Nearfield erosion at the steart marshes (UK) managed realignment scheme following opening

Abstract

Abstract The ingress and egress of water from managed realignment sites in macrotidal estuaries commonly causes erosion at the breaches and the formation of channels across the fronting intertidal areas. However, the rate of development of these features and their final dimensions have not been well documented. This case study paper details the development of the breach area and the erosional processes in the main and exit channels associated with the Steart Marshes managed realignment site in south west England. The paper shows type of data, along with its accuracy, which is currently available for this type of study. It uses a combination of UAV and LiDAR data that was collected as part of a monitoring campaign to record the development of scheme over a period of 3 years and 7 months following site opening. This data would normally reside within consultancy reports and thus be unavailable to the wider scientific community. The paper demonstrates that further work is needed to improve the prediction of erosion rates associated with managed realignment schemes. The monitoring data at Steart Marshes show the development of a large exit channel along the line of the former channel that led across the existing intertidal mudflats. The highest rates of erosion occurred in the first 3 months following opening. After this time rates declined markedly and, approximately 7 months after opening, the breach and exit channel region began to show periods of accretion as well as erosion. The non-linearity in erosion rates is similar to previously reported sites, but the rates of development were more rapid due to higher flow speeds generated by large tidal prism and the single breach design. Although the volume of the exit channel region appears to have stabilised, a secondary channel is starting to form, which suggests that further changes are likely. Within the site, further recession of the step feature is expected before a stable condition is reached. The paper documents these changes in more detail and draws out those behaviours that are likely to apply to other schemes.