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ABPmer
Quayside Suite
Medina Chambers
Town Quay
Southampton
SO14 2AQ
2016
Beneficial use of dredged sediment
United Kingdom
Blackwater Estuary (Essex)
0.71619719
51.725793
Saltmarsh
11.8 Hectares
Biodiversity Enhancement
Climate Change (Greenhouse Gas) Mitigation
Economic Need/Enhancement
Habitat Enhancement
Improve Flood Protection
This project is an ongoing one that involves beneficially using sediment that is dredged locally to maintain navigation access in the upper Blackwater Estuary.
In the years prior to this project, this locally-dredge sediment had been beneficially placed at other locations in the upper estuary (see Maldon Hythe Quay on the OMReg website as an example). This new project built on the lessons learned from this past work and adopted a new strategy to achieve greater benefits.
For this new project, the dredge sediment was used to reinforce a vulnerable 100-year old embankment and infill ditches on the west side of Northey Island. There was a risk of this embankment failing and of major tidal channel establishing through the bank from the ditch on its landward side. Protecting this embankment and enhancing an area of saltmarsh habitat helped to maintain the channel configuration and has had notable flood protection benefits.
This project was undertaken by the National Trust who own and manage Northey Island. The work was done by experienced local contractors who adopted similar techniques to other beneficial use projects in the upper Blackwater.
This project was distinctive because it involved a relatively novel double handling strategy. A fixed clam-shell excavator carried out the dredging and this filled a 90m3 hopper barge. The barge delivered the sediment to the receptor site where it was unloaded (again with the clam shell excavator) into a temporary containment feature (referred to as a "quay"). The sediment was dewatered in the quay before being placed onto the embankment and into the landward ditch with a long-reach excavator.
The placements were made at highwater on larger tides of +5.0m Chart Datum (CD) when the barge (with a 0.5m draft) could manoeuvre close enough to the bank. This meant the dredger could only have one uploading period per high tide. Each unload took 45 minutes and two unloads per 24-hour period were achievable when the tides were suitable. This involved some night-time as well as daylight working.
Due to the success of this project, consent was obtained to expand the deposit zone over time. The project has been, and is being, done over three phase as follows:
Over the first two phases (pending a report of the Phase 3 outcomes), a 200m length of embankment was raised and improved with these placements. At the completion of Phases 1 and 2, there was seen to little direct loss of placed sediment. There was a typical settlement and compactions followed, encouragingly, by accretion of 2,000m3 over the saltmarsh, fleet and creeks immediately east of the placed sediment. This was sediment being deposited by the tide and not sourced from the placed material (which remained stable once placed).
As is typical, the areas of initially bare placed mud were relatively quickly colonised by marsh plants (there was no seeding, planting or translocation). Samphire (Salicornia spp.) were present within two months of the first placement and there was then an increase in diversity of the vegetation including species such as or Golden Samphire (Inula crithmoides and Shrubby Sea-Blite (Suaeda vera). Following the July 2019 placement, for example, the deposit area was devoid of any vegetation. However this area was densely colonised with Samphire and Annual Sea-Blite (Suaeda maritima) by summer in 2020.
In Phase 1 and 2, the project addressed a flood risk issue; directly enhanced a 0.7 hectare area and indirectly improved a wider 4.5 hectare area.
The Phase 3 area is now 11.8 ha in size and will be used to receive dredged sediment in the future as an when that material is available.
This work at Northey Island is possible thanks to the Natural Flood Management (NFM) Fund, National Trust Neptune Funding and EU funding under the LIFE on the Edge Project.
ABPmer
Quayside Suite
Medina Chambers
Town Quay
Southampton
SO14 2AQ