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SuDS performance and evaluation

The SuDS approach to managing surface water has been in place since the 1990’s the parts of America use BMP (Best Management Practice) and this has been around since the 1970’s, or more recently they’re using the term LID (Low Impact Design). Australia and New Zealand use the terms Water Sensitive Urban Design (WSUD) and Low Impact (Urban) Design, (LIUD) and have a number of programmes to implement good practice.

SuDS performance

The SuDS philosophy is to mimic natural drainage that occurs prior to development and manage the water as close to its source as possible providing opportunities to manage flood risk, water quality and improve amenity/biodiversity. The benefits provided by SuDS are dependent on the opportunities and constraints of the site. The case studies provide a number of examples where benefits have been realised.
The Lamb Drove case study, has been monitored and evaluated since its completion and provides some good evidence on water quality and flood risk management performance as well as assessments on amenity and ecology.

Research into the benefits of SuDS components and schemes has been undertaken for a number of years in America and Europe, while there may be some differences in geography, urban design etc some parallels can be applied to the UK situation. Within the UK independent and objective research into the performance of SuDS components (both hard engineered and soft landscaped) components is relatively patchy.

While there is information on the broader benefits of SuDS, there is still a need for specific information and details on the performance of components, particularly with regards to water quality. The references below, provide a good summary of performance for components:

In 2009 CIRIA also worked with HR Wallingford, MWH, Pennine Water Group and WRc to collate information on the performance of SuDS components. The extract of the report can be downloaded by clicking here

In 2004 Professor Chris Pratt also undertook a literature review of the performance of certain components, It draws attention to those publications which have appeared in conference proceedings and journals which most frequently contain outputs of relevance to sustainable drainage in the UK and overseas. The report can be downloaded by clicking here.

Flood risk management benefits

SuDS are designed to meet specific performance criteria, much the same way as traditional drainage systems. SuDS components are generally volume based whereas pipe design is entirely focused on conveyance. The majority of SuDS components provide larger storage volumes than traditional drainage systems. Therefore, these systems will only become overloaded by events occurring over a longer duration, which generally means that “failure” results in less impact. With the majority of SuDS components managing water on the surface this increased visibility may also contribute to improved flood risk management, particularly during extreme events.

In addition to the type of failure being very different, SuDS schemes retain and attenuate the runoff for longer, while pipe based systems pass all this flow downstream. This means that areas downstream at risk of flooding receive all the water from an upstream pipe based system, but only a limited amount of water from a SuDS scheme. Flood risk is managed by SuDS reducing the volume, frequency and flow rate of surface water runoff and during extreme events exceedance can be managed and in many circumstances can be visually monitored.

Water quality benefits

The philosophy behind SuDS is that they also treat the surface water runoff often improving water quality as well as provide a drainage system. This is the main difference between traditional drainage systems based on the use of pipework and the SuDS philosophy. In general, the use of SuDS components, especially if a SuDS management/treatment train is used, can result in runoff water quality which is of a similar order to river water quality standards.

Amenity and biodiversity benefits

It is widely accepted that sustainable drainage, particularly landscaped SuDS can significantly contribute to the amenity value of an area and improve general quality of life. Several studies have identified the added value associated with the proximity of premises to open water areas. The resulting values range between 2% and 19% suggest that land values and house prices located adjacent to SuDS water features may attract a 10% premium on resale.

Evaluating benefits and costs

There are two main elements when assessing benefits and costs: the tangible and the intangible. It is possible to define some benefits and costs in monetary terms, such as capital outlay. Others, such as the attractiveness of new green areas are less straightforward. Ideas as to how to assign monetary values are only now emerging such as landscape/visual amenity as part of green infrastructure and ecosystem services and other approaches (e.g. categorised as contributing to ‘place and communities’. Intangible benefits (such as those associated with green infrastructure) may be important to stakeholders and a community, and are increasingly essential in proving the overall societal value of any scheme. However, evaluating intangible benefits can be challenging as many of these are subjective.

For a review of UK data and sources see Green Infrastructure North West (2011).

It is likely that the available information and approaches to evaluation will develop rapidly in the future. For the time being it is wise to be conscious of the current level of confidence for some of the data. Table 1.1 provides examples of the potential guidance that is available to assess benefits and costs.

