As part of the Solent Seascape Project, CHaPRoN are working in partnership with Land & Water/Earth Change to trial an innovative technique. This will see dredged sediment (generated from local maintenance dredging operations within Chichester Harbour) being used to restore saltmarsh habitat. If successful, the technique could radically transform the way dredged sediment is used for saltmarsh habitat restoration. This could help reverse the declining trend in biodiversity within the Harbour, restore the features of the SSSI (Site of Special Scientific Interest) and develop a circular economy model. Sarah Chatfield, CHaPRoN Manager explains why this is so important:
Why restore saltmarsh?
Saltmarsh is an important coastal habitat with many roles including supporting biodiversity. It also stores carbon – helping climate change, acts as a natural flood and coastal defence, improves water quality, and attracts tourism with people coming to enjoy the varied species which thrive on it.
The continued loss of saltmarsh in Chichester Harbour has led to the area being downgraded to ‘unfavourable declining condition’ by Natural England. Urgent action is needed to help counter this situation and reverse the rate and scale of deterioration.
Why is saltmarsh declining?
The reasons for the decline in saltmarsh are complex, and include pressures such as rising sea levels, coastal squeeze, lack of sediment supply, water quality, increasing frequency of storm events and ‘wave wash’ from increasing boat traffic. Through the CHaPRoN Coastal Resilience and Saltmarsh Restoration Focus Group we are exploring opportunities to reduce the pressures and help reverse the declining trend in saltmarsh, whilst seeking to increase the Harbour’s resilience to climate change and wider anthropogenic pressures.
How will the project help to restore saltmarsh?
The project will recycle clean dredged sediment sourced from routine maintenance dredging operations carried out by the marinas within Chichester Harbour and use it to help restore saltmarsh habitat. This approach is known as Beneficial use of Dredged sediment (BuDs).
On average, a total of 13,000m3 of sediment is dredged annually across the marinas within the Harbour. Currently, the majority of it is disposed of outside the Harbour at a licenced location east of the Isle of Wight, called Nabs Tower. This approach means that valuable sediment is being lost each year from the Harbour, that could be used to enhance, stabilise, and restore saltmarsh habitat.
This project will use dredged sediment to raise the intertidal ground level at a new, approved and licenced location, where saltmarsh has dramatically eroded over recent decades. Raising the elevation of the intertidal area to appropriate levels, improves the environmental conditions for saltmarsh plants to establish and grow, helping to support the natural regeneration of this coastal habitat.
Where is the project taking place?
The project site is located at West Itchenor on the intertidal area, where historic saltmarsh once existed. The site is now predominantly clay substrate fringed with a natural coastline. The trial will create an area of up to 0.7ha saltmarsh using approximately 4,500m3 of dredged sediment.
It is close to two thriving areas of saltmarsh, Chalkdock Marsh and a site west of Northshore boatyard. Both of these will provide a natural supply of seed to help colonise the new area of raised sediment. By restoring saltmarsh at this location, it will also help to protect the coastal footpath behind from erosional processes.
Where is the dredged sediment coming from?
The dredged sediment will come from a marina in Chichester Harbour, likely to be Northney or Chichester Marina. This is because their dredging programme fits in with the project delivery timeline. Other marinas within the harbour have also shown strong support towards trialling the initiative.
Dredging operations are highly regulated processes. All marinas have to obtain a licence to carry out dredging operations. The sediment is analysed for contaminant levels prior to dredging taking place. The sediment has to conform with national guidelines regulated by CEFAS. The dredged material can only be disposed of at appropriate and authorised sites.
Pictured right: A backhoe dredger – thanks to Colin Scott: Restoring Estuarine and Coastal Habitats with Dredged Sediment
Why has this approach not been used before now?
There are many financial, technical, regulatory and environmental reasons why dredged sediment has not been used to ‘recharge’ saltmarshes in the past. However, there is now growing impetus nationally to address some of these challenges, and we are learning from other sites, such as Boiler Marsh in Lymington, that have had success using BuDs over the last 10 years. Within Chichester Harbour, we need to explore different ways of working to improve the condition of the SSSI. This trial seeks to resolve some of the issues that surround the beneficial use of dredged sediment for coastal habitat creation.
How does the process work?
1] Existing tidal habitat
The project site has been thoroughly assessed for suitability prior to project commencement and become an authorised and licenced disposal site. Baseline monitoring takes place to record initial environmental conditions. The sediment is dredged from the marina as part of a routine maintenance dredging operation.
2] Sediment Delivery
The dredged sediment is transported and deposited by split hopper barges with bottom opening doors; these will have a capacity of around 300 tonnes (some 200 m3). The barges arrive at high tide and deposit the dredged sediment as high up the shore as possible.
