Surface water management planning: guidance (2018)

Guidance to assist the responsible authorities in preparation of Surface Water Management Plans (SWMPs) to help with the management of surface water flooding.

7. Option appraisal

Option appraisal: considerations and example outputs
Considerations Example outputs
  • Scoping the option appraisal, confirming objectives (e.g. high-level appraisal for all SWMP objectives or more detailed appraisal for fewer, high-priority objectives).
  • Making sure that the methods and level of detail of the appraisal are proportionate to the scale and complexity of the problem and the decisions to be made.
  • Developing and comparing options for each objective in order to choose preferred option.
  • Consultation and co-ordination:
    • Where there are links with other projects (or joint projects) then co-ordination and close working will be required to ensure that multiple benefits are realised.
    • Consulting land use planning is likely to be essential to ensure that any preferred structural options integrate with the urban landscape.
    • The local community may need to be consulted.
  • Understanding the degree of confidence in the outputs of the appraisal.
  • Communicating information clearly, considering different communication material for different audiences.
  • Section of SWMP report clearly communicating the outcomes of this stage (e.g. options considered, confidence in the appraisal, reasons for selecting preferred option).
  • Co-ordination and close joint working where required to develop options that will yield multiple benefits.
  • Other outputs, e.g. supporting information for the option appraisal, clear communication of information to aid decision making, consultation material for different audiences.

7.1 Introduction

This chapter provides guidance on how to appraise and select the most sustainable options to meet the objectives. The outcome of this stage should be an agreed set of feasible and sustainable options that meet the objectives for managing the risk of surface water flooding in an area (see Box 7.1 for a definition of options and actions). The appraisal process is iterative, which means that it can be progressively refined. Preferred options will go on to the next stage where they will be developed, have their funding confirmed and then be implemented (see Chapter 8).

This guidance is consistent with, and should be read alongside, Scottish Government guidance and policy on option appraisal for flood risk management and the HM Treasury Green Book (see Box 7.2).

The main stages of options appraisal are shown in Figure 7.1.

Box 7.1 Options and actions

An option is one or more flood risk management action(s) developed to meet an objective of
the SWMP. For example:
Option 1. Above-ground storage and conveyance.
Option 2. Above-ground storage and conveyance; below-ground storage and conveyance.

Actions can be structural or non-structural, and can be combined to make up options. A list of possible actions can be found in Appendix 4.

Box 7.2 Reference material for options appraisal

Public sector appraisal guidance

Scottish Government flood risk management guidance and policy

SEPA strategic appraisal methodology for flood risk management

CIRIA Susdrain website

  • Provides useful resources on sustainable drainage, including links to case studies and technical documents on the detailed design of actions.
Figure 7.1 Steps in option appraisal

Figure 7.1 Steps in option appraisal

These steps can be reviewed and refined as the process progresses to provide more detail if required.

7.2 the appraisal

Clarify the objectives

An SWMP may have many objectives for reducing flood risk in the areas at greatest risk, and these objectives may have been prioritised. A decision should be made on whether to conduct a high-level appraisal for all the objectives, or whether to carry out a more detailed appraisal for the priority objectives only. If an option appraisal is not carried out for all objectives, an action to the effect that one will be carried out for lower priority, longer term objectives at a later date, can be noted in the SWMP. See the example in Table 7.1 below.

Table 7.1 Example SWMP and what objectives to take forward for option appraisal

Location/hotspot History of flooding Confidence in data Initial objective Initial priority Include in option appraisal
Whole Town History of surface water flooding Avoid an increase in surface water flood risk in Whole Town. High Yes
Reduce surface water flood risk in Whole Town. Yes
Neighbourhood A History of significant surface water flooding but modelling shows low risk. Low confidence in modelled data. Improve understanding of surface water flooding in Neighbourhood A. High Yes - although action for this will be further modelling to improve understanding followed by option appraisal, so detailed appraisal of structural options not required at this stage.
Burn B Observed flooding matches modelled data. Good confidence in modelled data. Reduce surface water flood risk from Burn B. Medium Not at this time – action can be for future option appraisal.
Burn C Observed flooding in area but existing structure to manage risk not shown in modelled data. Maintain existing structure at Burn C. On-going No – existing action in place.
Neighbourhood D Observed flooding matches modelled data. Good confidence in modelled data. Reduce surface water flood risk in Neighbourhood D. High Yes – appraisal of structural options likely to be required.
Road E No history of surface water flooding but area could be at risk. Medium confidence in modelled data. Reduce surface water flood risk at Road E. Medium Not at this time – although a general action will be collecting data on observed flooding. Future option appraisal may be required.

Appraisal method
Scoping the approach for option appraisal will help to establish what information and resources are required. The type of appraisal and level of detail should be informed by risk and be proportionate to the scale and complexity of the problem, the planning stage (be it strategic or more detailed) and the level of detail needed to make a decision. A detailed benefit-cost analysis is not always required, nor is it always feasible. A simplified appraisal should not, however, be interpreted as one that lacks rigour.

Factors that will influence the type and detail of the appraisal include:

  • The level and complexity of flood risk.
  • The availability of, and confidence in, hazard and risk modelling and mapping.
  • The availability of, and confidence in, other data such as whole life costs and wider impacts.
  • The type and scale of the action and the information required to differentiate between options in order to make a decision in choosing a preferred option.

