Morphological Evolution of Creek Networks in Restored Coastal Wetland Schemes

Abstract

The restoration of coastal wetlands in the UK and worldwide often involves the excavation of artificial creek networks to encourage hydrological and ecological processes; however the evolution of these artificial creeks post-implementation is insufficiently monitored, and their design still lacks scientific guidance. This thesis analyses the morphological evolution of creeks in restored coastal wetlands in the UK to help inform future design strategies.
A new semi-automated creek parametrisation method was developed for lidar datasets, which is faster and less subjective than manual mapping. Morphological equilibrium was defined for natural saltmarshes; power-law relationships for creek dimensions and distribution at equilibrium were derived from the analysis of 13 mature natural saltmarshes to provide a range of potential end targets for MR creek design. A relationship was found between the overmarsh path length (mean distance to a creek anywhere in the marsh) and the mean site elevation within the tidal frame. This analysis also found that creek morphological equilibrium should be described as a range of potential states rather than as one quantifiable target.
The evolution of 10 MR creeks in their first 2-20 years post-breach was then studied. MR creeks evolved near-linearly towards a larger, more sinuous and better distributed system. These evolution rates towards the proposed equilibrium targets were then related to the initial conditions of the sites: low-lying sites with high accretion rates and large openings had faster-evolving creeks, while high, constrained sites displayed limited creek growth and required more extensive initial creek excavation. The 10 MR schemes and two accidentally realigned sites considered behave as though they should stabilise within 100 years into an alternative equilibrium state to that of natural systems, with a lower density distribution of creeks, mainly concentrated around the breach areas, while the further reaches of the site have fewer channels. Furthermore, the channels have a lower sinuosity due to inherited drainage ditches that remain visible even after 100 years, and a flatter substrate.
MR creek expansion may be hindered by the overcompacted soil and lack of small-scale topography inherited from the previous agricultural land use, which prevents creek incision and preferential deposition around topographic flow paths. Based on these findings, future studies should link the soil geotechnical properties, creek development and biodiversity of MR schemes to better understand creek-forming processes and improve the monitoring, management and design of MR sites.

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ORCID for Ivan Haigh: orcid.org/0000-0002-9722-3061

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