Infiltration SuDS: Reducing flood risk and creating water resources
Mark Fermor, GeoSmart Information
New mapping shows infiltration SuDS potential is good across 80% of England and Wales, supporting widespread adoption.
Water balance studies demonstrate that SuDS can boost water resources to match the increased demand represented by development of individual dwellings and this is illustrated by a Case Study for a house in Bedfordshire.
Data and reports are now available for initial site screening and preliminary assessment, enabling access to key information to support site selection when development is being considered.
Under the new planning regime introduced by the Flood and Water Management Act 2010, site reports should be accessed at the earliest stage, and ideally prior to site acquisition, so that appropriate drainage solutions can be factored into development proposals, and suitable assessments used to support planning submissions and consultations.
Water resources (and contribution towards ‘Good Status’ under the Water Framework Directive) is a material planning consideration. Quantifying impacts of SuDS on water resources is the flipside to flood risk assessment, and should also support a more holistic consideration of sustainable development within the catchment context.
Infiltration SuDS should be the preferred solution at most sites, and tools and resources to support each stage of your development project are available at www.susdrain.org.
SuDS is about water resources too!
There is widespread awareness that SuDS reduces flooding because it attenuates peak runoff so that flows do not exceed critical thresholds at downstream vulnerable locations. SuDS also enables effective drainage to prevent significant flood risk arising onsite. The role of SuDS in reducing flood risk is a key regulatory priority.
What is not so widely appreciated (and has hitherto had a lower regulatory priority) is the importance of SuDS in terms of its effect on water resources. There is not currently an overall catchment approach that places each site development into a coherent overall framework to demonstrate how much difference SuDS make on a catchment level, but as I shall demonstrate with a simple Case Study, SuDS is actually a far more powerful agent of change for water resources than may have been appreciated, and this is a material consideration at planning stage that will benefit from increased attention.
Recent flooding events have focused attention on one particular flooding source or another. Unfortunately, without an overall catchment framework these events distort our perceptions of what is important in the catchment. Flooding with its immediacy and dramatic effects will always divert our attention from underlying less obvious processes that are depleting our essential water resources and changing the long term behaviour of catchments towards flashier runoff.
In this article I present a simple Case Study to show that SuDS actually make a very big difference to both flooding and water resources. Because development leads to very significant increase in flooding, so the opportunity for including SuDS in all relevant development from now on leads to both significant reduction in flooding, and increase in water resources.
Attention is rightly given to change in climate as a critical driver in increased flood risk. However, just as significant but ‘below the radar’, man-made change has been happening far closer to home that has exacerbated flooding dramatically too. As an example, the Adaptation Sub-Committee of the Parliamentary Climate Change Committee reported that land used as front gardens in urban areas saw increase in paved area from 28% in 2001 to 48% in 2011. Less than 3% of this was permeable paving.
Paving of front gardens for parking was a major contributor. When you consider that paving can lead to a ten-fold increase in flood runoff compared to greenfield rates it is not hard to realise the effect of such widespread land use change on catchment flooding.
So, at the same time as spending Millions on flood defences in the flood plain, £Millions more is being spent promoting increased runoff from new developments that are built without SuDS, threatening those flood defences even more. In the meantime, water resources are under ever greater pressure in water scarce parts of the country.
The new SuDS regime implemented through the Flood and Water Management Act 2010 has a particular focus on flooding but it should be recognised that the impact on water resources can be just as significant. Due in part to the fragmented remit of the regulatory bodies’ involved, relevant guidance and procedures are all focused on flooding to the exclusion of water resources.
A word about site suitability
Government planning policy now include a presumption in favour of SuDS for all major development, and a hierarchy of drainage options promotes infiltration to the ground (Infiltration SuDS) as the first preference.
With the advent of Open Data and nationwide screening reports of the kind referred to here, there are many more sources of relevant information available at low cost to indicate where infiltration drainage potential is likely to be sufficient for new SuDS schemes. Under the new regime it is necessary to consider drainage at the earliest possible stage of any development, and preliminary indications should be obtained prior to acquiring sites to confirm that drainage solutions are available that will meet the LLFA and Planning requirements.
Initial assessment and relevant desktop studies are available now to facilitate better decisions prior to site testing, because the quality of maps and other data is radically improved and standardised reports are more widely available at minimal cost. Clearly the location of infiltration SuDS proposals should be screened using maps such as these to obtain a preliminary indication of site suitability. Early dialogue should also be held with relevant regulatory bodies. Reports focused on each stage of the project can be rapidly obtained to support relevant planning applications or preliminary discussions where previously it was often necessary to undertake expensive and time consuming site investigations.
In the example presented in Box A as a Case Study it was found that infiltration SuDS provided a suitable drainage solution, and significantly also led to the creation of additional water resources at the same scale as the increased demand represented by the typical household.
Overall new infiltration SuDS suitability mapping shows that across England and Wales close to 80% of land has moderate or high infiltration SuDS potential, and only about 5% of this area shows reduced potential due to high groundwater flood risk. Such results highlight the widespread potential for SuDS adoption, and support the presumption in favour of SuDS that is intended by the new regime.
Initial screening of sites should be undertaken ideally before site purchase because drainage is clearly a matter that has significant implications for cost of development and, in some cases, site viability. Subsequent stages of assessment will factor in potential groundwater protection, subsidence and geo-hazards, land contamination and other factors. Again, the availability of better data helps screen sites for such issues before funds are committed to more expensive site investigation and testing.
Selecting the right SuDS approach and components is vital for a successful development under the new regime. Infiltration SuDS is not suitable for all sites, and the vital thing is to get a clear appreciation of factors relevant to your particular site; the costs and the benefits; and understanding of project risks and how to mitigate them. Initial screening reports should be accessed prior to any property transaction. For sites where infiltration SuDS is less appropriate, attenuation SuDS provides the next alternative that may provide the right drainage solution. In all SuDS matters you can source great data, reports, and other resources that will help point you in the right direction and provide initial answers, at www.susdrain.org.
The simplified Case Study presented in this article demonstrates that infiltration SuDS not only reduces flood risk, but also creates additional water resource and ameliorates low flows problems that are widespread in our rivers and streams, giving a real boost to the development of new dwellings within water scarce areas.
Whilst the current focus in published guidance and regulatory attention is all directed at flooding issues, I hope this article helps to promote an appreciation that the water resources side of the equation should also be recognised when considering new development, and the overall objective of achieving more sustainable development benefits from consideration of the catchment context.