Less detail may be needed where the choice between options is clear, for non-structural actions or for a long-term strategic plan. Conversely, more detail is likely to be required where the situation is complex, differentiation is more difficult, a lot of resources are being invested or project planning is at the design stage.

The type and complexity of the options will also determine what expertise is required to carry out the appraisal. For example, a simple, localised problem and solution (e.g. raising a kerb to divert flood waters to a safe pathway) is unlikely to require a detailed assessment of costs and benefits. Professional judgment should be sufficient to design and implement this type of smaller-scale action with confidence.

Appraisal for a more complex flooding mechanism and/or larger-scale solution (e.g. using multiple actions above and below ground) will require greater detail and expertise. It may for example necessitate flood damages data to be manipulated or flood hazard maps remodelled, which may require the services of a multidisciplinary team including landscape architects, engineers, modellers and other specialists.

Further guidance on how to take a proportionate approach can be found in Scottish Government’s 2016 Appraisal guidance (see Box 7.2).

7.3 Identifying and screening long list of actions to get short list

7.3.1 Identifying the long list

For each objective, a long list of actions that could help to meet it should be identified. More strategic actions that may help to achieve more than one objective should also be considered. A list of possible non-structural and structural actions to use as a starting point can be found in Appendix 4.

A long list of actions should be identified with the following points in mind:

Meeting the objectives

  • Consider all actions (structural and non-structural) that could partially or completely achieve the objectives.
  • Consider actions that are effective at the property, neighbourhood or more strategic scale, as appropriate.
  • Bear in mind that actions being considered for objectives at a strategic scale may not need to be considered for more localised objectives (e.g. if land use planning, emergency response planning or review of maintenance regime actions are being considered for objectives for the whole SWMP area, they may not need to be considered for more localised ones).
  • Consider actions that can be implemented in the short term, as well as longer term, aspirational ones.
  • Consider whether there are opportunities to help meet objectives for managing river and coastal flood risk.
  • Consider opportunities to improve existing actions, e.g. change maintenance regimes or enhance or replace existing actions.

Promoting sustainable actions

  • Seek to apply the principles of sustainable surface water management ( Table 1.1).
  • Consider the impact of actions on surface water flood risk now and in the future – actions to manage flood risk should be flexible enough to meet the needs of future generations and be adaptable to a changing climate and other drivers of changing flood risk.
  • Consider actions that could realise wider benefits, such as creating better places for people, preventing deterioration of (and where possible improving) the water environment, improving biodiversity, or reducing the costs associated with water and waste water treatment.

Working with stakeholders

  • Consider actions that would be carried out by the full range of stakeholder organisations – identify the most sustainable actions and do not be constrained by responsibilities, funding concerns or delivery mechanisms.
  • Be aware that actions may be added to or refined by Local Flood Risk Management Partnerships or Local Advisory Groups, or through engagement with all stakeholders.

7.3.2 Screening the long list

Screening may be necessary to reduce a long list to a short list of actions. This will help to remove any actions that are unfeasible, leaving a smaller number for further appraisal. Sustainability must be a key consideration, and actions that are clearly unsustainable should be rejected early. Complex and integrated solutions should not, however, be shied away from.

Three main criteria are recommended for screening out unfeasible actions – technical, legal and economic ( Table 7.2). If necessary broad beneficial and adverse impacts can be identified for each of the actions, but detailed assessments should not be done at this stage and impacts do not need to be valued. Instead, experience and judgment should be applied. Where there is doubt, an action should be retained for further evaluation as part of the short list.

Screening of actions should not be constrained by concerns about funding or methods. Agreements on funding and responsibilities should be made once the most sustainable actions have been identified (see Chapter 8).

All decisions and reasoning should be clearly set out and recorded.

Table 7.2. Screening criteria

Criteria Considerations
1. Technical
Remove any actions that are not technically feasible, e.g. permeability of ground insufficient for infiltration, storage volume required and available space (above- or below-ground space).
Ground conditions – e.g. permeability, contamination (if considering ground infiltration).
Topography – areas set aside for temporarily storing surface water must be positioned down slope from the areas generating run-off, to allow water to flow by gravity.
Existing land use – this may affect the feasibility of some solutions, for example:
  • Using carriageways or entire roads for conveying or storing surface water may be incompatible with their use as strategic routes and for road safety.
  • Exceedance flow management may increase flood risk downstream.
  • Above-ground storage may not be feasible because of a shortage of space (or lack of safe access for maintenance). Innovative ideas for creating space or for combining public space with storage areas should not, though, be overlooked.
Flood forecasting – forecasts for surface water flooding are likely to remain probabilistic or have short lead times. Consequently, demountable (temporary) property-level protection may not be appropriate where no (or only a short) warning of a flood event is possible. (Permanent property-level protection or resilient property design/retrofit might be more appropriate in these conditions.)
2. Legal
Remove any actions that represent insurmountable legal challenges, including health and safety.
There are various legal constraints on what actions can be taken, or more specifically, the manner in which they are taken. They mainly deal with the impact on people and the natural or built environment. Specific legal obligations should be clarified early in the appraisal process and how such obligations can be met considered. Further guidance can be found in Scottish Government (2016) appraisal guidance (see Box 7.1).
3. Economic
Examine whether at this stage there is any evidence that the costs will be disproportionate to the benefits?
This may be done using professional judgment or by making a relatively rapid assessment of costs (at the lower end of the whole life cost estimate) and benefits (flood damages avoided to properties).
Whole life cost estimates can include any obvious significant additional costs, for example:
  • Some retrofit storm water actions (e.g. roadside rain gardens) may require costly disturbance or relocation of buried urban infrastructure (e.g. power and telecoms services).
  • Directing surface water to combined or foul sewer systems may increase flooding and pollution risks downstream unless infrastructure is upgraded.
Great care should be taken not to screen out actions that can significantly reduce flood risk for other (non-monetised) receptors, or actions that may bring about wider social, environmental and / or economic benefits.

