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A cost-benefit analysis model for
the retrofit of sustainable urban
drainage systems towards
improved flood risk mitigation
Oluwayemi A. Oladunjoye, David G. Proverbs, Beck Collins and
Hong Xiao
Faculty of Computing, Engineering and the Built Environment,
Birmingham City University, Birmingham, UK
Abstract
Purpose –The Environment Agency estimates that one in six homes in England (approximately 5.2m
properties) are at risk from flooding and 185,000 commercial properties are located in flood-prone areas.
Further, an estimate of 10,000 new homes are built on flood plains yearly. The UK has witnessed a significant
increase in flood events over the past 10 years. During this period, there has been growing research attention
into measures to mitigate the effects of flooding, including the benefits of deploying sustainable urban
drainage systems (SuDs) in new developments or as a retrofit. The purpose of this paper is to present the
development of a cost-benefit analysis model for the retrofit of SuDs focusing on the potential for improved
flood risk mitigation in the context of commercial properties.
Design/methodology/approach –A synthesis of flood risk management and SuDs literature is used to
inform the development of a conceptual cost-benefit analysis model for the retrofit of SuDs and focusing on
the potential for improved flood risk mitigation in the context of commercial properties.
Findings –SuDs have been applied successfully in different parts of the world; however, the uptake of SuDs,
in particular, the retrofit of SuDs, has been restricted by a number of issues including a lack of experience and
trust in their performance and a lack of understanding in their true benefits. In particular, there is the limited
experience of retrofitting SuDs and there are no well-established procedures for evaluating the feasibility,
value or cost effectiveness of doing this.
Social implications –This offers the potential to support the UK government’s flood risk management
policy by helping to increase the resilience of properties, whilst offering other benefits to communities such as
improvements in air quality and biodiversity and also presenting a clearer understanding of the monetary
and non-monetary implication to owners of commercial properties for a more informed and acceptable uptake
of SuDs retrofit.
Originality/value –The proposed model will allow a more comprehensive understanding of the costs and
associated benefits associated with SuDs retrofit, highlighting the flood risk mitigation benefits that might
accrue over a period of time for commercial property.
Keywords Flood risk, Commercial properties, Conceptual framework, Costs, Benefits
Paper type Research paper
1. Introduction
Globally, more than 80 per cent of the population living on land are prone to flooding
(Winsemius et al., 2018). In 2007, the worst flooding experience in the UK since 1947 coincided
with the start of the worst financial crisis since the 1930s. These had a major impact on people,
properties and businesses and brought the need for a better risk management system. In the
UK, the environment agency estimates that one in six homes in England (approximately 5.2m
properties) are at risk from flooding. Included in this statistic is an average of 10,000 homes
which are built on flood plains yearly, while 185,000 commercial properties are located in
flood-prone areas. These properties are valued at over £801bn which accounts for 15.8
per cent of the value of total buildings and 2.2 per cent of total assets in the UK (Bhattacharya
et al., 2013). One in six residential and commercial properties, which form about 2.4m in
England are at risk of flooding from main rivers or the sea.
International Journal of Building
Pathology and Adaptation
© Emerald Publishing Limited
2398-4708
DOI 10.1108/IJBPA-12-2018-0105
Received 10 January 2019
Revised 15 April 2019
7 August 2019
Accepted 3 September 2019
The current issue and full text archive of this journal is available on Emerald Insight at:
www.emeraldinsight.com/2398-4708.htm
Cost-benefit
analysis model
The commercial property sector plays an important role in the UK economy, both as a
direct or indirect employer, a generator of output, and in providing other sectors including
retailers, financial and business services, with a critical factor of production, that is, the
location from which their business is done. It is estimated that the total output in the sector
in 2011 was around £41bn which is equivalent to 3.2 per cent of UK gross value added and
total employment of over 800,000. The sector makes a substantial contribution to the
exchequer, with an estimated contribution in VAT and PAYE income tax of approximately
£6.5bn. Any disruption to the activities in this sector would be detrimental to the growth of
the UK economy in total (Bhattacharya et al., 2013).
Sustainable urban drainage systems (SuDs) are a system which could be used to protect
commercial properties from flooding. SuDs are uniquely designed to mimic natural
infiltration patterns, so that they can reduce the risk of flooding by reducing runoff volume
and attenuation peak flows. Kirby (2005) described SuDs as more sustainable than
conventional drainage methods because they are designed to manage flow rates, protect or
enhance water quality and are sympathetic to the environmental setting and the needs of
the local community by dealing with runoff close to where rain falls or attenuating flows
and controlling discharges downstream.
