Simon describing the swale |
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) |
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 |
Beneath our feet here is a tank |
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 |
Detention basin |
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 |
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 |
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 |
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.
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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.
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