7.4 Developing options from short list

Having removed any unfeasible actions through the screening exercise, the resulting short list should be used to develop viable options for each objective (some strategic options may apply to multiple objectives). This may involve providing further detail on particular actions (e.g. location, size, construction).

7.4.1 Baseline: the 'do nothing'/‘do minimum’ option

The starting point will be to develop a baseline against which the impacts and costs of other options can be compared. This is either the ‘do nothing’ or ‘do minimum’ option – see Scottish Government (2016) appraisal guidance for further information ( Box 7.2).

For the purpose of surface water management, a ‘do minimum’ option is likely to be the most appropriate baseline. This is because there is a statutory requirement to continue some activities (for example, there will never be a total abandonment of all existing surface water drainage infrastructure). The ‘do minimum’ option is therefore the minimum existing actions required to adhere to statutory duties and responsibilities, for example:

  • Adherence to Scottish Planning Policy.
  • Duties for emergency response planning.
  • Agreements between responsible authorities as a matter of policy.
  • Continuation of asset management.

The ‘do minimum’ option assumes that the baseline model drainage capacity (e.g. 1:5 year drainage capacity if using SEPA pluvial modelling) is maintained (through an effective schedule of clearance and repair or an inspection and maintenance regime of drainage infrastructure by the relevant authorities). Once existing structures reach the end of their design life, it should be assumed that they will be replaced.

Under the ‘do minimum’ option, flood risk is likely to increase over time as a result of climate change, urban creep and potentially new development.

7.4.2 ‘Do something’ options

The ‘do something’ options should be developed using one or more of the shortlisted actions. This may include developing further detail for particular actions (e.g. location, size, construction). Table 7.3 gives some examples of developing options for different objectives.

Opportunities for sustainable and best-practice actions that meet the principles of sustainable surface water management should be sought initially ( Table 1.1). Less sustainable options, e.g. below-ground actions, can then be considered only if the more sustainable options cannot meet the objectives. The process is likely to be iterative, with options being progressively refined.

Where discussions with other stakeholders are either required or desired, they should be planned at the outset and may take the form of:

  • Consultation with land use planning colleagues, to ensure that any preferred structural options integrate with and enhance the urban landscape.
  • Consultation with other departments or authorities likely to be responsible for any options (e.g. land use planners for any changes to land use planning policy; emergency response planners for any action relating to emergency response or roads departments if any changes to roads maintenance regimes are being considered).
  • Close co-ordination and joint working with relevant colleagues and authorities, which will be required if joint projects and opportunities for co-ordination to realise multiple benefits have been identified.
  • Discussions with SEPA and Scottish Water on actions to improve understanding of flood hazard and risk, in order to identify the scale and type of remodelling required, assign responsibilities and co-ordinate with any existing plans.
  • Consultation with communities at relevant points in the appraisal and detailed design stages.

Table 7.3. Example of option development

Objective: Avoid an increase in surface water flood risk in Whole Town
Option Description of actions
Do minimum (baseline)
  • Current situation for comparison, includes maintaining existing actions.
Option 1
  • Land use planning policy (adhere to existing).
  • Clarify responsibilities for new surface water management infrastructure.
Option 2
  • Land use planning policy (adhere to existing).
  • Clarify responsibilities for new surface water management infrastructure.
  • Clarify responsibilities for existing surface water management infrastructure (including SUDS).
Objective: Reduce surface water flood risk in Whole Town
Option Description of actions
Do minimum (baseline)
  • Current situation for comparison, includes maintaining existing actions.
Option 1
  • Review emergency response plans.
Option 2
  • Review emergency response plans.
  • Land use planning policy (implement more stringent policies where required).
  • Review drainage maintenance regimes.
Objective: Reduce surface water flood risk in neighbourhood D
Option Description of actions
Do minimum (baseline)
  • Current situation for comparison, includes maintaining existing actions.
Option 1
  • Property-level protection.
Option 2
  • Above-ground infiltration, conveyance and storage.
Option 3
  • Above-ground infiltration, conveyance and storage.
  • Below-ground storage.

7.5 Describing and valuing options

All the baseline and ‘do something’ options should be subjected to a robust and transparent appraisal of costs, benefits and impacts (both beneficial and adverse). The appraisal does not need to be complex or detailed, but it should provide sound evidence on which to base decisions. This process is likely to be iterative, with options being progressively refined over time.

