Hooks Marsh Riverbank: Natural solutions to water erosion

Hard engineering solutions such as gabions or walls are often turned to when seeking to de-escalate riverbank risk, but they are rarely the best options for promoting the healthiest river environment possible. Conversely, in keeping with ecological practices, soft engineering solutions often provide ample opportunity to simultaneously protect and enhance the area, particularly as they often incorporate the use of natural materials.

One such riverbank erosion scheme that our engineers have worked on was for the Lee Valley Regional Park Authority, which is responsible for managing the open spaces and venues along the 26-mile long, 10,000-acre Lee Valley Regional Park. Running in a generally southern direction towards the Thames, the River Lee forms the backbone of the park – but within one particular area of the park, the riverbank was under threat from its own waterflow.

The site in question was within River Lee Country Park at the Hooks Marsh area of the park. While the land on the east of the river in this area is woodland, the grassland on the west of the water has a previous life as a landfill site. Due to the layout and topography of the river, the western bank was being undercut and eroded by the water flow, resulting in parts of the bank becoming unstable and partially collapsing into the river.

Of course, with its history as a landfill site, the ongoing erosion of the bank raised concerns over pollutants and leachate from the landfill being discharged into the river and damaging the environment and wildlife in the area. So, a two-phased approach was taken to alleviating this problem – and to tie in with the country park setting and to protect and enhance the environment, we sought natural and soft engineering solutions.

The first step was to remove the existing debris building up in the river itself. This included removing dead trees and branches, those growing out of the riverbank that would interfere with the solutions to be installed, sediment within the river that had formed small islands, and vegetation that had started to grow in these areas. And this all had to be done while also not causing any harm or disturbance to a kingfisher nest on the west bank, nor obstructing the flow of water in the river.

Once this clean up was complete, our attention turned to the protective solutions to the ongoing erosion, which needed more than 120m of new protection along the riverbank. To remain in-keeping with the setting, we opted to install a hazel revetment spiling to act as the key form of protection.

A hazel revetment is a traditional technique for stabilising and preventing erosion of riverbanks, seeing hazel woven between staked uprights and then backfilled. It works most effectively when the revetment slows the flow of water, causing sediment to collect and other native plants to take root and flourish thereby binding the whole structure together.

The use of this natural technique creates a habitat both on the riverbank and within the water itself too – and as the years go by the structure gets stronger rather than weaker. This project included for a hazel revetment rather than a willow option in order to reduce the ongoing maintenance requirements in this fairly remote location.

Thanks to our strong working relationship and early engagement with the Environment Agency, we were also able to incorporate the use of brash berms as additional protection, as well as enhancing this particular stretch of the river. Brash berms work to change the flow of rivers, making them more habitable for fish and other aquatic life, and their use – along with the incorporation of a purpose-built Kingfisher nesting box - increased biodiversity in the area.

When installed in sequence between two banks, they also help to redefine the river line, creating a more meandering channel for gentler impact and capturing sediment in the water along the way.

It would have been easy to go in and look at installing more hard solutions to block the water from eroding the riverbank further, but it is crucial as engineering experts we examine the very best solutions for the bespoke circumstances. With the natural setting, these hard solutions would not have made any sense, and would have been a detriment rather than an enhancement to the area.

If you would like to discuss our civil engineering services or a potential project, please get in contact with Paul Cannaby here.