What Are Sustainable Drainage Systems & How Can You Use Them in Your Project?

Permeable Surfaces

For our purposes, this is about using driveway and paving materials that will allow water to permeate and soak through to the natural aquifers underground, replenishing the water table locally and reducing the burden on drains. The most common types of suitable material are gravel, blocks and grasscrete.

Some forms of block paving are more porous than others, mainly dependent on the size of gaps between the units. Large-nibbed blocks are specifically designed to allow the maximum amount of water to permeate through. Tarmac (asphalt) is normally thought of as being impermeable, but porous systems are now available.

Another way to lay hardstanding without adding to the burden on drainage systems is to go for wheel track strips (rather than full driveways) using gravel, concrete or grasscrete. Cars can then be driven in without causing wheel ruts, while rainwater drains off to surrounding grass or planting.

Just as important as the surface is the sub-base, which must be free-draining to accept and dissipate rainwater. Standard Type 1 hardcore has fine particles that prevent this; instead, you need a permeable sub-base, so water can pass through the gaps around the larger aggregates.

All these porous surfaces have the added advantage that ice is less likely to form in winter, which makes for a reduced slip hazard. It’s worth noting, however, that not all sites are suitable for permeable materials – if the subsoil itself is impermeable, the water will not drain away.

With the help of Welsh Oak Frame, Matt and Sarah Switzer self built their dream 1.5 storey oak home in the essex countryside. The gravel driveway suits the cottage-style home perfectly while adding a level of security with its crunch sound when driven on. Photo: Nikhilesh Haval

Holding Ponds

When rain falls on roofs or impermeable hard surfacing, it is collected and held in a pond, or in a series of ponds, drainage ditches or wetlands. It’s then released gradually into the drainage system in a controlled manner. This is known as attenuation. The basic principle is simple: large pipe in plus small pipe out equals slower flow!

Depending on topography, it’s possible to create large holding ponds for existing developments as a shared community resource – providing rainwater attenuation as well as amenity and a wildlife attraction. Native planting and careful design can therefore transform a potential flooding problem into a local environmental asset.

Holding ponds offer a natural solution for rainwater attenuation, and can be particularly useful on larger developments, such as group or custom build schemes, where they’ll also offer wildlife habitats and amenity. Photo: Nick Bear/shutterstock

Harvesting Water

Rainwater harvesting systems can have a significant impact as part of a SuDS strategy. The storage tanks tend to be substantial, from around 1,000L up to 5,000L in typical domestic installations. Usage of the water (mainly WC flushing) also continues throughout the year, meaning that tanks usually have at least some spare capacity to store rainfall at any given point.

Harvesting water for use in the garden is one of the cheapest ways of reducing both water demand and the burden on the drainage system. However, it’s worth noting that the capacity of a standard garden water butt is relatively low. Most store between 200L and 300L, which quickly fills up during a rainfall event – the exact time when the water is unlikely to be of much use. The overflow is usually connected to the original drain, too, so the role of water butts in SuDS is very limited.

Rules & Regulations

In Scotland, it is already a requirement for all new housing to have a SuDS strategy in place. For developments of fewer than 10 dwellings in England and Wales, however, there is still no absolute requirement for a SuDS approach to be used. So, if you’re self building a one-off home, sustainable drainage may not be mandated – though it might be on larger custom build or group self build sites, depending on how plot sales are arranged.

That said, some councils maintain their own planning policies and guidance, and they may have chosen to make SuDS compulsory in your area. These local rules are generally based on Defra’s non-statutory technical standards, which were first introduced 17 years ago, and on the guidance set out in The SuDS Manual by the CIRIA, which you can buy for £25. For instance, any new development in a flood risk area will need to give priority to the use of sustainable drainage systems. Check out the Susdrain website for a deep dive into the various technical considerations.

Even if it’s not compulsory, I would argue there’s no reason not to use SuDS in your project. Doing so will contribute to reducing flood risk and may even provide habitats to encourage wildlife. Note that there are rules around what you can do when replacing or laying new driveways.

SuDS & Surface Water Quality

Rainwater often gathers pollutants such as oils, salts, litter or even sediments during its journey from the ground to the drainage system. Water quality is now a recognised concern for housing developments, and this could well provoke a change in planning rules in future.

On that note, sustainable drainage is increasingly used to clean the run-off from commercial property. An extreme example is the petrol interceptor tank commonly used on filling station forecourts.

Lower quantities of impurities can be dealt with by more natural techniques, such as bioremediation (using living organisms to remove contaminants). On the domestic scale, combining several SuDS techniques could be an effective way to help improve the quality of water discharge.