Non-structural actions in particular are unlikely to need a detailed appraisal of costs and benefits. What they do require is an understanding of both their benefits and disbenefits, compared with those of baseline actions, and the resources needed to implement them. Structural actions, on the other hand, are likely to require a detailed cost-benefit appraisal.

For each option the following attributes should be described and valued:

Where appropriate and possible, flood risk impact and wider impacts should be assessed in quantitative or monetary terms. That is because in this form they can easily be compared with whole life costs to estimate the likely return on investment (by calculating net Present Values and benefit-costs) ( Section 7.5.5).

Some impacts may be difficult and / or may entail disproportionate effort to value in monetary terms. Nonetheless it is crucial that they too are included in the appraisal, so that all sustainable options are considered. This can be achieved by describing them in qualitative terms in the appraisal summary tables (see Section 7.5.3)

The decision on whether to try to quantify or monetise an impact will depend on:

  • Proportionality (relative to the complexity of the problem and the information required to identify a preferred option).
  • Availability and robustness of data.
  • Availability and robustness of the methodology.

Determining what a proportionate approach is can in turn depend greatly on the expertise of the appraisers. It is therefore essential that this information is also recorded.

The appraisal is thus likely to generate both qualitative and quantitative (including monetary) data. Appraisers must determine early on in the process how to deal with this mix of data. They may, for example, draw up a list of criteria against which to assess each option. Techniques such as benefit-cost analysis ( Section 7.5.5) and multi-criteria approaches may help. Scottish Government (2016) guidance and references therein contain further guidance.

The summing of costs and impacts for a particular option will also require a degree of judgment, as the amount of detail available for the individual actions that make up an option may vary. Thus a range of outputs, both detailed and summarised, may be helpful in informing decisions.

Enough information should be gathered at this stage to decide on the most sustainable and preferred option. If an option is obviously the most sustainable and there is sufficient confidence in the data, further appraisal and comparison of options should be unnecessary and the next stage (developing the preferred option in more detail) can begin.

Spatial scale and time period
The spatial extent of the appraisal should take into account the objective’s target areas as well as any other area that the options may affect. It may not be possible to assess all impacts, in which case a reasoned decision should be made on how far to purse the process.

When appraising options, a 100 year appraisal period should be used. If the anticipated lifespan of an option is less than 100 years, the appraisal should assume that capital maintenance occurs to make the lifespan up to 100 years and include this recurring cost in the appraisal (e.g. the cost of replacing electrical and mechanical equipment after 25 years). Consideration should be given to how the benefits, costs and wider impacts of actions might change over this period. This includes considering possible changes in future flood risk as a result of climate change and other drivers.

7.5.1 Impact on flood risk

This section provides guidance on how to estimate the impact on flood risk of the different options. In accordance with the FRM Act, the assessment should include the impact of flooding on the economy, society, cultural heritage and environment.

The assessment should show the flood risk management benefits of each option, along with any adverse impacts if flood risk is increased elsewhere. The assessed flood risk for each option can be compared against a baseline (e.g. to show the reduction in flood damages and number of properties at risk). Alternatively, as is often the case, the benefits for each option can be shown (e.g. showing the flood damages avoided and number of properties protected).

Approaches to assessing flood risk
Four approaches to assessing the impacts on flood risk are proposed below. The more sophisticated the approach the greater the certainty in the assessment but also the more detail required. The choice of approach will be influenced by a number of factors:

  • The level and complexity of flood risk
  • The availability of, and confidence in, hazard and risk modelling and mapping
  • Spatial scale
  • The type of actions being appraised.

Table 7.4 Approaches for assessing flood risk impacts

Approach Description
1. Simple Simple and pragmatic assessment of the number of properties or other receptors removed from flood risk. Economic benefits can be assigned using the Scottish Pluvial Annual Average Damage Estimates ( SPAADEs; see Box 7.3) per property. May be appropriate for relatively low risk and localised / simple flooding, e.g. raising road kerb heights to divert water away from homes to a safe route, or trash screen replacement. Based on professional judgment. Relatively quick and low cost.
2. Map-based Makes use of existing SEPA flood risk data to show receptors removed from flood risk and economic benefits of different options. Does not require additional modelling or mapping. High level assessment of the impacts of an option on flood risk.
3. Simplified modelling Involves re-running SEPA regional pluvial models for each option to show receptors removed from flood risk and economic benefits. The impacts of options on flood risk can be modelled and mapped by adjusting model parameters (e.g. varying assumptions about drainage capacity, run-off coefficients or digital terrain model ( DTM)).
4. Detailed modelling More detailed modelling for each option, e.g. more detailed pluvial modelling or integrated urban drainage modelling and the explicit modelling of actions and their effects. Useful for high-risk areas with complex flooding problems that may need larger-scale, more strategic solutions.

Approach 1: Simple
This approach will be suitable for many simple situations, where professional judgment can be used to link a straightforward option with a certain reduction in flood risk: for example, replacing a screen that is less susceptible to blockage to ensure the proper functioning of a culvert, thereby removing the risk of flooding from four properties. The flood risk impacts can be quickly and simply expressed by the number of properties (or other receptors) removed from flood risk. Assessment of economic impacts may be limited to the Scottish Pluvial Annual Average Damage Estimates per property (see Box 7.3).

