Guidance

The surface water investment model (SWIM) approach

Published 18 May 2026

Applies to England

Who this is for

You should use this approach if you:

• work for risk management authorities (RMAs) responsible for managing surface water flood risk
• are Environment Agency staff involved in reviewing applications for Flood and Coastal Erosion Risk Management (FCERM) grant in aid (GiA) funding to manage surface water flood risk
• are a project manager, project executive, project board, senior user, project sponsor or work alongside others involved in the governance of FCERM investment projects

You can email fcrm_investment@environment-agency.gov.uk for further information and support. You can also contact your local Environment Agency team.

What the surface water investment model (SWIM) is and when you should use it

Small scale resilience projects aiming to reduce surface water flood risk often struggle to demonstrate viability for FCERM GiA. This is because it can be difficult to quantify the benefits of small interventions like sustainable drainage systems (SuDS).

To be considered for funding, new project opportunities must initially follow the Flood and coastal erosion risk management funding policy guidance. The SWIM approach and its associated SuDS Programme Business Case (SPBC) should only be used when your project has been prioritised by the Environment Agency for development.

The SWIM approach accounts for the total benefits achieved by a package of small-scale SuDS measures. It does this by appraising the package of SuDS measures as a single intervention.

This approach:

• has the benefit of front-loading the approval of funding over a longer period and across multiple financial years
• enables greater flexibility for RMAs and means they can remain agile to opportunities that arise in-year - for example, you may be able to align your project timelines with other public realm works, which can lead to significant efficiencies

Some blue-green SuDS measures may also be eligible for funding through a standalone natural flood management (NFM) project. These projects follow a separate approval route, with their own single-stage business case template, benefits estimation methodology, and assessment tools. To find out if your project is eligible, please refer to the eligibility guidance for standalone NFM. More information on submitting a standalone NFM project opportunity can be found in the flood and coastal erosion risk management funding policy guidance.

SuDS programme business case

You should use the SPBC template to help you make the case for the effective and efficient delivery of a package of SuDS measures. Your programme should be large enough to realise a meaningful reduction in surface water flood risk. As a guideline, you should consider a programme of around 2-5 years.

1. Components of the SuDS programme business case

The SPBC contains several main components. The next sections explain these.

1.1 Identifying benefit areas

A benefit area is the place(s) that will be at reduced risk after you complete your package of measures. See the Defining which areas and properties benefit from FCERM investment guidance for more information on identifying your benefit area. Where relevant you should work in partnership across local authority administrative boundaries. This could include the development of flood risk mapping or modelling that covers multiple RMA areas. This will help to identify the optimal delivery approach.

1.2 Options appraisal

You should:

• document your thought process and options appraisal
• show how you have considered and deselected other options
• do this proportionately accounting for the likely size of investment

There may also be spatial limitations that mean other options are not reasonably deliverable. You must be able to explain how you concluded that SuDS would be the best option.

The options in your long-list should be broad enough to demonstrate that:

• you have explored a reasonable range of alternative potential measures to reduce flood risk in your benefit area
• your assessment has accounted for the flood risk impacts on the areas surrounding your benefit area

You will need to describe the outcomes arising from your project in your business case. These must align with the requirements of the Flood and coastal erosion risk management funding policy guidance.

1.3 Economic appraisal

You should consider the appropriate appraisal period and duration of benefits for your project. The nature of SuDS means that some elements of their design will have a shorter design life than others. Make sure you:

• consider the appropriate design life for your proposed SuDS features
• factor in any maintenance costs

It’s important that you take a proportionate approach in how you make the case for investment to avoid costs and delays. For smaller projects, you should use existing data like the national flood risk assessment (NaFRA) to determine where there is risk. Where appropriate, you should not produce new or additional mapping or modelling.

Baseline

Describe any baseline assumptions you have made in the calculation of your flood damages.

For example, you should assume that:

• there is a functioning highway drainage system operating to a 1 in 5 standard of protection, as per National Highways CG 501
• the sewerage undertaker is maintaining their surface water drainage to the appropriate industry standard

If you have used alternative assumptions or standards, you need to explain these in your SPBC. You should consider using sources like the risk of flooding from surface water (RoFSW) to appraise your baseline.

Benefits

You should use the Environment Agency’s RATE for your appraisal of flood benefits.

The CiRIA B£ST can help you estimate the wider benefits of your SuDS features. These can include:

• recreation
• education
• environmental improvements

However, your economic appraisal must follow the HM Treasury Green Book.  This means you must consider additionality—only counting benefits that would not happen without your project. You also need to account for displacement, where benefits are simply moved from one place to another rather than created. As a result, you should report the net benefit, not the total (gross) benefit. If most of your project’s benefits come from things other than reducing flood risk, then FCERM GiA is unlikely to be the main funding source. In that case, you should look for alternative funding.

Costs

To inform your cost estimates you can use:

• a unit-cost database
• quotes and estimates from a supplier
• examples of previous works

Actual costs may vary due to things like ground conditions or unexpected site limitations. Therefore, it is acceptable to quote your costs within a range. However, the higher end of the range must still deliver value for money. Provide as much certainty as you can in your business case; a smaller range is preferable to a larger one.

If you intend that a good proportion of your project will be aligned with other works to share costs, you can factor this into your cost estimates. You must ensure you apply an appropriate risk contingency and optimism bias to your costs. This will help to avoid foreseeable cost increases or unmitigated cost uncertainty.

See section 9 of the FCERM appraisal guidance for more information on costs.

Apportionment

You can achieve some or all of your SuDS as part of a larger, integrated project alongside other works. This can:

• improve efficiency
• increase overall benefits
• lead to better-quality outcomes

However, you must clearly separate (apportion) the costs and benefits of your SuDS elements from the rest of the project. You must only use FCERM GiA to fund activities permitted by the:

• flood and coastal erosion risk management funding policy guidance
• memorandum relating to capital grants for other risk management authorities in England (2020)

See section 9.6. of the Calculate GiA funding for FCERM projects for more information on apportionment.

1.4 Feature types

To realise the full complement of wider benefits achievable through a SuDS delivery, you should align with the 4 pillars of SuDS and the SuDS principles on the susdrain website. You should consider the following as examples of suitable feature types for your SPBC:

• SuDS trees, either
  • New tree pits
  • SuDS tree replacing existing trees
• surface-level bioretention, swales and attenuation ponds
• blue-green roofs

You must demonstrate the achievability and efficacy of any features you intend to provide for the FCERM investment programmes. For example, you are unlikely to receive FCERM GiA to deliver measures like:

• increasing drainage/sewer pipe size
• subterranean attenuation tanks

1.5 Design standards

To ensure consistency and appropriate use of measures, describe how you have designed your SuDS to an appropriate standard. This could include:

• designs used in other successful projects
• best practice like industry standard design approaches

Existing resources include:

• National Standards for SuDS
• CIRIA SuDS manual
• Examples from successfully delivered SuDS projects

1.6 Funding profile

Your funding profile should align with your planned project timetable. Overspending risks the FCERM GiA not being available when you need it. Meanwhile underspending risks undermining your programme. You may not be able to ‘roll over’ funding into the next financial year. Ultimately, your project may be de-prioritised if there is low confidence in delivering your projected outcomes.