Raised access roads and refuge platforms on flood sites

A raised access road flood site design is the answer to a question the flood risk assessment has already asked and failed: can people get in and out safely during the design event. National policy requires development to be safe for its lifetime, and safe access and escape is one of the things a council and the Environment Agency will not compromise on. Where the existing ground offers no dry route, raising the road, or providing a refuge, is the physical response. Both come with consequences that need to be designed rather than asserted.

The order of preference is worth stating plainly. Dry access first. Then access where the flood hazard along the route is low enough to be safe. Then, only where neither is achievable, refuge with an agreed evacuation plan. Moving down that list gets progressively harder to defend.

When raising the access is the only option left

Raising an access road makes sense on a site where the building platform can be kept dry but the route to it crosses a shallow flow path, and where the volume of fill involved is modest. A 60 metre stretch of driveway raised 400 millimetres to keep it above the design level is a proportionate solution.

It stops making sense in three situations. Where the route crosses the deeper part of the floodplain, because the fill volume and the flood storage loss become substantial. Where the road is the only conveyance route for flood flow, so raising it acts as a dam. And where the raised route has to connect to a public highway that itself floods, which achieves nothing beyond a dry first 60 metres.

Check the receiving highway before you design anything. The route has to work all the way to land outside the flood extent, and a modelled depth on the adopted road two junctions away defeats the whole strategy.

The depth and velocity criteria the route must meet

Safety along a route is assessed on the combination of depth and velocity rather than depth alone, because 200 millimetres of fast water is more dangerous than 400 millimetres of still water. The hazard rating approach from Defra and Environment Agency research combines flood depth with velocity and a debris factor to produce a hazard value, with bands running from caution through danger for some, danger for most, and danger for all.

Present the assessment along the route, not at a single point. A section along the centreline with modelled water levels for each return period, and a table of depth, velocity and hazard rating at intervals, is what a consultee can check. The vulnerability classification of the development matters too, because what is acceptable for a warehouse is not acceptable for housing, and housing for older or less mobile occupiers is treated more strictly again.

What raising land does to flood levels elsewhere

Fill placed in a floodplain displaces water. A raised causeway also alters where water can flow, which can raise levels upstream and concentrate flow at the ends of the embankment.

Three things belong in the design. Flow openings, sized by the model rather than by rule of thumb, so the raised road does not sever the floodplain. Compensation for the storage volume lost, provided level for level where the authority requires it. And an assessment of the effect on third party land, because an increase in flood level on a neighbour's property is the objection that ends applications.

Scour at the ends of an embankment and at the flow openings needs detailing, and the raised surface itself needs drainage that does not simply discharge into the floodplain at a concentrated point. Both details belong in the Drainage Strategy rather than being left to the contractor.

Where the scheme is in Flood Zone 3a and the exception test applies, the access strategy is part of the safety limb of that test, so the argument in the Exception Test has to be consistent with what the levels drawings show.

Refuge platforms and their limits

A refuge platform is a raised area within or adjacent to the site where people and vehicles can wait above the flood level until the event passes. It is a genuine solution in a narrow set of circumstances and an oversold one in many others.

What makes a platform work: an area large enough for the number of people and vehicles it serves, a dry route to it from every building, a level above the design flood level with freeboard, and an agreement with the emergency planning function that it forms part of a workable plan. What undermines it: no shelter, no services, no toilets, no arrangement for how long people might be there, and the simple human fact that people do not walk to a platform in the dark and the rain when their instinct is to drive out.

Refuge is not normally acceptable as the strategy for more vulnerable development. A care home or a school on a site where the only strategy is refuge is unlikely to be supportable, whatever the plan says, because evacuation of less mobile occupiers cannot be assumed.

Vertical refuge inside the building, meaning an upper floor above the flood level, is sometimes offered. Treat it as a residual measure for a rapid event, not as a substitute for access, and expect the Environment Agency to say the same.

Levels, freeboard and drainage of the raised surface

Get the levels arithmetic in one place and keep it consistent across every drawing. The chain runs: modelled flood level for the design event, plus the relevant climate change allowance, plus freeboard, giving the minimum finished level.

Freeboard requirements vary, with 300 millimetres above the design level a common expectation and some authorities asking for more. State the figure you have used and its source. Then check the finished floor levels, the raised road crest, the refuge platform level and the site drainage against the same datum, because a mismatch between the flood risk assessment and the levels drawing is one of the most common reasons a scheme goes back round the loop.

Raised surfaces also change the surface water design. A raised road sheds water to its edges, and the receiving swale or ditch has to have capacity when the floodplain itself is full, which is precisely when it cannot discharge. Basements on the same site add another layer of difficulty, as set out in basement development and flood risk.

Landscape and accessibility consequences

A causeway 600 millimetres above the surrounding land, with a retaining edge and a handrail, is a structure. On a rural site next to a river it is often the most visible element of the whole scheme.

Design it as landscape. Graded slopes rather than vertical retaining walls where space allows, planting that settles the edges, alignment that follows a hedge line or field boundary rather than cutting across it, and levels that tie back into existing ground rather than stopping abruptly at the boundary.

Accessibility is the constraint architects hit last and should consider first. An accessible approach needs gradients within the limits in the Building Regulations guidance, which on a 600 millimetre rise means a considerable length of ramp or a carefully designed profile. Steps with a separate ramped route often fit better than a single compromise gradient. Where a refuge platform is part of the strategy, the route to it has to be usable by the people who would need it.

The evidence to submit

A submission that gets through consultation without a holding objection usually contains the same package.

1. A topographical survey tied to Ordnance Datum, showing existing levels across the site and along the access route to the point where it leaves the flood extent.
2. Hydraulic modelling outputs giving depth and velocity along the route for the design event with the climate change allowance, and for a more extreme event as a sensitivity check.
3. A hazard rating assessment along the route, presented as a table and a plan.
4. Levels drawings showing the raised road crest, finished floor levels, refuge platform level and freeboard, all to the same datum.
5. A flood storage compensation calculation where land is being raised, with the volume accounting by elevation band.
6. A flood warning and evacuation plan, with evidence that the emergency planning function has been consulted where refuge is relied on.

Pull the narrative together in the Flood Risk Assessment and keep the figures identical to those on the drawings. Pre-application engagement with the Environment Agency and the lead local flood authority is worth the fee on any site where the access strategy is finely balanced, because a holding objection after submission costs far more time than the pre-application response does.

This is general guidance, not legal advice. Check the position with your local planning authority, the Environment Agency and a qualified engineer before relying on it.

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