Table 1.1 UK sources of guidance for benefit cost assessment

Name of guidance

What it includes

Other comments

SWMP guidance (Defra, 2010)

Guidance on the monetisation of costs and benefits and uncosted benefits and costs. Provides information on Net Present Value (NPV) and Benefit-Cost Ratio (BCR) assessments and dealing with uncertainty.

Straightforward introduction to benefit cost assessment. Limited actual data for value and cost of SuDS.

Valuing ecosystem services (Defra, 2007)

Guidance on valuing the natural environment. Usually applied to larger schemes.

Useful where there are significant environmental and ecological considerations.

Treasury Green Book (HM Treasury, 2003) and supplementary guidance Accounting for Climate Change (Defra, 2009)

The primary guidance for many assessments including the economic assessment for flood and coastal erosion risk investment (EA, 2010). The supplement explains how to evaluate benefits and costs for adaptation and flexibility. These are usually applied to larger schemes.

Use where there are significant flood risk reduction benefits.

Multi-coloured manual (Penning Rowsell et al, 2005) and The Benefits of Flood and Coastal Risk Management: A Handbook of Assessment Techniques - 2010 (Penning Rowsell et al, 2010).

Provides guidance on assessing the benefits of flood risk management options and the impacts resulting from flooding. It enables the practitioner to assess the relationship between costs and benefits.

The handbook is appropriate for most schemes. However, where the scheme is complex, use the more extensive Multi-Coloured Manual.

Exploring the cost benefit of separating surface water from combined sewers (UKWIR, 2009)

Guidance for cost benefit of separating stormwater from combined sewers to reduce overflows. Asset based investment water industry criteria are used in the assessments which are from a sewerage undertakers’ perspective.

Of applicability mainly to sewerage undertakers and does not include many of the wider benefits potentially accruing to other stakeholders

Sewer flood risk asset investment (UKWIR, 2007)

Methodology for cost benefit analysis for sewer flood risk asset investment. Complemented by Ofwat (2009) which deals with the use of NPV in asset investments.

Of applicability mainly to sewerage undertakers

Cost-benefit of SuDS retrofit (EA, 2007) Carbon related aspects of source control & related costs and benefits (EA, 2009)

Benefit cost assessment information for a reduced range of SuDS measures. Includes only a limited range of non-monetisable benefits. Relates costs to carbon abatement and sequestration for green SuDS.

The benefit cost assessment boundaries are limited in the analysis

SuDS manual (C697) (CIRIA, 2007)

Provides guidance on SuDS design, and whole life costing

The primary source for SuDS related cost data in the UK but more recent work should also be consulted

Guide to assess the whole life costs of SuDS
UKWIR/WERF (2005)
Also WERF (2009)

Guidance developed jointly between UK and USA providing a whole life costing methodology and data to assess the overall costs of SuDS. The database is recurrently updated by HR Wallingford as part of a whole life costing tool.

With the above, the primary source for SuDS related cost data in the UK

Scottish Water retrofit SuDS project (Atkins, 2004)

Provides a discussion of benefits to be considered and costed examples

Current thinking has advanced but still a very useful source of information

Sustainable drainage Cambridge design and adoption guide (Wilson et al, 2009)

Data on the maintenance costs of SuDS which, although set up for new build may also be applicable to retrofit

A typical example of Local Authority guidance for developers.

The primary sources of guidance in table 2.1 do not include the assessment of the full range of potential benefits of using nor how best to include future change, such as climate change. These aspects of benefits and costs are very important aspects of undertaking comparisons or retrofitting SuDS. Including multiple benefits especially relating to green infrastructure, the benefits become much more attractive.

Often a combination of ‘grey’ (piped) and green infrastructure approaches are the most beneficial as found in the plans for Combined Sewer Overflow control in many parts of America.

It is therefore important to find consensus between potential partners about the range of benefits to assess as well as the range of measures to practicably utilise. Table 2.1 outlines guidance that is currently available that include the wider benefits which is very new and, in some cases, incomplete when considering application to retrofit surface water management. This guidance is likely to evolve quickly over time.

As yet, none of the tools shown is readily applicable to the range and scale of SuDS. However, they do provide comprehensive details explaining the categories, criteria, assumptions and procedures to use to evaluate the wide range of benefits from using green infrastructure in conjunction with surface water management measures.

Table 1.2 Guidance for assessing multi benefits from retrofitting SWMM

Name of guidance

What it includes

Other comments

Green Infrastructure North West (2011)

A simple framework that can help identify and broadly assess the benefits of proposed green investments and existing green assets – whether those benefits directly contribute to a local economy, or provide
wider non-market returns for society and the environment.
Spreadsheet based tool provided for this.

