How to achieve sustainable drainage systems October 2nd 2018, by Ian Lishman

New housing developments are required to feature adequate drainage and manage surface water. A sustainable urban drainage system, or SUDS, is not defined by one individual measure, but a series of systems and controls – from site design to ongoing maintenance after completion.

As such, it is a collaborative process that should be engaged with from the early stages of a project. Sources and potential causes of flooding have to be identified from the outset, factoring in the topography of the site, its proximity to areas of flood risk, the soil type, and the water table level.

Sustainability has a broad definition, and a ‘sustainable’ drainage system is not just about the movement of water and preventing flooding. It is about the quality of the water that enters the water treatment process, and having a positive impact on the greenfield land around a development site – protecting it, and even potentially encouraging thriving wildlife habitats.

Why is water quality important?

As water runs across urbanised sites, particularly car parking areas, it picks up contaminants and pollutants that reduce the water quality. Allowing that water to enter the wider drainage system places additional burden on the treatment process, and increases the likelihood of lower quality water re-entering the water cycle.

Selecting and incorporating filters within a SUDS helps combat those pollutants at the site level, before they enter the existing drainage network.

Site investigations typically form one of the earliest processes in the development of land, especially on brownfield sites where understanding and dealing with existing contamination is a particular priority. The extent of mitigation measures required depends on how the site was used and the ground conditions identified.

Selecting SUDS solutions

Rather than simply allowing water to enter the drainage network unchecked, with little thought for its impact downstream, one of the main aims of a SUDS is attenuation. The act of slowing and controlling water flowing from a site, especially at times of peak rainfall, is designed to avoid overwhelming other parts of the drainage network, and reduce the chance of flooding.

It’s easy to think of drainage and stormwater as something to be dealt with below ground only, but attenuation can start right from roof level. For example, green roofs and rainwater harvesting (such as through the use of water butts) can provide an initial buffer. Although they’re not part of groundworks, an experienced drainage contractor can make sure they are properly incorporated within the wider drainage works on site.

Every site and development is unique, requiring different combinations of measures to achieve the best result. It should never be assumed that a solution shown to work on one site can be copied on another.

Common sustainable drainage techniques

Permeable surfaces, most commonly paving, are a development of traditional hardstanding materials that allow the passage of water. Rather than acting as a barrier and directing a large amount of water to a limited number of outlets, permeable surfaces quickly direct water to the construction (such as a perforated drain) or soil below, where it can be stored or directed in a controlled manner.

Alternatively, drains may be formed from trenches filled with permeable material that slow the rate of flow and even store water. Another attenuation technique is ‘filter strips’ – areas of grass or shrubs that break up hard surfaces, direct the flow of water evenly and filter contaminants.

There are a series of below-ground options, perhaps the most well-known of which are soakaways, and including infiltration trenches and basins, designed to act in a similar manner. The structures are excavated and filled with stones (or similar), providing a temporary – but potentially significant – store for rainwater before it percolates into the ground.

An experienced drainage contractor, familiar with all of these techniques, can make sure they are constructed to specification, in accordance with the agreed design, and will perform as required.

About FACE

For nearly two decades, FACE have delivered high quality civil engineering works on projects of all sizes. Our new turnkey service allows housing developers, whether small or large, to work with us and build a project-specific package of services that FACE then delivers.

The package can include highway works carried out under a section 278 agreement, drainage construction and groundworks. Acting as a single point of contact for multiple aspects of the infrastructure means we can offer improved coordination of works and minimise potential delays.

Find out more and enquire about our turnkey packages here.