Background

The A7 Auchenrivock scheme formed part of a 2.3km highway upgrade south of Langholm, delivered in response to a poor accident record along this section of route. The project aimed to improve road safety by increasing overtaking opportunities while also accommodating cyclists and pedestrians.

Alongside these objectives, careful consideration was required for surface water runoff and water quality management, given the scheme’s rural location and proximity to sensitive receiving waters.

The challenge

The original £8 million scheme design included three SuDS ponds to manage flood risk and treat pollutants and sediments. However, site investigation and topographical constraints highlighted significant challenges:

  • Limited availability of suitable level ground
  • The only viable location for a third pond was some distance from the new alignment
  • Constructing an additional pond would have required a 300m new sewer, major drainage works and potential road closures during construction

Given the environmental sensitivity of the area and the need to minimise construction impact, an alternative SuDS compliant solution was required.

The solution

Following a review of available options, the Downstream Defender® advanced vortex separator was selected to replace one of the originally proposed SuDS ponds.

The Downstream Defender® provides effective removal of sediments and pollutants within a very small footprint, making it well suited to constrained rural sites. The treated surface water discharges into the River Esk, meeting the required environmental standards.

By integrating the Downstream Defender® into the drainage strategy:

  • Two SuDS ponds were retained and redesigned to provide additional stormwater storage
  • The need for a third, remote pond and associated sewer works was eliminated
  • Environmental disruption and programme risk were significantly reduced

As part of the wider scheme, the original road is being de trunked and partially converted into a cycle path, supporting sustainable transport objectives.

James McLeod, Resident Engineer at Dumfries and Galloway Council’s in house design team (DGDesign), commented:

“The only level ground available for a SuDS pond was some distance away. Constructing a third pond would have meant building a new 300m sewer and carrying out major sewer works during the road construction phase.”

He added:

“It was identified that the Marshalls Downstream Defender®, developed with Hydro International, could provide a space saving solution while achieving the required level of pollutant and sediment removal before discharge into the River Esk.”

The outcome

The revised drainage strategy delivered a SuDS compliant, space efficient solution aligned with the project’s environmental and safety objectives.

  • Effective surface water treatment without additional land take
  • Reduced construction impact in a sensitive rural environment
  • Maintained flood risk mitigation through redesigned SuDS ponds
  • Improved road safety and active travel provision

The scheme demonstrates how compact treatment technologies can replace or complement traditional SuDS features where land availability is restricted.