Desktop FRA vs Detailed FRA: When Do You Need Modelling?
Understanding the difference between a desktop flood risk assessment and a detailed FRA with hydraulic modelling — and when each is appropriate.
Not all flood risk assessments are created equal. The scope of an FRA can range from a straightforward desktop review that takes a few days to a full hydraulic modelling study that takes several months. Choosing the right level of assessment at the outset is critical — commission too little and the Environment Agency will object; commission too much and you have spent money unnecessarily.
Understanding the distinction between a desktop FRA and a detailed FRA with hydraulic modelling will help you brief your consultant correctly, manage your programme, and control costs.
What Is a Desktop FRA?
A desktop flood risk assessment is a desk-based review of available flood risk information for a given site. It does not involve original modelling or fieldwork. Instead, it draws on existing datasets and published information to characterise the flood risk and determine whether the development can proceed safely.
A typical desktop FRA will include:
- Flood zone confirmation: Review of the EA’s Flood Map for Planning to establish the site’s flood zone classification.
- Surface water flood risk review: Analysis of the Risk of Flooding from Surface Water (RoFSW) mapping to identify pluvial risk.
- Historical flood records: Review of EA recorded flood outlines and any available historical flood data from the LPA or lead local flood authority (LLFA).
- Groundwater and sewer flood risk: Review of available BGS groundwater data and water company records.
- Climate change assessment: Application of current EA climate change allowances to assess future flood risk.
- Sequential and Exception Test considerations: Where applicable, commentary on the site’s position within the LPA’s sequential testing framework.
- Mitigation recommendations: Proposed finished floor levels, flood resilience measures, and access/egress arrangements.
A desktop FRA is appropriate where the flood risk is relatively straightforward and the available data is sufficient to characterise it. This is often the case for sites in Flood Zone 2, small sites on the edge of Flood Zone 3 where existing EA modelling is available, or larger sites in Flood Zone 1 where the primary concern is surface water management.
What Is a Detailed FRA?
A detailed FRA includes all of the elements of a desktop assessment but goes further by commissioning original hydraulic modelling to quantify the flood risk with greater precision. This typically involves building a computer model of the river, coastline, or drainage catchment that affects the site, running simulations for a range of storm events and climate change scenarios, and producing flood extent, depth, velocity, and hazard maps.
A detailed FRA may include:
- 1D hydraulic modelling: Modelling the river channel and floodplain using cross-section data, typically with software such as Flood Modeller Pro or HEC-RAS.
- 2D hydraulic modelling: Modelling surface water flow paths and floodplain storage using a 2D grid, typically with TUFLOW or similar software.
- 1D-2D linked modelling: Combining 1D channel modelling with 2D floodplain modelling for complex situations where both river and overland flow need to be represented.
- Breach modelling: Simulating the failure of flood defences to assess residual risk behind defences.
- Direct rainfall modelling: Applying rainfall directly to a 2D terrain model to assess surface water flood risk, particularly useful for sites where the RoFSW mapping is inadequate.
- Sensitivity testing: Running additional scenarios to test the sensitivity of the results to key assumptions such as roughness, inflow hydrographs, and blockage factors.
When Is Modelling Required?
The decision to commission hydraulic modelling is driven by several factors. In some cases the EA or LLFA will explicitly require it; in others it is a professional judgement call based on the complexity of the flood risk and the nature of the proposed development.
The EA or LLFA Requires It
The Environment Agency will typically require bespoke modelling in the following circumstances:
- No existing EA modelling covers the site: If the site is in Flood Zone 3 but there is no existing detailed model from which flood levels can be extracted, the EA will expect the applicant to commission one.
- Existing modelling is outdated: EA models older than 10 years, or models that pre-date significant changes to the catchment (such as upstream development or new defences), may not be considered reliable.
- The development could affect flood risk elsewhere: Large sites or sites adjacent to watercourses may need modelling to demonstrate that the development will not increase flood risk to third parties.
- Breach or overtopping assessment is needed: For sites behind flood defences, the EA will often require breach modelling to assess the residual risk in the event of defence failure.
Professional Judgement
Even where the EA does not explicitly require modelling, there are situations where a desktop FRA alone is unlikely to be sufficient:
- Complex surface water flood risk: Where the RoFSW mapping shows significant flow paths through or across the site, a direct rainfall model may be needed to understand the risk in sufficient detail to inform the design.
- Multiple flood sources interacting: Sites affected by a combination of fluvial, tidal, surface water, and/or groundwater flooding may require modelling to understand how these sources interact.
- Challenging topography: Steeply sloping sites, sites in valleys, or sites with complex built-up surroundings may require modelling to accurately map flood extents and flow paths.
- Large-scale developments: Major residential or commercial schemes often benefit from bespoke modelling to optimise the layout and reduce flood risk to future occupants.
Cost and Time Comparison
The cost and timescale difference between a desktop FRA and a detailed FRA is significant, and understanding this upfront helps with programme planning.
| Desktop FRA | Detailed FRA | |
|---|---|---|
| Typical cost | £750 - £3,000 | £5,000 - £15,000+ |
| Typical timescale | 1 - 3 weeks | 6 - 16 weeks |
| Data requirements | Publicly available data | LiDAR, survey, EA data request |
| EA pre-application | Usually not needed | Often recommended |
| EA review time | 2 - 4 weeks | 4 - 8 weeks |
These figures are indicative and will vary depending on site-specific circumstances. A complex coastal site with multiple defended frontages will cost more to model than a straightforward fluvial site. Similarly, a site where high-quality LiDAR and survey data is already available will be quicker and cheaper to model than one requiring new topographic surveys.
Common Mistakes
1. Commissioning a Desktop FRA When Modelling Is Needed
This is the most expensive mistake. If you submit a desktop FRA and the EA responds by requesting bespoke modelling, you will need to commission the modelling study, wait for it to be completed, submit the revised FRA, and then wait for the EA to review it again. This can add three to six months to your planning programme.
The best way to avoid this is to engage with the EA’s pre-application advice service before commissioning the FRA. The EA will confirm what level of assessment they expect, what modelling approach they consider appropriate, and what data they can make available. This conversation costs nothing and can save you months.
2. Over-Specifying the Modelling Scope
Conversely, some consultants default to detailed modelling when a desktop assessment would suffice. If your site is in Flood Zone 2 with published EA flood levels available, and the proposed development is a minor residential scheme, a desktop FRA may be perfectly adequate. Spending money on bespoke modelling that the EA has not requested and does not need is a waste of your client’s budget.
3. Not Allowing Enough Time
A detailed FRA with hydraulic modelling cannot be completed in a week. The data acquisition, model build, calibration, scenario runs, and reporting take time. Factor in EA data requests (which can take four to six weeks) and EA review (another four to eight weeks) and the total programme from instruction to EA sign-off can easily be four to five months.
If modelling is required, instruct it as early as possible — ideally at the pre-application stage, not after submission.
Making the Right Choice
The decision between a desktop FRA and a detailed FRA should be made early, based on a clear understanding of the site’s flood risk, the EA’s expectations, and the nature of the proposed development. A good starting point is to commission a preliminary site screening that reviews the available data and recommends the appropriate level of assessment.
At Aegaea, we provide honest advice on what level of FRA your site genuinely needs. We will not recommend modelling unless it is necessary, and we will not submit a desktop assessment if we believe the EA will require more detail. Get in touch for a no-obligation discussion about your site.