Charmouth Sewage Treatment Works

Charmouth Sewage Treatment Works

Project Brief

The River Char is a spate river with a short, highly-erosive and steep-gradient catchment which transports large volumes of eroded stone and gravel to the point where it empties into the sea on the Dorset coast at Charmouth.

Wessex Water Plc operates a local sewage treatment works, the settlement tank of which is perched close to the edge of a highly-erosive outside bend of the river, shortly before it enters sea.

The settlement tank only needed to fall out of alignment by a few millimetres to render it inoperable. Its position on the leading edge of a vertical and rapidly eroding 3-metre-deep bank face and erosion hole placed it in a precarious condition with water seeping out at its base.

Cain Bio-engineering consultants were engaged to investigate and survey the site with a client brief to design/build an urgent long-term and cost-effective re-instatement of the eroding undercut bank.

The Solution

Before

A 3-metre-deep bank face and erosion hole was undercutting the bank immediately adjacent to the settlement tank with the perimeter fence already collapsing into the erosion hole now dangerously close to the sewage treatment works settlement tanks.

Undercut bank close to settlement tank

Perimeter fencing collapsing into erosion hole

Bank cliff face with settlement tank visibly close

During

Coffer dam formed to isolate working area

New bank formed from hazel faggots and geotextiles

New sloping bank face created

View of tiered bank structure

Structure top-dressed

Site won willow stakes driven into bank

Completed new sloping bank

Riffle creation

Before work could commence the left hand bank back channel was excavated to direct flows into an extended outside meander thereby taking flow pressure off the repair site. A Coffer dam was then formed from site-won gravel to isolate the working area (around the undercut bank) from river flows and sediment release.

Work could then begin on creating the new bank toe, that was constructed from hazel faggots, retaining geotextiles and hydraulically-driven hardwood posts. An entire new sloping bank face was created using a four-tiered geotextile structure back-filled with site won stone and gravel from the emergent inside bend point bar depositions.

The completed structure was then top-dressed with a sacrificial layer of gravelly growing medium containing native root stock and seed bank, before site-won Willow stakes were cut from bankside saplings and driven into the new sloping bank. These provided the structure for the bio-element of the engineering.

In order to attenuate (or moderate) flood flows over the new structure to provide time for protective vegetation to grow, a hazel faggot riffle top-dressed with loose gravels was installed immediately downstream of the project site. The riffle was deliberately created to be partially sacrificial with the intention of the structure adjusting to form a natural long-term spawning feature.

Three years after

The Cain Bio-Engineering team visited the site three years after construction completion to see how the construction work and new bank was faring.

Natural & heavily vegetated new bank

Settlement tank visible at a safe distance from bank

New riffle

New backwater channel

Four years after completion

The new structure was heavily vegetated with naturally-seeded sedge species. The willow stakes have sprouted with new whips growing up to 2m providing the additional benefit of structural binding via the living root structures.

The riffle attenuator, constructed from natural materials, has proven remarkably resistant to a succession of winter flood events, providing in the meantime additional protection to the new bank whilst creating a natural spawning habitat and resting pool for migratory fish.

The new backwater channel (dug initially to redirect flows) was left intact on project completion to provide a deeper discreet backwater refuge for migratory fish. This eventually became the primary flow path with the opposite repaired bank serving only as a flood relief channel during times of high flow.

The final image in the above gallery was recorded during May 2009 (four years after completion) showing an upstream view of the fully vegetated, bio-engineered bank. The original erosion hole and undercut bank, has in-filled with water-borne gravel to form a flood relief channel, meaning that the sewage treatment site is protected into the long term from any future threat of erosion.

Finally, no engineering project can be considered successful without demonstrating, where possible, benefit to the environment. With the narrow backwater channel excavated at the start of the project, to divert main-stream flows away from the project site. It has since formed an ideal and long-term refuge pool and spawning site for migratory fish.