Surface water and the Code

In recent years many parts of the UK have experienced major flooding. Our climate is changing, with rainfall intensities expected to increase. To help reduce future dangers from flooding, by Spring 2012 the National Standards for Sustainable Drainage Systems (SUDS) should be introduced. Until then, the Code for Sustainable Homes is setting a standard, and includes surface water issues Sur 1 and Sur 2, containing some mandatory elements. These elements are mandatory for all Code levels; therefore they are potential “show-stoppers” and can prevent the issuing of a Code certificate. The following information is based on the November 2010 version of the Code, although older versions are similar.

For a site which was or is occupied by a permanent structure (the Code definition of “brownfield site”) the mandatory elements are simplified if it can be shown that the man-made impermeable area after development will be no more than that before development. Evidence of achieving this can be in the form of drawings from which areas can be calculated.

For all greenfield sites, and others where the impermeable area will be increased, it is necessary to ensure as far as is feasible that the peak runoff rate is reduced, and that the runoff volume for an extreme storm is also reduced. Proving that these criteria are achieved or that it is not feasible to achieve them will require calculations performed by an appropriately qualified professional. These calculations involve working through what would happen in various storm scenarios.

For all sites it is mandatory to demonstrate that the SUDS design takes account of the possibility of local drainage system failure (whether caused by extreme rainfall or lack of maintenance). There must not be an increased risk of flooding in these circumstances to the development site or to any other property as a result of the development.

The principle of SUDS is to dispose of the surface water as close as possible to its source. SUDS techniques include green roofs, permeable paving, rainwater harvesting and soakaways. Where ground conditions do not allow a soakaway to be effective, other techniques are available, such as ponds, which can be designed to meet safety criteria. Many SUDS features have amenity value. Furthermore, under the guidance of an ecologist, many features can contribute towards credits in the Ecology category of the Code. Another benefit which improves Code scores is the reduced mains water consumption resulting from rainwater harvesting.

Surface water credits can be earned by incorporating SUDS techniques to:

	improve runoff quality by removing pollutants, and
	prevent any water discharging off the site for the more common UK rainfall events, which bring up to 5mm of rain.

Further credits can be earned by providing evidence that there will be a low risk of the development flooding from any source – not just rivers and the sea, but also groundwater, surface water and infrastructure failure. This evidence will be in the form of a Flood Consequence Assessment (FCA) in Wales, or a Flood Risk Assessment (FRA) in England. These assessments will entail enquiries to the Environment Agency, the water authority, sewerage undertaker and local authority. If there is a significant flood risk a credit can still be gained by meeting certain criteria of flood resistance and resilience, which must be documented in the FCA/FRA.

SUDS design, FRAs and FCAs are specialised tasks that require a suitably qualified professional – a person or team with the required skills, knowledge and experience.

As can be seen, meeting the mandatory requirements and gaining credits in these issues is not a task that can be ignored. Bearing in mind the possible effect of the chosen SUDS features on the site plan, Sur 1 and Sur 2 should be among the first issues to be tackled in a Code pre-assessment.

Finally, here are a few tips:

	Start early.
	If you are using a landscape architect, find out whether they have sufficient experience of SUDS techniques to carry out your SUDS design and calculations.
	Identify your source(s) of expertise and get written confirmation of their qualifications and experience. Contact us if you would like some suggestions.
	If you are engaging an ecologist consider getting them to talk to your SUDS expert in the interests of synergy: Otherwise you may end up with conflicting recommendations.

Copyright © Eco-Rating Services Ltd | 2010-11

Home| Code | Code detail | EcoHomes | EcoHomes Detail | Our Customers | Glossary | Contact Us