However, the uptake of SuDs and the retrofit of SuDs has been restricted by a number of
issues including a lack of experience and trust in such schemes, and a lack of understanding
of their wider benefits (Oladunjoye et al., 2017; Malulu, 2016; Ossa-Moreno et al., 2017;
Ellis, 2013; Lamond, 2016). There has been limited research on the retrofit of SuDs and there
are no well-established procedures for evaluating the feasibility, value or cost effectiveness
of doing this (Ossa-Moreno et al., 2017; Alexander et al., 2016). Lead (2018) revealed that
68 per cent of construction professionals felt there was a lack of understanding of
SuDs among key decision makers. However, there remains growing interest in the
introduction of this technology (Stovin, 2010), and stakeholders and researchers have
sought to develop modalities on how to make SuDs more acceptable and relevant in the UK
(Carboni et al., 2016). This study aims to help facilitate a deeper appreciation of the monetary
and non-monetary value of SuDs retrofit with the development of a cost-benefit analysis
(CBA) model towards improving uptake and thereby improving flood risk mitigation in
commercial properties in the UK. The results of this study will help to develop a better
understanding of the long-term viability of SuDs retrofit and its monetary value, in order to
encourage uptake by key decision makers involved in the development and redevelopment
of buildings.
2. Sustainable Urban drainage systems
SuDs is a generic term that refers to various measures used to control the effect of surface
water runoff in the environment (Locatelli, 2016). Charlesworth and Booth (2017) defined
sustainable drainage as the management of rainwater which includes snow and other
precipitation, with the aim of reducing damages caused by flooding events, improving the
quality of water, the improvement and protection of the environment, the improvement and
safety of the health of the residents and the process of ensuring the effective stability and
durability of drainage systems. Baron and Petersen (2016) further described SuDs as an
important contribution to urban climate change adaptation.
SuDs replicate the natural drainage processes of an area through the use of vegetation-
based interventions such as swales, water gardens and green roofs, which increase localised
infiltration, attenuation and/or detention of stormwater. Hence, SuDs improves flood
alleviation capacity in any community. SuDs are essential for the reduction in the quantity
of surface water that flows off a surface and are useful for different purposes including
reduction in runoff by, for example, the installation of green roofs or other SuDs which could
result in savings in wastewater disposal (Wilkinson, and Dixon, 2016). The most well-used
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SuDs are rainwater harvesting systems, green roofs, swales, rainwater beds and permeable
surfaces as well as open drainage waterways and reservoirs for excess rainwater (Baron
and Petersen, 2016). All these solutions aim to absorb, evaporate and/or channel rainwater
so it does not end up in the sewage system.
2.1 Retrofit of SuDs
SuDs retrofit is a stormwater management process which is aimed at addressing urban
water quality and the problems associated with flooding (Walsh et al., 2016). Retrofit is used
when SuDs are proposed for the replacement or augmentation of an existing drainage
system (Smith and Mijic, 2016). Examples of retrofitting SuDs can be seen in the installation
of green roofs, the diversion of roof drainage from a combined sewer system into a garden
soakaway, and the conveyance of road runoff via roadside swales into a pond sited in an
area of open space (Ellis, 2013). These represent alternative ways of influencing the quality
of the water downstream and the problems relating to it, thereby providing a more effective,
resilient and sustainable approach. For SuDs to be acceptable, three core attributes are
essential: economic viability, resilience and sustainability (Lead, 2018).
In considering the economic viability of SuDs retrofit, it is important that SuDs retrofit is
affordable in costs and benefit for it to be accepted as a potential replacement for the
existing drainage system. Fisher-Jeffes and Armitage (2011) identified that impacts such as
aesthetics, economic value, rainwater harvesting, influences the choices made by
individuals and organisations as it affects the cost and sale of properties. Ossa-Moreno
et al. (2017) opined that for SuDs retrofit installation to be economically feasible or viable, the
wider benefits would need to be taken into account.
Resilience is multifaceted and is defined in different ways by various professionals.
Adedeji et al. (2018) define resilience as the ability of a system to return to its stable state
after a momentary disturbance. Therefore, the acceptance of SuDs retrofit in terms of its
resilience requires that the installation of SuDs retrofit would help to restore any flood
affected area into a more attractive place.
According to Sieker et al. (2008), SuDs are known to be more adaptable and flexible than
traditional solutions, allowing future modification to cope with climate and other changes in
urban areas. Therefore, if SuDs retrofit can be incorporated into existing developed areas,
the opportunities for delivering sustainable solutions that offer multiple benefits will be
much greater.
2.2 The implementation of the retrofit of SuDs
In this section, various benefits and barriers to the uptake of SuDs retrofit have been
identified. The implication of the impact of these factors are further discussed.
