05/08/2010
GLASGOW FLOOD PREVENTION SCHEME INCORPORATES NOVEL EMBANKMENT PROTECTION SYSTEM.
In the hills above Glasgow, engineers are working on phase one of a £53 million project designed to protect the city from millions of pounds of flood damage. Project Scotland takes a look at part of Glasgow City Council’s White Cart Water Flood Prevention Scheme, where a novel approach to prevention of embankment scour by high water velocities, has been adopted by main contractor, VolkerStevin.
Existing flood defences along the White Cart Water corridor were piecemeal and isolated and the threat of repeated flooding presented major insurance difficulties for householders, businesses and the local economy.
The principle behind the Flood Prevention Scheme is one of catchment management and involves, as part of phase one, the formation of three flood storage areas upstream of the city. These are intended to temporarily hold back the bulk of floodwater generated by extreme rainfall and control the release of water passing downstream through the city, to an acceptable level.
At the downstream end, where the river passes through the residential and business areas of Cathcart, Langside, Shawlands, Pollokshaws and Auldhouse, a network of flood defence walls which, as part of phase two of the project, are being installed to provide containment to the river and to provide protection for the surrounding areas.
At the Cathcart section, Consulting Engineers Halcrow Group Ltd, were faced with particular challenges because of high river velocities and the ground conditions peculiar to that area. Alan McGowan, Halcrow Project Manager explains.
“The original intention was to use conventional steel sheet piles to create a physical barrier where possible. At the Cathcart section, we found that the underlying rock head was very shallow and the depth of penetration for the pile could not be achieved. Our design engineers’ alternative solution was a 0.5m thick cantilevered, reinforced concrete flood defense wall, built in 5.0m long sections, 2.5m high”.
Existing land use on the 'protected' side of the flood defense wall dictated the alignment of the wall. The alignment was fairly close to the river which is 5-10m wide at this point, and the existing river bank was fairly steep due to the natural processes of river formation. In order to maintain a similar river bank profile, a reinforced earth slope was required to accommodate the steepness of the reinstated river bank.
Although the wall was designed to be more than capable of preventing flooding of the surrounding areas, erosion of the toe of the reinforced slope was identified as a potential risk due to the extreme velocities of water in this reach of the river. Consequently, Perth based erosion control specialist, Maccaferri was brought in to advise.
Their solution, at three sections of the river, was a reinforced soil structure comprising stone filled gabion baskets in combination with high strength polypropylene geo-grid overlaid with protective erosion matting.
Maccaferri Technical Sales Engineer, Drew McCartney, explained. “Conventional gabions would not have achieved the required visual appearance. Instead, tapered or trapezoidal gabions were used which would match the profile of the original river bank.”
The galvanized woven wire gabions were supplied with a PVC coating to provide additional protection against water immersion. These were modified on site to create the tapered profile by main contractor, VolkerStevin. The gabions were placed and hand filled with locally sourced stone to provide the necessary resistance to water-scour.
Each course of gabions was laid in combination with Maccaferri Enkagrid MAX 30 biaxial polypropylene geo-grid, sandwiched at 500mm lifts with compacted granular backfill between the gabions and the river-side face of the concrete wall.
According to the manufacturer, Enkagrid Max has a laser-welded structure of chemically inert extruded polypropylene bars which has high resistance to UV degradation and excellent passive bearing capacity resistance. Its reinforcing function can also reduce the amount of granular materials needed for a specific bearing capacity. This reduction in back-fill requirement is better for the environment through reduced aggregate extraction and fewer truck movements, to and from site.
When combined, these components form a high strength reinforced soil structure which, not only protects the toe area of the reinforced concrete wall from water attack, but also creates a visually pleasing embankment slope of roughly 40 degrees, in keeping with a natural riverbank appearance.
With construction under way since 2008, phase one is due for completion early in 2011 and phase two towards the end of 2011
For further information on the progress of the scheme see: www.whitecartwaterproject.org
