Cross referencing a drainage strategy and a flood risk assessment

A drainage strategy flood risk cross reference problem is the most common avoidable reason for a second round of consultation on a small site. The two documents are written by different people, sometimes different firms, and they share perhaps a dozen numbers. When those numbers disagree, the Environment Agency and the lead local flood authority each spot it, and the response asks which figure is correct rather than approving either.

Nobody is confused about the engineering. They are confused about which document to believe.

Why the two documents are usually separate

They answer to different consultees and different guidance. A Flood Risk Assessment addresses flood risk to and from the development from all sources, and the Environment Agency is the consultee on fluvial and tidal risk. A Drainage Strategy sets out how surface water will be managed on the site, and the lead local flood authority is the statutory consultee on that for major development.

The split is also commercial. A flood risk assessment on a site in Zone 2 is often produced by a flood specialist, while the drainage design comes from the civil engineer doing the levels and the road layout. Each writes to their own brief and neither reads the other's document carefully before submission.

So the overlap is real and it is narrow. Surface water flood risk, the levels, the discharge rates, the climate change allowances, the finished floor levels and the exceedance routes appear in both. Everything else belongs to one or the other.

Figures that must be identical in both

Build a shared parameter table before either document is written, and give it a revision number.

The failure that recurs most often is the discharge rate. The strategy calculates 4.7 litres per second and the assessment says "restricted to 5 litres per second", because somebody rounded. An officer reading both will ask which it is, and the condition will be drafted against whichever number the report quoted rather than the one the system was designed to.

Round once, agree the rounded figure, and use it in both.

Levels, datum and benchmark consistency

Mixed datums cause more wasted time than any other single error here.

A topographic survey delivered on an arbitrary site datum, where the manhole cover is 100.00, cannot be compared with a modelled flood level in metres above Ordnance Datum. If one document works in site datum and the other in Ordnance Datum, the freeboard calculation is nonsense even when both documents are internally consistent.

The discipline:

1. Use Ordnance Datum throughout both documents, and say so in each.
2. Reference the same topographic survey by drawing number, revision and date in both.
3. Name the benchmark or the control station used, and its value.
4. Where the survey was supplied on a site datum, state the conversion applied and put it in both documents.
5. Reissue both documents when the survey is revised, rather than only the one that changed.

Point five is the one that gets forgotten. A revised topographic survey picking up a 250 mm discrepancy on the road level changes the drainage design and the finished floor levels, and an assessment still quoting the old levels is now wrong.

Discharge rates quoted twice

Beyond the rounding problem, there are three ways the discharge rate goes wrong between documents.

The first is which return period is being discussed. Greenfield run-off is usually expressed for several events, and a strategy that restricts to the mean annual flow while the assessment refers to the one in one year rate is describing two different numbers. State the event alongside every rate.

The second is whether the figure is total or per outfall. On a site with two outfalls, the permitted total has to be apportioned, and an assessment quoting the total next to a drawing showing one outfall creates an apparent inconsistency.

The third is the treatment of existing development. On a brownfield site the baseline is the existing run-off rate rather than the greenfield rate, and the betterment offered is the key figure. A strategy claiming a reduction against greenfield while the assessment claims betterment against existing is quoting two different comparisons as if they were one.

Where the sequential and exception tests are engaged, the same figures feed the safety argument in the Exception Test, and an inconsistency there is more damaging because the test turns on whether the development will be safe for its lifetime.

Climate change allowances are not one number

This catches experienced practitioners. The Environment Agency publishes separate allowances for peak river flow and for peak rainfall intensity, and they are used for different purposes.

The assessment applies the peak river flow allowance for the relevant river basin district, management catchment, epoch and allowance category, to the fluvial modelling. The strategy applies the peak rainfall intensity allowance to the design storm for the drainage system. They are different percentages, derived differently, and quoting one in place of the other is a substantive error rather than a presentational one.

Both documents do need to agree on two things: the development lifetime assumed, which drives the epoch used, and the allowance category selected where a choice exists. If the assessment assumes a 100 year lifetime for residential development and the strategy designs to a shorter horizon, the two are not describing the same scheme.

State in both documents the lifetime assumed, the epoch, the allowance applied and the version of the published allowances used, with the date it was accessed. Those allowances have been revised, and a report citing an older version invites a request to rerun.

Exceedance routes in which document

Exceedance belongs primarily in the drainage strategy, because it is a consequence of the drainage design: where water goes when the system is full or a control blocks. The strategy should show the routes on a drawing with depths, velocities and the receiving area.

The assessment then needs a short section confirming two things. That exceedance does not increase flood risk to third parties, and that exceedance flows do not combine with fluvial or tidal flooding in a way that makes the site unsafe. That second point is the one most often missed on a site in Zone 2 or 3, where the exceedance route runs towards the river and the river is already in flood when the storm occurs.

Do not duplicate the exceedance drawing in both documents. Reference it from the assessment by drawing number and revision. Duplicated drawings diverge the moment one is revised.

Where the flood risk sequential test is in play, the Sequential Test is a separate document again, and it should use the same flood zone and modelled level figures as the assessment rather than the ones from the published mapping.

Avoiding contradictory conclusions

The contradictions consultees actually find, in order of frequency:

An assessment stating that surface water will infiltrate while the strategy discharges to a sewer because infiltration testing failed. The assessment was written first and never updated.

Finished floor levels quoted in the assessment and drawn differently on the strategy's drainage layout. Usually a datum problem or a late level change.

An assessment stating that the site is in Flood Zone 1 based on the published map while the strategy's levels show the low corner below a modelled level the assessment itself quotes elsewhere.

A strategy relying on a flood storage compensation area that the assessment's site layout shows as a parking court.

Attenuation volumes that differ by a few cubic metres between the assessment's summary table and the strategy's calculation output, usually because the strategy was revised and the summary was not.

Every one of those is a cross-checking failure rather than an engineering failure, and every one costs several weeks.

Cross reference conventions that help a reviewer

What works, and it is mostly clerical.

A shared parameter schedule as an appendix to both documents, identical, with its own revision number and a change log. When a figure moves, it moves in one place and both documents are reissued.

A short statement at the front of each document saying what it covers, what the companion document covers, and which governs where they overlap. Three sentences that save a consultee from guessing.

Drawing references with revision letters, used consistently in both, and no duplicated drawings.

A single levels schedule, in Ordnance Datum, listing existing ground levels, proposed levels, finished floor levels, modelled flood levels and freeboard, referenced from both documents.

And a cross-check signed off by one person who has read both documents end to end before submission. On a small scheme that is half a day. It is the best value half day in the whole submission, and the person who does it should not be either of the authors.

One honest uncertainty. Where the two consultees want different things on the same point, which happens on sites with both fluvial and surface water risk, there is no national arbiter. Raise it at pre-application stage and record the answer in both documents.

This is general guidance, not legal advice; check the position with your local planning authority, the lead local flood authority or a qualified professional before relying on it. To see which flood and drainage documents your site is likely to need, enter the postcode on PlanScope.

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