2.3 Processes of the implementation of the retrofit of SuDs
A number of benefits that cut across various positive improvements in schemes and the
lives of people have been identified. For example, Malulu (2016) found that a common SuDs
intervention scheme entails the carrying out of works to rivers with the aim of increasing
their capacity to carry flood flows. Friberg et al., identify a further scheme involving channel
maintenance or enlarging the channel cross section and thereby increasing the flow of
surface water by extending the capacity. The mitigation of the heat island effect and noise,
the improvement in air and water quality and the provision of sites for recreation or urban
amenities are various ways by which the ecosystem is sustained through SUDS retrofit
(Demuzere et al., 2014; Ellis, 2013; Kazmierczak and Carter, 2010).
Other benefits of SuDs retrofit is in the reduced cost of infrastructure by the introduction
of green infrastructure (GI). Ellis (2013) argues that conventional drainage systems cannot
Cost-benefit
analysis model
provide the expected solution to any flood mitigation process but an extended approach
based on the introduction of retrofit SuDs, in the likes of micro-and meso-vegetative SuDS
systems into a wider GI framework, can effectively address on-site and catchment urban
surface water issues. Foster et al. (2011) identified the importance of the aesthetic value of a
building or location which is increased by the installation of SuDs retrofit by way of GI,
creating habitat for wildlife, by constructing swales and other forms.
Health improvements from the use of SuDs is also an important benefit to every citizen.
Lamond et al. (2015) affirm the importance of an improved flood risk management system
to manage the growing pressure of the effect of flooding events on the health of the
occupants of any community. Greenough et al. (2001) address the health effects of flooding
which are typically associated with disasters. These are direct morbidity and mortality
and secondary or indirect health impacts. A direct impact includes an impaired public
health infrastructure, reduced access to health care facilities, and psychological and
social effects, whereas indirect effects could result in the alteration of ecologic systems
which may result in land covers (i.e. grass, asphalt, trees, etc.) being damaged, and the
abundance and distribution of disease-carrying insects, rodents and some other vectors.
An improved health system through the application of SuDs retrofit helps to address these
health issues.
Economically, the security of the reputation of a business is very important. In recent
years, the importance of reputation has become increasingly apparent with the rising effect
of damages which are caused by flood events. A good reputation for an organisation will
inform the consumer’s preference and external support for an organisation in critical times
and enhance the value of an organisation in the market place (Epstein, 2018). Economic
growth can also be stimulated by SuDs retrofit, through attractiveness of an area to new
businesses, creation of jobs from the installation and maintenance of SuDs, and the
improved productivity of workers when the environment is positively impacted by
aesthetics, improved health conditions, improved air quality and many others (West, 2009;
Kruger, 2014). Carpenter (2012) found that an aesthetically improved environment with the
installation of SuDs can improve tourist attractions and recreation centres, with the aim of
attracting visitors from both locally and internationally. GI has been credited in the UK with
a significant impact in job creation (Chegut et al., 2014). Also, in the USA, shoppers tend to
stay longer when visits are made for business purposes, owing to the presence of green
structures (Yi et al., 2014).
In addition, in projecting the cost of installingSuDsretrofitandthefutureeffectona
community, most importantly when considering a value-oriented structure, a conducive
whole life costing (WLC) is guaranteed for a clearer understanding of the required costs.
Lamond (2016) explains WLC as a methodology that gives a systematic economic
consideration of all costs associated with SuDs retrofit. In considering this methodology, a
number of factors are measured –finance, business costs and income from land sale,
user costs. In order to deliver the best value for money, these factors are essential
when measuring the economic implications in terms of the cost effectiveness of SuDs
retrofit in a community.
Despite the increased flooding events in the UK, the uptake of SuDs retrofit as a flood
risk management measure is still largely being ignored (Ossa-Moreno et al., 2017). The lack
of experience of, and trust in some of the approaches, is a major setback for the
implementation of SuDs retrofit (Backhaus et al., 2016). Convincing stakeholders about the
implementation of a new scheme is difficult when consideration is given to failed flood risk
management schemes (Kundzewicz et al., 2017).
Flood management in England and Wales is currently seen differently to water supply
and water quality management terms (Kangalawe, 2017). In Wales, Natural Resource Wales
provide an oversight role in relation to all flood and coastal risk management in Wales and
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are responsible for managing flooding from main rivers, reservoirs and the sea including the
provision of technical advice and support. On the other hand in England, The Department
for Environment, Food and Rural Affairs is the policy lead for flood and coastal erosion risk
management in England. These bodies have got their individual policies which differ in
some ways and hinders the possibility of collaborating efforts and budgets across these
regions through major solutions which are able to manage existing challenges in a
cost-effective way (Cousins, 2017).
The responsibility for the cost of maintaining and implementing SuDs retrofit is also a
major barrier. Like all drainage systems, SuDs retrofit should be inspected and maintained.
