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 the second phase was facilitated through a combination of reserves and a fixed rate HM Treasury loan of £2.007m facilitated by New Forest District Council over a ten year term.
Towards the end of 2014 New Forest District Council and the Channel Coastal Observatory updated their analysis and projections for salt marsh loss due to erosion. The revised long-term projections for the extent of the vegetated and high mud, non-vegetated salt marsh indicate that the marsh is now expected to still make a significant contribution to harbour protection in 2050.
In the light of the revised forecasts in 2014 the Commissioners asked their engineering consultants to review the implications for the timing of future phases of the breakwater project. The projected timing for extension to the breakwater on the western side of the river (Phase 3) remained broadly unchanged and will need to be completed before 2030. A projected construction window between 2024 and 2028 has been identified. The Phase 2 breakwater on the east side of the river is forecast to provide the required level of protection until at least 2039 before requiring extension. Beyond that the timeline for constructing future Phases is considered to be too long to be meaningful given the variables that prevail in the marine habitat.
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 at least 2024. 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. To determine whether this intervention has had a positive effect in slowing down localised erosion, longer term monitoring will need to take place.
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.
This will include exploring whether there are other local sites where mud can be strategically placed to slow down erosion of marsh that provides shelter to the Harbour. This will become important as the Boiler Marsh area 'fills up' to the extent that the barges can no longer gain safe access.
Funding & Financing
Since 2006, the additional funds required to pay for harbour protection have come through charging all harbour users a Harbour Protection Levy. This is a separately identified element of the harbour dues which will continue to be charged until all phases of the scheme have been completed. This is a long term project (approximately 40+ years) and the Commissioners will remain alive to the opportunities to generate additional revenues from new and previously unexplored sources including grant funding.