CANAPE: First sediment dug at Hickling Broad

26 October 2018 - Published by Harry Mach
The end of October marks the beginning of work in the CANAPE project to restore the reedswamp at Hickling Broad. Over the next 3 winters Broads Authority teams will work to construct a 1ha area of reedswamp on the edge of the broad, in an area that has been named “Chara Bay.” This in honour of a genus of plants that grow in the broads, and should be able to develop in the area around our work.

This marks the next stage in the management of Hickling Broad and the Upper Thurne River by the partnership of the Broads Authority, the Norfolk Wildlife Trust, and others.   

The aim is to restore this area back to its original banks based on aerial photographs taken in 1946. In addition the project will improve the water quality of Hickling Broad. The work will achieve this in a number of ways;

  1. Increasing the area of reed in the broad – the reed acts as a natural filter for the water, removing the excess nutrients that are currently harming the wildlife, helping a more normal balance return to the water and improving the habitats.
  2. By dredging the marked channel in the Broad, we will increase the distance between boats and the bottom of the lake. This reduces the impact propellers have on the loose sediment at the bottom of the lake, and therefore reduces the amount of mud that gets stirred up, increasing the clarity of the water.
  3. The new reedswamp area will shelter the bay behind it from the wind; protecting plants such as stonewort (Chara) that currently have limited places to thrive on the broad.

What will this look like?

Chara Bay, Hickling Broad, Summer 2018

Chara bay in the summer of 2018, looking southwest towards Catfield Common

Back in the 1940s there were reeds growing out on a peninsula that covered most of the entrance to the bay. Over the years this has eroded away to a small stub, visible in the left-hand corner of the picture above. The project will rebuild the banks of Hickling Broad to their original lines.

Artist impression of Chara Bay in the future

An artist’s impression of how Chara bay will look once the work is completed and the natural reedbed has established itself.

Carbon Savings

Currently Hickling Broad has an excessive level of nutrients (nitrates and phosphates). A study published earlier this year[1] suggested that shallow peatland lakes with high concentrations of nutrients will become major methane sources as the planet warms. As methane is a powerful greenhouse gas, this presents a risk that lakes such as Hickling could accelerate global warming as the planet warms in response to carbon already in the atmosphere.

Fen acts as a natural carbon sink, so the creation of new Fen will already act to help combat global warming. At a larger scale, reducing the level of nutrients in the lake may help to prevent Hickling Broad become a major methane emitter as the planet warms. 

How it will be built?

Simply piling mud into a heap and hoping nature takes care of the rest will not work. To restore the area requires more substantive engineering.  

First, to construct the new banks geotextile tubes will be laid in a rough cashew shape. The Geotextile tubes are essentially 50m long sandbags that can be pumped full of mud. These will be held in place with alder poles driven into the bed of the broad, and filled with mud dug from the marked channel.

Diagram of Chara Bay construction

Design produced by NETICS. Background mapping is under crown copyright 2018 OS 100021773. 

Filling the tubes will use a combination of our excavators and concrete pumps. The excavators will remove the sediment from the bottom on the broad, and load it into a hopper. The hopper will then feed the mud into a concrete pump that has been repurposed for the project. The concrete pump will pump the mud through a pipe into the bags, inflating them like a balloon.

Construction method at Hickling

Photo Credit: NETICS - Method for filling a geotextile tube with material dredged from the bottom of a lake. 

Once they are inflated and held in place by the alder poles, the bags will act as a solid barrier to prevent the new reedbed being swept away by the wind and waves in the Broad. This will allow the new banks to form a permanent part of the environment. The bags are permeable, allowing water to pass through them creating a naturally wet area of swamp behind them. Reed will be planted on top of the bags, forming a bank similar to the natural banks of the broad.

 Design of the finished product

Photo credit: NETICS - Finished cross section at Hickling Broad. 

Once the bags are filled, which should be completed by the end of this winter, 10 thousand cubic metres of sediment will be pumped into the area behind, filling it to a level just below the average water level of the broad. This will create an area of wet land on which reed can take hold, mirroring the natural fen around the edge of the Broad. This process will be kick-started by sowing nursery plants to form a perimeter, and scattering seed across the remainder of the area.

 

[1] Synergy between nutrients and warming enhances methane ebullition from experimental lakes, Davidosn et al.