Flood Risk Assessment

Getting the levels right is most of the job

An assessment stands or falls on one number: the design flood level at your site. Read it off the public flood map and you have an estimate. Request the Environment Agency’s modelled output for that reach and you have a defensible figure, tied to a datum, that a case officer can check.

We ask for that data as a matter of course where it exists, and we say plainly in the report where it does not. A topographic survey levelled to Ordnance Datum then lets us set finished floor levels with freeboard rather than guessing at them from the drawings.

Safe access and egress — the point most often missed

A building can sit comfortably above the design flood level while the only road out of it goes under half a metre of water. Authorities look closely at this, and it is a common reason for a report to come back.

We assess the route itself: the depth and velocity along it, the hazard rating that produces, and whether it stays usable for the people who would need it. Where a fully dry route cannot be achieved, we say so and set out what would make the residual risk manageable — which is usually where a flood risk emergency plan comes in.

Compensation storage and third parties

If your proposal puts built form into the floodplain, that volume of water has to go somewhere. Where compensation is required we work it out on a level-for-level basis, so the storage lost at each elevation is replaced at the same elevation, and show it on a plan rather than asserting it in a paragraph.

The same logic applies to your neighbours. Demonstrating that the development does not increase flood risk elsewhere is a policy test in its own right, not a courtesy.

Climate change over the lifetime of the development

Flood risk is assessed over the lifetime of what you are building — commonly one hundred years for housing, less for commercial uses. That means applying the Environment Agency’s peak river flow and peak rainfall allowances for the right catchment and the right period, and being explicit about which allowance was used and why.

Choosing an allowance that is too low is one of the quicker ways to attract an objection. We set it out in the open so a reviewer can agree with it or challenge it.