A newly-published assessment study of the evidence around SuDS potential in London undertaken by the Environment Agency, in conjunction with Defra, has suggested that institutional barriers are inhibiting wide SuDS uptake in the UK - compared to other countries, the institutional framework of roles and responsibilities in the UK are diverse.

The study was designed to answer the following question:

"Do we have sufficient evidence and knowledge to be confident that Sustainable Drainage Systems (SuDS) could or could not be reasonably implemented at a scale that achieves the water quality standards for the tidal Thames?"

The review looked at available evidence whether SuDS alone can meet the reduction in spill frequencies that would allow compliance with the water quality objectives. The report uses the Rapid Evidence Assessment (REA) approach (HM Treasury Magenta Book) and brings together existing evidence (both national and international) concerning the ability of SuDS to achieve the water quality standards for the tidal Thames. The REA includes around 70 sources.

The REA found that there are only a small number of key primary sources of evidence pertaining to wide-scale retrofit of SuDS in London. These include a specific study assessing the viability of SuDS as a solution to the sewer overflow problems in London, namely, work by the Thames Tunnel Strategic Study (2005) and the Thames Water Needs Report (2010). A more recent key source of evidence is work from the British Geological Survey (2012) assessing the potential for the use of infiltration. The available evidence finds there are substantial constraints that mean SuDS alone would not achieve the Thames Tideway objectives and standards and that SuDS in new housing developments alone will not make a significant contribution to improving water quality.

The REA also said that while the most detailed and relevant study for London shows that intensive retrofitting of SuDS measures will substantially reduce the number of events and volume of spills, SuDS measures alone cannot reduce spills from the Combined Sewer Overflows (CSOs) sufficiently to meet required standards, and would therefore fail to meet the requirements of the Urban Waste Water Treatment Directive.

The REA said the Needs Report had found that the level of service provided by even the highest likely level of SuDS intervention did not match that of the proposed tunnel option. It had also found that there is a “paucity of data” to remove uncertainties concerning costs of wide scale implementation of retrofit SuDS across London and that due to the focus on hydraulic performance of the Thames Water Needs Report, the wider benefits of SuDS were not detailed.

Across the body of work, the REA concludes that there is a “lack of consistent cost and benefit methodology for wide-scale retrofitting" and that the only work to explore the costs of wide-scale SuDS implementation was carried out for the Needs Report. The Report had concluded that an estimate of £13bn (+50% to -30% at 2010 prices) was the likely costs of implementing SuDS across the combined sewer network in London – however, the figure is an estimate based on this one study and has a low level of confidence. The current cost of the Thames Tideway Tunnel is estimated at £4.1-4.2bn (2011 prices).

With regard to the potential for retrofitting of SuDS in London, the REA says that British Geological Survey (BGS) evidence highlights the limited scope for implementing wide-scale infiltration SuDS in the combined sewer network area.

The REA was peer-reviewed by independent civil engineering and environmental hydraulics consultancy HR Wallingford -  the organisation has agreed with the assessment conclusion that SuDS alone would not achieve the Thames Tideway objectives and standards.

Institutional barriers to SuDS

The REA study makes some interesting points about institutional barriers which currently inhibit the uptake of SuDS in the UK, commenting:

“There are policies and proposed legal frameworks being put in place for the incorporation of SuDS into new build and to maintain those SuDS into the future, but the diffuse stakeholder roles for stormwater, distributed benefits of SuDS, and a regulatory framework that was not designed to deliver SuDS has inhibited wide uptake.”

“Since the review was completed the Water Bill has been introduced to Parliament and includes a provision to remove any doubt that sewerage undertakers can construct and use SuDS to fulfil their statutory duty to drain an area, where it is cost effective for them to do so. Once law this should help promote the retrofitting of SuDS.”

The REA says that while policies and proposed legal frameworks are being put in place for the incorporation of SuDS into new build, there is nothing in place to effectively promote the retrofitting of SuDS.

It also flags up current gaps in the available evidence and knowledge base, including:

i. National and London-specific reports highlight the following as areas of uncertainty:

• quantification of benefits of SuDS
• monitoring performance of wide-scale SuDS
• understanding the extent to which urban diffuse water pollution could be controlled cost effectively

ii. There is a paucity of data on wide-scale application of retrofit SuDS to remove uncertainties of cost/ benefits.

iii. There is no work on a pan-London extensive use of SuDS as a strategic solution.

The REA says that compared to other countries, the institutional framework of roles and responsibilities in the UK are diverse. This, along with the distributed benefits of SuDS not necessarily going to those owning or constructing them, acts against their implementation.

It concludes by suggesting that organisationally, for an extensive strategic programme of retrofitting to occur, changes in legislation, the establishment of a single responsible institution in London and extended timescales for delivery, may be required.

Click here to download An assessment of evidence on Sustainable Drainage Systems and the Thames Tideway Standards in full.