Approach 2: Map-based
This approach makes use of existing SEPA flood risk data to show receptors removed from flood risk and economic benefits. It does not require additional modelling or mapping. A worked example is given in Appendix 5. It uses professional judgment to estimate the effectiveness of an option and links it to a standard of protection. For example, professional judgment may estimate that changes to maintenance regimes will improve drainage to achieve no flooding in the 1:30 year event.

It is suitable for many situations where SEPA’s regional pluvial mapping shows good validation with observed flood events. It may also be suitable for more detailed local authority flood hazard and risk data where this is available (Appendix 3 has information on further modelling).

Approach 3: Simplified modelling (using SEPA regional pluvial models)
This approach requires re-running SEPA’s regional pluvial models for each option to determine the impact on flood risk. It is suitable where SEPA’s regional pluvial mapping shows good validation with observed flood events and has been used to provide the baseline. It may also be suitable for more detailed local authority flood hazard and risk data where this is available ( Appendix 3 has information on further modelling).

The effectiveness of certain options can be re-modelled by adjusting parameters in the pluvial models. For example, drainage capacity can be increased locally to simulate the effect of an improved drainage system providing a 1:10 year or 1:30 year standard of protection; run-off coefficients can be varied locally to account for the use of permeable surfaces or green space in urban neighbourhoods; and local topographical changes (such as high kerbs or bunds) can be represented by altering the DTM.

Based on the new flood hazard for each option, the adverse impacts of flooding (flood risk) will need to be re-assessed. Flood risk under the FRM Act includes impacts on the economy, society, cultural heritage and the environment. If using the SEPA regional pluvial as the baseline option, flood risk will need to re-assessed using SEPA’s method. [13] This will show the impact of each option on flood risk and allow it to be compared with the baseline.

Approach 4: Detailed modelling
This allows for the most comprehensive representation of options and calls for an experienced modeller and engineer working together. It can be applied where more detailed models (e.g. more detailed pluvial models or integrated urban drainage model) are required to replicate known flood hazard and has been used to assess the adverse impacts of flooding (flood risk), achieving good agreement between observed and modelled floods. These models may be developed earlier in the process to provide a more detailed picture of flood hazard and flood risk, and can be used as the baseline in the appraisal.

The effectiveness of each option can be re-modelled showing new flood hazard (e.g. extents and depths) with the option in place. Based on the new flood hazard for each option, the adverse impacts of flooding (flood risk) which include impacts on the economy, society, cultural heritage and the environment), should be assessed. The method used to assess the adverse impacts of flooding should be the same as that used to assess the baseline impacts. This will allow the impact on flood risk of each option to be compared with the baseline.

A detailed model can be used to optimise the effectiveness of options, such as the capacity of individual sewers and storage areas, the management of flow moving across the surface or the effect of source control at property level.

Assessing flood risk
To be able to compare the flood risk management benefits of each option, adverse impacts of flooding (flood risk) on the economy, society, cultural heritage and environment should be determined for the baseline and the ‘do something’ options, using the same assessment method. A range of flood risk indicators should be used to do this, including economic impacts described in monetary terms and other impacts described in monetary and non-monetary terms.

Scottish Government’s 2016 guidance on appraisal gives further information on assessing flood risk. The Flood Hazard Research Centre Flood and Coastal Erosion Risk Management: Handbook and Data for Economic Appraisal also provides detailed guidance on monetising the impacts of flooding (see Box 7.3).

Where SEPA pluvial flooding data is used (approaches 2 and 3), it is anticipated that the range of indicators applied will reflect those used in SEPA’s pluvial risk data ( Table 7.5). If the SEPA data is being used as the baseline, then any reassessment of flood risk for each option should be done using SEPA’s method. If a local authority has more detailed pluvial modelling and is assessing flood risk to establish a baseline, then the same method should be applied to assessing the flood risk for each ‘do something’ option to allow baseline comparisons.

Economic impacts ( Box 7.3) will be represented as an estimate of the economic cost of the damages caused by flooding. This is then used to compare against the whole life costs of the option in order to assess the balance of benefits and costs and compare value for money ( Section 7.5.5).

Box 7.3. Assessing economic flood damages

The Flood Hazard Research Centre, Flood and Coastal Erosion Risk Management: Handbook and Data for Economic Appraisal (Multi-Coloured Handbook and Data) provides guidance and data for assessing the economic damages of flooding. In general it presents three approaches, each with different levels of detail for assessing economic damages ( NB: the approach used may differ for the different receptors being assessed):

  • Overview appraisals – less complex (for example, the use of weighted annual average damages ( WAAD)) and used where less detail is required and / or there is little or no information on the depths of flooding. WAAD can be applied to residential and nonresidential properties.
  • Initial appraisals – require more information, e.g. the depth of flooding for each property for different return periods. Can be used to assess damages in each return period for each property. Annual Average Damages ( AAD) can then be assessed for each property based on all return periods over long periods of time. They are based on the relationship between flood damage and the probability of incurring that damage in any one year. Economic damages for properties can then be summed at different spatial scales.
  • Full-scale appraisals – require more detailed information on the receptors being flooded, e.g. type of house and social demographics of the occupants. Damages for each property in each return period are assessed and AAD for each property then calculated.

