In several recent FloodList articles relating to the Los Angeles River in the USA, and revitalising natural landscapes to reduce flood risk, how changes to our rivers and Floodplains have exacerbated flooding, reforestation of uplands to help tackle rural flooding and Slow the Flow project in the UK, references are made to ‘natural flood risk management’, creating ‘more absorbent landscapes’ and ‘slowing the flow’ in order to reduce the intensity of flood events.
These approaches epitomise the concept of ‘Low Impact Development’ (LID) as implemented in the USA, and a concept described as ‘Sustainable Drainage Systems’ (SuDS) in the UK.
LID (or SuDS) represents a new approach to storm-water management that is aimed at reducing capital costs and ecological intrusiveness, whilst providing a wider range of benefits than traditionally-engineered storm-water systems. Although these approaches have been in existence for decades, they have only become accepted best practice in recent years.
In this series of articles, the background and concept of the SuDS approach in England (Wales, Scotland and Northern Ireland have their own policies) will be examined, then two brief case studies of its effectiveness will be outlined. This will be followed by an examination of the US LID approach (often combined with the ‘Green Infrastructure’ concept, as LID/GI), and finally a summary of some benefit-cost evaluations of LID/GI will be given, including a project in Los Angeles, California.
Sustainable Drainage System Provisions in England – A Background
Immediately following devastating floods in the UK in June and July of 2007 – in which 13 people died and 7,000 had to be rescued, while 55,000 properties were flooded and nearly 500,000 people were left without electricity and water – the British Government commissioned Sir Michael Pitt to carry out a review focussing on flood risk management, emergency planning, the resilience and vulnerability of critical infrastructure, the emergency response and the recovery phase. Pitt described this as “…one of the widest ranging policy reviews ever carried out in the UK”.
More than 1,000 pieces of written evidence were received during a three month consultation period. The final Pitt Review report, consisting of 505 pages, was published in June 2008 and contained some 92 recommendations, which were evaluated and accepted by Parliament by the end of that year.
One of the resulting actions was the introduction of the Flood and Water Management Act in 2010, in terms of which the UK Department for Environment, Food and Rural Areas (DEFRA), in December 2011, published proposed National Standards for Sustainable Drainage Systems (SuDS) for consultation purposes (DEFRA had, in 2009, already published PPS25, a Planning Policy Statement on ‘Development and Flood Risk’).
The National Standards were proposed in terms of Schedule 3 of the Act, which envisaged a 3-year phasing-in period, with only major developments initially required to seek approval from applicable SuDS Approving Bodies (SAB’s) for sustainable drainage proposals, but by October 2015, all developments of more than one house would require SAB approvals.
The government’s final progress report, published on 27 January 2012, indicated that 43 of the 92 Pitt Review recommendations had been implemented and that a further 46 were progressing (although no deadlines were given for these remaining recommendations). March 2012 saw the publication of a single National Planning Policy Framework (NPPF) to replace 44 planning policy documents for England (including the PPS25 Practice Guide, withdrawn in 2014). It was accompanied by Planning Practice Guidance documents, with one entitled ‘Flood Risk and Coastal Change’, which states, at para 079, that new development “should only be considered appropriate in areas at risk of flooding if priority has been given to the use of sustainable drainage systems”.
In December 2014 the government announced that existing planning authorities would, from April 2015, be given authority to approve SuDS proposals (in accordance with one of the key Pitt Review proposals), and that a threshold of ten or more dwellings, or equivalent non-residential or mixed development, would apply. There are, however, still some significant shortfalls in implementing the Pitt Review proposals, according to a recent (January 2016) Institute of Civil Engineers article, for example, that the right to connect stormwater outflow to the public sewer system has been retained “despite a clear recommendation by Pitt that this should be repealed.”
The Construction Industry Research and Information Association (CIRIA) published ‘The SuDS Manual’ (CIRIA Report C753) in November 2015, prompted by a ministerial statement by the Secretary of State for Communities and Local Government “which made it clear that SuDS must play a greater role in the planning system”. The CIRIA website describes the manual as ”the most comprehensive industry SuDS guidance available in the UK”.
Clearly, SuDS has eventually become mainstream in the UK.
So What Are Sustainable Drainage Systems (SuDS)?
