Civil Engineering Flood Review – Commercial Site, Leicestershire

FPS were appointed to investigate a site which had experienced repeated surface water flooding affecting a commercial warehouse. The building was of portal frame construction with profiled metal cladding. Without a continuous masonry or reinforced concrete wall, any flood water would pass through the cladding, so conventional Property Flood Resistance at door thresholds alone would not prevent ingress. Achieving resistance would have required significant alteration to the building fabric or the creation of an internal bunded arrangement, neither of which was considered a proportionate starting point.

The instruction was therefore to step back and understand how water was moving across the wider site and whether the problem could be addressed before it reached the building.

Establishing the Flood Mechanism

The initial stage combined a walkover with a review of historic incidents, surface water mapping, a review of LiDAR, ground conditions, and the available drainage information. That work confirmed that the warehouse sat in a local low point and that overland flow paths were routing directly towards it, with very little opportunity for attenuation or controlled exceedance during higher return period events.

At that point the priority was not to move to a solution but to establish what further information was required so that any intervention would be based on evidence. CCTV drainage surveys, verification of levels and a review of the existing network were scoped to support the next stage.

One of the key considerations at this stage was an area of unused grassland within the ownership boundary. That changed the direction of the project because it provided somewhere for water to be intercepted and stored as part of a SuDS-based approach. The focus moved from trying to exclude water at the building line to understanding and managing runoff across the wider site in line with the way the catchment functions.

Developing a Site-Wide Response

The emerging approach was to reprofile part of the land and introduce attenuation so that surface water could be collected and stored, instead of impacting the warehouse. In the longer term this also created the potential for the mapped surface water low point to move away from the building footprint and into the new storage area as datasets are updated, which is important in terms of insurability.

Although this represented a capital intervention, it provided a route to a permanent reduction in risk rather than repeated short-term measures.

Modelling and Option Testing

FPS developed a hydraulic model using a storm event for a 1 in 100 year plus 40% climate change allowance. This allowed the required storage volume to be quantified and different configurations to be tested, including basin storage, geocellular attenuation, regrading and revised drainage routing.

The site-wide model also allowed potential future changes, such as extension of the warehouse or alterations to access and parking, to be tested. This ensured the preferred arrangement remained compatible with the way the site may develop.

Coordination and Detailed Design

The preferred solution involved earthworks and new attenuation features. Structural input was obtained to confirm that the proposals were achievable and could be taken forward.

Following agreement of the strategy the scheme was progressed into detailed design, including levels, basin geometry, drainage connectivity and a coordinated construction package.

Outcome

The project moved from repeated flooding and reactive measures to a modelled and buildable site-wide solution.

The completed design:

• Reduces flood risk to the warehouse without altering the building fabric
• Provides resilience for future rainfall conditions
• Introduces SuDS features that add operational and amenity value

Most importantly, the client now has a clear technical basis for long-term management of the asset rather than a series of isolated interventions.

It also illustrates the point at which flood risk becomes a civil engineering problem. That is always site and client specific, but where it is appropriate it can provide a permanent and deliverable solution. It is the type of project we are increasingly instructed to deliver.