Surface elevation change in natural and re-created intertidal habitats, eastern England, UK, with particular reference to Freiston Shore

Abstract

Quantification of processes contributing to overall surface elevation change is critical to the accurate assessment of saltmarsh sedimentary status, particularly when predicting system stability in relation to accelerated sea level rise. Rates of short-term (up to 5.5 years) surface elevation change and accretion on a temperate saltmarsh were measured at an open coast managed realignment (habitat creation) trial, and the surrounding intertidal zone using surface elevation table–marker horizon (accretion plate) methodologies. Mean surface elevation changes on vegetated saltmarsh control sites, at some distance from the hydrodynamic disturbance of breaches in a former sea defence line, showed rates of change compatible with marsh surface dynamics seen elsewhere in the region, exceeding rates of regional relative sea-level rise. Within the managed realignment, sites close to the breaches showed exceptionally high rates of both accretion and surface elevation change, most likely as a result of high localized sediment supply from breach and channel enlargement and the presence of surfaces left unnaturally low in the tidal frame. Positive surface elevation change on the landward side of the breaches reached >30.4 mm a−1, up to one hundred times greater than rates of surface elevation change at locations 1 km from the breach entrances. Shallow sub-surface processes, or ‘surface subsidence’, was an important process on natural marshes outside the managed realignment but not seen within the site, due to the consolidated nature of the underlying substrate. The Freiston Shore managed realignment site shows remarkably similar time variation in mean elevation to that of the longer record from the Tollesbury managed realignment site, Blackwater estuary, Essex coast, UK. It is not clear how long it will take for re-created intertidal habitats at both Freiston Shore and Tollesbury to approach an ‘equilibrium’ elevation within the tidal frame but modelling suggests that in natural wetlands on the coast of eastern England this process takes at least 150 years.