sustainable drainage

Relieving the pressure

Stuart Pearce of GeoSmart Information explains how SuDS can help mitigate the flood risk.


mproving the country’s resilience to flooding is driving the need for a new approach to managing surface water runoff. Existing drainage systems and sewers have a limited capacity and

the increase in surface water from development means that we can no longer rely on this infrastructure to cope with surface water on site. Add to this the impact of climate change and the risk and severity of flooding increases. Planning policy requires that development must accommodate additional runoff on site, and only rely on the existing drainage network as a last resort where it is not practical to manage the additional water within the site. This calls for a new approach and help is at hand for developers in the shape of Sustainable Drainage Systems (SuDS) that are designed to replicate the natural drainage from the site (pre-development) to mitigate the flood risk. They provide areas for water storage and drainage as close as possible to where the rain falls. Far less then goes into traditional sewers, taking pressure off existing drains and river systems elsewhere.

Clearer SuDS legislation

Currently, SuDS legislation is a complex and messy picture. Schedule 3 of the Water Management Act 2010 required the establishment of statutory approval boards to scrutinise drainage proposals. These have yet to be enacted. In part, concern over introducing further hurdles during planning had dampened the Government’s appetite at a time when housing supply is in high demand. The Chartered Institution of Water and Environment Management

(CIWEM) undertook a survey in 2016 on SuDS implementation on developments in the planning system. Their “A Place for SuDS” report concluded that the main barriers to widespread SuDS uptake in England are not from cost or practicality, but from policy and institutional barriers (progress on SuDS legislation is far more advanced in Scotland and Wales). While the Government is unlikely to remove the automatic right

to connect to the main sewer network, the report makes three clear recommendations: • Make discharge to the sewer system conditional on high-quality SuDS in new developments

• Update standards, focusing on the wider benefits of SuDS: amenity, biodiversity and water quality

• Clear direction on SuDS adoption and funding maintenance This will feature strongly in the House of Lords Spring Review of the

2010 Act Schedule 3 with repeated calls for the government to amend the 2016 Housing Act further to ensure SuDS legislation gains more bite.

Existing drainage guidance

That said, a drainage hierarchy already exists in both the Building Regulations and National Planning Policy Framework (NPPF) which

“Ideally, surface water should infiltrate into the soil directly but this may not always be possible as a single solution. Clay based ground conditions, such as those in parts of London, make solely relying on infiltration far harder”

prioritises drainage through soil infiltration rather than to a sewer. Since 2015, all developments in a flood risk area and all major developments (10 or more dwellings) need to demonstrate that the project will not increase flooding elsewhere or downstream. NPPF also states that Local Authorities must scrutinise development for

flood risk and to liaise with Lead Local Flood Authorities (LLFAs), created from the 2010 Act, to ensure that surface water runoff calculations meet the Authorities’ guidance. Increasingly, LLFAs and Local Authorities are more robust in their approach – albeit that the guidance varies frequently between Authorities. Ideally, surface water should infiltrate into the soil directly, but this

may not always be possible as a single solution. Clay based ground conditions, such as those in parts of London, make solely relying on infiltration far harder. Authority planning teams and LLFAs will be looking for infiltration SuDS schemes to be quoting adequate rates of water infiltration that do not increase flood risk on site or elsewhere. They will also look to set the water discharge rate as close to those of a Greenfield site.

Planning ahead Developers, architects and planning consultants are increasingly being | HMM March 2017 | 47

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