Rainfall data choices and the design storm profile

The rainfall dataset and storm profile chosen for a drainage design will move the required attenuation volume by a wide margin before any climate change allowance is applied, so the choice has to be stated and justified in the calculation rather than left buried in the software defaults. Most reviewers in England now expect FEH13 depth-duration-frequency rainfall, a summer profile for small piped networks and a winter profile for volume-critical storage, and a clear statement of which rainfall model, which grid point and which profile produced the numbers in front of them.

A drainage strategy that does not say which dataset it used is not reviewable, and a lead local flood authority is entitled to ask for it again.

Where design rainfall data comes from

Point rainfall statistics for the United Kingdom come from the Flood Estimation Handbook family of depth-duration-frequency models, derived from the national rain gauge record and supplied for a specified location. Design software requests a grid reference and returns rainfall depths for a range of durations and return periods.

Before the Handbook, the Flood Studies Report of 1975 provided the equivalent statistics, and FSR rainfall still appears in older drainage calculations and in some legacy models of existing networks. The two are not interchangeable.

The point to carry through the whole design is that the rainfall data is location-specific. A grid reference taken from the wrong corner of a large site, or from an office address, produces a defensible-looking calculation that describes somewhere else.

Dataset versions and why they differ

FEH13 rainfall superseded the earlier Handbook model and changed estimated depths across much of the country, up in some regions and down in others. The differences are not uniform, and they are largest at the longer return periods that matter for attenuation sizing.

Three consequences follow for practice:

• A scheme modelled against the older dataset and resubmitted years later may need more storage than the original design provided, with no change to the layout.
• Comparing a proposed scheme against an existing network modelled on a different dataset is not a like for like comparison, and the strategy should say so.
• Where an authority holds an integrated catchment model built on earlier data, its outputs and yours will not match, and explaining why in advance saves a round of comments.

Where a dataset or an allowance has been updated since the original design, say which version you used and when it was downloaded. That one line removes an entire category of reviewer query.

Summer and winter profiles compared

A storm profile describes how a given rainfall depth is distributed over the duration of the event. Two standard profiles are used in United Kingdom drainage design, and they test different failure modes.

The summer profile produces a high instantaneous intensity, so it drives pipe sizes and surface flooding in short-duration events. The winter profile produces more total volume arriving while the outflow is already restricted, so it usually governs tank and basin sizing.

Run both. A design checked only against a summer profile will often undersize storage, and a design checked only against a winter profile will often undersize the network upstream of it. Where the software offers a range of durations, the critical duration is the one that produces the largest storage requirement, and it has to be found by testing rather than assumed.

Return periods tested and why

The non-statutory technical standards for sustainable drainage systems set the flood risk expectations within a development, and local authority guidance commonly builds on them. In ordinary practice a design is tested at three levels:

1. A frequent event, to confirm that the network operates without surcharge in normal conditions.
2. A 1 in 30 year event, to confirm that surcharge does not cause flooding at ground level.
3. A 1 in 100 year event with the appropriate climate change allowance, to confirm that no flooding occurs to buildings and that exceedance flows have a safe route.

The climate change allowance is applied to rainfall depth and is set by river basin district and management catchment. Allowances are revised periodically, so cite the version used with the date you took it, and check the current figures rather than reusing an allowance from a scheme completed two years ago.

Exceedance deserves more attention than it usually gets. Showing a 1 in 100 year plus allowance result with no flooding is not the same as showing where water goes in a larger event. Officers increasingly ask for an exceedance flow route plan alongside the calculation.

Stating the data source in the calculation

A reviewable set of drainage calculations names its inputs on the face of the output. What a lead local flood authority looks for:

• The rainfall model and version, with the date the data was obtained.
• The grid reference used, and confirmation that it sits within the site.
• The storm profiles run, and the range of durations tested.
• The return periods assessed and the climate change allowance applied, with its source.
• The assumed permissible discharge rate and where that rate came from, whether a greenfield calculation, an existing positive connection, or a limit agreed with the undertaker.
• The infiltration test results and the method used, where infiltration forms part of the design.

Put those on a single summary page at the front of the appendix. It makes the difference between one round of comments and three.

Where a dataset update changes a result

A scheme designed in one year and built three years later can fail its own calculation after an update, and this comes up in two recurring situations.

The first is a reserved matters or condition discharge submission made long after outline consent. The outline drainage strategy used the data available then. The detailed design uses current data, and the storage volume has grown. Whether the condition can still be discharged on the original parameter depends on the condition wording, which is a good reason to resist a condition that locks a numerical volume rather than a performance outcome.

The second is a section 73 application to vary a scheme. Changing the layout reopens the drainage design, and the updated dataset comes with it.

Neither is a reason to avoid updating. Using superseded rainfall data because it produces a smaller tank is indefensible if the scheme later floods, and a reviewer who spots it will ask for the whole calculation again.

Avoiding a mismatch between documents

The commonest avoidable comment on a drainage submission is an internal contradiction. The Flood Risk Assessment quotes one climate change allowance, the Drainage Strategy uses another, and the microdrainage output in the appendix shows a third because it was run first.

Four checks before submission catch almost all of it:

1. One table of design parameters, written once and referenced by every document rather than retyped.
2. The same grid reference, allowance and discharge rate quoted in the assessment, the strategy and the calculation appendix.
3. Drawing annotations checked against the model, particularly storage volumes and control device settings.
4. A version date on every appendix, so a superseded run cannot be submitted alongside a current one.

Where the proposal relies on infiltration, the test results should appear in full rather than as a quoted rate, because a reviewer will want to see the method, the depth and the location of each trial pit.

Readers working through the surface water evidence base will also want the note on reading the surface water flood risk map without overstating it, and where the outfall is a ditch or stream, land drainage consent for works to an ordinary watercourse covers the separate approval that runs alongside planning.

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