Flood-resilient structural design

There are two honest strategies and the right one depends on the flood depth and how often it returns. Water resistance keeps the water out, using tanked slabs, waterproof render, non-return valves on drainage and flood barriers to doors and airbricks. This works where the depth is modest and short lived, but above roughly 600mm the pressure difference across a wall becomes large enough that keeping water out risks structural damage to the wall itself, so the strategy has to change.

Water recoverability, sometimes called wet-proofing, lets water in and out in a controlled way and concentrates on fast drying and low damage. That means solid floors instead of suspended timber, closed-cell insulation that does not hold water, services and sockets raised above the design level, and lime or cement finishes that survive wetting. On many sites the sensible answer is resistance to a sensible depth and recoverability above it, and we set that threshold from the flood data rather than guessing.

Flooding changes the ground as well as the building. Saturated soil loses strength and can settle, scour around foundations removes support at the very moment loads are highest, and a rising water table reduces the effective weight of the ground that was helping to resist uplift. We assess these effects in line with Eurocode 7 for the geotechnical design, and where scour or softening is a real risk we deepen or widen foundations, or move to piles that carry load below the affected zone.

We also look beyond the footprint. Raising a floor level, adding flood-resistant construction or building a resistant wall can push water somewhere else, so the design has to sit within the flood risk assessment for the site and any planning conditions attached to it. We coordinate the structural design with the flood risk work and, where a property is being repaired after a flood, with insurers and loss adjusters, so the rebuilt structure is measurably better than what it replaced.