Ashley, R.

Baylis, A.

Digman, C.

Gersonius, B.

Horton, B.

Shaffer, P.

Smith, B.

I-Revues

NOVATECH 2016 
1 
Using the multiple benefits of SuDS tool (BeST) to 
deliver long-term benefits 
Utilisation de l'outil d'évaluation multicritère des solutions 
durables de gestion des eaux pluviales (BeST) pour offrir 
des avantages à long terme 
 
Richard Ashley*,**,****, Christopher Digman**,***; Bruce Horton***, 
Berry Gersonius**** Brian Smith*****, Paul Shaffer******; Adam 
Baylis*******;  
 
*Ecofutures Ltd., 3 Greendale Court, Honley, Holmfirth, West Yorkshire & **University Of 
Sheffield, Mappin Street, South Yorkshire, UK. r.ashley@sheffield.ac.uk 
***MWH, Red Hall Avenue, Paragon Business Village, Wakefield, WF1 2UL, UK 
Christopher.J.Digman@mwhglobal.com; bruce.horton@mwhglobal.com 
**** UNESCO-IHE Institute for Water Education, 2611 AX , Delft, Netherlands 
B.Gersonius@unesco-ihe.org  
***** Yorkshire Water, Western House, Halifax Road, Bradford, West Yorkshire BD6 
2SZ, UK brian.c.smith@yorkshirewater.co.uk  
***** CIRIA, Griffin Court, 15 Long Lane, London, EC1A 9PN, UK paul.shaffer@ciria.org  
****** Environment Agency, Horizon House, Deanery Road, Bristol, BS1 5AH, UK 
adam.baylis@environment-agency.gov.uk 
 
RÉSUMÉ 
Cela a été monnaie courante d'affirmer que les systèmes de drainage durables (SuDS) peuvent 
apporter un large éventail d'avantages sans mettre en évidence ou quantifier ces avantages. 
L'Association d'information sur la recherche de l'industrie de la Construction (CIRIA) a mis au point un 
outil appelé BeST. Cet outil aide les praticiens au Royaume-Uni à évaluer une vaste gamme 
d'avantages économiques de l'utilisation des SuDS. Toutefois, l'avenir étant incertain, il est difficile 
d'évaluer les avantages à plus long terme avec certitude. L'approche présentée ici, fondée sur la 
planification de scénarios, permet d'envisager les futures incertitudes dans l'analyse des SuDS. La 
méthodologie présentée et les conclusions qui en sont tirées montrent comment considérer le futur de 
cette façon peut aider à éclairer la prise de décision et la sélection des options de SuDS pour la 
robustesse future et la valeur maximale. 
 
ABSTRACT 
It has been commonplace to state that sustainable drainage systems (SuDS) can bring a wide range 
of benefits without evidencing or quantifying these benefits. The Construction Industry Research 
Information Association (CIRIA) has developed a tool called BeST. This helps practitioners in the UK 
to evaluate a wide range of economic benefits from using SuDS. However, as the future is uncertain it 
is difficult to evaluate the longer-term benefits with any certainty. The approach presented here helps 
to consider the future uncertainties in the analysis of SuDS based on scenario planning. The 
presented methodology and conclusions drawn show how considering futures in this way can help 
inform decision pathway planning and selection of SuDS options for future robustness and maximum 
value. 
 
KEYWORDS 
Adaptive management, Drainage, SuDS, stormwater, sustainable, uncertainty, urban, benefits 
SESSION 
2 
 