The only publicly available green infrastructure valuation toolkit in the UK that also includes surface water management. Focus is mainly, however, on GI rather than surface water.

Center for Neighborhood Technology (2010)

As above, this US based guidance and tool has been in use since 2006 and recently updated.

The main emphasis is on managing surface water using green infrastructure.

Comparisons of costs and benefits

There is a dearth of information available on costs and benefits on sustainable drainage and even less on comparisons between SuDS and traditional drainage.

However, the Lamb Drove example in Cambridge has been evaluated. The exercise primarily based on evaluating the tangible costs and benefits suggested around a 10% saving on capital costs with the SuDS scheme. It’s been suggested that the savings could have been greater if the SuDS layout had been considered earlier in the development process.

Defra as part of the work on the Flood and Water Management Act has also undertaken a number of comparative studies on the costs and benefits of traditional drainage and SuDS. Clients, designers, engineers and quantity surveyors compared the capital (and sometimes the maintenance) costs for draining sites using sustainable drainage and more traditional approaches. The sites they looked at included:

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Name of guidance	What it includes	Other comments
SWMP guidance (Defra, 2010)	Guidance on the monetisation of costs and benefits and uncosted benefits and costs. Provides information on Net Present Value (NPV) and Benefit-Cost Ratio (BCR) assessments and dealing with uncertainty.	Straightforward introduction to benefit cost assessment. Limited actual data for value and cost of SuDS.
Valuing ecosystem services (Defra, 2007)	Guidance on valuing the natural environment. Usually applied to larger schemes.	Useful where there are significant environmental and ecological considerations.
Treasury Green Book (HM Treasury, 2003) and supplementary guidance Accounting for Climate Change (Defra, 2009)	The primary guidance for many assessments including the economic assessment for flood and coastal erosion risk investment (EA, 2010). The supplement explains how to evaluate benefits and costs for adaptation and flexibility. These are usually applied to larger schemes.	Use where there are significant flood risk reduction benefits.
Multi-coloured manual (Penning Rowsell et al, 2005) and The Benefits of Flood and Coastal Risk Management: A Handbook of Assessment Techniques - 2010 (Penning Rowsell et al, 2010).	Provides guidance on assessing the benefits of flood risk management options and the impacts resulting from flooding. It enables the practitioner to assess the relationship between costs and benefits.	The handbook is appropriate for most schemes. However, where the scheme is complex, use the more extensive Multi-Coloured Manual.
Exploring the cost benefit of separating surface water from combined sewers (UKWIR, 2009)	Guidance for cost benefit of separating stormwater from combined sewers to reduce overflows. Asset based investment water industry criteria are used in the assessments which are from a sewerage undertakers’ perspective.	Of applicability mainly to sewerage undertakers and does not include many of the wider benefits potentially accruing to other stakeholders
Sewer flood risk asset investment (UKWIR, 2007)	Methodology for cost benefit analysis for sewer flood risk asset investment. Complemented by Ofwat (2009) which deals with the use of NPV in asset investments.	Of applicability mainly to sewerage undertakers
Cost-benefit of SuDS retrofit (EA, 2007) Carbon related aspects of source control & related costs and benefits (EA, 2009)	Benefit cost assessment information for a reduced range of SuDS measures. Includes only a limited range of non-monetisable benefits. Relates costs to carbon abatement and sequestration for green SuDS.	The benefit cost assessment boundaries are limited in the analysis
SuDS manual (C697) (CIRIA, 2007)	Provides guidance on SuDS design, and whole life costing	The primary source for SuDS related cost data in the UK but more recent work should also be consulted
Guide to assess the whole life costs of SuDS UKWIR/WERF (2005) Also WERF (2009)	Guidance developed jointly between UK and USA providing a whole life costing methodology and data to assess the overall costs of SuDS. The database is recurrently updated by HR Wallingford as part of a whole life costing tool.	With the above, the primary source for SuDS related cost data in the UK
Scottish Water retrofit SuDS project (Atkins, 2004)	Provides a discussion of benefits to be considered and costed examples	Current thinking has advanced but still a very useful source of information
Sustainable drainage Cambridge design and adoption guide (Wilson et al, 2009)	Data on the maintenance costs of SuDS which, although set up for new build may also be applicable to retrofit	A typical example of Local Authority guidance for developers.