Scottish Pluvial Annual Average Damage Estimate ( SPAADEs)
Where there is no reliable estimate of the depth of flooding, SEPA’s Scottish Pluvial Annual Average Damage Estimates ( SPAADEs) should be used in place of the WAAD. SPAADEs provide an estimate of the average pluvial flood damages to an individual property per year, based on SEPA’s 2013 regional pluvial data. Economic damages for properties can then be summed at different spatial scales.

For a residential property, the SPAADE is £1,100; for a non-residential property, the SPAADE is £1,700. (These estimates are for 2010 and so an uplift will be required to convert them to present values.) SPAADE estimates are lower than the corresponding values of WAAD in the Multi-Coloured Handbook, as the latter tend to be used to estimate fluvial flood damages. This reflects the differing characteristics of pluvial flooding, which is often shallower and more localised than fluvial flooding.

SPAADEs are not based on observed pluvial flood damages; instead they are derived from strategic national modelling. See SEPA (in preparation) Flood Risk Management Appraisal Methodology for more information.

Table 7.5. Flood risk indicators assessed in SEPA’s 2013 pluvial risk data

Category Receptors Flood risk indicators Applied to:
Economic Non-residential properties Count of non-residential properties flooded National and regional models
Non-residential properties Direct economic damages (£s) (including SPAADE for national and AAD for regional models) National and regional models
Residential properties Direct and indirect economic damages (£s) (including SPAADE for national and AAD for regional models)
Roads Direct damages to road infrastructure (£s) and AAD Regional models only
Vehicles Direct damages to vehicles (£s) and AAD
Emergency services Additional assessment of indirect damages (£s) and AAD
Social Residential properties Count of residential properties flooded National and regional models
Residential properties Social flood vulnerability score for each residential property National and regional models
Community facilities Count and type of community facilities National and regional models
Cultural heritage Cultural heritage Area and type of cultural heritage sites affected National and regional models
Cultural heritage Count and type of listed buildings flooded National and regional models
Infrastructure Utilities Count and type of utilities National and regional models
Transport Length of roads flooded National and regional models
Transport Length of rail flooded National and regional models
Transport Count of airports flooded National and regional models
Environment SEPA’s pluvial risk data does not include an assessment of the adverse impact of pluvial flooding on environmental designated sites, because the risk from pluvial flooding was deemed low. A more detailed assessment of impacts on the environment was not feasible at a national strategic scale.

7.5.2 Adaptability to future flood risk

Taking future change in flood risk into account when considering surface water management options is essential if sustainable actions are to be selected that will stand the test of time. Drivers of future change include climate change, urban creep and demographic change. Every option should always be assessed on its adaptability to climate change, and to the other factors where required. Appendix 6 contains further information on adapting to future flood risk.

Climate change
Climate change in particular poses serious challenges and risks for managing flooding in Scotland. Its impacts include the potential rise in intensity and frequency of rainfall events increasing the risk of surface water flooding.

This section provides guidance on assessing the impact of options on climate change adaptability. Two adaptation approaches are described in the Defra 2009 policy statement. [14] Both approaches could also be applied to urban creep and population growth. For each option, an indication of which approach is likely to be implemented should be given:

  • Managed adaptive – this approach allows for adaptation in the future by planning multiple ‘phased’ interventions over time. The first phases can initially use lower allowances for climate change over the shorter term, with further interventions implemented if and when required. Change in risk is monitored over the lifetime of the actions and any change managed through multiple ‘phased’ interventions (often interventions are implemented after a trigger point, indicating that the risk will become unacceptable, is reached). This approach is flexible enough to manage future uncertainties associated with climate change during the whole life of a flood risk management system. Other benefits of managed adaptive approaches are:
    • They can be less costly – as future adaptation phases have been planned from the start and can be implemented if and when required, the need to introduce new or significantly change existing actions to manage future changes may be avoided.
    • They use a risk-based decision framework – enabling risk to be monitored and managed at periodic intervals during the design life of a development.
    • They are usually more sustainable over the long term – presenting opportunities for enhancing the environmental, societal benefits and cost savings that cannot be achieved through precautionary approaches.
    • They can take advantage of innovative advances over time and are sufficiently flexible to cope with changing climate change projections that may differ from those available to us today.
  • Precautionary – in some circumstances, future adaptation may be technically unfeasible or too complex to administer over the long term. Hence this approach, resulting primarily in one-off interventions with a higher allowance in the design for climate change over the longer term, may be the only feasible option (such as in the design capacity of a major culvert or underground storage).

For each option a short description of the level of intervention, the costs and feasibility associated with ensuring that the action can respond to changing conditions should be provided. The assessment may include:

  • A description of likely impacts of climate change and other drivers.
  • A description of adaptability to climate change – e.g. whether a managed adaptive or precautionary approach is being taken and how it will be implemented.
  • A description of adaptability to other future flood risks – urban creep and demographic change (managed adaptive or precautionary approaches may also be used for these factors).
  • Information on the level of intervention, costs and feasibility associated with ensuring that the option can respond to changing flood conditions.

It may be helpful to present this information as a class or score in an appraisal summary table.