1 INTRODUCTION 
Estimating the value of the benefits arising from non-piped drainage systems has advanced 
considerably in the last decade. The development of a spreadsheet tool, known as BeST (Benefits of 
SuDS Tool), to evaluate the benefits that SuDS provide has been managed by the UK’s CIRIA (with 
support from industry). Its technical development was led by MWH (Ashley et al, 2014; Horton et al, 
2015) with the support of EcoFutures, CH2M and UNESCO IHE in Delft. The tool utilises an impact 
pathway methodology framed within an ecosystem services valuation alongside other specific 
valuation criteria to estimate the monetised benefits arising from the use of SuDS and other non-
quantifiable benefits. This paper considers how to evaluate the longer-term benefits when using BeST, 
including the management of uncertainties, illustrated using a case study example. 
2 CONSIDERING THE LONGER-TERM IN DECISION MAKING 
In an uncertain world it is essential to consider both today’s uncertainties and how the future may 
unfold in regard to the implications for infrastructure selection and performance (Casal-Campos et al, 
2015). As awareness of and knowledge about the changing climate has grown, new approaches to 
plan the infrastructure systems needed to cope with predicted more extreme climate conditions have 
been developed. There are many other changes which need to be considered in today’s decisions 
when selecting the best infrastructural systems to meet future anticipated and expected services. 
Today’s decisions will have long-lasting consequences. Due to the longevity of built assets it will be 
necessary to adapt or modify systems in the light of changes in external factors such as climate; 
population and development growth; regulatory drivers; government priorities and legislative change. 
Social demands and needs will also change and this will be reflected in the performance criteria and 
standards expected in the future. Planning for and designing SuDS requires a rather different 
approach compared to previous practice. Traditional engineering used safety-factors, headroom or 
other means to create redundancy for anticipated imperfect knowledge, so giving new infrastructure 
systems a life often beyond the expected 30 to 50 years (e.g. Dessai et al, 2013). Drainage designs 
now rely on observed data collected over decades about system performance and exogenous factors. 
Standards and codes tell designers the criteria to use. Environmental loadings are known to no longer 
be statistically stationary (Milly et al, 2008). Hence we typically rely on climate model predictions, 
assume rainfall uplifts and try to interpret imprecise predictions of the future. Decision makers are also 
worried about other things such as: What happens if the scheme isn't properly maintained?  
Therefore, if considering the future, users of tools such as BeST need to be able to consider how 
SuDS and the context within which they are set will need to change. There are many examples of 
applying scenario planning to consider such future uncertainties (e.g. Maier et al, 2016). This 
comprises the definition of plausible future states against which to assess the likelihood of the 
proposed development being able to continue to deliver the expected performance (e.g. Dessai et al, 
2013; Casal-Campos et al, 2015). A way to consider the future and other relevant uncertainties when 
using BeST for scenario planning is set out in forthcoming guidance from CIRIA and outlined here. 
3 MANAGING UNCERTAINTIES IN BEST 
3.1 Confidence and sensitivity 
The first version of BeST includes the option to assign confidence scores (25, 50, 75, 100%) to the 
quantified data and monetised values. A sensitivity analysis option facilitates the exploration of the 
effects of assumptions across the complete range of confidence scores. However, decision making for 
the longer-term requires more than this. It needs to consider the spatial and temporal changes that 
may occur during the SuDS lifetime, especially as many benefits will not occur immediately after 
commissioning, but take time to come to fruition and others will become less important over time.  
3.2 Temporal and spatial considerations 
‘Robustness’ is commonly used to define how likely it is that a system can cope with future changes. 
Whereas ‘flexibility’ and ‘adaptability’ are characteristics that define how a system can deal with future 
changes without failure and can be modified successfully. There are a number of qualitative and 
quantitative ways to estimate these characteristics (e.g. Brisley et al, 2015; Casal-Campos et al, 
2015). SuDS are inherently more adaptable and flexible than piped drainage systems (Eckart et al, 
