Beneficial use of dredged sediment
Lymington Estuary (Hampshire)
This work was done by Wightlink Ltd. to mitigate for the possible (and uncertain) effects on the Solent Estuary European Marine Site from the navigation of new passenger ferries between Lymington to Yarmouth.
The aim of the recharge was to delay the loss of the deteriorating marsh within and surrounding the area where dredge sediments were placed.
Maintenance dredged silt from the upper Lymington Channel and Yacht Haven were placed on an eroding area of saltmarsh ('Boiler Marsh').
The site was remote from the sediment sources so the maintenance arisings had to be 'double handled'. They were initially dug using a back-hoe dredger and placed in 70m³ hoppers and transported to the site where they were pumped through a 100m pipe to the deposition area.
There were two campaigns in February and March 2012 and 2013. A total of 1,330m³ and 3,120m³ were delivered in these two years. Relatively consolidated clay was also present in some hoppers and this presented challenges for moving and pumping the sediment.
The pumped sediment was retained in situ using fences made of stakes, willow and straw. These were built across the mouth of the main (eroded) drainage channel and also at intervals throughout the area to create weir-like features and stilling basins.
The final monitoring report for this project was completed in 2020. It found that much of the dredge sediment was still there after eight years, the site was still a good mix of mudflat and saltmarsh (an improvement on the baseline conditions) and the fences retained a good degree structural integrity.
At the end of 2020, the project was signed off as having delivered the requisite mitigation by an environmental panel overseeing the work.