This will ensure efficient operation and prevention of future failures. However, the lack of
understanding of the cost of maintenance and implementation of this relatively new scheme
is exacerbating its uptake.
3. Cost-benefit analysis approaches
In approaching real-life problems, a number of decision-making tools have been developed
such as CBA, cost-effectiveness analysis (CEA), cost-utility analysis (CUA), risk-benefit
analysis (RBA), economic impact analysis (EIA), fiscal impact analysis (FIA) and social
return on investment analysis (Greco et al., 2007; Snell, 2011).
CEA is an economic analysis tool, distinct from CBA which assigns value to the measure
of effect. It compares relative costs and outcomes of different actions (Price et al., 2018). CEA
is applied to planning and management of many types of organised activities, and it is used
in many aspects of life which includes the health sector where it may be inappropriate to
monetise health effects. Campillo-Artero and Ortún (2016) defined CEA as a measure of
health that developed countries use in making funding decisions, which is aimed at publicly
funding health technologies that produce the greatest health gain at a given cost. CEA has
faced some setbacks due to the fact that it reflects mistrust of the underlying methods or the
motives of the parties conducting the analyses, or a desire on the part of many to deny or
downplay the underlying problem of resource scarcity in health care due to the ethical
difficulty in monetizing health effects. However, CEA is widely accepted as a useful tool for
resource allocation. CEA is also not applicable in the context of this research, Brock (2004)
opined that there are important ethical and value choices to be made in constructing and
using CEA; these choices are not merely technical, empirical, or economic, but moral and
value choices as well. These moral choices explain why it may be difficult to quantify
outcomes of health issues and its suitability for a CBA model.
CUA is similar to CEA; it is mostly used in pharma economics especially health
technology assessments. It estimates the ratio between the cost of a health-related
intervention and the benefit it produces in terms of the number of years lived in full health
by the beneficiaries. CUA are estimates of health outcomes and costs of competing
alternatives and is widely accepted as a useful tool for resource allocation. Health outcomes
are commonly summarised as quality-adjusted life-years (QALY), which are a combination
of quantity and quality of life (Kuntz, 2016). There are continuing controversies about the
QALY unit, which is used to measure the outcome of the findings from CUA. One very
important aspect of the CUA is the term “quality”. Richardson (1994) identified the fact that
there are varieties of meanings to the “quality”aspect of CUA, with different scaling
techniques and concepts which makes CUA inappropriate as an idea economic tool.
A similar principle that governs the ethical and moral implication of using CEA in this
research context is also applicable with CUA because of the difficulty associated with
monetizing its outcomes.
RBA seeks to quantify the risk and benefits by employing the ratio of the risk of an
action to its potential benefits (Guo et al., 2010). RBA for a clinical trial is provisionally based
on the preclinical phase of the medicinal product. The sponsor–investigator team needs to
Cost-benefit
analysis model
evaluate the toxicological tests and results as well as submit the data to the competent
health authorities, with a projection of all the possible risks for the proposed trial subjects
(Fortwengel, 2011). This tool does not have the facility to determine the cost of a product
which is required under the determination of the cost effectiveness of SuDs retrofit because
it is not a financial-based tool. This is therefore outside the scope of the research on the cost
and benefit of the installation of the retrofit of SuDs.
Furthermore, the EIA examines the effect of an event on the economy in a specific
location; this ranges from a single neighbourhood to the entire globe. EIA measures changes
in business revenue, profits, personal wages which can lead to the suggestion of policies and
laws that could improve the economy. Drucker (2015) described EIA method as an analytical
technique that is predicated on economic stability, yet commonly applied to situations that
violate this condition with little consideration of the implications. EIA is basically a tool
which is useful for the wider economy of a nation and in determining the political and
economic stability.
FIA is a tool that is used to compare project or policy change, changes in governmental costs
against changes in governmental revenue. Moore (2015) describes the FIA tool as a revenue-to-
cost relationship which explains the implication of a proposed revenue to be generated from a
new development in any location. This can either be positive or negative, depending on whether
the revenue generated is greater or lower than the cost. For example, Town A is a major
residential development project that requires new services and facilities such as fire and police
protection, libraries, schools, parks and others. At the same time, Town A will as a result of this
project receive new revenues such as property tax revenues, local sales tax revenue, and other
taxes and fees. FIA therefore compares the total expected costs to the total expected revenues to
determine the net fiscal impact of the proposed development on Town A.
The CBA approach has been selected in this study in order to undertake an economic
appraisal of the monetary and non-monetary benefits of the uptake of SuDS retrofit.
The CBA approach suggests that any new initiative or investment decision should only be
adopted if its expected benefits (political, social, environmental and moral) exceed its costs
(Wildavsky, 2018).
