Sequential approach on a single site explained

The sequential approach to flood risk is the requirement to steer development within a site to the land at lowest risk. It is a different exercise from the sequential test, which compares your site with other reasonably available sites in the area, and from the exception test, which asks whether a development at higher risk can be justified at all. The three are often collapsed into one paragraph in a flood risk assessment, and that paragraph is usually the reason the Environment Agency asks for more information.

Once the site is chosen, the sequential approach is the part still in your control. It operates on the layout, the levels and the arrangement of uses inside the red line, and it is judged on a drawing rather than on an argument.

Three tests, three questions

The sequential test asks whether the development could reasonably be located on land at lower flood risk elsewhere. It is applied by the decision maker, informed by evidence from the applicant, and it is about choosing between sites. Where the site is allocated in an adopted local plan and the test was applied at plan making stage, that work may not need repeating, though the authority will want to know the allocation covered this use.

The sequential approach applies inside the boundary. It asks whether the more vulnerable parts of the development sit on the lower risk parts of the land.

The exception test comes last and only where policy requires it. It has two parts, and both must be passed: that the development would provide wider sustainability benefits to the community that outweigh the flood risk, and that it will be safe for its lifetime, will not increase flood risk elsewhere, and will where possible reduce risk overall.

Set them out in that order in the Flood Risk Assessment, with headed sections, and the consultee can find what they came for.

Matching vulnerability to the land

Planning Practice Guidance classifies uses into five flood risk vulnerability categories and sets out which are compatible with which flood zone. Essential infrastructure and highly vulnerable development sit at the top. Dwellings, student accommodation, residential institutions and hospitals fall into the more vulnerable band. Shops, offices, restaurants and most commercial uses are less vulnerable. Open space, outdoor sport, docks and some agricultural uses are water compatible.

The mechanics of the approach follow directly. Take the classification of each element of the scheme, take the flood zone and the modelled level under each part of the site, and arrange them so the most sensitive uses sit where the risk is lowest.

On a mixed use scheme this is usually vertical as well as horizontal. Commercial floorspace at ground level with residential above is the classic response on a constrained urban site in Flood Zone 3a, and it works because the sleeping accommodation is above the modelled level rather than because the ground floor is somehow protected.

The horizontal moves are the familiar ones. Car parking, access roads, landscaping, attenuation basins, play space and drainage features are the things you put on the wettest land. And they are useful there, because a basin in the floodplain can serve two purposes at once if the Drainage Strategy is designed alongside the layout rather than after it.

Levels, not just zones

Flood zone boundaries are a planning tool drawn at a coarse resolution, and a large site will contain variation the zone map does not show.

Where a hydraulic model exists, work from modelled levels and depths rather than from the zone outline. A site wholly in Flood Zone 3 can have a metre of difference in modelled level across it, and hazard ratings that vary from low to dangerous for some. Steering within that variation is exactly what the approach requires.

Watch for flow routes and for the functional floodplain, which is the land where water has to flow or be stored in times of flood. Building on a flow route displaces water somewhere else, which engages the separate requirement not to increase risk elsewhere.

Finished floor levels then come from the modelled level for the design event, with the climate change allowance applied and whatever freeboard the authority requires on top.

Access, egress and what happens on the night

Safe access and egress is the part of the approach that most often defeats a layout that otherwise works.

The test is whether people can leave the site, and emergency services can reach it, during the design event over the lifetime of the development. A dry route to land outside the flood extent is the strongest answer. Where the route is wet, the assessment has to give depths and hazard ratings along it, and describe how occupants would be warned, by whom, and what they would do.

Refuge above the modelled level is sometimes accepted for less sensitive schemes, and rarely accepted for accommodation housing people who cannot self-evacuate. Care accommodation in a flood risk area is the hardest case in the whole subject, and the honest answer is often that the use should be somewhere else.

Where the local authority emergency planner is a consultee, send them the evacuation plan early. Their objection at week six is much harder to unwind than their comment at week one.

Documenting it so it is not challenged

The approach is a design decision, so show it as one.

Include a plan with the flood zones, or the modelled extents for the relevant events, overlaid on the proposed layout with the uses annotated. Add a section through the site showing existing ground, proposed levels, the modelled level and the finished floor level. Then write two paragraphs explaining what was moved and why.

Say what was considered and rejected. A layout that started with houses on the low ground and moved them is a stronger story than a layout that claims it always had them in the right place. And where a constraint outside flood risk drove a compromise, such as a root protection area or a highway access position, name it.

Where the exception test is engaged, keep the two limbs separate under their own headings, because they are answered by different evidence: the sustainability benefits by the planning case, the safety limb by the technical assessment. The Sequential Test and Exception Test are commonly submitted as short standalone documents alongside the flood risk assessment, which suits authorities that check them off a validation list.

An illustrative example

This example is illustrative. A 0.8 hectare site slopes from a road at the north down to a small watercourse at the south. The northern third is Flood Zone 1, the middle is Flood Zone 2 and the southern strip is Flood Zone 3a with a narrow band of functional floodplain along the channel.

A workable response places the dwellings on the northern third, the access road and parking court across the middle, and public open space with an attenuation basin and a wildlife margin on the southern strip. Finished floor levels of the nearest plots are set above the modelled level for the design event with the climate change allowance and the authority's freeboard. Pedestrian and vehicular access runs north to the road, dry in the design event, and the drainage discharges at the existing greenfield rate.

That is not a clever scheme. It is the obvious one, which is the point.

Enter your postcode on PlanScope to check whether your site falls in Flood Zone 2 or 3 and which flood documents it is likely to need.

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