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Sustainable Drainage Systems - another piece of the jigsaw

Oct 23 2020

You may have come across the phrase 'sustainable drainage systems', also known as 'SuDS', but be unsure what it means, and just how important it is as part of the overall flood risk management picture. So here's a quick explanation, in non-technical language.

On Saturday 3rd October, the UK experienced the highest total rainfall for a single day since records began back in 1891, according to the MetOffice*. Householders in Northumberland, Oxfordshire, Hertfordshire and parts of Wales all suffered the misery of floodwaters invading their homes (and, as always, my heart goes out to all to those affected).

The only reason that flooding wasn't even more widespread was because most of September had been unusually dry – so, for once, the rain actually had somewhere to go – i.e. the ground was able to absorb much of the deluge.

It could easily have been a very different story, as our habit of paving, asphalting and concreting over large areas in recent years has come at a cost. Not only have much-needed homes and businesses been built on green fields, but also fewer people in towns and cities have gardens devoted to lawns and flowerbeds – instead there has been a trend for patios, decking, and car-parking spaces. All of those hard surfaces mean that rainfall runs off very quickly, which can overwhelm the drainage system – and yes, that DOES include those that are kept clean and clear of leaves/road grit etc, because when a pipe is full, it's full!

The good news is that there are tried and trusted ways to reduce the risk of this type of flooding – and they all involve deliberately creating places for the rain to go - at least temporarily. This is what 'SuDS' do - instead of using traditional asphalt for a driveway, for example, it is possible to use 'permeable paving' which allows the rain to trickle through and safely soak into the ground below.

The soil may still become saturated after a while but, crucially, the run-off starts later than it otherwise would – and there's a bit less water heading for the drains overall. The more houses that have this type of drive/car-parking space, then the more rain that will be held back – and so the lower the chance of the drains being filled to capacity within minutes of a storm beginning.

Many more sophisticated types of SuDS also exist, but quite a lot of them need to be created as part of new developments at the outset for maximum effectiveness. Some of them can even help reduce pollution reaching the rivers as an added benefit. To encourage their adoption the 'National Planning Policy Framework', which now guides English local authorities when considering planning applications, states that permission should only be given if it can be demonstrated that a scheme:

"… gives priority to the use of sustainable drainage systems."

This is good news for reducing the flood risk from new developments, but how about existing homes and small businesses?

There are other ways to help to 'slow the flow' of rainwater hurtling on its way downhill, and some of them are quite inexpensive. The humble water butt has long been used by gardeners to collect rain from the roofs of their sheds and greenhouses for use in dry spells (especially on fussier plants like rhododendrons that dislike tap water). Any water that can be captured in this way has the added advantage of helping to delay the start of the run-off process – and 'every little helps' is very true in this case – just think how much water could be collected if every household that could possibly manage it installed at least one water butt?

Some other ideas you might like to consider are covered in our Homeowners Guide (pp 43-44). For those interested in delving deeper into the topic, then I can recommend the independent organisation 'Susdrain', whose website*** has lots of case studies, as well as the technical details I've (deliberately) omitted from this blog!

* Saturday 3rd October – details here: https://www.bbc.co.uk/news/science-environment-54561601

** Planning permission – para 103: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/810197/NPPF_Feb_2019_revised.pdf

*** Susdrain: https://www.susdrain.org/case-studies/