Joseph et al. (2014) successfully applied the CBA concept to property level flood
adaptation (PLFRA) measures, incorporating recognition of the intangible benefits. This
provided a robust mechanism for decision making on investments in property level floor
adaptation measures by homeowners. This modelwasdesignedtoadvisehomeownersof
the potential benefits of investing in PLFRA measures. Another example can be seen in
the analysis carried out on the report on the CBA of Western Cape climate change
response (Parmesan, 2006). The Western Cape Government recognised the risks posed by
climate change to its economy, population, ecosystems and infrastructure and sought for
measures to mitigate its effect by the use of the CBA model. The use of the CBA model
helped lead to a better and more informed implementation process of activities that are
economically valuable in terms of reducing climate change risks.
The CBA tool enables a clear monetary comparison of the costs and benefits of the
installation of SuDS, thereby facilitating the decision-making process and providing an
appreciation of the cost effectiveness of the range of alternative solutions. A unique feature
in this study is the application of the Choice Modelling Method (CMM). The CMM Method is
to be employed to elicit willingness to pay (WTP) values from property owners to obtain the
non-monetary benefits of the installation of SuDS retrofit. The advantage of using CMM is
that respondents are presented with various alternative descriptions of non-monetary
benefits, differentiated by their attributes and levels, and are asked to rank the various
alternatives, to rate them or to choose their most preferred (Hanley et al., 2001). By including
price/cost as one of the attributes of the non-monetary benefits, WTP can be indirectly
recovered from people’s rankings, ratings or choices.
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4. Evaluation of the costs and benefits of suds retrofit
One of the important recommendations from the Mayes (2008) review was to encourage
property owners to take up the responsibility of reducing the effect of flood events on their
properties by the uptake of available flood risk measures. Although the uptake of SuDs
remains challenging owing to the complexity in its monetary and non-monetary benefits,
there is continuous growth in public interest (Ossa-Moreno et al., 2017). The proposed CBA
conceptual framework has taken into consideration the key components associated with the
cost and benefits of SuDs retrofit and could help support further uptake.
4.1 The costs of SuDs retrofit
In the development of the CBA conceptual framework, the method for estimating additional
costs of the measures adopted is to proceed stage-by-stage from the beginning to the end of
the estimation activity. This method, whilst time consuming, tends to be the best because it
outlines a detailed and more informed process in handling the breakdown of the costs of
installing SuDs.
Figure 1 is a representation of the stages that a typical SuDs retrofit installation is
expected to go through for the full costs (and benefits) to be determined. These stages
include the preliminary stage, implementation stage and the maintenance stage.
According to Merz et al. (2004), in estimating the cost of flood adaptation measures, the
ability to categorise the flood mitigation measures by building design and construction
process in the required order has the potential to lead to a better outcome. The decision to
invest in SuDs as a retrofit is determined by the type of property and the level of impact
flood events have had and will have on the property. Therefore, it is important to consider
the impact of flood characteristics in this evaluation process.
According to Soetanto and Proverbs (2004), the damage caused by flood events on any
property is a function of variables including flood characteristics such as the depth of flood
water, velocity, history, duration, probability and the source of the flood. Among these, flood
depth, duration, probability, history, velocity are essential characteristics because they play
significant roles in the extent of damage experienced by any property. They also help to
determine the additional costs before installation as a result of the extent of damage and the
repair work required.
Flood Characteristics
Flood duration, depth and probability Costs from the installation of SuDS retrofit
Implementation Maintenance Phase
Performance
Enforcement
Duration
Inspection
Post-
Construction
Fulfilling local planning
policies
Design Criteria
Pre-application
discussions
Site
Survey
Structural
Investigation
Process
Determination of
the type of
vegetation/
substrate depth to
be grown
Problem
Understanding
Water
Quantity
Water
Quality
Amenity
Biodiversity
Preliminaries
Control runoff
quantity
Support the
effective management
of flood risk
Protect
morphology
and ecology
Preserve and protect
hydrological systems
on the site
Drain the site effectively
Manage flood risk
Prevent
pollution
System resilience
to cope with
future change
Multi
functionality
Enhance
visual
character
Deliver safe
surface water
system
Resilience/adaptability
to future change
Maximise legibility
Support community
environmental learning
Support and protect
local habitat species
Contribute to local
biodiversity objective
Contribute to habitat
connectivity
Diverse self-
sustaining
ecosystem
Design
Set Surface
water
management
objectives
Conceptual
design. (Initial
design and
layout)
Outline design
(Sizing and
optimisation)
Detailed
design (testing
and finalising
scheme)
Development of
a master plan
Outline planning
permission (+ conditions)
Full Planning
permission
Building regulations
and approval
Construction,
Aesthetic Cost,
inspection and
approval
Figure 1.
