Discussing planning-led sustainable drainage at the CIWEM Surface Water Management Conference

Anna Beasley, Principal Analyst, is presenting at the CIWEM Surface Water Management conference today with Barry James, Strategic Manager at Somerset County Council.

The pair will discuss ‘Evaluating the effectiveness of planning-led sustainable drainage in Somerset’ in session one at 10.20 – 10.40. Their presentation summary, detailed below, highlights the key aspects their talk will discuss.

Assessing the effectiveness of planning-led sustainable drainage in Somerset

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). Through the SRA, Somerset County Council (SCC), along with its partners, is working to deliver the actions contained in the Action Plan.

There was a perception that Sustainable Drainage Systems (SuDS) installed on new developments were ineffective or poorly implemented, monitored and maintained. One action was to conduct an SRA funded review of recent SuDS delivered through the planning process in Somerset to test this perception and identify weaknesses.

The objective was to evaluate the effectiveness of the 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?

Desk-based assessments of each site were undertaken using planning applications, supporting documents and sewer maps. Designs were compared against relevant standards and good practice.  An assessment was made of the impact (if any) of new methodologies and standards, and what difference this may have made to the design. A high-level assessment of any ‘multiple benefits’ offered by the design (intentionally or unintentionally) was made, including water quality, biodiversity, landscape, amenity.

Site inspections using GISmapp identified SuDS/drainage features on the ground and assessed their compliance with design and their condition/evidence of maintenance.

A partnership approach was taken with consultation and review by the SRA, including Wessex Water, the Environment Agency, Highways England, IDB, and CIRIA.

In addition to providing valuable information about each site, the work provided an overview of planning-led SuDS provision in Somerset. It was found that there was inconsistency in planning documentation and level of detail of evidence submitted, making it difficult to evaluate scheme designs or to quantitatively assess whether these developments have impacted flood risk.

The sites varied in design, from traditional piped systems with underground attenuation to more recognisable ‘SuDS’ providing multiple benefits, e.g. swales, reed beds and bio-retention planters. Most schemes considered climate change, although there was little evidence that designs had considered exceedance or urban creep. Several sites included features that would improve water quality, although the water quality treatment train was not explicitly included in design.

Only a few of the sites included source control features, and there was little evidence of deliberate provision of biodiversity and amenity benefits. Site inspections showed poor construction site management practices with respect to managing sediment and pollution entering surface waters.  Maintenance plans were not evident in planning documentation, and while just over 50% of SuDS features were assessed as being in ‘Good’ or ‘Very Good’ condition, there were several sites where maintenance was lacking.

Somerset County Council are working on a standardised and consistent approach to SuDS consultation across the County, and we have made initial recommendations based on our findings.  The project has just been extended for 2017/18 to widen the sample of development sites and increase the scope of the review.

Specialist in flood risk consultancy

Anna Beasley has 15 years’ experience in flood risk consultancy. Her background is in hydrology and hydraulic modelling, focussing on fluvial, surface water and groundwater risk projects for clients including local authorities, the Environment Agency, water companies, and private developers. Anna has been involved in a wide range of projects including flood mapping and warning, feasibility studies, river and wetland restoration, assessing flood risk for strategic planning and development, sustainable drainage, and wastewater management. She has a particular interest in encouraging a holistic approach to sustainable drainage, to reduce flood risk while maximising environmental and social benefits.

Want to know more?

If you are at the CIWEM Surface Water Management conference, make sure you talk to Anna about this topic and her presentation. If you can’t make it, you can email Anna or visit our Flood and Water Management web pages.

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