Flood evidence for a one hectare site in Flood Zone 1

A site of one hectare or more needs a site-specific flood risk assessment even where the whole of it sits in Flood Zone 1. That is the rule in the national guidance on flood risk and coastal change, and it surprises developers every year, because Flood Zone 1 is the low probability zone and the instinct is that low probability means no document.

Flood Zone 1 describes one thing only. It means land assessed as having a less than 1 in 1,000 annual probability of flooding from rivers or the sea. It says nothing at all about surface water, groundwater, sewers, reservoirs or canals.

Why a Flood Zone 1 site still needs an assessment

The purpose of the assessment on a Zone 1 site is different from its purpose in Zone 3. There is no sequential test to pass and no exception test to satisfy, so the document is not about site selection. It is about the other sources of flooding and about what the development does to run-off.

Four questions have to be answered. Which sources of flooding affect the site, now and over the lifetime of the development with climate change applied. Where surface water will go, at what rate, and whether the receiving system can take it. What happens in an event that exceeds the design standard. And whether the development increases risk anywhere else.

That is why the Flood Risk Assessment on a Zone 1 site reads as a drainage document with a flood risk chapter rather than the other way round.

Where the one hectare trigger comes from

The guidance lists the circumstances in which a site-specific assessment is required. Development proposals of one hectare or more in Flood Zone 1. All proposals in Flood Zones 2 and 3. Development in an area with critical drainage problems notified by the Environment Agency. Proposals involving land raising. And proposals where other sources of flooding have been identified.

Measure the figure from the application site boundary rather than from the built area, which means access strips, attenuation land and public open space all count. A scheme of twelve houses on 1.04 hectares is caught. The same twelve houses on 0.94 hectares are not, although they will still need drainage evidence.

The sources that Zone 1 says nothing about

Run the check properly, because the flood map for planning covers rivers and the sea and stops there.

Surface water, from rainfall ponding and flowing overland before it reaches any drain. The long term flood risk service shows surface water risk bands, and a Zone 1 site crossed by an overland flow route is a real constraint even though the fluvial map is blank.

Groundwater, which matters on chalk, on shallow superficial deposits and on former workings, and which is addressed through the strategic flood risk assessment prepared for the local plan plus any borehole data.

Sewer flooding, where the undertaker's records of historic flooding incidents and the capacity position from a pre-development enquiry are the evidence.

Artificial sources, meaning reservoirs, canals and other raised water bodies, with reservoir inundation mapping available and a canal in embankment deserving specific attention.

The method for identifying overland routes and local designations is set out in more detail in surface water flood risk and critical drainage areas.

Critical drainage areas and local designations

Two different things carry similar names. Areas with critical drainage problems are notified by the Environment Agency to local planning authorities and bring the assessment requirement regardless of site area. Critical drainage areas are usually defined in a strategic flood risk assessment or a surface water management plan, and carry whatever local requirements that document sets.

Find out which applies before you design. Inside one, expect a tighter discharge rate, sometimes a requirement for betterment rather than no worsening, and occasionally a restriction on development in the parts of the site at highest surface water risk.

And read the strategic flood risk assessment itself rather than the summary on the authority's web page. It frequently contains the discharge rates, the climate change position and the local standards that the lead local flood authority will apply, which saves a round of comments.

The discharge hierarchy and the rate you will be given

The hierarchy is settled. Infiltration first, then discharge to a watercourse, then discharge to a sewer, with a combined sewer at the bottom. Each step down needs the step above ruled out with evidence rather than assertion.

Infiltration has to be tested, not assumed. Soakaway testing to the recognised method, at the depth and location of the proposed features, with the groundwater level recorded and a factor of safety applied. A single trial pit on a 1.5 hectare site will not support an infiltration-based strategy.

For greenfield sites the discharge rate is normally tied to the calculated greenfield run-off rate for the site, with long-term storage provided for the additional volume generated by the development. For brownfield sites the starting point is often the existing positive discharge rate with a reduction, and the amount of that reduction is negotiated. Get the number agreed in principle before the attenuation is sized, because the volume is sensitive to it.

Apply the current climate change allowances for peak rainfall intensity, checking the published figures for your management catchment and the relevant epoch, since the allowances are revised. Then put the full calculation set in the Drainage Strategy.

Finished floor levels, exceedance and resilience

On a Zone 1 site the floor level argument is about surface water rather than river levels. The usual expectation is floor levels set above surrounding ground by a stated margin, with external levels falling away from the building and no low spots against a threshold.

Exceedance is the part most often missing. Draw the route that water will take when the design event is exceeded or a gully blocks, and show that it runs to open space or a highway rather than into a dwelling. A plan with arrows on it answers the question better than a paragraph.

Resilience measures earn their place where the surface water risk band is moderate or high across part of the site. Raised electrical sockets, water-resistant construction to the lower part of walls, and no habitable basement without a specific case. None of it substitutes for keeping buildings off the flow route.

What the lead local flood authority actually reviews

On major development the lead local flood authority is the statutory consultee for surface water, and its comments follow a predictable pattern. Eight items come up repeatedly.

1. The discharge rate and its derivation, with the calculation shown.
2. Infiltration test results, including depth, method and groundwater level.
3. Attenuation volume for the design event with the climate change allowance applied, and the critical duration identified.
4. The treatment train for water quality, not just the storage volume.
5. Half drain time and the time to empty for infiltration features.
6. Exceedance routing on a plan.
7. Maintenance, with a named responsible party, a schedule and an access route for plant.
8. A commitment to a verification report after construction.

Items four and seven are the ones that generate holding objections. Water quality treatment has to be designed rather than asserted, and a maintenance regime has to identify who will do the work in twenty years.

Submitting the two documents together

Produce the flood risk assessment and the drainage strategy as a matched pair, whether bound together or separately. They share the same levels, the same areas, the same climate allowance and the same exceedance plan, and a mismatch between them is the fastest route to a request for further information.

A workable sequence for a one hectare scheme: desk study of all flooding sources, infiltration testing on site, pre-development enquiry to the undertaker, discharge rate agreed with the lead local flood authority, then the drainage design, then the layout adjusted to it, then both documents written against the final drawings. The order matters more than the page count.

Illustrative example: fourteen houses on a 1.1 hectare former paddock entirely within Flood Zone 1, with a surface water flow route crossing the lower corner. Infiltration tests fail in clay, so discharge goes to a ditch at the greenfield rate with long-term storage, the flow route is kept open as public open space with the attenuation basin in it, floor levels are set 150 mm above adjacent ground, and the exceedance route runs along the estate road to the same basin. The constraint shaped the layout rather than the reverse.

Browse the 66 report types to see which flood and drainage documents your site needs, and the wider picture at the ten unit major threshold sets out what else changes at this scale. Have the drainage calculations prepared or reviewed by a drainage engineer. This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it.

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