Cost parameter
for the installation
of SuDS retrofit
Cost-benefit
analysis model
5. The benefits of SUDS retrofit
The uptake of SuDS as a retrofit could be of benefit to different stakeholders including
property owners and users, insurance companies, flood management professionals and the
government. The benefits of SuDS retrofit can be grouped into tangible benefits (Monetary)
and intangible benefits (non-monetary) as shown in Figure 2.
To evaluate these benefits, several considerations needs to be involved, such as taking into
account the benefits accruing to and the cost incurred by the property owner (Penning-Rowsell
et al., 2005); selecting appropriate prices for evaluating the benefits and costs in monetary
terms and adjusting the future prices of benefits to present values to make them comparable
with the costs ( Joseph & University of the West of England, 2014). This means that despite the
fact that the benefits and costs are from different sources, it is important that a systematic
procedure is established to allow the proper evaluation of every parameter.
Bozman et al. (2015) described tangible benefits as quantifiable especially monetarily;
these are identified as reduced cost of infrastructure, improved aesthetic value, reduction of
surface water charges, flood risk reduction and improved market value of the property.
The intangible benefits are subdivided into the benefits accrued by the property owner and
the benefits accrued by the wider community. For the accrued benefits by the property
owner, this includes rainwater harvesting, reduced post-flood recovery inconvenience,
security of business reputation, reduced interruption to business activities, reduced cost of
business assets and values, reduced insurance claim, increased property protection,
reduced/elimination of property content evacuation and reduction in energy usage.
In terms of the benefits accrued by the wider community, this includes economic
improvements, air and water quality, reduced loss of life, reduction/elimination of
diseases, reduction/elimination of infections, reduction/elimination of muddy part ways,
reduced loss of ecological and cultural values, reduction/elimination of depression,
reduction/elimination of anxiety, reduction/elimination of stress, reduction of G.P. visits,
habitat for wildlife.
Value of Benefits
Actual Market Value
Willingness to pay (WTP)
Tangible Benefit accrued
Reduced cost of infrastructure
Improved Aesthetic Value
Reduction of surface water charges
Improved market value of the property
Flood risk reduction
Intangible Benefits accrued
• Economic improvements
• Air and water quality
• Reduced loss of life
• Reduction/elimination of diseases
• Reduction/elimination of infections
• Reduction/elimination of muddy part
ways
• Reduced loss of ecological and
cultural values
• Reduction/elimination of depression
• Reduction/elimination of anxiety
• Reduction/elimination of stress
• Reduction of G.P. visits
• Habitat for widelife
Wider Community
Property Owner
• Rainwater harvesting and amenity
• Reduced post-flood recovery
Inconvenience
• Security of business reputation
• Reduced interruption to business activities
• Reduced cost of business assets and values
• Reduced insurance claim
• Increased property protection
• Reduced/elimination of property content
evacuation
• Reduction in energy useage
Flood Characteristics
Flood duration, depth and probability
Figure 2.
Value of benefits
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Figure 2 shows the stages in evaluating the value of benefits that will accrue with the
uptake of SuDS. These are in two forms, the actual market data for tangible benefits and
the WTP values on the intangible benefits. This is important because the application of the
concept of CBA requires that both the costs and benefits have to be in the same unit of
measurement before any decision can be made on whether a project is cost-effective or not
( Joseph & University of the West of England, 2014).
6. Willingness to pay (WTP)
Intangible benefits by their subjective nature are difficult to quantify and are said to be
more personal to the victim of a flood event ( Joseph & University of the West of England,
2014). This impact depends on the relationship of the individual to the loss or damage which
had been experienced from the flood. Therefore, ignoring the intangible benefits of SuDS
retrofit can lead to an incomplete understanding of the full benefits. Non-availability of
locations, where intangible benefits of flooding are being considered, makes its evaluation
more difficult ( Joseph & University of the West of England, 2014; Markantonis et al., 2012),
this is why it is usually left out of the CBA appraisal for flood adaptation measures.
The intangible benefits of SuDS retrofit can be evaluated by using one of the stated
preference methods of valuation referred to as CMM, this can be used to elicit WTP
estimates from property owners.
6.1 Choice modelling method (CMM)
The CMM is a family of survey-based methodologies for modelling preference for different
options of choices. CMM gives more detailed options to respondents enabling a more explicit
understanding of their needs. With the CMM, respondents are presented with various
alternative descriptions of the intangible benefits with different levels of financial
commitments which would then be ranked from most preferred till the less preferred. By
including price as one of the rankings, WTP can then be indirectly recovered from the
ratings or choices (Snell, 2011).
