Northey Island NW Saltmarsh Restoration

Year Implemented

2016

Project Type

Beneficial use of dredged sediment

Country

United Kingdom

Location

Blackwater Estuary (Essex)

Longitude

0.71619719

Latitude

51.725793

Habitat(s) Created
  • Saltmarsh

Size

0.4 Hectares

Why Undertaken
  • Biodiversity Enhancement

  • Climate Change (Greenhouse Gas) Mitigation

  • Economic Need/Enhancement

  • Habitat Enhancement

  • Improve Flood Protection

Project Description Summary

This project involved beneficially using sediment which is dredged locally from the upper Blackwater Estuary for navigation access.  This dredged sediment was used to reinforce a vulnerable embankment on the west side of Northey Island.  There was an imminent risk of this embankment failing and of major tidal channel establishing through the bank from ditch on its landward side.  Protecting this embankment and enhancing an area of saltmarsh habitats had notable flood protection benefits.  

This project was undertaken by the National Trust who own and manage Northey Island.  It was made possible through contributions from the Natural Flood Management (NFM) Fund. The work was done by experienced local contractors who adopted similar techniques to other beneficial use projects in the upper Blackwater.  However,  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 bund  The sediment was dewatered in the bunded area before then 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 24hour period were achievable when the tides were suitable.  This involved some night-time as well as daylight working.

A 200m length of embankment was raised and improved with these placements.  For the final placements almost 3,000 m3 (around 1,000m3 per year) were placed.  This covered a 4,225m2 area with a thickness of sediment ranging from 0.1m to 2.5m (the average depth of placement was 0.7m).

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 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 Glasswort and Annual Sea-Blite by summer in 2020.

There was little direct loss of placed sediment.  There was settlement and compactions and also  addition 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).

This project addressed a flood risk issue, directly enhanced a 0.7 hectare area and indirectly improved a wider 4.5 hectare area.  

Documents/References uploaded


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