Surface Water Investigation – Evidence-Led Drainage Appraisal, Cumberland

FPS were instructed by a charity responsible for a significant parcel of open land where surface water was reported to be collecting and affecting neighbouring properties during heavy rainfall.

The concern from all parties was understandable. Recent changes to the land were being viewed as a possible cause, and there was pressure to move quickly toward physical mitigation. Before discussing solutions, we needed to understand how the site functions within the wider catchment and whether the observed behaviour represented a change or the continuation of a natural drainage pattern.

A site walkover was undertaken alongside a detailed desk-based review of topography, surface water mapping and ground conditions. When the LiDAR data is considered together with the on-site levels, the site’s position within a natural surface water flow path becomes clear. The predicted ponding shown in the national mapping aligns with the physical low point on the ground. In other words, the presence of water at this location is consistent with the baseline drainage behaviour of the landscape rather than being an anomalous condition.

We prepared project-specific GIS plans to demonstrate mapped flow routes, relative depths and hazard. This allowed the client to visualise how water interacts with the land during an exceedance event and provided a far more robust basis for discussion than reliance on the online mapped flood extents alone.

Ground conditions were then reviewed using British Geological Survey data and verified against site observations. Although the surface soils appear permeable, the underlying clay-rich glacial deposits form an infiltration-limiting layer. The standing water observed during the visit provided field confirmation of this behaviour. This has direct implications for drainage strategy: infiltration-led solutions are unlikely to perform reliably without detailed testing, and shallow surface storage and controlled conveyance become more appropriate design principles.

An existing drainage chamber was identified within the site. At the time of the visit its route, capacity, condition and outfall were unknown. In practical terms this introduces a significant constraint. Introducing new drainage or increasing inflows to an unverified system risks transferring flooding elsewhere and creating a new liability for the landowner. We set out a staged programme of investigation, records requests, connectivity tracing and CCTV survey, to establish whether the asset is hydraulically relevant before any connection is considered.

As part of the appraisal we also considered the potential for an artificial water source. In previous investigations apparent surface water flooding has ultimately been traced back to clean water supply leakage, which can follow natural surface water flow routes and therefore present in the same locations as rainfall runoff. In this instance there was no indication that this was the primary mechanism, however the option of simple water sampling was identified as a proportionate step to conclusively rule this out if required.

Several third-party mitigation measures had been proposed prior to our appointment, including the construction of an earth bund and the installation of new land drainage. Our advice was that neither should proceed at this stage. A bund in this location would function as a hydraulic structure, raising retained water levels and introducing residual risks associated with seepage, overtopping and failure. Similarly, connecting into a drainage system of unknown performance could exacerbate downstream flooding.

Instead, we identified that the site’s natural characteristics, shallow gradients, available open space and low permeability soils, lend themselves to shallow vegetated conveyance features designed to slow and temporarily store water at the surface. Any such intervention would follow confirmation of levels, ground conditions and flow paths and would be developed as part of a staged design process.

The outcome for the client was clarity. The investigation demonstrated that the observed ponding is consistent with the site’s topographic position and ground conditions, that recent land management works are not the primary driver of the behaviour, and that premature construction works could have increased the risk both on and off site. Most importantly, it provided a defensible technical basis for future decisions.

Client Feedback

“We are a charity owning a large parcel of land where gathering water is then affecting neighbouring properties. Received expert flood risk and drainage advice from FPS Environmental Ltd. All dealings with them were timely and professional. Site visit arranged where we were able to fully discuss causes and solutions. Great peace of mind obtained from engaging their expertise.”

Engineering-Led Approach

This project is a good example of the difference between describing flood risk and investigating it.

Our role was not to produce a generic report. It was to:

• Understand the physical behaviour of water across the site
• Test whether perceived changes had caused impact
• Identify constraints within existing drainage infrastructure
• Ensure that any future intervention is proportionate and does not transfer risk elsewhere
• Prepare a briefing note

That evidence-led process is what allows landowners, designers and stakeholders to move forward with confidence.