5 minute read
Mersea Harbour and Horsey Island
Essex, United Kingdom
Protecting against erosion and creating habitat by beneficially using dredged material. In the United Kingdom, less than 1% of dredged sediment is currently used, particularly as a tool to support estuarine and coastal habitat restoration, with the majority being disposed of offshore. To capitalize on this resource, during winter 2021, a series of beneficial use sites was delivered by the Mersea Harbour Protection Trust (MHPT) and the Royal Society for the Protection of Birds (RSPB) along the Essex coast. The project was designed to create habitat for little terns (Sternula albifrons; a protected species), protect local salt marsh from erosion, and help maintain natural harbors by providing shelter from wave action. Using sand and gravel arising from the Harwich Haven Authority (HHA) channel-deepening project, sediments were hydraulically pumped ashore via pipeline and deposited at Horsey Island in the Hamford Water embayment and at four locations across Mersea Harbour in the Blackwater Estuary via pipeline and rainbowing. The material at Horsey Island was an instant success for little terns, with an increase of nests and fledged juveniles from 2021. The shelter afforded by the recharge at Mersea Harbour is already transforming the damaged salt marsh, supporting sediment accretion and vegetation development in the lee of the deposits, which themselves have evolved into a more natural form.
Producing Efficiencies
A nature-based sand and gravel recharge approach was selected, with dredged material that emulates the Pleistocene gravels forming natural beach ridges within Hamford Water and the Blackwater Estuary. Coarser sediments over finer were preferred, as they provide more protection to the eroding marshes and harbors and are favored by the target species of nesting birds. The recharged sediments could also respond to the local tides and waves, creating a more natural and resilient coastal profile. Where safe and navigable access allowed, rainbowing via dredger was used to discharge the sediments. This avoided pipeline costs and reduced the cost of operations.
Using Natural Processes
No reprofiling of the sediments was undertaken once discharged. Instead, coastal processes were allowed to rework them to create a natural morphology. This approach was based on similar beneficial use projects undertaken at the same sites. Following placement, the sediments also help reduce wave energy, encouraging the deposition and accretion of fine, soft sediments. At Mersea, the first-year increase in elevation of the intertidal mudflats landward of the deposits shows the potential, in time, for conditions suitable for the colonization of pioneering salt-marsh vegetation, further stabilizing the sediments and providing ecosystem service benefits associated with salt-marsh habitat.
Broadening Benefits
The project’s primary aims were environmental, providing suitable habitat for bird species and protecting salt-marsh habitat from erosion. The gravel and sand recharge areas have been hugely beneficial to nesting birds like little terns. These benefits have included much-needed winter roosting habitat and the support of marine invertebrates, a year-round food source, by the accreting soft sediments. The project also has significant socioeconomic value. Improved flood defenses increase the sustainability of Mersea Harbour, which directly supports approximately 80 full-time jobs throughout the year. The project also similarly safeguards wildlife and boating interests, supporting area tourism contributing approximately £276 million to the local economy.
Promoting Collaboration
At the project’s core was a strong collaboration between MHPT on Blackwater Estuary and RSPB on Horsey Island with funding via the European Union’s L’Instrument Financier pour l’Environnement (LIFE) Programme, LIFE on the Edge project. The partner and major funder was HHA, providing the source of gravel and sands and marine construction experience and managing the contractors, Boskalis and Van Oord. As the flood and coastal risk authority, the Environment Agency also provided a financial contribution. Finally, community support was critical. Public talks and exhibitions during design and construction helped identify issues, address concerns, agree on monitoring, and develop solutions, while postrecharge engagement allowed the successes of the project to be shared.
