Field investigation and numerical modelling of drainage systems in coastal regions

Among the many concerns raised by scientists regarding the effects of climate change and sea level rise is that related to the capacity of existing land drainage systems to mitigate flooding in low-lying coastal areas. Increased storm intensities imply a need to drain water away from residential areas at a time when higher mean sea level implies a lower hydraulic gradient at high water, especially during spring tides. We use a numerical model, together with field data collected at Portsmouth Harbour, UK, to show how land drainage outlets effectively carve and maintain channels through intertidal areas of fine sediment, exporting large amounts of sediment and water into Portsmouth Harbour, an area of great strategic interest. We show how flap gates, which open automatically at low tidal levels, encourage automatic export of fine material, and do so to a far greater extent on spring tides and under high fresh water flows. Using a simple model, we investigate the sensitivity of net tidal sediment transport to varying tidal range and flow rate, thus enabling lessons to be learned about the effects of these simple drainage systems under a changing climate, and after changes to local land drainage strategies.