Abstract
Sea walls provide vital flood protection for lowland coastal property. We investigated the integrity of a cost-effective method of repairing sea defences, which has potential to create habitat for coastal and salt marsh flora. Experimental stone-gabion and clay-filled terraces were installed as a soft engineered approach to repair damaged sea walls in estuarine embayments in south east England. Changes in the surface heights of sediment and vascular plant colonisation were monitored over a 22 month period. Seven of the 12 terraces were colonised, by 12 species of plant, reaching a maximum of 85% cover. The main drivers of plant colonisation were sediment stability, elevation, exposure and sediment shear strength. Terraces with least change in the surface height of sediments were favourable for plant colonisation. Ordination (Canonical Correspondence Analysis) showed 72% variation in plant distribution explained by elevation (37%), exposure (30%), terrace length and sediment shear strength (5%). Elevation was the most influential variable; recruitment increased as terrace height approached the height of existing marsh (r2 = 0.43). This cost-effective approach has the potential to provide protection to sea walls and create additional habitat for wildlife. Key considerations for the improvement of terrace design and construction are discussed.
Highlights
Salt marshes are important coastal habitats (Ford et al, 2013), supporting a range of terrestrial and marine species and feeding, roosting and nesting sites for various species of shorebirds (Norris, 2000)
Over the three growing seasons, a pattern of sedimentation was seen at seven of the 12 terraces where net sediment deposition occurred towards the sea wall and was eroded towards the edges of the seaward edges of the terraces (Figs. 3 and 4; were between 6.24 m (W2), W3, W4, L1, L3, L4 and L5)
Terrace L2 had a pronounced (U 1⁄4 À11.02, p < 0.01) þ13 cm gain in surface height during this period, which was concentrated towards either end of the terrace (Fig. 4)
Summary
Salt marshes are important coastal habitats (Ford et al, 2013), supporting a range of terrestrial and marine species (acting as nursery grounds for various fish species, Green et al, 2009) and feeding, roosting and nesting sites for various species of shorebirds (Norris, 2000). In response extensive coastal defences were created, which included the raising of approximately 2,100 km of earthen sea wall embankment around the coasts of England and Wales (Gardiner et al, 2015) These conventional defences will eventually need to be replaced because they are approaching the end of their design life and are challenged by rising maintenance costs (Temmerman et al, 2013). The success of the trial would be assessed on the ability of the terraces to protect the “toe” of sea wall blockwork revetment over time, and by the recruitment of salt marsh vegetation onto the new surfaces, in areas where the marsh had been lost to erosion. The outcomes of this research could inform further trials of this approach to sea wall repair, and determine whether gabion terraces can provide a cost-effective solution to sea wall maintenance with the additional benefit of providing biodiversity gains for salt marsh communities
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