Cullompton Flood Channel

Cullompton Flood Channel

CEEQUAL Very Good – Whole Project Award
Version 4, Mar 2011 | Cullompton, England

Client: Highways Agency
Design: Balfour Beatty / Mott MacDonald
Construction: Balfour Beatty / Mott MacDonald

Assessor: Bruce MacFarlane

Project Summary

The flood channel adjacent to the M5 at Cullompton was constructed in 1968 to compensate for the area of flood plain lost when the M5 was constructed. As part of the land purchase agreement, it became the responsibility of the Highways Agency (HA) to maintain the channel. Over the pasts 40 years it has been subjected to periods of flooding, leading to large amounts of silt being deposited. This has reduced the capacity of the channel and increased the risk of flooding in the surrounding area.

After consultation with the Landowner and the Environment Agency (EA) a design solution was agreed where the amount of silt to be removed was reduced from 16,000m3 to 6,500m3. This allowed the desired profile to be achieved and return the channel to its original design. With the employment of a local contractor to do the works and recycle the removed silt, the project team were able to reduce the distance travelled by the lorries removing the silt.

The project was designed and built by the Balfour Beatty Mott MacDonald (BBMM) Joint Venture, under the Highways Agency Area 2 Enhanced Managing Agent Contractor Contract (EMAC) and totalled £870,000 in project value.

The decision was to use the Cullumpton project as the first Area 2 Joint Venture project to be assessed under CEEQUAL to help the drive of environment and sustainable methodologies within the participating companies. During the project a new carbon tool was used to record energy, materials and transport used throughout the scheme which will be used in future works carried out by BBMM.


Consideration of the Environment

Extensive environmental checks were carried out and drainage consents were obtained, due to the proximity of the River Culm and a stream crossing the flood plain. This lead to comprehensive mitigation measures being put in place such as:
• noise, dust and working hours restrictions;
• fencing to protect a potential otter holt;
• works to take place outside fish breeding times;
• and measures to prevent pollution of the water courses. This included temporary earth bunds (constructed out of the retained top soil) with straw ‘sluice gates’ installed should there be a flood event during works, to catch washed away silt. The stream across the site also had silt booms installed.

Part of the works included the construction of a new lay-by on the motorway slip road to provide access to Cullumpton channel and provided an area for arising to be loaded onto wagons clear of the motorway traffic. This avoided the need for lorries to access the flood channel via a listed historic bridge and reduced the amount of compaction of retained soil, as well as reducing the likelihood of mud being tracked onto the carriageway.

Following the creation of the lay-by, approximately 6500m3 of silt was removed from the flood channel and taken to a local facility were it will be stored and recycled. Due to the presence of Himalayan Balsam the arisings will be stored and treated for three years before being re-used. Retained topsoil was re-spread and grass seeding took place to help return the flood channel into pasture land.

During the silt removal works, rock armour was installed to the southern bank of the stream which crosses the channel. This bank was badly eroded and the protection works are intended to prevent further erosion during future flood events.