Oxfordshire: From Flood Investigation to Surface Water Drainage Strategy

Flood Investigation and Site Assessment

This project began as a Flood Investigation and Management Strategy following ongoing concerns about flooding at a property located close to a main river.

Historic records showed the property had flooded during a 2007 flood event. More recently, however, the main issue was persistent water ingress into the cellar.

The pattern suggested something more complex than groundwater, so the first step was to properly understand what was happening around the property.

During the initial site visit a number of contributing factors became apparent.

The cellar contained a small sump chamber with a pump installed. However the system had been installed without a non return valve. Because the pump discharged vertically by around two metres before exiting the property, water sitting within the rising main simply flowed straight back into the chamber every time the pump switched off.

This meant the pump would almost immediately switch on again. The result was rapid cycling, unnecessary electricity use, and premature wear on the pump. Installing a simple non return valve to the pump outlet resolved that first issue.

If you are unfamiliar with these systems, this guide explains how sump pumps work and how they are typically installed: What is a sump pump and how do you install one?

Investigating the Source of Water Ingress

The next question was whether the cellar was experiencing groundwater ingress or whether another water source was present.

A small on-site comparison test was undertaken, comparing water from the sump with the clean mains supply. This helped rule out the possibility of a hidden clean water leak within the property.

On-site water dip test carried out during the flood investigation.

Alongside this investigation we prepared a bespoke scope for a CCTV Drainage Survey to understand the drainage layout serving the building. The results proved extremely valuable.

CCTV Drainage Survey Findings

Drainage inspection as part of the flood investigation. Simon Crowther from FPS Environmental Ltd, pictured.

At the front of the property the survey showed that a historic flood wall had inadvertently blocked the original downpipe outfall which once discharged to the road. With no functioning outlet remaining, the pipework had simply been cut back, leaving roof water discharging into gravel beside the house, without any drainage connection. It was highly likely this was contributing to water reaching the cellar.

The findings at the rear of the property were also significant.

A former brick culvert which historically carried surface water away from the site had deteriorated and collapsed in sections. With no functioning route for water to escape, surface water was effectively accumulating around the building and discharging into the ground around the cellar walls.

“This project also reinforced the importance of CCTV drainage surveys, regardless of flood risk, when purchasing heritage properties. Drainage defects are often hidden and the cost of repair can be substantial.”

Simon Crowther BEng (Hons) FCIWEM C.WEM MIET – Director at FPS Environmental Ltd

Surface Water Drainage Strategy

Once the investigation stage had established how water was interacting with the property, FPS Environmental developed a new surface water drainage strategy and progressed this into a detailed drainage design for construction.

The proposed works included:

• A new outfall for the roof drainage.
• A replacement surface water route for the collapsed culvert.
• Infiltration testing, runoff calculations and soakaway sizing.
• Backflow protection within the drainage system.

Surface water drainage strategy design, following site inspection, prepared by FPS Environmental Ltd with proposed soakaway.

Detailed Drainage Design and Implementation

By undertaking a new drainage design we were also able to consider additional risk-reducing measures. Channel drains were proposed in front of the low threshold doors so that surface water is intercepted before it reaches the building.

The Surface Water Drainage Strategy was then taken forward into a Detailed Drainage Design for construction. The final design addressed both the cellar water ingress and the wider drainage issues around the property.

Example channel drain installed along a building edge to intercept surface water runoff.

Engineering-Led Flood Risk Management

What became clear during the investigation was that no single issue was causing the problem. Instead, several small factors were interacting with one another – levels, surface runoff, drainage arrangements and how water was collecting around the building.

By taking the time to properly review the site first, rather than jumping straight to a single solution, it was possible to design a drainage approach that was appropriate for the property.

Infiltration Testing for Soakaway and Drainage Design

Infiltration testing is an important part of designing surface water drainage systems. The results help determine how quickly water can drain into the ground and are used when sizing soakaways and developing a Surface Water Drainage Strategy.

The video below shows how a percolation test is undertaken on site and how the results are interpreted during drainage design.

Example infiltration testing carried out to inform soakaway sizing and surface water drainage design.