Between 2014 and 2017 Lymington Harbour Commissioners undertook a three-year trial to beneficially use mud dredged from the river to recharge an intertidal area in a bay within ‘Boiler Marsh’ where the saltmarsh has been lost through erosion. To achieve this, dredging barges navigate close to the marsh where they directly discharge their loads.
The plan was to create an unconfined intertidal reef within the bay that will provide shelter to the marsh from wave action or a source of sediment to feed the marsh in the immediate vicinity. It is hoped that by reducing the amount of wave energy reaching the marsh in this area, this will slow down erosion.
The trial involved the placement of 2,380 tonnes of sediment in November 2014 building up to 6,900 tonnes in 2015 and 10,000 tonnes in 2016. During the trial monitoring was undertaken to ensure there was no detriment to water quality or the benthic communities that inhabit the mud, the latter with the support of Natural England and the Environment Agency. Bathymetric surveys were also undertaken before and after each survey to establish the persistence of placed mud over time.
The picture below shows the differences in bed levels between the 2014 baseline and June 2017, some 6 months after the final year placement of mud. The dark red area towards the north of the bay is where most mud was placed. The yellow areas further south show smaller rises in bed levels where placed mud has dispersed.
The final monitoring report concluded that the trial had no adverse effect on water quality or the saltmarsh environment and, encouragingly, bed levels had risen by an average of 0.66m in the area of disposal when compared to the 2014 baseline data. This information was used to support a successful application to the Marine Management Organisation for a new extended licence. The new licence authorises the placement of up to 10,000 tonnes per year until 2024 and it is hoped that these larger quantities will make a material difference to slowing marsh erosion in this area.
Looking forward, Lymington Harbour Commissioners are members of the Solent Forum ‘Beneficial Use of Dredging in the Solent’ (BUDS) project to identify wider opportunities for beneficial use of dredged mud within the Solent. As well as trying to identify wider opportunities around the Solent, this work will include assessing whether there are other strategic areas where mud can be placed to slow down marsh erosion where the marsh provides shelter to Lymington Harbour. This will become important as the Boiler Marsh area ‘fills up’ to the extent that the barges can no longer gain safe access.
For more information on BUDS go to www.solentforum.org/services/buds