3] Sediment is dragged up the shore
Once the sediment has been deposited, it will be moved from the low shore areas to higher elevation surfaces as soon as possible after the barges have left. This process will be carried out using the novel Saltmarsh Restoration Drag Box (SRDB) designed by Land & Water. The SRDB sits on a set of skis and is connected to a winch that runs between a temporary pontoon moored at the edge of the sub-tidal channel and a land-based excavator.
4] Sediment shaped to restore saltmarsh
Once in place on the upper shore, and during low tide periods, the sediment is moved and shaped using an amphibious excavator with GPS guidance, into a predetermined profile that will support saltmarsh regeneration.
What is the Saltmarsh Restoration Drag Box (SRDB)?
The saltmarsh restoration drag box (SRDB) is a novel piece of engineering that has been designed by Land & Water to place the dredged sediment in position on the upper intertidal area. The SRDB sits on skis, with several sets of skis (of different widths) available to facilitate adaptation to local conditions, and minimise sinking/compaction. It has been tested on Rainham Marsh, but not in a tidal system.
The saltmarsh restoration drag box being tested at Rainham Marshes (c Uwe Dornbusch)
Other projects within the Solent that have used BuDs, such as the one at Boiler Marsh, have pumped the sediment into place using long pipes. However, the sediment has to be mixed with a very high percentage of water (approx. 90% water) to facilitate this transfer. Consequently, it has been difficult to retain the sediment in a precise location without using costly retainment bunds.
Using the SRDB approach will mean that dredged sediment can be placed on the upper intertidal areas in a much firmer consistency and can be shaped into the desired profile to support saltmarsh regeneration. It has the potential to transform the way dredged sediment is beneficially used for coastal habitat restoration, not only in terms of sediment placement, but also by creating a way of ensuring the efficiency of dredging and disposal operations is not hampered.
How long will it take before saltmarsh starts to establish at the site?
It is expected that the site will be seeded by the nearby saltmarsh sites and that the pioneer species will to start to grow within the first year and begin to colonise the site.
How will the success of the project be monitored?
There will be a number of measures in place to monitor and assess the progress of the project. These include:
- Bathymetry / hydrographic survey of the adjacent subtidal channel: this will stretch from around 0.1 km west of the site to the Quay at Itchenor, and also slightly into Bosham Channel. A baseline survey will be undertaken immediately before the works, and another immediately after the works (to confirm no substantial changes in subtidal area).
- Topography: Readings will be taken before the works commence, and as soon as the shaping works are completed. Then, after three months, in June 2023, a repeat survey is proposed, to help determine how much of the material remains, and what level of compaction has taken place. Subsequent monitoring frequency is to be determined in an adaptive manner, in collaboration with relevant government agencies.
- Accretion: once the trial works are complete, stakes, feldspar marker horizons, or similar mechanisms, will be put in place to facilitate monitoring of accretion at the restoration site.
- Vegetation: A baseline survey has been carried out to confirm saltmarsh extent along the frontage and plant communities. In June 2023, a vegetation survey is to be undertaken to determine the level of pioneer vegetation which has established. Subsequent monitoring frequency is to be determined in an adaptive manner, in collaboration with relevant government agencies.
- Overwintering Birds: Monthly overwintering bird surveys are to be undertaken at the site, commencing in November 2022, to confirm usage of the site. These will focus on the site frontage, as well as a 0.3 km buffer zone around it. The necessity/timing/frequency for surveys during subsequent winters is to be determined after the trials, in consultation with regulators.
- Benthic Invertebrates: Samples were taken in July 2022 to analyse the numbers and distribution of different benthic invertebrate species at the site. After the first winter (in June 2023), sampling is to be repeated at the site. It is proposed that benthic sampling of the restoration site is repeated in Years 2, 3 and 5, to investigate colonisation rates.
- Water clarity: measures will be taken at the site prior to project commencement to assess water clarity. This will be repeated at intervals as the site continues to evolve, to help assess the impact saltmarsh has on improving water clarity.
- Nutrient levels: core samples have been taken at the trial site and two reference sites either side of the trial, to support academic research into the nutrient absorption ability of saltmarsh
The CHaPRoN partnership will assist regulatory bodies in formally assessing the outcomes of the trial project.
Who is funding the project?
The project is co-funded by East Head Impact, Earth Change and the Endangered Landscape Programme as part of the Solent Seascape Project.
(The Endangered Landscapes Programme is managed by the Cambridge Conservation Initiative in partnership with Arcadia, a charitable fund of Lisbet Rausing and Peter Baldwin.)