survey licence number :100017897/2007

3. © NERC All rights reserved Braunton Burrows North-south oriented dunes (38 maoD) Discontinuous belt of slacks with ephemeral wet weather pools Rain fed groundwater system Designated area: SSSI/ Biosphere Reserve

4. © NERC All rights reserved Braunton Burrows Geology: Blown sand/ marine beach deposits overlying marine clays

5. © NERC All rights reserved Braunton Burrows Golf course Military training area Designated area status: SSSI/ Biosphere Reserve Piezometer transects in central / southern part of dune system Water level monitoring since 1966 Contains Ordnance Survey data © Crown copyright and database rights 2010. Ordnance Survey licence number :100017897/2007

6. © NERC All rights reserved Background Less frequent flooding of dune slacks Concern of ‘drying out’ of the dune system due to  Land drainage in adjacent Braunton Marsh  Increased ET caused by dune vegetation/ encroachment of various scrub species  Changes in effective precipitation Burden, 1998

7. © NERC All rights reserved Modelling aims To gain better understanding of dynamics of sand dune systems by investigating effects of  Changing precipitation pattern and  Changing vegetation cover on dune hydrology

8. © NERC All rights reserved Conceptual Model Robins (2007)

9. © NERC All rights reserved Conceptual Model

10. © NERC All rights reserved 1 layer 200 x200m cell size Leakage nodes Sea level Impermeable / no flow Numerical Model ? ? ?

11. © NERC All rights reserved Results

12. © NERC All rights reserved Results Y-scale: -1 – 15(25)m Green= observed Blue = modelled

13. © NERC All rights reserved Sensitivity Analysis Structural sensitivity Parameter sensitivity Boundary location Base level elevation Boundary conditions Hydraulic conductivity Transmissivity DrainageLeakage Storage

14. © NERC All rights reserved Field observations

15. © NERC All rights reserved 1 layer 200 x200m cell size Leakage nodes Sea level Impermeable / no flow Numerical Model

16. © NERC All rights reserved Results

17. © NERC All rights reserved Results Y-scale: 8 – 15m Green= observed Blue = modelled

18. © NERC All rights reserved Known unknowns T distribution - Base topography of aquifer - K distribution T= transmissivity (m 2 /day), K = hydraulic conductivity (m/day) ?????? Drainage/ recharge at golf course

19. © NERC All rights reserved Known unknowns T distribution - Base topography of aquifer - K distribution (zonation?) T= transmissivity (m 2 /day), K = hydraulic conductivity (m/day) Drainage/ recharge at golf course (historically/current)

20. © NERC All rights reserved Consult Stakeholders Define Problem Data collection Conceptual u/s Decide on approach Construct model Prediction simulations Refine model

21. © NERC All rights reserved Initial modelling....  provided valuable tool for testing conceptual understanding of the Braunton Burrows dune system  supported identification of additional mechanisms that control gw levels in the dune system  helped to constrain the unknowns / to identify important controls/ to focus data acquisition  allows better informed calibration based on (geological) understanding of the dune system

22. © NERC All rights reserved References Burden, RJ (1998): A hydrological investigation of three Devon sand dune systems: Braunton Burrows, Northam Burrows and Dawlish Warren. Unpublished PhD Thesis, University of Plymouth. Robins, N. (2007): Conceptual flow model and changes with time at Braunton Burrows coastal dunes. BGS Commissioned Report, CR/07/072N