Grassholme Reservoir, Durham: Collaborative Working Delivers Engineering and Biodiversity Solution


The need for remedial works to Grassholme Reservoir had the potential for significant adverse effects on important ecological features including woodland, freshwater habitats, protected species and statutory designated sites.

BSG Ecology worked with nature conservation stakeholders and the Northumbrian Water project team to minimise ecological impacts and achieve no net loss of biodiversity.


Significant remedial works are required at Grassholme Reservoir, County Durham, to address safety concerns raised during a statutory inspection by the Reservoir Panel Engineer.

Unavoidable impacts of these works will include:

  • The need to re-route a 280 m section of watercourse
  • The loss of up to 0.6 hectares of ancient semi-natural woodland

Potential loss of foraging and roosting habitat for bats, loss of supporting habitat for bird populations associated with the North Pennine Moors Special Protection Area (SPA), and changes to hydrology resulting in impacts on a groundwater-fed Site of Special Scientific Interest were other key considerations for the planning application.

The challenge was therefore to identify how to reduce impacts on these ecological features to an acceptable level and achieve no net loss of biodiversity without compromising a viable engineering solution.


The approach to the work was informed by extensive consultation with stakeholders that included the Environment Agency, Natural England, Durham County Council and the North Pennines Area of Outstanding Natural Beauty, and confirmed through liaison with the Northumbrian Water Conservation team.

Detailed baseline data were then gathered for the site. Key considerations were characterisation of the stream, the woodland and the bat community of the site.

  • The stream was found to be a good example of a natural upland watercourse, with minimal human alteration, and supporting a range of fish and fresh water invertebrate species.
  • The more common mature trees within the woodland were silver birch and hazel; rowan and ash were also present but occasional. The woodland was considered a relatively poor example of its type due to overgrazing affecting the quality of the ground flora and evidence of recent infill tree planting to supplement the more mature trees.
  • Initial ground level and climbed assessment identified considerable opportunities for roosting bats in the woodland. Extensive capture and radio tracking of bats was therefore carried out alongside standard emergence / re-entry surveys to determine the extent to which the woodland to be lost acted as a roosting resource. This established that while a minimum of seven bats species used the site, only small numbers of pipistrelle bats roosted in the wood; most bats commuted to the site to forage from areas of valley woodland downstream of the reservoir (along the River Lune corridor). The results made the situation with regard to European Protected Species Licensing more straightforward.

The mitigation solution outlined in the planning application was developed in conjunction with detailed discussions with the project engineers and hydrologists, and included:

  • The creation of a replacement stream section including many of the boulders from the stream that was lost, incorporating riffles and other features, and supplemented by the planting of riparian vegetation.
  • The translocation of many of the trees / their root balls (using skylines) and the soil from the ancient semi-natural woodland to a nearby receptor area as part of the creation of approximately four hectares of semi-natural woodland.
  • The creation of a linear strip of riparian woodland, providing approximately 2 km of woodland cover along the edge of the reservoir. This is likely to provide a valuable local resource for bats.
  • The erection of bat boxes in suitable areas of nearby habitat to compensate for the roosting opportunities lost.

A shadow Habitats Regulations Assessment (sHRA), informed by desk study and field survey data (a habitat appraisal) concluded there was unlikely to be a significant effect on North Pennine SPA bird populations as a result of the proposals. The conclusion of the sHRA was accepted by Natural England and Durham County Council.


Planning permission was received for the project in November 2020.

The legacy of the project will be both a safety compliant dam structure and the delivery of a far more extensive and better managed area of semi-natural woodland habitat than was present prior to the works taking place. Habitat creation will be secured through a long term management plan which will be implemented by Northumbrian Water.


Throughout the design, consultation and planning phases of this project BSG Ecology has delivered a complete ecological service, providing guidance and high-quality advice to both NWL and the wider project team”.

Mark Morris, Conservation team, Northumbrian Water

Our work in support of the Grassholme Reservoir project provided BSG Ecology with a considerable technical challenge; to strike a delicate balance between the need to deliver high quality ecological mitigation and a robust engineering solution. To achieve this we worked closely with the project engineers, hydrologists and construction contractors and drew from expertise across the BSG Ecology team. The outcome is a project that delivers in terms of dam safety and for biodiversity.”

Neil Beamsley, Principal Ecologist and BSG Ecology project manager


Northumbrian Water

Arboriculture, Bats, Biodiversity, Birds, Habitat Management Planning, HRA, Infrastructure, North East, Projects, Protected Species