Land use - urban creep
Urban creep can significantly compound surface water flooding; therefore options (and costs) should be developed that allow for its occurrence (see Appendix 6). The allowance to include may vary (e.g. on the type or location of option, or whether a managed adaptive or precautionary approach is being taken). Note that both managed adaptive and precautionary approaches can be applied to urban creep.

Some actions may mitigate the impact of urban creep, e.g. de-paving strategies that make urban areas more permeable and green over time. Other factors, such as planning policy, permitted development and local housing stock, will influence the rate of urban creep and its subsequent impacts on flood risk, all of which can be taken into account.

It is important that land use planning policies are adhered to, to ensure that new development is not at risk of surface water flooding and does not increase the risk elsewhere. If deemed necessary, projections for news homes required could be taken into account to help inform appropriate options to mitigate or adapt to any impact (further information can be found in Section 5.4). Information, including projections for new homes, is available from local authority land use planners and National Records of Scotland.

7.5.3 Assessing wider beneficial and adverse impacts

The wider impacts of the options on the economy, society and environment – such as improvements to water quality or the urban landscape – that are not related to changes in flood risk, should be assessed. Understanding these wider impacts is an important component of managing flood risk in the most sustainable way. It is therefore essential that they are identified and assessed alongside flood risk reduction benefits (see Appendix 7 for further information).

The assessment should focus on impacts that are likely to be significant and have the potential to affect decisions. The following questions may help to determine which to assess and how to establish their significance:

  • What is the economic, social and environmental baseline against which wider impacts will be assessed? Information being developed for the Flood Risk Management Strategies and Local Flood Risk Management Plans will help with this, as will other sources of information such as River Basin Management Plans and Scotland’s Environment Web (
  • What is the magnitude and direction (large or small, beneficial or adverse impact) of change?
  • Will the option help to mitigate potential future economic, social and environmental pressures (e.g. climate, land use (urban creep) or demographic change)?
  • How important is the receptor that is affected: is it locally, regionally, nationally or internationally important?
  • Do impacts occur along the flow pathway?
  • What is the predicted duration of the impact?
  • What steps can be taken to mitigate any adverse impacts?
  • How important are the impacts likely to be to local stakeholders and communities?
  • Are the impacts important enough to affect the final decision?

Deciding which impacts to assess should be proportionate and based on risk. For example, simple, small-scale options are unlikely to require extensive assessment, whereas large and complex ones may necessitate more detailed consideration. Table 7.6 gives examples of the types of impact most likely to arise; further guidance can be found in Appendix 7. The guidance does not provide an exhaustive list and any other significant impacts must be identified and described.

The wider impacts will usually be described in non-monetised terms, such as short descriptive statements, rather than being quantified in detail. That is because they may be unsuited to or difficult to define in monetary terms. Classifying impacts on a five-point scale (e.g. from ‘Significant Negative’ to ‘Significant Positive’) may be a useful way of summarising the information. That said appraisers may choose to monetise some impacts if it is considered important and approaches for doing so are available.

Some adverse impacts may be avoided or minimised through appropriate mitigation measures. Any such mitigation should be specified, costed and included as an integral component of the option.

Table 7.6 Likely significant wider beneficial and adverse impacts of surface water management actions

Receptor Assessment may include:
Human health and wellbeing
  • Description of significant impacts on making better places for people to live (e.g. enhancing the urban landscape, active travel, recreation, outdoor access, wildlife watching opportunities).
  • Description of significant impacts on opportunities to promote healthy lifestyles. This should consider both the physical and mental health benefits of access to green space and promoting active travel.
Local economy
  • Description of the potential for providing wider economic benefits, for example:
  • o Boosting local economic opportunities (e.g. attracting businesses and investment) by enhancing the urban landscape and providing better places to live and work.
  • o Enabling redevelopment by freeing up capacity in waste water systems.
  • o Reducing water purifying and treatment costs.
Water quality
  • Description of significant impacts on the water environment (in particular, water quality and physical habitat).
  • Identification of opportunities to help meet River Basin Management Planning objectives: e.g. restoring habitat / straightened channels; opening up culverts; preventing deterioration of the water environment (including bathing waters) by avoiding a rise in the frequency of CSO spills.
Biodiversity, habitats and species
  • Description of any significant impacts on habitats and species, such as degradation of habitats or improvements to habitat connectivity.
  • Identification of opportunities to help achieve objectives of Local Biodiversity Action Plans.
  • Description of significant impacts on ecosystem health.
Climate change mitigation
  • Description of whether the option gives rise to greenhouse gas emissions during construction and maintenance / repair stages (e.g. through the use of building materials, construction traffic or change in land use).
  • Description of any beneficial, longer term reduction in greenhouse gas emissions (e.g. through reducing water treatment or pumping).
Any other relevant impacts
  • Assessment of any other significant relevant impacts, e.g. on air quality or on the historic environment.

7.5.4 Estimating the whole life cost

Whole life costs are the total costs of an option over its whole life. They take account of design costs, initial capital costs (including mitigation), operation, maintenance and repair, and, where significant, disposal costs. They do not include costs already incurred, such as investment in preceding studies or defences; these are defined as ‘sunk’ costs and cannot be recovered whatever decision is subsequently taken.

