Sustainable Drainage – and why you need drought tolerant plants in a rain garden

On Saturday 28th May, Simon Bunn from Cambridge City Council took us on a tour of sustainable drainage features on the new housing estates around Trumpington. It may have been a Saturday morning but this was definitely worth getting up for. Thanks to Dawn and the Staying Dry project for organising this! There is nothing like getting out into the field to understand how things work at a landscape scale. Here are some photos to show you what we saw.

Simon describing the swale

This is Simon telling us about the first feature, which is a swale. But first, you need to know what is the point of sustainable drainage.


The land under these housing developments was previously farmland draining into Hobson’s Brook, and ultimately into the River Cam. Hobson’s Brook is a particularly important site for biodiversity as it is a chalk stream which is quite rare, especially in this part of the country. It is also sensitive to changes in water flow – too much could cause erosion and too little could lead to stagnation. Also too much silt or other pollution would be a problem. So the goal of sustainable drainage on this site is to manage flood risk, of course, but also to minimise the impact on Hobson’s Brook and the River Cam.

Ideally, drainage from the new development should mimic the old farmland both in terms of overall water flow and the rate of flow. When it was farmland the soil would have absorbed a lot of the water until it was saturated. Then the extra would seep through slowly into the brook. The sustainable drainage system has to mimic this behaviour by reducing and slowing the flow.

The ditch behind Simon is called a swale. Surface water drains feed into it from a road and from the residential area behind. (The sewers are a completely separate system). The swale has weirs along its length to slow the water flow and encourage silt to settle. The rate of flow into the swale is also limited by valves in the drains, to avoid flooding. When properly planted up, the vegetation on the sides of the swale support useful bugs and things that clean the water by consuming nutrients. So the swale doesn’t just hold the water, it also improves the quality. Plus it supports biodiversity and (should be) nice to look at. For more imformation see Susdrain.org.

Permeable paving (left) with crinkly edges and and normal paving (right)

We saw permeable paving in quite a few areas of the developments. The gaps between slabs are not sealed up with cement, so water can flow between and seeps slowly through the sand and gravel into the soil beneath. Simon is particularly keen on this where there are cars standing for long periods because bacteria living under the paving can ‘eat’ hydrocarbon pollutants that washes off the cars and tires.

This permeable paving has crinkly edges because the slabs need to touch at some points, so as to lock them in position, but have gaps at other points allowing water to flow.

There are other designs with different shapes available – for example Marshalls has a variety of designs, some with more room for grass and plants between the blocks.

A green roof over the porch

There were very few green roofs in any of the developments. The developers have a view that people do not like them. They do require some maintenance, as does any bit of garden and flat roofs are not exactly maintenance free either. Green roofs hold water when it rains. Also, in places where there are a lot of green roofs they have been shown to reduce the heat island effect.

Beneath our feet here is a tank

This is a grass play area, good for running around and ball games. However, 75cm below the surface is the top of a large ‘tank’ that can hold large amounts of rainwater in case of a big storm. The drainage system is designed to handle a storm that is as bad as a 1 in 100 year event, plus 30% to allow for climate change.

Attenuation tank construction using modular ‘crates’. Photo from http://www.fine-turf.co.uk/projects/sports-pitch-construction/attenuation-tank-installation-priory-academy-lsst-lincoln

Tanks like this are not the ideal solution as they only hold the water. They do not improve quality or supply any other amenities or biodiversity.

Detention basin

Dawn and Poppy (the dog) are standing in the middle of another play area that has a second function for flood management. I hope you can see that the grassy area is in a slight depression. This acts as flood plain, in case of a severe storm. This feature is called a detention basin. Yes really. You can read more about them here on Susdrain.

Downstream Defender – hydrodynamic separator. This is actually 1.2m wide and several metres tall. Picture from http://www.hydro-int.com/uk/products/downstream-defender

As well as reducing the flow rate into Hobson’s Brook, sustainable drainage needs to ensure good water quality, hence the need for devices such as this hydrodynamic separator. When the rain is heavy it can wash a lot dust and silt etc. into the drain system but this device cleans it up, removing 90% of the solids. It acts a bit like a Dyson vacuum cleaner. As the water flows through it is forced into a vortex that pushes the silt to the outside. It also separates out oil.

This device will need occasional emptying just like your vacuum cleaner. But it does not use electricity and it has no moving parts. None of the drainage systems installed here need power which means that if a severe storm causes a power cut they will continue to work as normal.

Rain garden in Hobson’s Square

This is a rain garden. It takes rainwater from the surrounding area that would normally be diverted into drains. Like the permeable paving, the purpose of the garden is to hold the water for a while and slow the flow in the piped drainage system when there is heavy rain.

Most of the time the garden is dry, because it does not rain that often in Cambridge. So the garden needs drought tolerant plants like these oriental grasses.

The white channel coming across the front of the picture is a rill that is aligned with a bronze age field boundary. The rill also feeds the rain garden. This square is not yet complete and we were the first members of the public allowed in. It will be known as Hobson’s square (more details here).

Attenuation pond

There is a large area next to the housing developments and Addenbrookes that is given over to a wildlife reserve but the ponds there are also part of the drainage system. They are called attenuation ponds. This is quite a small one, and fairly shallow. It already looks very natural in the wildflower meadow although both are entirely manmade.

The attenuation ponds hold water and slow its flow. The plants and bugs in the water clean it. There are four ponds on the site and they will be adopted by Anglian Water. The city council will adopt the land around them as a nature reserve.

Wildflowers and reeds on the nature reserve

Bird hide, built from wood reclaimed on site, looking onto the main attenuation pond.

The reserve is already home to a variety of wild birds. We saw Canada geese and several different kinds of ducks, while the air over the grassy areas was loud with the cries of skylarks. There are paths for people and we saw quite a few joggers and walkers. Being there you felt you were in a grand space for nature. You could still see Addenbrookes on one side and the housing estates on the other, both views punctuated by cranes. But from where we stood the meadows and sky felt much bigger than either of them.

Climate change brings increasing flood risk to existing homes too - what can we do?
The drainage systems we saw on this tour integrate flood protection into the landscape in subtle ways so that you hardly know they are there. Some of them support biodiversity and provide other amenities as well. It was great to see them in new housing development around Cambridge. However, sustainable drainage is not just an issue for new homes. Climate change means we can expect more severe storms and increasing flood risk to existing homes too.

Fortunately, there is stuff we can do. In our own homes we can adopt sustainable drainage tricks like turning flat roofs into green roofs, installing permeable paving instead of asphalt and making rain gardens to absorb the water from drain pipes. Transition Cambridge Staying Dry group has made a pilot rain garden at Romsey Mill Community Centre that you can visit. See Staying Dry for more information and to see how you could get involved.