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Back up flood plan

Giant cones are being installed in dams along White Cart Water to control river flows in an area of Scotland with a history of serious flooding. NCE reports.

For most its course the river White Cart Water travels peacefully through the Scottish uplands to the south of Glasgow. It then makes a 10km journey north-west, first through agricultural land before reaching the city’s urban South Side and passing through Cathcart, Pollok Country Park and Paisley before joining the River Clyde at Renfrew.

But White Cart Water has cast a turbulent shadow over the Glasgow’s South Side for more than a century. Prolonged and intense periods of rain signal trouble for residents and businesses along its urban banks and they are used to waiting and watching anxiously when water levels rise. As little as 12 hours of heavy rain can cause water levels to rise by 6m, turning the river into a raging torrent as it gathers momentum downstream towards the vulnerable suburbs of the city.

20 floods

Since 1908 there have been more than 20 significant floods caused by relatively minor storms. One of the most memorable was at Hogmanay 1984 when 500 homes in Battlefield and Langside were inundated and millions of pounds of damage caused. Only 12 days later floods inflicted further misery on the same area. In 1990, residents were struck again. In 1994, the river burst its banks once more in various places including Paisley, and in 1999 families living in Cathcart and Langside suffered thousands of pounds of damage when the water reached waist height in their homes.

The ongoing White Cart menace was something that Glasgow City Council’s Iain Macnab remembers well.
“As a small boy living in the area I often saw the river flooding. I thought to myself: ‘I would like to sort that river out.’ And it looks like by the time I retire it will be sorted.”

“Our designers had to pioneer ground breaking methods of manufacture and to work closely with Carillion”

Alex Stephenson, Hydro International

Macnab is now head of project management and design at Glasgow City Council and leading the £53M flood alleviation project that will provide flood protection for 1,750 homes and 40 businesses.

Central to the entire scheme was the installation of giant cone-shaped Hydro-Brake flow control devices into the three dams. The system’s internal geometry is designed to enable water to flow unrestricted through it for as long as possible before a self-activating vortex is created when the water upstream reaches a pre-determined height; in a flood situation, this throttles back the water, and releases it at a measured, controlled rate.

“Using the Hydro system enabled the storage capacity within the attenuation area to be used most effectively, because flows can pass unheeded through them for longer than other flow control devices. Alternatives such as orifice plates, weirs or gates would start to hold back the water at a much earlier stage,” says Halcrow project manager Alan McGowan.

“This will now reduce frequency and duration of flooding of the storage area, which is important given that most of the land upstream is agricultural.

“As a small boy living in the area, I often saw the river flooding and said to myself ‘I would like to sort that river out’”

Iain Macnab, Glasgow City Council

 

“It also offers an efficient fail-safe, passive method of flow control. They do not need power to operate, and once installed, they can be left alone to do their job with minimal maintenance for the foreseeable future.”
Manufacturer Hydro International was challenged with building the world’s five largest-ever Hydro-Brake flow controls for the White Cart scheme. New manufacturing and installation techniques were developed for the largest of all, two 8m long, 6m diameter flow controls.

The manufacturer worked closely with Carillion to perfect new installation processes to fix them into the Blackhouse Dam in July and the Kittoch Dam in August. In June three more flow controls 6.5m long and 4m in diameter were positioned in the dam at Kirkland Bridge.

“In order to cope with the flow rates and velocities of the water in a peak storm event, we needed to design Hydro-Brake flow controls to a size never before attempted,” says Hydro International stormwater division director Alex Stephenson.

“The size of the two biggest controls meant that our designers were challenged to pioneer ground-breaking methods of manufacture and to work closely with Carillion in planning and executing their successful installation.

Close working relationship

“Hydro and Carillion worked closely together to pioneer installation methods for the new design. Constructing a dam with a device of this size and type has never been attempted before and a close working relationship between the manufacturer and ourselves was essential to the successful outcome,” says
Carillion project manager Mark Johnston.

Public awareness of 1980s and 1990s floods, coupled with increasing awareness that meteorologists predict more intense periods of rainfall as a result of climate change had made finding the solution for a scheme a major priority for the city council.

In addition, insurers began threatening to charge higher premiums or even to refuse flood cover for high-risk areas. The prospect that residents might be unable to sell their properties or that businesses would become unsustainable was a portent of neglect for an area desperately in need of regeneration.

But finding a solution to tame White Cart Water and its tributaries was not straightforward. Initial studies revealed that the height and force of the floods meant that engineering conventional flood protection using flood defences along the river’s urban banks would require flood walls over 2m high.

“The extent of construction along the river would have been enormous. It was environmentally unacceptable. We would essentially have had to close the natural river system down in some places, leading to a real loss of amenity for residents, as well as important natural habitats,” says Macnab.

“We had to think outside of the box to find a sustainable solution and so we came to consider an attenuation project that held back water upstream during a storm, making the best use of the natural environment.”

In 2002 Halcrow was appointed to design a flood alleviation scheme based on a holistic catchment management principle. Main contractor Carillion was appointed by the city council in February 2008 to construct the three flood storage areas integral to the scheme.

Building flood storage areas upstream to hold back the water during storm events would enable flood walls and defences along the river downstream in the city to be reduced to an average height of about 1m.

Halcrow began by looking at potential sites for flood storage areas in the uplands. In all, 33 sites were considered for size, topography and geotechnical ability to take a dam.

Halcrow established that toconstrain the flow would require attenuation basins on the White Cart Water and its two major tributaries - Earn Water and Kittoch Water.

Various considerations

“There were various considerations to the selection of the final three storage areas and dam design, including the engineering requirements, environmental considerations, the social impact of the scheme and costs. Two of the three dams were constructed using locally-sourced materials avoiding the need to import material and reducing the number of vehicle journeys,” explains McGowan.

The completed scheme will protect against a 1 in 200 year flood, or 0.5% annual exceedance probability. Peak flows will be reduced by up to 45%.

Carillion is constructing three flood storage areas and associated dams. Blackhouse (Earn Water) is located in East Renfrewshire and Kirkland Bridge (White Cart Water) and Kittoch Bridge (Kittoch Water) straddle East Renfrewshire and South Lanarkshire. All three should be complete in January.

Together they have the capability to hold back more than 2.95bn litres of flood water. They will also provide 90,000m2 of rich and diverse wetland habitats. As part of the scheme’s design, plans were made to enhance biodiversity and wildlife habitat of the area with the creation of woodland, scrub, and species-rich wet grasslands, shallow scrapes and ponds and other artificial habitats.

While the flood storage areas are being completed upstream, a second part of the scheme involves the construction of flood defences along the river bank in the city. The project involves the construction of further flood defences in selected parts of the river corridor downstream, including 4.5 km of urban flood walls, pumping stations and the raising of two footbridges.

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