A precious part of London’s remaining medieval common land has been saved from the threat of pollution from nearby highways with a simple solution using Hydro International’s Downstream Defender^®.

The Situation

The precarious wetland habitat at Shortwood Pond off the A308 by the Crooked Billet Roundabout is home to endangered species such as the Brown Galingale plant and the Little Whirlpool Ramshorn snail.  It is part of Staines Moor, located at the eastern end of Colne Valley Park, which contains many Sites of Special Scientific Interest.

The Victorian-dug Shortwood Pond is a chalk spring-fed water with no outlet. Road drainage work in the 1970s directed stormwater runoff into the pond and, in recent years, water quality has been declining, with an increase in black globular sediment smelling strongly of hydrocarbons," said Project engineer David Funchal, of Mouchel Parkman. "Increasing traffic contributes to this pollution, and there was also no protection from a major spillage event. Working with Hydro International, we have found the Downstream Defender® is ideally suited to protecting the pond, because it separates out and retains the sediments. The entrapped solids and sediments are not washed out by high storm flows as they would be in conventional gully pots. The hydrocarbons and floatable portions are also retained.

Staines Moor is one of the remaining pastures of the manor of Staines, having been unworked for at least 1,000 years and common land since 1065. Given its precarious location between Staines and other conurbations, preservation of the quality of the habitats in this river valley is of high importance to the Spelthorne Borough Council, Plantlife UK, Colne Valley Park, Groundwork Thames Valley and the Environment Agency.

Shortwood Pond, close to the River Ash, was dug in the 1850s to provide water from the chalk aquifer for cattle. In fact, there are many unique SSSIs located within the Staines area, thanks to the rich Thames Valley floodplain and the high water table with good quality water.

Balanced management policies with close collaboration between all interested parties are key to the survival of these unique legacies.

The Problem

The increase in the fine black globular sediment was causing particular concern.  A pond without outlets acts like a sump, and can accumulate sediment which blankets the plants and algae, leading to very poor, oxygen depleted water conditions. 

The Downstream Defender® and the Up-Flo™ Filter from Hydro International, which targets other micro pollutants, nutrients and heavy metals assist with high rate treatment of stormwater runoff from highways.

The Outcome

The project was completed within a tight budget and kept to a small footprint," said David Funchal, "Apart from chamber emptying, maintenance is minimal. In fact we were able to oversize it to allow for up to 50 l/s and help protect the site against future large storm events.