A £4.5m scheme that involved engineers battling with the second strongest tidal range in the world has been completed reducing the risk of flooding in Bristol city centre. For more than three years Wessex Water has been working alongside the River Avon to construct a reinforced concrete overflow structure that will release screened waste water into the river during times of heavy rainfall.

The scheme involved construction of a water tight pumping station on the river’s edge, near the A4 Portway, that would house equipment to screen and pump water into the river during any tidal condition. However, constructing next to the Portway proved challenging for engineers who had to contend with a tidal range that sees the river rise half a meter every 10 minutes during spring tides.

Nigel Butler, of Wessex Water who was responsible for overseeing the project, said:

“We were aware before we began work that we had to be careful not to affect the stability of the A4 Portway, which partly slipped into the river during a landslip following a burst water main in 2001. To enable this to take place the structure is piled and rock bolted into the rock under the A4.

“It meant we had to take a cautious approach constructing the cofferdam for the new outfall structure to ensure it would hold up against the second highest tidal range in the world yet not affect the Portways structural integrity. We worked very closely with Bristol City to achieve a suitable design.

“There were times when we had to plan to literally down tools to allow for the river to flood the construction site. As soon as the tide went out we would then be in the position of carrying on with the work.”

The project, known as the Plimsoll Combined Sewer Overflow, is the second flood alleviation scheme Wessex Water has completed in Bristol this year. In May a £9m tunnel under the city centre was completed to provide a new sewer from the St Augustine’s area to Tyndall’s Park Road, near Clifton.

This latest project provides relief during storm conditions to a trunk sewer that takes away waste water from the centre to Bristol’s sewage treatment works in Avonmouth. When the sewer reaches capacity the new overflow is used to release screened water into the River Avon protecting the city from flooding.

Mr Butler added:

“One of the main benefits of the project is that it also reduces the likelihood of pollution entering the Floating Harbour.

“The Plimsoll CSO scheme is one of a number of projects that has taken place in recent time to ‘clean-up’ Bristol as part of Wessex Water’s commitment to ensure our operations do not impact on the environment.

“Areas such as The Waterfront along Bristol’s floating harbour are now attractive places to live, work and visit – a significant contrast to 40 years ago when Bristol’s sewerage system was not fit for purpose when millions of gallons of sewage were discharged into the river everyday.”

Although the construction work took place alongside one of Bristol’s major commuter routes which carries more than 38,000 vehicles a day to the M5 there was no major impact on traffic as Wessex Water ensured the main working site was off the A4 which meant only one lane had to be closed.

Mr Butler added:

“For many commuters our construction site has been a regular feature on their way to and from work. Although cranes and machinery have disappeared, below the ground a number of complex chambers and pumps will be working hard during times of intense rainfall to protect our city from flooding.”