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Slowing the flow

New biodiverse wetlands and recreational enhancements are a useful by-product from the Environment Agency’s major flood protection works in Wakefield reports Adrian Greeman.

When extraordinary summer rains swamped Yorkshire in 2007 news cameras were mainly focused on the plight of residents forced out of their homes in Sheffield and Hull. But the streets of Wakefield, just south of Leeds, got their share of flooding too.

“Wakefield was not hit quite as dramatically, but on the other hand it does get routinely flooded, every couple of years perhaps,” says Environment Agency project manager Jamie Wolstencroft. As a result, the Agency has been running a programme of small-scale alleviation works in the town for more than two decades.

Once and for all solution

Wolstencroft, who works for the Agency’s national capital programme management service, is now managing a much bigger scheme, worth £12M and involving £7M of construction work, which is attempting to deal with the problem once and for all in the northern part of the town.

This area has suffered continuously from water backup along two watercourses - “becks” in Yorkshire parlance - which run into the river Calder as it winds through the centre of the town.

The two streams drain a local watershed that forms a “bowl” in the landscape in which a number of Victorian and early 20th century housing estates and a small village are situated. Run-off can collect very quickly, and the little becks can become a torrent, sometimes up to a metre in depth within minutes.

“Wakefield was not hit quite as dramatically, but on the other hand it does get routinely flooded, every couple of years perhaps”

Jamie Wolstencroft, Environment Agency


“Just before they run into the river they join together and then pass through a double brick arch culvert and along a 400m channel to the river junction,” says Wolstencroft. The twin culverts, running under the main dual carriageway into the town centre from the M1 motorway, rapidly choke and the water backs up. Other sections of the becks upstream can also overtop separately causing local flooding.

This latest scheme is being carried out by the Agency’s term contractor Morrison Construction Civil Engineering, part of the Galliford Try group. It comprises a number of separate elements to improve the culverted watercourse lower down and create water retention buffer areas upstream. Two dams are being created, with wetland reservoirs behind. These can fill in in a sudden downpour and slowly release their water afterwards.

Separate sections of work

“These are separate sections of construction and there are various other small improvement works too,” says Morrison senior agent Steve Hamer. They include renovating and enlarging beck channels in parts of the town, including through the car parks of a local retail park and in a green open area just outside Wakefield high security prison.

The two dams and culvert expansion are the largest individual elements of the job. First of these is the Fenton dam, where a very small retaining structure currently exists that is, ostensibly, capable of throttling flow, “but not working very well”, according to Wolstencroft. A new, much larger, earthfill dam is being built here, measuring 400m in length and up to 50m in width, that will enable 233,000m3 of flood water to back up into a reservoir area behind.

“The difficulty is that the council insists that three of four lanes must remain open on the main road at all times”

Steve Hamer, Morrison


Fill material for the 40,000m3 compacted embankment is coming from an upstream field, which the Agency purchased along with the dam area itself. The field is being excavated in a series of undulations so that a marshy pond area will be left behind, with varying depths of water. Conveniently, this fits with the Agency’s policy of trying to create wetland and open habitat as part of its projects, and the landscaped area will eventually be added as a spur to the Pennine Way.

The borrow material is mainly clay with a small amount of granular material, and is compacted in layers using sheep foot rollers. The Agency’s designer, Jacobs, originally asked for a highway embankment specification, but Morrison employed its own consultant, PTS, to tailor this to the ground in the borrow pit, which had a higher clay content than thought.

Joint working

This move was typical of the joint working with the Agency and the ongoing team effort to value engineer the project as it unrolls and save costs, says Hamer.

A 100m wide spillway area of vegetated concrete grid on the downstream face will finish the structure, and prevent erosion in the event of overtopping.

On the other side of town a second dam is being created from an old disused railway embankment. This runs for 250m at a height of 4m across what is now a highly scenic parkland area of grassland, trees and ponds.

The embankment already acts as a flood retention structure since an existing culvert through it limits flows, but it will be beefed up by driving a sheet pile wall along the centre to ensure watertightness. The “near collapsed” brick culvert is being replaced with a new concrete structure, fitted with a simple orifice plate, and an overflow culvert in case the first should be blocked. Two 900m diameter overflows owned by Yorkshire Water for its combined storm and sewer system nearby are also being incorporated into the scheme.

Curved embankment

Downstream of the park a new curved embankment will enclose a small field to protect housing that surrounds it. This embankment will direct overflow from an overtopping spillway area on the main dam in the event of high floods. Upstream of the park, at the small village of Wrenthorpe channel widening and deepening works have been undertaken. To do the finickity excavations, “in people’s gardens and tight spaces”, says Hamer, the contractor has been using,a device that works like a giant vacuum cleaner. It allows delicate work to be carried out around tree roots and service lines without damage.

The most complex work, which began in early January, involves the expansion of the downstream channel, the Ings Beck, where the two streams merge and then take a right-angled turn into two brick arch culverts under the main road.

The capacity of these 20m long “tunnels” is to be increased by adding two new concrete culverts alongside, and expanding the open channel section of the becks where they run on into the river Calder.

Open channel increased

The open channel is also being increased by 20m just south of the main road culverts where four old buildings stood previously on a steel platform built over and enclosing the watercourse. They were dilapidated and have been purchased and demolished.

The new open space will eventually form a small open park, but serves in the meantime as a worksite to help with the complex construction of the culverts. These comprise two flat concrete box sections with a profile kept low enough - at 1m high and 3m wide - to fit beneath a clutch of major services in the road above.

“The difficulty is that the council insists that three of four lanes must remain open on the main road at all times,” explains Hamer. “The narrow space left is just too little for the work, which means dropping in and fitting together precast concrete units 1.5m long.”

Craned units

Morrison’s solution is to use another 30 of the precast units for a temporary culvert that will be made by craning units into the channel just before the turn under the road. The culverts will be installed in three excavated sections, 8m, 4m and 8m long respectively. The first will be constructed on the south side using a working excavation in the future “park” area with a concrete “landing platform” at the bottom. Precast units that connect with female and male endings will then be dropped onto steel rails and pushed forwards under the services, which will be supported by beams.

The second, short, excavation is easier, as there are no services to worry about and units will just be lifted in and pushed together. The final section may also involve a landing stage and pushing forwards. But that is to come later this spring.

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