Planning and Sustainable Drainage

Warwickshire County Council Flood Risk Management team first published its flood risk and sustainable drainage (SuDS) guidance document in 2015. This was soon after the LLFA was made a statutory consultee to the planning process for Major Development.

It is now eight years since the LLFA commenced our statutory consultee role and in the intervening time there have been increasing expectations regarding how development provides good design and manages the environment. We believe SuDS can play an important role in all facets of a development: managing flood risk; improving water quality; providing habitats and increasing bio-diversity; and aiding placemaking and creating attractive community orientated spaces. It is therefore time for an update to our guidance.

Our latest Flood Risk Guidance for Development provides advice regarding flood risk and surface water drainage and should be used by masterplanners, consultants and other stakeholders in designing developments. We hope this updated guidance provides a vision of holistic surface water management in new developments, allowing space for watercourses and SuDS from the outset.

Throughout, we have tried to provide examples of different scales and features to provoke thought and innovation, driving a greater uptake of SuDS across a development, not just in single/large features at the end of pipe runs. 

The changes within the guidance are extensive but should not be considered new or different to high-quality SuDS design. The below provides a high-level description of some of the changes.

• A new Foreword sets out more of a vision of SuDS across the county, synthesising how SuDS can meet multiple aims and objectives including good design and bio-diversity.
• Section 1 on planning policy & legislation has been shortened but instead a matrix is provided summarising various Local Plan policies.
• Section 3 merges a number of disparate flood risk elements from our previous version including our expectations regarding deculverting & hydraulic modelling.
• Section 4 provides various site-specific constraints and how we expect these to be managed. Included here are details regarding solar farms and educational sites with examples provided from a site visit to Bewdley School.
• Section 6 clarifies our position on a number of discharge rate elements.
  • We have seen some applicants/consultants calculate greenfield runoff rate based on ‘development area’ but use this only for the contributing area without pro-rating it. As the LLFA, we are concerned about how this ‘double counting’ could increase flood risk elsewhere.
  • Our brownfield 50% betterment is a minimum and we want to be clearer to applicants they should approach this in the opposite direction to determine “what is the most feasible discharge rate” rather than the current “minimum required”.
• Section 7 is a brand new and significant section that sets out our vision for SuDS and tries to provide more examples of where they could be incorporated into development
  • Sections are provided covering ‘micro-SuDS’ documenting where simple changes can be instituted such as raingardens instead of raised planters.
  • Examples are provided of different inlets and outlets along with consideration for conveyance and strategic attenuation design.
• Section 8 combines a number of different elements relating to maintenance, easements etc.
• Section 9 sets out our Micro Drainage / Causeway Flow parameters of what we expect in drainage design modelling.
  • Importantly, we will be expecting calculations to use Cv values of 1 for both summer & winter storm events. The default values in Micro Drainage are historic and incorrect, justification is provided within the report.
  • We’ve been increasingly challenging MADD factors, the guidance is now brought in line with what we’ve asked for previously.
• Section 10 changes the style of what we as the LLFA typically ask for at planning. We have tried to show how the same information is required but increases in detail as a strategy moves through the stages of strategy, scheme and detailed design.
• Appendix A – Our old pro-forma has been removed as this was rarely used. Instead it has been replaced with a template/pro-forma for reviewing all SuDS features.
  • We’re hoping that this tries to spur more thought about all SuDS features
  • We expect some applicants to use their own templates/reports to do this but it’s important that they provide justification. Too often we see reports that provide a generic explanation of what SuDS features are but not justifying why they aren’t included.

We do accept an element of transition will be required for existing developments and will apply this on a case-by-case basis, sensitivity testing may be used in the interim.

This update can be seen as an interim revision prior to enactment of Schedule 3 of the Flood & Water Management Act (2010) later next year where it is expected further design guidance / information will be provided.