Drainage Strategy

Scope of the Project

Sustain Quality was commissioned to provide a Sustainable Drainage Strategy for a proposed development on a greenfield site in rural Herefordshire. The Hillside Glasshouse is a uniquely designed sustainable house, which will introduce 646 metres square of hardstanding to the site in the form of residential roof area, access and parking and footpaths. The drainage strategy will be used to support a planning application for the development. It aims to identify water management measures, including Sustainable Drainage Systems (SuDS), and to provide surface water runoff reduction and treatment in line with CIRIA SuDS (C753) Guidance.

Tools for Task Implementation

As an integrated service, the Sustainable Drainage Strategy incorporates a report delivered by an external agency. Sustain Quality coordinated the report, and reviewed and evaluated its results to form the drainage strategy. The report takes into account national and local water guidance policies, and reviews information from external sources such as British Geological Society (BGS) Interactive Map, and Magic Interactive Map.

Challenges for Task Implementation

Some elements of the report were identified as being not representative of site-specific conditions. For example, identifying the site’s underlying geology relied on a historic borehole that was located 300 metres away. Also as the development is not yet built, all measurements and recommendations were based on plans provided by the client. The proposed site is near a river and in close proximity to a farm, which means high phosphate and other minerals contamination need to be brought to acceptable levels.

Solutions

The Sustainable Drainage Strategy gave a detailed report of the site, which was on greenfield, undeveloped land close to a river, and included existing conditions, geology and local drainage. It also examined planning policy and guidance relevant to the proposed development. This information informed the strategy’s assessment of the following:

  • Surface water discharge to soakway
  • Surface water discharge to watercourse
  • Surface water discharge to sewer
  • Soakaways
  • Wetlands, swales, detention basins and ponds
  • Rainwater harvesting
  • Green roofs
  • Porous/permeable paving
  • Underground attenuation tanks

To ensure the increase in surface water does not increase flood risk elsewhere, the drainage strategy recommended the use of flow control and the provision of attenuation on site to accommodate storm events and climate change event. It also recommended surface water be attenuated to the greenfield rate and discharged into the nearby river via a new gravity-fed outfall.

Recommendations were also made about the provision of attenuation within the the permeable paving incorporated in the hardstanding areas, and a wetland area located on site.

The strategy also identified the treatment of foul flows, and recommended using a Klargester BioDisc sewage treatment plant, before discharging into the wetland system for further treatment of phosphates and other pollutants. Following the required treatment, the report found the effluent could be discharged into the nearby river at a controlled rate.

Key Achievements

The drainage strategy makes a comprehensive assessment of the site and identifies key recommendations to reduce surface water runoff and treatment in line with the highest national standards. This will provide strong support to the proposed development’s planning application, and help deliver a highly sustainable project with drainage management that allows for storm and climate change events.

Benefits for Clients

Adopting the recommendations of this drainage strategy will demonstrate the client’s strong commitment to sustainability. Together with the Sustainable Development and Net Zero Home reports, the Sustainable Drainage Strategy reflects the project’s holistic approach, and provides a solid foundation from which the client can create a truly unique, ambitious development that pushes the boundaries of sustainable and environmental design.