Simulation of wetland water-level manipulation using coupled hydrological/hydraulic modeling

Hydrological modifications frequently result in wetland loss and degradation while wetland management, restoration, and creation schemes rely upon further hydrological manipulations. These schemes can benefit from models which can accurately represent often complex wetland hydrological situations. A coupled hydrological/hydraulic model of the Elmley Marshes, an archetypal example of U.K. lowland wet grassland, has been developed using the MIKE SHE/MIKE 11 system. Long-term monitoring, supplemented by secondary sources, provided the necessary data. Calibration and validation were based upon comparisons of observed and simulated water-table depths and ditchwater levels for two consecutive 18-mo. periods. Results were generally consistent with the observed data and reproduced the seasonal hydrological dynamics. The close association between flooding and both water-table and ditch-water levels was demonstrated. The effects of modifying the heights of the drop-board sluices installed within the ditches were investigated. Ditch-water levels and the extent of inundation declined as the sluices were lowered until significant spills from the ditches onto the marsh surface were terminated. However, since the sluices are currently maintained at a high level, only limited gains in ditch-water level and flood extent could be achieved by raising the sluices further. The influence of alternative ditch-water levels upon the water table was limited due to the low hydraulic conductivity of the clay soil. These results have important implications for the management of the Elmley Marshes and similar wet grassland environments.