Restoring Saltmarsh Habitats to Prevent Erosion

The Port of London Authority requested assistance to restore saltmarsh habitats and prevent further erosion at West Thurrock lagoon and marshes, in Essex.

The reserve, which is a site of special scientific interest (SSSI), was designated as “unfavourable – declining”. The saltmarsh had suffered from significant erosion as a result of wave action from a large fetch and boat activity on the River Thames.

Hard engineered flood defenses to the rear of the saltmarsh also contributed to coastal squeeze.The restoration works needed to retain sediment while also encouraging saltmarsh vegetation to re-establish. Healthy saltmarsh should show zonation of species from lower reaches to upper reaches, natural colonisation of saltmarsh species does occur however planting with plug plants would aim to reduce the time taken to full colonisation, and increase rates of sediment deposition and help to compete against invasive species. 

Undertaking the works involved use of specialist equipment to traverse the soft mudflats whilst also working in between the tides. Limited access from the north bank meant launching from the south bank of the Thames to reach the project site. 

The Design

The project encompassed 600m of eroding saltmarsh and the design made use of brushwood fascines as a biodegradable method to retain sediment, whilst allowing water levels to rise and recede unimpeded.

The brushwood slows the flow as the tide goes out, depositing sediment out of suspension and build back the saltmarsh levels, to provide a more suitable environment for vegetation to establish. The establishment of saltmarsh vegetation will provide long-term, sustainable erosion protection, whilst also improving its SSSI status.

Fish passes have been incorporated into the polder design to ensure that continued use of saltmarsh habitat as a nursery and refuge area for smaller and juvenile fish will remain, and this habitat retains more of its ecological value. The fish passes had to be designed and built to allow free movement, whilst also retaining sediment without creating channels that would reroute receding tides.

Plug planting of arrowgrass and sea aster was undertaken to stimulate the establishment of saltmarsh pioneer species. Invasive common cordgrass is present on the site and so expected to spread further into the lower reaches once sediment retention creates suitable conditions. 

The Benefits

Retaining sediment on the saltmarsh and reducing erosion helps maintain this critical habitat, the soft mud provides feeding grounds essential for wading birds, fish and invertebrates. Saltmarsh sequesters carbon, improves water quality, and slows the flow during tide cycles to provide a natural flood defence through vegetation establishment. 

Sea club rush, sea arrowgrass and sea aster are establishing within the restored saltmarsh area. This creates habitat for invertebrates and nursery refuge for small fish during high tide.

The physical characteristics of the vegetation further slows the flow allowing for sediment deposition. With increased vegetation on the lower reaches of the saltmarsh there is further facilitation of colonisation by other saltmarsh species.