Sediment Removal
The project has completed planned sediment removal restoring Hoveton Great Broad and Hudson’s Bay to an average depth of 1.1m. The removed sediment has been used on site to create new fen areas.
Why was sediment removal required?
High rates of sedimentation, with rich fine sediment, had reduced the depth of Hoveton Great Broad and Hudson’s Bay to less than 0.5m in areas. This high rate of sediment build-up was the result of historic nutrient pollution in the broads. High nutrient levels resulted in large algal blooms, when these algae died they fell to the lakebed creating a very soft nutrient rich sediment. This sediment was releasing pollutants into the water and was too soft to allow aquatic plants to root effectively.
By removing this sediment Hoveton Great Broad and Hudson’s Bay have been restored to an average depth of 1.1m. This has several benefits. By removing the very soft nutrient rich sediment we have exposed older sediment more suitable for the growth of aquatic plants. This sediment also contains propagules (seeds and spores) from when the site contained a diverse macrophyte (aquatic plant) community. Some of these propagules will still be viable and will help Hoveton Great Broad and Hudson’s Bay re-establish a diverse community of aquatic plants following biomanipulation. Studies have also shown that the highest diversity of aquatic plants are observed at depths between 1m and 1.4m. Aquatic plant diversity is key in achieving a resilient and stable plant community following biomanipulation, with biomanipulate sites supporting more than 10 species exhibiting greater stability.
The increased depth will also reduce sediment disturbance caused by wind fetch. When high winds blow across these broads it can create waves and turbulence which re-suspends the sediment, resulting in increased turbidity and release of nutrients. Increasing the depth of Hoveton Great Broad and Hudson’s Bay reduces this effect.
Sediment removal can also have an impact on in lake nutrient cycling, the process where nutrient rich sediments will leach nutrients into the water, particularly during the warmer summer months when algal blooms are most common. Sediment removal has been shown to reduce the rate of in lake nutrient cycling, however, recent studies suggest that these gains may be short lived (2-3years).
Creation of new fen areas
The sediment removed from Hoveton Great Broad and Hudson’s Bay was to create new in lake fen areas. This method was chosen:
- due to the reduce carbon cost compared to drying and transporting sediment off site.
- to replace floating fen vegetation once present on site but lost since the 1940’s.
- to avoid the spreading of high nutrient fine sediment on land where it could find its way back into the system.
These new fen areas are currently dominated by bare sediment and shallow pools. This has created feeding areas for ducks and waders, with our first records of avocets on site. This bare sediment will slowly be colonised by fen vegetation, creating a rich fen habitat.
Fen turf has been gathered within the Bure Marshes National Nature Reserve, from fen adjacent to Decoy Broad, creating wildlife rich fen ponds. This fen turf has then been used on top of the geobags in Hoveton Great Broad to quickly establish a rich fen vegetation community, with 28 species recorded so far. Along with the other areas of fen surrounding Hoveton Great Broad, this will provide a seed source for the new fen areas to establish. We will then see plants such as Purple-loosestrife, hemp agrimony and many other wetland loving species start the process of colonising the new fen.
