CANAPE: Looking back at Hickling Broad
In that time the Broads Authority construction team have moved over 31,000 m3 of sediment, enough to fill 12 Olympic Swimming pools. The sediment was taken from selected navigable areas within the upper Thurne, working delicately to avoid damaging the peat layer below the more recently accumulated mud.
Re-use of the sediment has been at the centre of this project. Allowing dredged sediment to try out has a potential to release significant amounts of Carbon Dioxide, as this sludgy sediment is formed from peaty soils that have been eroded from the banks of the lake by the action of wind and waves. By keeping it wet and in an anaerobic environment we prevent the oxidisation process that releases this CO2.
Photograph taken from a Drone overlooking Hickling Broad.
Hickling Broad is owned by the Norfolk Wildlife Trust so by working in partnership, we have re-constructed a former reedbed area on the western side of Hickling Broad, at a place known as Chara bay. This creates an additional hectare of reedbed, and then a sheltered area behind the lagoon which will create still water that can be a habitat for the rare plants found in this part of the Broads. These re-created habitats aim to help protect the special plant, invertebrate and bird communities that make this Site of Special Scientific Interest their home.
This project also saw the first use by the Broads Authority of Biofuel to power its operations. As there are no electric vehicles that could carry out the work (although the use of electric excavators, either battery or Hydrogen powered, is the long term aim for the authority), the Broads Authority looked for other ways to reduce its carbon footprint. In the CANAPE project a large amount of the work has been fuelled by Hydrotreated Vegetable Oil (HVO), which is a “drop-in” replacement for diesel. All HVO used in the Broads is certified as being produced from waste cooking oil, so this is reusing a waste product for energy.
The Construction Process
The first stage of constructing the new peninsula was to build a solid barrier that could hold dredged material in the centre. This was done with geotextile tubes that were anchored in place by alder poles driven into the bottom of the Broad. Each of the 50m long tubes was filled with sediment, pumped in by a repurposed concrete pump loaded with sediment from the first dredging’s in the Broad.
Once this was in place the area behind the tubes has been gradually filled each winter, with an aim to create a level surface that is suitable for the establishment of a natural reedbed. This has been a particular challenge at past restoration methods as a surface that is too high will be colonised by willow, and too low will not lead to the establishment of any plants.
As a final step, at the end of 2021, matts of reed were added to the top of the geotextile tube to accelerate the natural establishment of reedbed. This was delayed from the original planned date due to higher than usual water levels covering the tops of the site.
Construction works underway in the winter of 2018
The Dredging Process
The removal of sediment was carried out by large construction excavators, which rest on floating pontoons held in place by “spud” legs.
These remove the sediment from the broad bottom, and load it onto mud wherries, each of which can hold 60 cubic metres of sediment – so in total we have loaded 500 wherry loads! To prevent disturbance outside of the dredged area, a series of floating barriers are placed around the dredging area, with sheets hanging down to the bottom.
In addition to using excavators for most of the work, for the removal of sediment from Catfield dyke a mud-pump was used. This works essentially like an oversized vacuum cleaner, sucking loose sediment off the floor of the channel. This was able to be piped directly into the construction area.
The wider context
The work at Hickling Broad comes at the end of a decade of ecological restoration and enhancement at Hickling Broad. It is the largest Broad in the Broads National Park and is important both for wildlife and tourism, and a valued part of the navigable area in the Broads.
From comparisons of aerial photographs from the 1940s to those taken today, we can see that there has been a steady erosion of parts of the banks of the Broads. This has been driven in part by the loss of water plants in the Broads due to poorer water quality between the 1960’s and 1980’s, which has thankfully been reversed. For example, a 2022 survey found 68% coverage of water plants across the bed of Hickling Broad. This previous loss of submerged plants, which anchored the sediment, combined with the long reach of open water (the fetch) from the western side meant that wave action had broken down the banks and distributed sediment around the broad.
Aerial image taken in the RAF 1940s survey - hosted by Norfolk County Council on http://www.historic-maps.norfolk.gov.uk/
The reed fronting the bay has been entirely lost. has disappeared by 2017, when this aerial photograph was taken. (c) Getmapping plc licenced to Norfolk Authorities
By 2020 we were halfway through the restoration, with the outer perimeter in place and a significant amount of sediment placed within the boundary. (c) Broads Authority
In addition to supporting the general responsibility of the Broads Authority to adequately maintain the navigation, the delivery of the project in this manner will have a number of benefits for the local environment;
- Creating a new area of reedbed expands the habitat available to wetland species in this area
- The still water area behind the new peninsula will be a good location for the establishment of various rare plants, some of which are only found in the upper Thurne area.
- The deepening of the marked channel through which boats navigate will increase the distance between their propellers and the bottom of the Broad – this will reducing the stirring of water & sediment in the Broad, reducing the amount of suspended sediment.