In a typical CMM technique, individual preferences are uncovered in the survey by asking
respondents to rank the options presented to them, to score them or to choose their most
preferred. These different ways of measuring preferences correspond to different variants of
the CMM approach. There are four major variants: choice experiments, contingent ranking,
contingent rating and paired comparisons (Olazak and Mach-Król, 2018).
An example of the application of CMM in the context of this study is shown in Table I.
6.2 Discounting
In considering the value of the benefits accrued from the installation of SuDS retrofit, it is
important to apply a discount rate. Szekeres (2011) argues that it is useful to address how
the discounting paradigm fares in the long run, especially as it affects climate change and
environmental policy, to see if it suffers from any special limitations that need to be taken
into account. Ackerman and Heinzerling (2001) described discounting as a tool used in CBA
to compare the present costs and benefits and the implication for the future. This is the
SuDs retrofit type (Green roof ) Intangible benefit WTP value (£)
1. 20 years maintenance Economic improvement 20,000
2. 15 years maintenance 15,000
3. 5 years maintenance 5,000
4. 2 years maintenance 2,000
5. 1 year maintenance 1,000
Table I.
Example of the
application of
the CMM to
intangible benefits
Cost-benefit
analysis model
reduction in the value of future costs or benefits at a pre-specified rate, which depends on its
temporal distance from a common time.
Given the value of money, a pound is worth more today than it would be worth
tomorrow. Therefore, discounting is the primary factor used in pricing a stream of
tomorrow’s cash flow. Very often, decisions have to be made about whether to incur costs in
the present, in return for benefits in the future, as in the case of investing in SuDS retrofit.
Every investment requires this type of decision at one point or the other.
Since individuals and organisations have their preference as it relates to receiving
benefits or incurring costs, time preferences also have to be accounted for through the
process called discounting. The advantage of discounting is that it enforces consistency and
it makes the assumptions explicit (Charness et al., 2013).
In presenting the costs and benefits of SuDS retrofit in monetary terms, CBA follows
standard economic practice in discounting future benefits and converting them to their
equivalent value today, or present value. In the Economist view, when the time span is
long and different generations are required to be involved in the costs and benefits of a
particular project, the analogy to an individual investment decision breaks down (Keynes &
SpringerLink 2018). Ackerman (2001) therefore suggested that when setting a discount rate
for a project, it must be set to a very low level, so that an enhanced benefit is generated.
6.3 Flood probability
One major factor in determining the cost effectiveness of SuDS retrofit is flood probability
(flood return period). Destro et al. (2018) described it as the estimate of the likelihood of the
occurrence of a flood event. It is a key determining factor in the installation of SuDS retrofit,
as it affects the accrued benefits. A study by Thurston et al. (2008) determined that a flood
resistance measure could be said to be worth an economical value for properties with a
50-year return period. However, properties that experience flooding events more than once
in every 10 years, the benefits outweigh the up-front investment. Also, Joseph et al. (2014)
found that the adoption of resilience measures will be more economical for properties which
are located in areas with up to 25 years return period. However, properties that experience
flooding events more than once in every five years, the benefits are said to outweigh the
up-front investment. Therefore, in considering the accrued benefits from repeat flooding in
high risk areas, flood probability (flood return period) is an important variable which should
be included in the CBA conceptual model.
7. The conceptual framework
Figure 3 represents the CBA framework for comparing the costs and benefits of SuDs
retrofit in commercial properties. This CBA framework gives a detailed description of the
monetary and non-monetary value of the installation of SuDs retrofit in a commercial
property. Oladunjoye et al. (2017) identify this as a gap that has resulted in a reluctance
towards the uptake of SuDs retrofit to mitigate flood risks, hence the need for a detailed
framework that will give a robust understanding of the monetary values.
The framework is represented by a pivoted depiction representing the implications of the
impact costs and the benefits accrued from a typical SuDs retrofit. Stovin et al. (2013) and
Lamond and Penning-Rowsell (2014) opined that when considering the decision for the
uptake of an element like SuDs retrofit, it is important that if the cost of installing SuDS
retrofit is less than the benefit, then investment in it is advised but if it is otherwise, it is not
advisable to go ahead with its uptake.
In a typical CBA model, it is important that costs and benefits are well defined. Snell
(2011) described CBA as a formal technique adopted for clear, systematic and rational
decision making especially when faced with complex alternatives or uncertain data. Hence a
detailed CBA model will make it easy for clarity and a rational decision to be attained.
IJBPA
Although CBA is a well-established tool, its application in this context is quite unique.
In this framework, in a bid to derive a robust outcome, consideration was given to the
involvement of indirect property users in terms of the benefits accrued from the installation
of SuDs retrofit.
