Project Brief

On the outskirts of Bristol in a busy residential area, Wessex Water were tasked with providing a solution to a historic surface water flooding problem that could provide storage during heavy storm conditions. Several options were considered during the design stage, but with the restrictive location, a solution had to be developed which could provide the necessary storage volume within the existing carriageway as there were no available plots of land to locate a large storage tank.

Marshalls Civils & Drainage Solution 

It was decided that two large diameter precast concrete pipelines would be installed on-line to provide the necessary storage, the first tank on Downend Road was to be the deepest tank, 1600mm in diameter concrete sewer at a length of approximately 60m, at the top end of this tank a backdrop manhole was installed in order to maximise storage in the 1600mm tank and to reduce the depth of dig further upstream.

Up from the 1600mm tank, the surface water sewer was then reduced to 225mm, which acted as a flow control. In addition to this throttle pipe a mechanical flow control was installed at the top end of the 225mm pipe.

Further up the line, another storage tank was installed, this time 1800mm in diameter and approximately 70m in length.

Due to the nature of this scheme, the depth at which was needed to work in a residential road (6.5m at the deepest point) and the sensitive location the contractor opted to use preformed side entry chambers provided by Marshalls Civils & Drainage. These chambers/ access points were used in several locations along the length of the 1600mm and 1800mm tanks.

The works were such that the whole scheme had to be built under a full road closure so it was in the interest of Wessex Water and the local residents that the scheme was built to a high standard but within as short a timescale as possible.

On the large diameter tanks side entry manholes were used, these are inserted at regular intervals along the length of the tank sewer. Under normal circumstances, 3m diameter manholes would need to be built along the length of the tanks but installing the side entry manholes meant that 3m chambers were no longer required reducing the construction time, materials and the footprint of the storm tanks (especially important within the carriageway where the new tank had to share it’s space with the existing utilities already crisscrossing the road).