According to the England’s (now withdrawn) PPS25 Development and Flood Risk Practice Guide (2009), the Environment Agency had indicated that 70% of the UK’s main flood event in 2007 was caused by surface water overwhelming the capacity of drains and sewers, noting that “the critical factors in surface water flooding are the volume of rainfall, its intensity, where it falls, topography and the permeability of the surface onto which it falls”. It further noted that conventional surface water drainage largely utilizes underground piped systems to remove surface water from a site as quickly as possible, which may result in simply shifting the flooding problem downstream, whilst also reducing the recharging of groundwater. Conventional systems may also create a direct route for pollutants from urban areas to enter watercourses and groundwater.
The Practice Guide went on to say that the best way of reducing flood risk within a new development is to control the water at source through sustainable drainage systems (SuDS), and to consider what flow paths will be taken by excess surface water when the capacity of the drainage system is exceeded (‘exceedance’). Sustainable drainage systems are described as “a sequence of control structures designed to drain surface water in a more sustainable fashion than conventional techniques (in order to) mimic natural drainage and reduce the amount and rate of water flow by:
- Infiltration into the ground
- Holding water in storage areas
- Slowing the flow of water
PPS25 itself interpreted the term SuDS as covering “the whole range of sustainable approaches to surface water drainage management including:
- source control measures including rainwater recycling and drainage;
- infiltration devices to allow water to soak into the ground, that can include individual soakaways and communal facilities;
- filter strips and swales, which are vegetated features that hold and drain water downhill mimicking natural drainage patterns;
- filter drains and porous pavements to allow rainwater and run-off to infiltrate into permeable material below ground and provide storage if needed; and
- basins and ponds to hold excess water after rain and allow controlled discharge that avoids flooding.”
According to the CIRIA SuDS Manual, SuDS “should not be thought of as an individual component (such as a filter strip, swale or detention pond …), but as an interconnected system designed to manage, treat and make best use of surface water, from where it falls as rain to the point at which it is discharged into the receiving environment beyond the boundaries of the site”.
The use of multiple components in a single system is expressed in the central design concept of the SuDS Management Train, which the Manual describes as “the use of a sequence of components that collectively provide the necessary processes to control the frequency of runoff, the flow rates and the volumes of runoff, and to reduce concentrations of contaminants to acceptable levels”.
The Manual contains a useful list of SuDS components and their function (Box 1.3, p28):
“Rainwater harvesting systems – components that capture rainwater and facilitate its use within the building or local environment.
Pervious surfacing systems – structural surfaces that allow water to penetrate, thus reducing the proportion of runoff that is conveyed to the drainage system, e.g. green roofs, pervious paving. Many of these systems also include some subsurface storage and treatment.
Infiltration systems – components that facilitate the infiltration of water into the ground. These often include temporary storage zones to accommodate runoff volumes before slow release to the soil.
Conveyance systems – components that convey flows to downstream storage systems. Where possible, these systems also provide flow and volume control and treatment, e.g. swales
Storage systems – components that control the flows and, where possible, volumes of runoff being discharged from the site, by storing water and releasing it slowly (attenuation). These systems may also provide further treatment of the runoff, e.g. ponds, wetlands and detention basins.
Treatment systems – components that remove or facilitate the degradation of contaminants present in the runoff.”
Sustainable drainage systems are aimed at achieving multiple objectives, including removal of pollutants from urban run-off at source, ensuring new developments do not increase downstream flood risk, controlling water run-off from developments, and combining water management with green space which can increase amenity, recreational and biodiversity value.
As mentioned earlier, SuDS has become mainstream, being recognized as a valuable alternative or addition to the ‘hard infrastructure’ normally associated with stormwater management. A number of engineering and landscape architecture firms now specialize in SuDS design and implementation, assisting developers, local authorities and community organizations.
In the following articles to be published over the coming weeks, two case studies of SuDS implementation will be outlined, the first being Manor Field Park in Sheffield, where SuDS has been successfully integrated with the redevelopment of waste ground into an urban park, and the second, Pickering in North Yorkshire, where low-tech attenuation (‘slowing the flow’) averted flooding of the town during the December 2015 floods. The former case study was sourced from the PPS25 Practice Guide, the Landscape Institute’s website, and the SuDS consultant’s website, the second is summarised from an Independent newspaper article as well as well as other sites such as the BBC News, and is referred to in a recent Institute of Civil Engineers website article.