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How to stave off a flood invasion

In January 2005 the centre of Carlisle was devastated by flooding. Now engineers are reaching the end of a project that should ensure its residents keep dry. By Paul Thompson

Scheme: Carlisle Flood Alleviation – Warwick Road relief sewerage/wastewater upgrade
Client:United Utilities
Contractor:DCT Civil Engineering
Contract Value: £3.5million

It is difficult to comprehend just how much water surged through the streets of Carlisle during some of the city’s darkest days. Thanks to its strategically important location close to the border between Scotland and England the town has faced and driven off challenges from would-be invaders throughout its history. But the watery challenge it faced during the first few days of January 2005 even a city with such a proud heritage could not repel.

During the first week of the new year storms lashed across north west England. At the rain’s peak during the early hours of 8 January as much as 25cm fell on the already sodden ground of the Lake District and Cumbria, most of this ending up surging down the River Eden which pours through the centre of Carlisle.

By midday the same day some 90,000 tonnes of water was trying to force its way through the river channel, a volume that the normally sedate river and the aging Victorian brick-lined sewer system that served the town was utterly incapable of handling.

Thousands of residents had their homes ruined and estimates have put the total cost of the flooding to the city at £500 million. Now though, engineers are close to completing the flood defence and sewage system improvements that should prevent a repeat.

Three-pronged attack

While flood alleviation work involving raising and replacing flood defence embankments alongside the river’s banks has already been completed, engineers and contractors working for sewerage and water network provider United Utilities (UU) are well on the way to boosting the capacity of Carlisle’s sewerage network.

As part of a three phase £20 million upgrade the company is set to install and commission new trunk sewers, pumping stations and combined sewer overflow chambers across the city to help protect it from future flooding.

The £2.2million first phase of the sewer system upgrade focused around installing 1200m of new 1800mm diameter trunk sewer system in the Willowholme area to the north of the city centre and was completed shortly before Christmas in December 2008. The second phase with an overall budget of £7.2 million focuses on sewer improvements around the Warwick and Strand Road areas to the north east of the city centre in full view of Carlisle United’s Brunton Park football stadium. This area was badly hit by flood waters that galoshed through people’s homes, leaving them to clean up the mess caused by a cocktail of flood water and raw sewage.

A further £6million or so is due to be spent back in the Willowholme area providing a new pumping station under phase three of the project but that will be the final piece of the overall jigsaw which sees Oldham based contractor DCT Civil Engineering pick up the baton to deliver the vital second phase through its Scotland Region office in West Lothian.

Flooding potential

“The whole programme is due to be completed by March 2010,” says John Parr, senior project manager at United Utilities. He concedes that there has always been the potential of flooding in the town but explains that it wasn’t until the Environment Agency decided to build up flood defences around the Warwick Road stretch of the River Eden that United Utilities looked at the capacity problems that the network in the area faced.

“Normally river flooding events and capacity problems occur at different times of the year. Rivers tend to flood during the winter whereas our network problems are generally in summer thanks to flash flooding,” he says, adding “The Environment Agency rushed to site with the first phase of its flood defence work which included heightening defence bunds alongside the river to accommodate a 1 in 200 year event. That was when we realised we may have capacity problems.”

The existing system is typical of a Victorian network. It has been adapted many times during its life and has dropped and sagged in some areas but to ensure the best results from any renewal scheme a complete manhole survey and flow model was carried out by the UU team with the results plugged into a flow model and designed around a 1 in 30 year storm. The design team also took river data into consideration too which forced a rethink on some of the facilities.

“Our overflow goes into the river and our outfalls get blocked as its level rises. That is why we will have had to build the new pumping station, so that should the outfalls get blocked by the rising water we can pump out over the top of the embankments,” he says.

Within the second phase of the scheme DCT is building a large combined sewer overflow (see box) at Strand Road close to the pinch point where the River Eden runs closest to the properties and schools in the catchment area of the project. This will screen out some of the debris from the flow before it is piped directly into the river through a strengthened and beefed-up reinforced concrete outfall structure.

“There are no environmental issues with the effluent from the outfall. In storm conditions it is classed as stormwater,” explains Mr Parr, adding “But building the outfall head wall structure can’t start until later in the year because of the salmon running through to spawn and otter activity in the area.”

Normally work of this ilk starts at the lowest end at the outfall and works its way back up along the line of the sewer pipe until it reaches the CSO. By fixing the height of the outfall first the rest of the system can be built with exactly the right falls and gradients throughout the network. Unfortunately that is not the case at the Strand Road CSO thanks to the breeding habits of Atlantic Salmon and the DCT team will install the new outfall later in the year.

By then the residents of the ‘Border City’ should be able to sleep soundly knowing that the disaster that struck in January 2005 is nothing more than a fading memory.

Cutting through Carlisle

“Some of these roads were the worst hit,” says Craig Shuttleton, site agent for contractor DCT Civil Engineering, pointing at tidemarks that are still visible on walls halfway up the sides of houses close to the scheme’s Strand Road site compound. That the project he is helping to deliver is benefitting so many local residents is one of the many reasons why there have been no complaints so far.

“Everyone is very understanding. They realise that although the work is disruptive the benefits will be huge,” he says.

Nevertheless upgrading some 3 km of combined foul and stormwater sewer is a difficult task, particularly when as much as 95 per cent of its length is sited in the highway.

During the project the DCT team will carry out online replacement of the existing oval 750mm x 500mm brick lined sewer with new concrete 500 – 1800mm diameter pipes, each weighing as much as 3.5 tonnes. These will be laid at a range of depths of between 2m and 6m and set on clean stone pipe bedding at grades dependant on their diameter. Placed so that the new pipe crown sits at the same level as that of the existing the pipes will be fitted with integral rubber gaskets on the female end, allowing the male end to settle in the fit before the whole section is backfilled using type 1 sub-base in 300mm layers and compacted before the final road construction is reinstated.

Each of the two pipe laying gangs will over pump effluent from one manhole to the next and install new pipes along that section length in bays lengths of 6m. The gang is protected by a closed strut trench frame to a full temporary works design which is installed in three sections. It is slow going even on a good day admits Mr Shuttleton.

“Some days, on the deep sections, we will get about 2 bays or 4 pipes installed. It takes time but the ground is not that good – it’s mostly river bed sands and gravels on top of solid clay – it’s not very consistent and we are getting lots of groundwater and running sands along the excavation,” he says.

Screen goddess

The huge 18m long by 8m wide by 5m deep excavation at the DCT site team’s Strand Road compound to the north east of Carlisle’s city centre is the site for the massive combined sewer overflow chamber. It is this chamber that will hold the screens which will sift out some of the larger solids in the effluent during storm conditions. Despite not being structurally loaded to any great degree its 108mm thick C40 concrete walls are reinforced using 12mm diameter steel bars while the 56mm thick concrete base features 32mm rebar.

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