Hydrodynamics and sedimentary processes in the main drainage channel of a large open coast managed realignment site

Managed Realignment (MR) is becoming increasingly popular with many coastal managers and engineers. Monitoring of MR sites has provided growing evidence that many of the saltmarshes created in these environments have lower biodiversity than naturally formed intertidal marshes, and may not fully deliver the anticipated ecosystem services such as carbon sequestration and coastal flood defence. Despite the importance of the sedimentary environment in developing an intertidal morphology suitable for plant establishment and succession, the evolution of the sediment erosion, transportation, deposition and consolidation cycle in newly breached sites is rarely examined. This study evaluates the hydrodynamics and concentration of suspended sediment exported and imported along the main drainage channel within the Medmerry Managed Realignment Site, West Sussex, UK, the largest open coast realignment in Europe (at the time of breaching). Measurements were taken over a one year period (November 2015–October 2016) at the breach, at the landwards extremity where freshwater drains into the site, and in an excavated channel in the centre of the site. At the latter site, 1.7 cm of sediment accreted over the study period. Suspended sediment concentration (SSC) measurements indicate that, under ambient conditions, sediment is imported into and exported from the Medmerry site, although similar concentrations of sediment were recorded being internally redistributed around the site (typically 0.11 g/l measured in the breach area compared to 0.12 g/l measured in the centre of the site). Sediment is removed from the site following large (1–2 mm/hour) rainfall events, which take several tidal cycles to drain through the site. Peaks in SSC corresponding with lower intensity rainfall events, especially during periods when the intertidal mudflats have been exposed, have also been observed. Analysis of the hydrodynamics and patterns of sedimentation during and following storm occurrences (the 2015-16 Storms Eva, Imogen and Katie) however demonstrate the relative resilience (i.e. rapid recovery and minimal disturbance) of the site to extreme storm events.