The framework is divided into two parts representing first, the details of the costs of
installing a typical SuDs retrofit for a commercial property and second, representing
details of the accrued benefits. The CBA conceptual framework is developed by
introducing required elements of the costs vs the tangible and intangible benefits as it
affects commercial properties and reflects the hypothesised relationship between costs
and benefits of a typical SuDs retrofit installation. The costs and benefits of the SuDs
retrofit are linked together to produce a CBA conceptual framework which incorporates all
necessary parameters.
A clear and well-detailed process of installing a typical SuDs retrofit has been employed,
to form the framework. This framework represents a contribution to the study of SuDs
retrofit in the context of commercial property. In terms of the benefits accrued, these are
considered in the context of both direct and indirect users of a commercial property. This is
important because consideration is needed to be given to both the property owner and other
users of the property such as customers, employees and suppliers.
Included in the framework are the flood characteristics which have influence
on the outcome of the costs of installation and also the benefits. Flood duration, depth,
velocity, probability and history are vital determinants in the determination of the
outcome of installing SuDs retrofit. Soetanto and Proverbs (2004) opined that the
damage caused by any disaster is highly dependent on the scale and nature of that
disaster. In this context, the damage cause to a commercial property is dependent on the
flood characteristics.
8. Conclusions
The development of a CBA conceptual framework for the costs and benefits of SuDs
retrofit has been discussed and presented. This framework highlights the essential elements
of the costs and benefits of SuDs retrofit which need to be examined in the context of
commercial properties. The CBA framework provides an in-depth means of assessing the
actual cost and benefits of the installation of SuDs retrofit. By combining the relevant
elements in each section of the framework, the full costs and benefits of retrofitting SuDs
can be established. This would help in the decision-making process when faced with
choosing to invest in any type of SuDs retrofit.
Tangible Benefit accrued
(Actual Market Value)
Intangible Benefits accrued
(Willingness to pay)
Wider Community
Property Owner
PRELIMINARIES
IMPLEMENTATION
MAINTENANCE
SUDS
RETROFIT
INSTALLATION
PROCESS
DIRECT AND
INDIRECT
PROPERTY
USERS
Design Criteria
Site layout Survey
Design
Reconnaissance Survey
Performance
Enforcement
Duration
Inspection Post-Construction
Fulfilling local planning policies
Pre-application discussions
Development of a master plan
Outline planning permission
Full Planning permission
Building regulations
and approval.
Construction, Aesthetic Cost,
inspection and approval
Flood duration
FLOOD CHARACTERISTICS
Flood depth
Flood history Flood
Flood velocity
COST
Decision stage
(If the cost of
installing SuDS
retrofit is less
than the
discounted
benefit, then
investment in it
is advised)
BENEFITS ACCRUED
Figure 3.
CBA conceptual
framework for
comparing the costs
and benefits of suds
retrofit in commercial
properties
Cost-benefit
analysis model
The conceptual framework presented gives the much-needed understanding about the cost
effectiveness and benefits of the installation of the retrofit of SuDs which is previously
lacking in the literature. The framework draws on the various approaches used in
estimating costs and benefits of SuDs retrofit which will assist decision makers and end
users in deciding how best to reduce the impacts of flooding.
A full understanding of the costs and benefits of retrofitting SuDs will help to inform
better decision making in choosing the most appropriate and cost-effective means of
retrofitting SuDs for any given location. The proposed model is expected to be used by flood
risk management professionals, property professionals and commercial property owners of
the potential benefits of investing in the installation of SuDS. This study will help develop
our understanding of the full costs and benefits accrued from the retrofit of SuDs and so
lead to an increase in uptake. Also, details about the benefits accrued by indirect users of the
commercial property will inform a robust understanding of the advantage that these set of
users will derive from the uptake of SuDs retrofit. The model developed here is specifically
for commercial properties but many of the principles applied would be equally relevant to
other types of property.
However, one major challenge with this research is with quantifying the intangible
accrued benefits from installing SuDs retrofit. Putting a value to these parameters is very
important but difficult for most professionals to accomplish. Quillin (2010) described
intangible benefits as hidden jewels that do exist and need to be accepted as valid. However,
being able to validate this parameter stands as a major difficulty.
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Agwuele, A. (2013), Development, Modernism and Modernity in Africa, Routledge, London.
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opportunities for developing a green industry”, doctoral dissertation, University of Cape Town.
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Ashley, R.M., Walker, A.L., D’Arcy, B., Wilson, S., Illman, S., Shaffer, P., Woods-Ballard, P. and
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Chofreh, A.G., Goni, F.A. and Klemeš, J.J. (2018), “Evaluation of a framework for sustainable
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IJBPA
Vargas Renzi, F. (2018), “BIM application in construction management”, master’s thesis, Universitat
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Corresponding author
Oluwayemi A. Oladunjoye can be contacted at: yemmyola2@gmail.com
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Cost-benefit
analysis model
