- On 2nd September 2017
- In Ecology, Engineering, Sediment Management
- By William Coulet
With the Brightlingsea Dredging and Restoration Year 2 work package fast approaching, Miles Water Engineering have been contracted to undertake groundworks across the St Osyth Borrow Pits restoration site, installing a series of control structures in preparation for their upcoming restoration and storage of arising dredged material.
The site consists of twenty three (23) pits, approximately 30x30x1.5m in size. They were originally excavated in the 1960’s along the seaward face of the local coastal flood defences following the 1953 floods, in order to strengthen the damaged structures and improve resilience of the area to future storm events.
As subsequent accumulation has not occurred, they have now been incorporated into the USAR project and provide a potential storage volume of approximately 12,000m3. However, due to the extensive saltmarsh along the seaward side of the pits, the arising dredged material will need to be hydraulically pumped from Brightlingsea South Channel.
Whilst this is the most efficient method to support this beneficial use application, it does have several important implications. First, is that the addition of water in order to pump the sediment means that the actual sediment storage is likely to be approximately 60-80% of the internal volume of the pits. Still a significant proportion of the USAR project as a whole!
The second consideration is that in order to maximise the fill volumes achieved, repeated fill cycles will be required, whilst the sediment will also need to be retained to allow the long-term dewatering of the sediment to facilitate consolidate and subsequent succession by the local saltmarsh vegetation.
This is where Miles Water come in. Using specialist plant suitable for use on sites with low ground bearing capacity, a series of ground works will allow the control of fill and drainage cycles, whilst also allowing long-term storage and an efficient works program to be achieved.
First, a series of excavations will join individual borrow pits together where they are not already joined by existing, natural rills. Using material from these excavations, dams will be installed in strategic areas and will create seven distinct groups of borrow pits, ranging from a two to five pits per group. Finally, one drop-board timber sluice will be installed per borrow pit group.
These seven sluices are crucial to the works. Closed during fill cycles, the sluices contain the sediment-water mix, whilst following settlement of the newly placed material, the removal of the drop-boards will allow the drainage of clean supernatant back into Brightlingsea Creek during periods of low tide. Repeating this cycle across successive neap tides will allow the maximum sediment storage volume to be achieved, whilst following completion of the Year 2 works, the sluices will also support the natural tidal regime during the long-term dewatering of the sediment, thereby allowing colonisation and stabilisation of the sites.
Following the natural deterioration of the timber sluices, all that will be left is approximately 1.6ha of restored, protected, saltmarsh habitat!