What a consultee checks in a drainage model output
Drainage model outputs are reviewed quickly and rejected for predictable reasons. Here is what a consultee reads first, the checks to run before submission, and how to present results so they can be followed.
A drainage model output is reviewed in about twenty minutes by someone who has read a hundred of them, and they look at the same six things every time. Rainfall inputs and allowances. Whether the critical duration was actually searched. The flood volume and surcharge results across the design events. Half drain time. Whether the modelled areas match the drawn layout. And whether the version submitted is the version the drawings describe. A submission that presents those clearly gets agreed. One that presents 180 pages of storm-by-storm printouts with no summary does not.
The summary tables a reviewer reads first
Every hydraulic package produces a simulation summary listing, for each storm event, whether any node flooded, the flood volume, the maximum surcharge and the status of the network. That table is the first thing opened.
Alongside it sit the manhole schedule, with cover and invert levels, and the pipeline schedule, with diameters, lengths, gradients and roughness. A reviewer reads these three together to see whether the network is plausible before looking at the hydraulics at all. Inverts that rise downstream, pipes with gradients flatter than anything that will self-cleanse, or a 150 mm pipe draining two hectares are all visible in seconds.
Put those three tables at the front of the calculations appendix, with a one-page network schematic. The full storm output belongs behind them, not instead of them.
Checking rainfall inputs and climate change factors
The rainfall model, its version and the point or catchment it was generated for are checked first, because everything downstream depends on them. State which rainfall dataset was used and its version, the location it was extracted for, and the summer or winter profile applied.
Then the allowances. Climate change uplift comes from the Environment Agency's published allowances for peak rainfall intensity, which vary by river basin district, by epoch and by the vulnerability of the development, and which are revised. Cite the version you used and the date you took it, because a reviewer comparing your figure with the current table needs to know why they differ. Urban creep is a separate allowance for future paving of private areas, commonly applied at around 10 per cent for residential development, and it is frequently omitted.
The two most common findings at this stage are an allowance taken from a superseded table and a scheme where the climate change uplift has been applied to the 1 in 100 event but not to the events used to size the flow control.
Confirming the critical duration was searched
A model run for a single storm duration proves nothing. The critical duration for a network is the one that produces the worst result, and it differs between the pipe network, the attenuation volume and the outfall, so it has to be found by running a range.
Look for evidence that durations from short, intense events through to long, low-intensity events were simulated for each return period, and that the reported result is the worst case rather than a convenient one. On sites with large attenuation storage the critical duration is often several hours or more, which surprises people used to short pipe-network durations.
Present it as a table of duration against flood volume or peak storage for each return period. It takes half a page, it shows the search was carried out, and it removes the most common request for further information.
Flooded volume, surcharge and half drain time
For the 1 in 30 event, the usual expectation is no flooding from the network. For the 1 in 100 plus climate change event, some surcharge is acceptable and any flooding has to be shown to be contained safely on site, in designed exceedance routes or in areas where depth and duration do not create a hazard.
Report flood volume by node rather than as a single total. A cumulative figure of 4 m³ spread across a network is a different proposition from 4 m³ at one manhole in a rear garden.
Half drain time matters for attenuation and infiltration systems: storage that empties too slowly is not available for the next event, and the technical standards expect systems to recover within a stated period. Report the half drain time and the time to fully empty for the critical event, and where infiltration is proposed, report the infiltration rate used, the test method, the number of tests and the factor of safety applied.
Free-standing exceedance drawings help here. Show where water goes when the design event is exceeded, with flow arrows and the finished floor levels it is being kept away from. This is the part the reviewer worries about, and it connects directly to the case made in the Flood Risk Assessment.
Matching the model to the drawn layout
The single most common cause of rejection is a model that does not describe the scheme on the drawings.
Run four checks before submission. Do the node references in the model match the manhole references on the drainage layout. Does the total contributing impermeable area in the model equal the impermeable area shown on the drainage plan, allowing for whatever is draining elsewhere. Are the flow control settings in the model the same as the device specified on the details sheet. And does every attenuation structure in the model appear on the layout at the same volume and invert level.
Where the site has an agreed discharge rate, state it, state the modelled peak discharge, and show them side by side. Reviewers should not have to hunt for that number, and our note on what the lead local flood authority checks sets out how much weight it carries.
Version control and unmodelled changes
Layouts change. Plot positions move, a road is realigned, an attenuation basin is reshaped to fit a tree, and the drainage model does not always follow.
Give every model file a reference and a date, quote that reference in the Drainage Strategy and on the drainage drawings, and keep a short revision log recording what changed and why. When a reviewer asks whether the model reflects revision P07 of the layout, the answer should take one line.
Where the layout has changed since the model was run, say so and explain why the change does not affect the result, or re-run it. A reviewer who finds an unmodelled change goes back through everything else with more suspicion than the change deserved.
Presenting outputs so a reviewer can follow them
Order the appendix as a reader would use it: a short method note, the input assumptions, the network schedules, the critical duration search, the summary results, then the detailed output. Number the pages. Include a schematic.
And write two paragraphs of narrative explaining what the model shows and where the residual risk sits. Long term maintenance is the other half of the answer, as our note on SuDS adoption and maintenance explains, because a model that works on paper depends on a system somebody is actually cleaning.
This is general guidance, not legal advice; check the position with your lead local flood authority, your sewerage undertaker and your local planning authority before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Which tables should go at the front of the calculations?
The simulation summary showing flood status and volume for each event, the manhole schedule with cover and invert levels, and the pipeline schedule with diameters, lengths, gradients and roughness, plus a one-page network schematic. Detailed storm-by-storm output belongs behind those, not instead of them.
How is the critical duration demonstrated?
By running a range of storm durations for each return period and reporting the worst case, with a table of duration against flood volume or peak storage. A single duration proves nothing, and on sites with large attenuation storage the critical duration is often several hours, which is longer than pipe-network intuition suggests.
Which climate change allowance applies?
The Environment Agency publishes peak rainfall intensity allowances that vary by river basin district, by epoch and by development vulnerability, and they are revised. State the version used and the date it was taken, so a reviewer comparing your figure with the current table can see why they differ. Apply urban creep as a separate allowance.
What is half drain time and why is it checked?
It is the time for an attenuation or infiltration system to release half its stored volume. Storage that empties too slowly is not available for the next storm, so the technical standards expect recovery within a stated period. Report it for the critical event along with the time to empty fully.
What causes most drainage submissions to be rejected?
A model that does not match the drawings. Node references that differ from the layout, contributing areas that do not add up to the impermeable area shown, flow control settings that differ from the specified device, or attenuation volumes that changed when the layout was revised. Run those four checks before submitting.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.