2012) and would therefore be expected to be modified during their typical 25-50 year lifetime. This 
NOVATECH 2016 
3 
should be recognised in any valuation assessment, via for example, staged intervention analysis. 
These characteristics are further considered in the case study in Section 4. There are two main 
approaches to plan and design SuDS and other water infrastructure to cope with an uncertain future. 
Firstly the precautionary approach: traditionally used to construct large and ‘too big to fail’ systems. 
These continue to provide the expected service levels over their functional lifetime, before eventually 
being replaced. Secondly, the adaptive approach: here a more modest investment (compared with 
precautionary) is made initially, assuming that as more knowledge unfolds over time, a series of other 
investments (adaptation interventions) can be made over the lifetime of the system in stages. There 
are important considerations regarding the timing of adaptive interventions, of which major 
maintenance may be timed to coincide with (e.g. Brisley et al, 2015). In BeST only some of the 
benefits change in a non-linear way with time; the majority of benefit categories have linear and, 
reducing, present value over time.  For the scale of the development, BeST currently accounts for the 
benefits linearly, i.e. additional benefits accrue uniformly as more of the same SuDS are implemented. 
However, there may be step-changes in benefit value, for example, where there are sufficient SuDS 
fitted to transform the quality of runoff to such an extent that there is a step change in river water 
quality class.  
Figure 1 shows the process for assessing the robustness of SuDS options and also determining the 
Net Present Value (NPV) under a range of scenarios. It is expected that most analyses will follow the 
‘simple approach’ in Figure 1, without fully detailed determination of the costs and benefits under each 
scenario being considered, as shown in the ‘detailed approach’.  
There are two types of adaptive 
approach, managed or reactive, together 
with a staged intervention approach.  The 
adaptive approach requires making 
changes over time; i.e. decision points in 
time may be pre-selected or they may be 
in response to changing external drivers, 
including climate, performance 
deterioration, major maintenance needs 
etc. (e.g. Brisley et al, 2015). Adaptive 
management requires careful 
consideration of how futures may unfold 
over time. This is typically undertaken 
using scenario planning as shown in 
Figure 1 (e.g. Henriques et al, 2015). In 
each case, the robustness is defined as 
how well the option will continue to 
provide the expected performance into 
the future under each scenario 
considered.  
4 CASE STUDY 
An English study aimed to reduce combined sewer overflow (CSO) spills by either retrofitting SuDS or 
traditional below ground storage (see Woods-Ballard et al, 2015). The Baseline and Business-as-usual 
(BAU) scenario analysis (Figure 1) found that it was significantly cheaper to reduce CSO spills using 
underground storage rather than a SuDS conveyance/storage option. However, BeST showed that 
SuDS bring a far higher financial return. Scenario planning has been used to evaluate the relative 
robustness of the options. This comprised the four socio-economic scenarios defined in the UK 
Foresight programme, together with the UKCP09 climate predictions (Thorne et al, 2007), ‘A’ in Figure 
1. Assessing the robustness qualitatively of all initial options, a stakeholder group, assigned a ‘1’ to an 
option considered robust under a particular scenario and time epoch, and a ‘0’ where the option was 
not considered robust. In this assessment the storage tanks scored 1½ and the SuDS option 3½ (‘B’ in 
Figure 1). The simple method was applied in Figure 1 for Stage C. An analysis of flexibility, using the 
COFAS internal homogeneity score (Eckart et al, 2012), showed the tank option had 0 flexibility, 
whereas the SuDS option scored 47%; 52%, 41% respectively when considered over the time periods 
2015-2030; 2015-2055 and 2015-2095. Given the disparity, further analysis of the tank would not be 
required if robustness and flexibility were considered important. 
Figure 2 shows the distribution of the £ benefits for the SuDS option for three periods of time by using 
Figure 1 Process showing the inclusion of scenario planning for 
robustness assessment of Options when using BeST 
SESSION 
4 
BeST. Between 2015 and 2030, there are four significant benefits, with amenity being the greatest due 
to a rise in property prices (suggesting it was a nicer place to live).  From 2030 and 2055, the same 
benefits still dominate but their distribution is different for the various scenarios. Flooding is still 
