Flood modelling where no published model exists

Where a site sits on an ordinary watercourse rather than a main river, there is often no detailed hydraulic model at all, and the hydraulic modelling cost flood risk question becomes a real budget line rather than a mapping exercise. The Environment Agency's Flood Map for Planning covers main rivers and the sea in detail. It does not model every brook, ditch and culverted stream in England, and on those catchments the published mapping is either absent or derived from a national-scale screening model that is too coarse to fix finished floor levels.

The practical consequence for a small developer is a choice between commissioning bespoke modelling, which runs into five figures on a complex site, or finding a route that avoids it.

When published mapping is not enough

The published flood zones are a planning tool, not a design tool. They are good for the sequential test and for telling you whether you are in Zone 2 or Zone 3. They are not good for setting a floor level or sizing a flood storage compensation volume.

The situations where bespoke modelling becomes hard to avoid:

• The site is on an ordinary watercourse with no detailed model behind the mapping.
• The mapping shows the site on a flood zone boundary, so the answer depends on a line drawn at a coarse resolution.
• The scheme needs finished floor levels set to a design flood level plus freeboard, and no modelled level exists.
• Floodplain compensation is required, which needs a level-for-level volume calculation.
• A culvert, bridge or weir on or near the site controls water levels and is not represented in the national model.
• The proposal changes the channel, the floodplain or the conveyance route.
• Surface water flood risk from the national mapping shows a flow route through the site that needs resolving.

On the other side, a site in Flood Zone 1 with no surface water risk, no ordinary watercourse within a few hundred metres and no history of flooding does not need a model. It needs a proportionate Flood Risk Assessment that says so and evidences it.

Requesting modelling data and what arrives

Start with a product request to the Environment Agency rather than with a modelling quote. The Agency supplies flood risk data for a site, and for many catchments that includes modelled levels and extents behind the published mapping.

What a request can produce:

• Modelled flood levels and flows for a range of annual exceedance probabilities, where a detailed model exists.
• The model extent and the date it was built, which tells you whether it predates current climate change allowances.
• Historic flood outlines, which are observed events rather than modelled ones.
• Detailed river network data and gauging station records.
• Confirmation that no detailed model exists, which is itself a useful answer.

Some products are free and some are charged, and the charges are published, so check the current price list. Lead times are measured in weeks rather than days and should be requested before the planning programme is set.

Where a model does exist, the least costly route is often a model enquiry: asking for the existing model to be run or interrogated at your site rather than building a new one. Where the model is held by the lead local flood authority or a water company for an ordinary watercourse or a sewer network, ask them instead.

Survey input a model needs

A hydraulic model is only as good as the geometry put into it, and survey is the cost most often underestimated.

Structure surveys are where the surprises are. A culvert that needs a confined space entry, or a blocked trash screen that nobody has measured, can add weeks and a specialist contractor to the programme. Walk the watercourse before you commission anything, and photograph every structure.

Climate change allowances change. The published peak river flow and rainfall allowances are revised periodically and vary by river basin district and by the development's lifetime, so take the current figures from the published guidance at the time of modelling and state the version used in the report.

Ordinary watercourses and the gap in coverage

An ordinary watercourse is any watercourse that is not designated a main river. The lead local flood authority, usually the county or unitary council, is the risk management authority for it, and consent for works in or affecting the channel comes from them under the Land Drainage Act 1991 rather than from the Environment Agency.

The gap in coverage follows from that split. Main rivers have been modelled extensively because the Agency has a statutory programme. Ordinary watercourses have been modelled where a local authority has commissioned a study, often as part of a Strategic Flood Risk Assessment or a Surface Water Management Plan, and not otherwise.

Which means the first enquiry on an ordinary watercourse site is to the lead local flood authority, asking three questions. Is there a model for this watercourse. Is it covered by a Strategic Flood Risk Assessment with modelled outputs. And what does the authority expect for a site of this size on this watercourse.

That third question is the one that saves money. Some authorities accept a simplified approach on small sites. Others require full 1D-2D modelling regardless. Ask before you budget.

Review by the lead local flood authority

The lead local flood authority is a statutory consultee on major development for surface water drainage, and in practice comments on smaller schemes too where an ordinary watercourse is involved.

What a reviewer looks for in a bespoke model:

1. A stated and justified hydrological method, with the catchment descriptors used.
2. Model schematisation shown on a plan, with the extent, the boundaries and the structures represented.
3. Calibration or verification against an observed event where any record exists, or an honest statement that none does.
4. Sensitivity testing on roughness, blockage and inflow, because an uncalibrated model needs it.
5. Climate change allowances applied, with the version and the basin district stated.
6. Blockage scenarios for culverts and trash screens, which is the single most common omission.
7. Results presented as levels at defined points as well as extents.

Agree the modelling approach with the authority before the model is built. A pre-application discussion that confirms the hydrological method, the scenarios and the blockage assumptions costs nothing and removes the main cause of a model being rejected after the fee has been spent.

Where a simpler approach is accepted

Full modelling is not always proportionate, and the National Planning Policy Framework and the Planning Practice Guidance on flood risk both support a proportionate approach.

Routes that authorities commonly accept on small sites:

• A qualitative assessment with a conservative floor level, setting levels well above any credible flood level and accepting a precautionary freeboard rather than modelling to find a precise one.
• Using modelled levels from a neighbouring or upstream study with a stated justification for transferring them.
• A simple 1D model of a short reach where the hydraulics are straightforward and no structure controls levels.
• Reliance on historic flood outlines plus a topographic survey, where observed events give a defensible upper bound.
• A surface water only assessment, where the risk is pluvial rather than fluvial and the answer is a flow route and an exceedance design.

The conservative approach is often the commercially sensible one. Raising the floor level by 300mm costs less than modelling to prove that 150mm would do, and on a single plot the extra blockwork costs a fraction of a modelling fee. A Drainage Strategy can carry that reasoning where the authority accepts it.

Programming the work

The sequence that avoids a stranded fee:

1. Request Environment Agency flood risk data and ask the lead local flood authority what exists, at the same time. Allow several weeks.
2. Walk the watercourse, photograph every structure and note anything that needs a survey.
3. Hold a pre-application discussion on the modelling approach, and record what was agreed in writing.
4. Commission topographic and channel survey only once the model scope is agreed.
5. Build the model, run the scenarios agreed, and include blockage and sensitivity runs.
6. Submit the assessment with the model schematisation and the agreed scenarios visible.

Budget the modelling as a range with the survey separated out, because survey is the element most likely to move once the watercourse has been walked. PlanScope's pricing is fixed for the drafting side, which makes the specialist modelling and survey lines easier to see clearly against the rest of the appraisal, and the approach to water and drainage connection costs follows the same early-enquiry logic.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice. Climate change allowances and data charges are revised, so check the current published figures before relying on them.