Case Study – Floodplain Impact Calculations

When we undertake a Flood Survey, the first objective is always the same: develop a clear understanding of how water behaves at a site, and where the true risks lie.

In this instance, the property was a high‑value detached home with several potential routes for water to enter. The homeowner was open to significant investment if it meant creating a robust, long‑term solution, and a permanent flood wall was one of the early considerations.

Before progressing to design, we recommended, scoped, and commissioned a CCTV Drainage Survey. This proved critical. The survey identified that a manhole within the driveway wasn’t isolated to the property, it carried a connection from the neighbouring plot.

This meant a boundary‑line wall would never provide true protection; even if the perimeter were sealed, water could still enter the defended area via the shared drainage system.

With this in mind, we reviewed alternative alignments for a wall positioned closer to the dwelling. Our Director, Simon Crowther, visited the site with our Construction Project Consultant, Anton Richards, to assess buildability. Access constraints, the manoeuvring of machinery, and the depth required for stable foundations all presented challenges.

We also had to consider how any new structure would alter the displacement of water, an essential part of scheme feasibility that is sometimes overlooked.

To answer those questions definitively, FPS Environmental were instructed to undertake formal floodplain impact calculations. These helped quantify whether the proposed concept was viable, what level of displacement might occur, and whether it could be mitigated without increasing risk elsewhere. These calculations could be used to support an Environmental Permit application where necessary.

From this stage, the project progressed to structural wall calculations, based on the depths identified and mapped in our initial risk assessment.

This case highlights why flood surveys must start with understanding rather than assumptions. Every site is unique, and only by interrogating drainage, access, ground levels and water behaviour before a detailed flood risk management strategy can be developed.

Written February 2025