Physicochemical changes in sediments at Orplands Farm, Essex, UK following 8 years of managed realignment

Managed realignment (MR) is a ‘soft’ engineering technique that involves the deliberate landwards retreat of the existing line of coastal defence and subsequent tidal inundation of land. Managed realignment has been established worldwide for over 30 years and its goals may include habitat restoration, recovery of biodiversity and sustainable coastal defence. In southeast England in particular, an increasing number of MR sites (20e 110 ha) have been commissioned in the last decade in response to increasing coastal habitat loss and sea-level rise. Following initial sea wall breaching and site flooding, monitoring of these sites is usually carried out for a period of 5 years and during this time changes in ecosystem structure can be easily observed. However, there is a poor understanding of the long-term effects of flooding on soil physicochemical parameters including sediment geochemistry and geochemical cycling, nutrient fluxes and soil maturation processes. Such physical and chemical changes may continue to take place over time-scales exceeding 5 years and therefore current monitoring practices may not be sufficient. This paper examines the changes in the physicochemical parameters of sediments in vertical core sections at the Orplands Farm MR site, Blackwater Estuary, Essex, 8 years after its flooding in 1995. Post-breach sediment accumulation rates at the site have been determined and a multi-proxy approach has been used to identify a pre-breach land surface. Soil development and the vertical distribution of saltmarsh plants following breaching have also been examined. The species saturation index approach has been used to provide an indication of the success of MR at this site. The pre-breach land surface was identified at 4e6 cmdepth indicating a sediment accumulation rate ca. 0.75 cm a 1 since the site was breached in 1995. This sedimentation rate exceeds the regional sea-level rise of 0.1e0.3 cm a 1 and hence marsh development at the site is expected to continue in the short term. The examination of physicochemical parameters with depth indicates that the pre-breach land surface forms a barrier that may inhibit vertical tidal flushing, nutrient transfer and contaminant removal. The vegetation was mostly a pioneer and low-mid marsh assemblage typical of the area, although the species saturation index is low compared to other European sites and this may be attributed to a range of factors including poor drainage at the site and reduced seed availability.