Let’s have an argument

- 8:34 pm - November 29th, 2015

A discussion on Linkedin some time ago suggested that the best way to start an argument is to invite a bunch of engineers into a room and ask them “what is a raingarden?”   At the green infrastructure conference in Glasgow (www.sgif.org.uk ) last month, an excellent presentation by Tony Barrett of AECOM had a nice analogy with breeds of dog:  every size, shape and purpose, from a chihuahua (he only had to say that, not spell it however) to a great Dane.  In Australia, and there are examples too in West Coast USA (and probably elsewhere), there is a range of raingardens from DIY houseplot units for utilising roof runoff from a house, to large scale community raingardens.  The unifying characteristic seems to be that they are all green infrastructure drainage features, with multiple benefits including stormwater management, amenity and in some instances wildlife interest as well.

Another common characteristic of many raingardens in North America and Australia has been the design rationale; typically they are biofiltration features.   The under-drained swales also familiar in many countries are also examples of biofiltration technology.  There a planted topsoil material is laid onto an engineered or artificial subsoil/drainage filter media, from which water can flow into a collection pipe, or infiltrate where natural soil conditions and water table allow.  The Rummelsburg housing development in Berlin, and the Hoppegarten industrial estate just outside the city are well known examples in Germany.  At that scale, the biofiltration idea, especially if combined with in-built additional storage capacity, allows a smaller land-take (narrower swale), important in dense developments.   The German examples are 20 years old and studies of their continuing performance have recently been published (D’Arcy & Sieker, Water 21, February 2015, pp 26-28; and Somer et al 2014, 13th ICUD conference, Sarawak, Malaysia, 7-12 September 2014).

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Large scale raingarden near sports facility, and small scale unit in a car park, Hobart, Australia.

The use of multiple layers of filtration material and a biological treatment zone in topsoil, is easily managed at a houseplot scale, which has attracted most interest in the UK.  Here raingardens are often assumed to mean house plot gardens, not public parks and gardens as might be seen in Melbourne and Hobart for example in  Australia, or in Seattle or Portland in USA.    

At a larger scale it remains to be seen if the biofiltration complexity in vertical profile can be successfully managed and maintained where extended arm excavators and diggers are required for sediment removal at some point, especially in hybid systems involving a depth of water above the treatment/filtration layers (there’s at least one such challenging example in Greater Melbourne which will be interesting to monitor).

For a raised bed raingarden, biofiltration design is very similar to good practice advice for gardeners building raised beds, in terms of soil depth and drainage layer/s.  It is the introduction of rainwater from a downpipe which makes a raised bed a raingarden, and the consequent modifications to design to avoid soil wash out and flooding of the foundations of the property from overflowing water.  C & D Associates have been developing prototype raised bed units that combine planter functions with flow attenuation, and examples are being tested at several sites now, and a programme of installation and evaluation is planned for 2016 (contact brian@enviroexperience.co.uk ).

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Raised bed raingarden unit at Taylor Wimpey show house (short-listed for VIBES Awards).

Telling the householders, and members of the public in general, that their raingardens are biofiltration SUDS – which will always be essential for conversations and written proposals from engineers and regulators and other technical professionals  – is unlikely to help gain understanding and interest.   Arguably, for most types of green infrastructure SUDS, from houseplot raised beds, green roofs and walls, swales and filter strips, to detention basins and detention ponds (see first blog on raingardens) we should be introducing them to the public simply as raingardens, and learning from experience in Melbourne that the public will respond to such an idea.  The engineers and architects will still need to be asking what kind of features (i.e. what type of SUDS techniques are to be used, and looking at SUDS manuals for design advice).

BJD, 22nd November 2015