6 Dec 2013
Despite all the good work underway to deliver SuDS in the UK at the moment we are still only on the foothills of a very tall mountain. Welcome legislation has already made SuDS compulsory for new development in Scotland and soon England and Wales will follow suit.
But new development is just the start of our climb to the summit. While there have been some opportunistic and small-scale retrofit SuDS, there are currently no plans for any broader institutional programme to retrofit SuDS on a catchment or regional basis in the UK.
The issue was highlighted in a recent Environment Agency (EA) report commissioned by Defra “An assessment of evidence on Sustainable Drainage Systems and Thames Tideway Standards”. It collected together a wealth of available evidence around the challenges and barriers to wide scale SuDS implementation. It includes evidence cited from Engineering Nature’s Way own report into “SuDS in the Urban Landscape”.
The clear inference of the EA report is that, based on the evidence, we cannot be confident SuDS alone could adequately protect a wider catchment from CSO spills into the Thames without the need for piped underground systems like the Thames Tunnel. Leaving aside the specific arguments about the Tunnel, this conclusion surely challenges us to go back to some ‘first principles’ of SuDS?
Sustainable Drainage Benefits
I have been involved with the development of SuDS for more than 30 years and, in my view, the crux of their sustainable benefit is that they deal with surface water as close to the point where the rain falls to prevent the need to pipe stormwater away in large quantities. They should do this by mimicking natural paths and processes.
If we are ever really to get off base camp then there are three fundamental ‘building blocks’ without which I believe we will fail to make progress.
1. What it means for drainage to be sustainable
In the EA report (page 4) there appears to be an underlying assumption that SuDS should always be ‘natural’: For example it states that in London “many of the surface water management techniques employed would have to use underground storage attenuation schemes (which are not SuDS) and do not provide water quality treatment, or the benefits of wildlife, air quality and reduce heat island effects, and amenity.) This statement demonstrates the danger of confusing the need to achieve sustainable surface water objectives through source control with an imperative for biodiversity. I fully support the need for natural features, but in retrofit situations they may be difficult – even impossible – to achieve. We should not preclude using proprietary systems – with or without some natural features – to achieve sustainable source control through attenuation and infiltration. It may also be possible to use underground storage to achieve other sustainable benefits, for example, for recycling rainwater for re-use in public or private settings.
2. SuDS performance must be predicted through robust evidence.
The EA report demonstrates the lack of good quality data on the effectiveness of SuDS features, especially for water quality. To successfully engineer a drainage system, you must be able to predict how it will perform over the whole of its working life. Performance data is much easier to demonstrate for proprietary systems, especially when they have independently verified certification such as BBA or WRc approvals. But how do you scientifically predict the effectiveness of a pond, swale or a wetland? There is little reliable, objective data available based on actual experience and I think the whole industry would welcome the funding of an independent databank, for example by a university or trade association. Coupled with this must be a universally-accepted definition of what a stage or level of treatment is. The new National Standards could help provide a more robust framework for England and Wales, but Scotland still lacks clarity on what is a level of treatment, according to measurable parameters e.g. sediment loadings, or key pollutant removal targets.
3. Through-life maintenance must be planned from the outset
We are getting better at SuDS design and development, including SuDS from the earliest stages of masterplanning and involving landscape architects where appropriate. But planning and funding for long-term maintenance of SuDS also has to be built in to the initial SuDS specification. As our survey showed, in Scotland well-designed SuDS features appear to have been installed without robust arrangements for who will maintain them in the longer term. Indeed, as the Round Table debate highlighted, sometimes the responsible authority may lack the skills or resources to maintain SuDS features like wetlands and swales. It’s therefore vital that clear, predictable and achievable maintenance schedules by the designated owner should be subject to approval from the outset whether the SuDS is natural or proprietary, new or retrofit.
What an achievement it would be if we could achieve a culture change for sustainable drainage just like we have done with domestic waste recycling in the UK. There can be very few people that would not support the principle of at-source, sustainable SuDS. How long will it be before we all have SuDS features on our street corners?