The Impact of Erosion Protection by Stone Dams on Salt-Marsh Vegetation on Two Wadden Sea Barrier Islands

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Abstract

ABSTRACT

This paper describes and quantifies the effect of low stone dams on the extent and composition of salt-marsh habitats on two Dutch Wadden islands: Terschelling and Ameland. The stone dams were built to prevent erosion of the salt-marsh edge. Analyses of a series of aerial photographs taken between 1949 and 2010 show a strong reduction in retreat of the marsh edge on the island of Terschelling, from an average rate of 1.3 m per year before construction of the dam to 0.2 m per year after dam construction. Within 20 years of construction of the dam, sedimentation raised the mudflats between the dam and the former cliff, creating a broader foreshore and new marsh area with typical salt-marsh vegetation cover. The dam on the island of Ameland was built on the remnants of a previous low coastal defense. This reinforcement stopped cliff retreat and led to restoration of the eroded salt-marsh strip. Vegetation surveys along transects perpendicular to the coastline revealed that at both sites, typical pioneer salt-marsh vegetation had developed in the raised area between the erosion protection works and the former marsh edge. These habitats were not found in the reference transects without erosion protection. Based on these findings, we conclude that under favorable conditions for sedimentation, erosion protection by low stone dams may bring about a strong reduction in retreat of the salt-marsh edge while helping to restore an ecological attractive foreshore zone.

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