by Richard Steward and Robin Whittle
Estuary saltmarshes are an ecologically unique tidal area of marshland which is regularly flooded with salt or brackish water and then drained by the lowering tide. The River Deben has the highest amount of saltmarsh of any Suffolk River. It extends 18km from Felixstowe Ferry up to Melton. The width of saltmarsh between the river channel and bank varies from nothing to over 600m between Hemley and Waldringfield. Salt-tolerant plants stabilise incoming sediment creating a marsh environment that supports the aquatic food chain, provides essential coastal flood protection and improves carbon storage.
The Deben saltmarsh started life when the sea entered the river near Felixstowe Ferry some 6500 years ago converting the fringing freshwater marsh into mature saltmarsh. During the time since then the level of the saltmarsh has risen 14m from the deposition of sea borne sediments. This has kept in line with rising sea levels and Mean High Water Springs. The area of saltmarsh has also grown and has reached 18km upstream and 3km to the sides of the estuary, the lower reaches of the river have widened from about 8m to 550m through wind fetch and surge flow erosion – the latter from increasing surge volumes that flooded the growing area of open (unembanked) saltmarsh.
Between the 16th and 17th centuries large areas of saltmarsh were embanked to create arable land and grazing marsh. This resulted in a significant drop in surge flow erosion and caused an increase in mudflat and saltmarsh growth. However, in common with many Suffolk and Essex estuaries a new and serious problem has arisen – the loss of saltmarsh within the area away from the river edge (see above photo). The cause of this needs to be identified. This area is not affected by wind fetch or sea level rise and a possible cause is shore crab burrowing. This is currently under investigation by the RDA Saltmarsh Research Group which was set up in September 2016 to try and determine why and how saltmarshes on the river Deben erode and regenerate.
In 2014 Robert Simper catalogued <the changes> that had taken place over the past fifty years.
Section through Saltmarsh and River Edge
The section through a typical area of saltmarsh shows that the level of external saltmarsh has risen over 1m in line with sea level rise since the grazing marsh was embanked 300 to 400 years ago.
In recent years the value of saltmarshes has been recognised and promoted by the Government. Local groups are being encouraged to find ways to help conserve saltmarsh (find ways to slow or reverse the current erosion), including restoration with the beneficial use of dredgings.
The value of saltmarshes includes:
- providing essential protection to the <river walls>.
- providing a high biodiversity supporting a wide range of flora and fauna, <ecosystems and marine habitats>.
- providing carbon storage.
A note written in October 2017 ‘<Saltmarsh Restoration>’, considered the current state of the saltmarshes on the River Deben and gives some possible reasons for their deterioration together with other observations. Proposals are made for a possible trial to help restore badly eroded areas. Since the note was written more emphasis is being given to the use of <dredgings> for saltmarsh restoration.
A report has now been written that describes the research work carried out by the River Deben Association Saltmarsh Research Group (RDA SRG) in three different areas of the saltmarsh of the river Deben.from Waldringfield and Loder’s Cut Island between 2014 and 2020.
The purpose of this work is to measure:
- Changes in saltmarsh level
- Changes in mud level in the drainage channels
- The effect of ragworm exclusion plates on mud levels in the drainage channels
- The effect of shore crab exclusion cages on mud levels in the drainage channels
- The erosion rate at the edges of the saltmarsh
- The effect of creating an impermeable sill in two of the channels
- The effect of placing crab exclusion netting along the edge of the saltmarsh
Special equipment has been developed to measure the mud level in the saltmarsh channels and lagoons. Work is ongoing at the time of this report (December 2020). So far, the results confirm that:
- The saltmarsh is rising at a similar rate to that of Mean High Water Springs (MHWS) sea level rise (about 3.5mm/year).
- In addition to the erosion of the saltmarsh taking place along the river edges of 0.6ha/year measurements show that 0.83ha/year erosion is taking place within the heart of the saltmarsh (increasing the size of channels and lagoons)
- The rate of accretion in an artificially trapped lagoon area of the saltmarsh has resulted in a rise of the mud level of 15mm/year
- New saltmarsh can be created from the use of dredgings
- Rag worms may help to prevent mud loss in the saltmarsh drainage channels and lagoons
This is an important research project being carried out by the RDA. The full report can be read here: ‘Saltmarsh research on the River Deben’.
Additional reports supporting this work can be found here:
- Saltmarsh Restoration Report Sep 17
- Appendix A to SR Report – Dredging
- Appendix B of SR Report – Changes