Review of Developer Sustainable Drainage Schemes in Somerset

Challenge

Somerset Sustainable Drainage Schemes

Following the flooding in Somerset in 2012 and 2013/14 the Local Authorities prepared a 20-year Flood Action Plan, which led to the formation of the Somerset Rivers Authority (SRA). One action was to conduct an SRA funded review of recent SuDS delivered through the planning process in Somerset.

The objective was to evaluate the effectiveness of the SuDS schemes post-construction:

  • Were they adequately designed?
  • Were they constructed as designed and agreed through planning?
  • Are there any deficiencies?
  • Are they adequately maintained?
  • Are they operating as intended?

A partnership approach was taken with consultation and review by the SRA (including five District councils, Somerset County Council, Environment Agency, Internal Drainage Boards, Wessex Regional Flood and Coastal Committee and Natural England), Highways England and CIRIA.

Following the success of Phase 1 of the project, covering seven sites, further funding was made available and in total twenty sites were assessed in detail.

Solution

JBA Somerset Sustainable Drainage Schemes GISmapp
Somerset Sustainable Drainage Schemes GISmapp

Desk-based assessment of each site was undertaken following a CIRIA-style checklist using information from the planning applications, supporting documents and sewer maps. Designs were compared against relevant standards and good practice including:

  • Management train
  • Discharge destination
  • Runoff/volume estimation
  • Climate change allowance
  • Discharge rate
  • Storage volumes
  • SuDS component suitability
  • Exceedance routes.

Multiple benefits offered by the design were assessed, including water quality, biodiversity, landscape and amenity. Impact on any existing flood risk was examined, and maintenance and adoption arrangements were noted.

The desk-based assessments were followed up with site inspections using GISmapp®, our location-aware mapping system, to identify and record SuDS/drainage features on site and assess their compliance, confirming: presence of critical assets (scale, form and construction) in accordance with approved drawings, dimensions, levels, condition and evidence of maintenance. Many of the site visits were carried out alongside Wessex Water inspecting engineers allowing their knowledge of the sites to be included.

In addition to the detailed site assessments, a high-level analysis of trends across 84 major development planning applications was also carried out, to find out whether the availability of evidence documents and calculations, and the type and variation of proposed SuDS features, varied spatially or over time.

Benefit

JBA Somerset Sustainable Drainage Schemes
Somerset Sustainable Drainage Schemes

In addition to providing valuable information about each site, the work provided an overview of planning-led SuDS provision in Somerset.

Findings from each development were drawn together highlighting common trends, and recommendations were made in terms of:

  • Type and detail of evidence included in planning applications
  • Detail and replicability of runoff and volume calculations
  • Design for exceedance
  • Consideration of the water quality treatment train
  • Inclusion of source control features, and provision of biodiversity and amenity benefits
  • Construction site management practices and quality of construction of SuDS features
  • Impact of adoption and maintenance arrangements.

The findings of the project will help the SRA, Lead Local Flood Authority (LLFA) and Local Planning Authorities (LPA) work together to standardise the approach to SuDS across Somerset. It will support them in their aim to develop robust bespoke SuDS guidance and planning responses for Somerset, to engage with developers on SuDS, and to monitor and enforce SuDS implementation in the future to ensure the delivery of multiple benefits. It will also provide an evidence base for the SRA to consider further funding initiatives to address weaknesses in the current process.

Want to know more?

Somerset Sustainable Drainage Schemes GISmapp

Email Anna Beasley for more information on this project, and on SuDS review and guidance.

You may also be interested in finding out more about our SuDS Site Checker GISmapp® app, which enables LPAs and LLFAs to quickly check flood risk and SuDS requirements for a site at the click of a button, as well as generating and pre-populating Pre-application advice or consultation response letters.

Find out more about GISmapp®

You can also find out more about these services on our Local Flood Risk Management web pages.



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