Following receipt of environmental and regulatory consents, the first phase construction of a 100m long breakwater on the western side of the channel was completed in 2010. Construction of the second phase on the eastern side of the channel took place over the summer of 2014 and was completed in October at an overall cost of £2.32m. Funding of breakwater construction (and any loans needed) is through a harbour protection levy on users of the harbour.
Approximately every 5 years the Commissioners commission a review of the most recent data for saltmarsh erosion and to measure settlement of the first two phases of rock breakwater to help inform Commissioners on when future phases of breakwater construction are likely to be needed. This information informs and updates our financial planning.
The most recent review was undertaken in June 2020 and the projected timing for the extension to the breakwater on the western side of the river (Phase 3) remains broadly unchanged and it is currently planned for this to take place by 2030. It is also anticipated that at the same time it will be necessary to 'top up' the rock on the Phase 1 breakwater due to the predicted settlement. The Phase 2 breakwater on the east side of the river is forecast to provide the required level of protection for the foreseeable future.
Attempt to Reduce Marsh Erosion through Beneficial Use of Dredged Material
LHC’s overwhelming priority for the foreseeable future is to ensure the Harbour remains viable as protection is lost as the marshes retreat. The existing strategy to construct rock breakwaters in phases as the marsh recedes is the ‘default option’ to ensure the harbour remains protected.
However, as noted above, Phase 3 will not be required until around 2030. In the meantime LHC wish to explore whether there are low cost sustainable options to beneficially use sediment dredged from the harbour in order to replenish salt marsh and slow down the rate at which it is being lost. If successful this could delay the rate at which future phases of the breakwater will be required.
In 2012 and 2013 both Wightlink and LHC undertook separate marsh recharge works as mitigation to offset possible impacts of the introduction of new ferries and the harbour protection project. These works raised the area within the tidal frame by pumping mud on to poor areas of marsh with sediment retention being aided by using brushwood polders (fencing). In both cases, subsequent monitoring has shown that much of the sediment deposited has consolidated and stayed in place and that the sites were functioning well ecologically with marsh plants growing in the upper sections and birds feeding in other parts.
Although the techniques adopted for the both schemes were effective at delivering relatively small amounts of mud in a beneficial way, these schemes proved very expensive. In LHC’s case the cost was approximately 13 times as expensive as disposal at sea and in Wightlink’s case, because of the sites more remote location, costs were considerably higher. Although LHC were fortunate to secure grant funding from the Crown Estate to undertake their trial, the high cost makes this method of recharge unsustainable as a long term option.
To bring the costs down it is necessary to dispense with retaining structures and use the techniques used for disposal at sea, but close inshore thus ensuring there is no additional cost of disposal. The barges used by Berthon Boat Company and Lymington Yacht Haven for maintenance dredging are unusual in that the hopper doors open within the hull, without increasing the draft. It is therefore possible to discharge the load in very shallow water. A further advantage is that the sediment deposited is of a denser consistency as there is no requirement to mix with water to facilitate pumping through a pipeline. This aids retention following placement.
In 2014, the Commissioners were granted a marine licence to undertake a three year trial to beneficially use 19,380 tonnes of mud dredged from the river to recharge an inter-tidal area in a bay within ‘Boiler Marsh’ where the salt marsh has been lost through erosion.
The plan was to create a sacrificial reef reef within the bay that will shelter the marsh behind from wind wave action and provide 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 demonstrated that this type of beneficial use in this area was able to successfully form a sacrificial bund to provide shelter to the marsh behind with no adverse effects for the local environment and in 2017 LHC successfully applied for a licence to beneficially place up to 10,000 tonnes a year until 2024.
The latest monitoring report confirms that around 40,000 m³ of mud has been placed over the last seven years, and around half of this was still in place at the time of the most recent surveys. It also concluded that the ongoing and regular recharge placements have been effective in creating a raised bed feature which is around 1.5 ha in size which will be acting as ‘sacrificial bund’ feature that will be protecting parts of the inner marsh and helping to retain sediment in the area. The latest monitoring report (May 2021) is available HERE.
A video of the process for placing mud has been posted by one of our dredging partners and can be viewed HERE.
LHC is a member 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.
The project has completed a feasibility review (May 2020) for sediment recharge sites in West Solent marshes (including Lymington). It is now in its final phase to identify preferred locations and technical options which can be taken forward to an application for a marine licence This will become important as the Boiler Marsh area 'fills up' to the extent that the barges can no longer gain access.
More detail on the BUDS scheme (including supporting reports) is available on the Solent Forum website.