Present Value
The whole life cost will be expressed in Present Value ( PV) terms. It will be assessed over a 100-year time period (with reinvestment in actions taken into account if their anticipated lifespan is less than 100 years- see Section 7.5). The discount rate used to determine Present Values will be assigned according to the ‘social time preference’ discount rate recommended in HM Treasury Green Book (see Box 7.2). Under this system a discount rate of 3.5% is applied to years 1 to 30, of 3% to years 31 to 75 and of 2.5% to years 76 to 100.

Optimism bias
When estimating costs, contingencies should be built in to account for the likelihood of costs being under or over estimated. An optimism bias of 60% is typically used for projects (including strategies) at an early stage of consideration. At the more detailed project stage, a figure of 30% is commonly used. The adopted optimism bias should ultimately reflect the uncertainty of construction costs for a particular element, and may therefore vary depending on the proposed approach.

The HM Treasury Green Book recommends that final whole life costs be subject to sensitivity testing for key variables such as levels of capital costs, duration of works and levels of operating costs.

Estimating costs
The approach to estimating whole life costs should be proportionate and informed by risk: in some cases professional judgment may be sufficient, whereas in others a more detailed estimate may be required ( Table 7.7). Appraisers should select the most appropriate source of information, including those listed below, and consider costs from previous studies and works (e.g. estimates from local authority departments). The source of cost estimates should be recorded.

Costs for run-off reduction strategies should be calculated on the basis of the unit cost for impermeable surfaces or disconnections.

Table 7.7 Potential sources of information on whole life costs

Type of estimate Sources
Professional judgment
  • Local authority or Scottish Water officers
  • Consultants
Strategic estimates
Previous studies and works
  • Studies and works commissioned by local authority or Scottish Water officers
  • Studies and works completed by consultants
Detailed estimates
  • AECOM (Eds.) (2015) SPON’s Civil Engineering and Highway Works Price Book 2016. CRC Press
  • CIRIA (2015/2016) Benefits of SuDS Tool. Links to tools and supporting resources:

7.5.5 Valuing the economic benefits and costs

Where the impacts of an option have been assessed in monetary form, the costs of implementing the option can be assessed against the costs of the economic flood damages avoided for each. Net Present Value and Benefit-Cost Ratios can be used to assess the balance of benefits and costs over a longer period and compare value for money of different options.

Net Present Value ( NPV) works out the net benefits of an option in order to demonstrate the magnitude of the economic benefits and whether they outweigh the costs. It is the Present Value of the benefits minus the Present Value of the costs ( Section 7.5.4).

A Benefit-Cost Ratio ( BCR) examines the relative return on investment for every pound spent. It is the Present Value of the benefits divided by the Present Value costs ( Section 7.5.4). Assuming all (significant) benefits and costs have been valued in monetary terms, an option with a benefit-cost ratio greater than one represents value for money. Appraisers should always consider significant non-monetised impacts too, as these are important for identifying sustainable solutions. The benefit-cost ratio should therefore not be the sole criterion for decisions.

For further guidance see Scottish Government (2016) appraisal guidance and HM Treasury (2003) The Green Book ( Box 7.2).

7.6 Comparing options and choosing the preferred one

Flood risk management decisions should be underpinned by a thorough appraisal of economic, social and environmental impacts, whole life costs, risk and uncertainty. It is by balancing all these factors that the most sustainable solution can be found. Decisions should therefore be based on robust information and presented clearly and transparently so that they can be easily understood by those affected.

A range of outputs, both detailed and summarised, is likely to help in making and communicating decisions. A well-designed appraisal summary table with supporting information will assist with this. The Environment Agency appraisal summary note provides an example of a summary table, at:

When deciding on which option(s) to implement, several questions should be borne in mind:

  • Will the option meet the objectives?
  • Does the option represent best value for money?
  • Will the option yield multiple benefits and what are the adverse impacts?
  • What are the uncertainties and robustness in the appraisal and what are the risks in implementation?

It is important that all impacts (both beneficial and adverse) of the options are taken into account when making decisions. It is therefore necessary to weigh up those impacts that have not been valued in monetary terms and assess whether they are significant enough to change the option that would be preferred on the basis of economic criteria alone.

There are many ways of making these decisions. See Scottish Government’s 2016 appraisal guidance for further information.

7.7 Degree of confidence in the appraisal

The degree of confidence in the appraisal should be recorded as it may influence the outcome of the next stage, i.e. confirming funding and developing the preferred option in more detail.

Innovative solutions should not be compromised in favour of more traditional solutions, just because there might be less confidence in the results of the appraisal. If more innovative options have potentially greater benefits, further trials and evidence-gathering on their costs and benefits should be considered. Most actions (with the exception of the simplest ones) will require detailed design and assessment prior to the implementation stage.

Any uncertainties raised during the appraisal can be examined using sensitivity analysis to test the implications of alternative assumptions. For example, by exploring a range of costs and benefits a sensitivity analysis can help to determine whether the preferred solution still measures up if the costs and benefits are different from those originally estimated. This technique avoids the need to collect additional data, thereby helping to keep the effort and cost of carrying out the appraisal proportionate.


Gordon Robertson:

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