significant in terms of the benefits provided for the two subsequent periods and under each of the 
scenarios. The uniformity of the value of the flood damage reduction across all scenarios shows that 
these benefits are likely to be robust whatever the future. The amenity benefits fall dramatically and 
become a net cost rather than a benefit from the second period of analysis for the Global Stewardship 
scenario and also the National Enterprise scenario in the third epoch. This is based on the view that 
the property values increase no longer applied after 2030 under first the Global Sustainability scenario 
and then both that and the National Enterprise scenario from 2055. From this analysis, the SuDS 
option was found to be the more robust and provide a range of individual benefits sustained (apart 
from the amenity benefits) over the three time periods of analysis in Figure 2. 
5 CONCLUSIONS 
SuDS are important measures in tackling future 
drainage needs. They are more flexible and 
adaptable than traditional piped drainage 
systems and can be added to or even 
abandoned relatively cheaply in the light of 
advancing knowledge about climate and other 
changes. Tools such as BeST can provide the 
necessary framework to value the widest range 
of benefits from using SuDS. This includes 
testing the relative robustness of planned 
schemes for retrofit or new build if scenario 
planning and adaptation pathways are 
considered for longer term plans for staged 
interventions whereby a portfolio of SuDS is 
introduced over time and spatially within a 
catchment. Understanding the uncertainties 
associated with doing this and how certain the 
benefits will become more or less important 
over time, can help to make the right decisions 
now in order to ensure that performance over 
time is maintained as required.   
Disclaimer 
The views expressed by the authors in this 
paper are their own and do not necessarily 
represent the views of their organisations. 
LIST OF REFERENCES 
Ashley R.M, Digman C J., Horton B., Gill E., Dudley 
E., Baylis A., Shaffer P., Simmons S. 2014. Seizing 
multiple benefit opportunities from stormwater 
management schemes. Proc. 13th Int. Conf. Urban 
Drainage. Kuching. 
Brisley R., et al (2015). Techniques for valuing 
adaptive capacity in flood risk management. Proc. 
ICE Water Management. Paper 1400070. P1-9. 
Casal-Campos A., Fu G., Butler D., Moore A. 
(2015). An Integrated Environmental Assessment of 
Green and Gray Infrastructure Strategies for Robust 
Decision Making. Environ. Sci. Technol. 49, 8307-8314 
CIRIA (2015). Susdrain. http://www.susdrain.org/ [accessed 30-11-15] Construction Industry Research and 
Information Association, London. 
Dessai J., Browne A., Harou J J. (2013). Introduction to the Special Issue on “Adaptation and Resilience of Water 
Systems to an Uncertain Changing Climate”. Water Resour Manage 27:943–948. 
Eckart J., et al (2012). Flexible design of urban drainage systems: demand led research for Hamburg-
Wilhelmsburg. Rev Environ Sci Biotechnol  11:5–10. 
Figure 2 Estimated breakdown of benefits provided by 
the SuDS option for periods: 2015-2030; 2030-2055; 
2055-2095 (Scenarios: WM – World Markets; NE – 
National Enterprise; GS – Global Sustainability; LS – 
Local Stewardship).  
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Henriques L., et al (2015). The future water environment — Using scenarios to explore the significant water 
management challenges in England and Wales to 2050. Science of the Total Environment 512–513 (2015) 
381–396. 
Horton, B., et al., (2015). BeST (Benefits of SuDS Tool) Technical Guidance. CIRIA W045c RP993 © CIRIA 2015 
ISBN: 978-0-86017-769-2. http://www.susdrain.org/resources/best.html. 
Maier H R., et al (2016). An uncertain future, deep uncertainty, scenarios, robustness and adaptation: How do 
they fit together? Environmental Modelling and Software, April.  
Milly, P.C.D., et al. (2008). Stationarity Is Dead: Whither Water Management? Science, 319 (5863), 573-574 
Thorne, C R, et al., Eds. (2007) Future Flooding and Coastal Erosion Risks, Thomas Telford, London, UK, ISBN 
978-0-7277-3449-5, p320-339. 
Woods Ballard, B, et al., (2015) The SuDS Manual, C753, CIRIA, London, UK. ISBN 978-0-86017-760-9. 
http://www.susdrain.org/resources/SuDS_Manual.html 


Utilisation de l'outil d'évaluation multicritère des solutions durables de gestion des eaux pluviales (BeST) pour offrir des avantages à long terme

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Utilisation de l'outil d'évaluation multicritère des solutions durables de gestion des eaux pluviales (BeST) pour offrir des avantages à long terme

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