New biodiverse wetlands and recreational enhancements are a useful by-product from the EA’s major flood protection works in Wakefield near Leeds. Adrian Greeman reports.
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 (EA) project manager Jamie Wolstencroft.
As a result, he says, the EA has been running a programme of small-scale alleviation works in the town for more than two decades.
Wolstencroft, who works for the agency’s national capital programme management service, is now managing a much bigger scheme, worth £12M in total 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, usually just a few centimetres deep, 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,”
“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, being carried out by EA term contractor Morrison Construction Civil Engineering, 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 them that can fill in the event of sudden downpour and slowly release their water afterwards.
“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.
“All these works need to be synchronised, because each section depends on the flows created by the others,” explains Hamer, whose site office for the two-year project is a converted former bakery in the town.
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.
“In the centre there is a concrete structure with a steel water vortex flow control, a hydrobrake with a 0.86m3/sec capacity from Hydro International,” explains Hamer.
“Alongside, there is also a remotely operated penstock for emergencies, for example if there is a blockage.”
Fill material for the 40,000m3 compacted embankment is coming from an upstream field, which the EA 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 exactly with the EA’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 a Pennine Way walking route.
“Jacobs accepted the bespoke spec, which allowed us to reduce the testing regime,”
The borrow material is mainly clay with a small amount of granular material, and is compacted in layers using sheep foot rollers.
The EA’s designer, Jacobs, originally asked for a highway embankment specification, but Morrison employed its own consultant, PTS, to tailor this specifically to the very particular ground in the borrow pit, which had a higher clay content than thought.
“Jacobs accepted the bespoke spec, which allowed us to reduce the testing regime,” says Hamer.
This move was typical of the joint working with the EA, he says, and the ongoing team effort to value engineer the project as it unrolls and save costs.
However, work on the dam has thrown up some interesting challenges.
The area was a Second World War “dummy site” to fox German bombers, and so had to be examined beforehand for unexploded munitions, though none were found.
There was also a danger of old bell workings, or pit-and-stall mine cavities, in this coal mining area, and a 6m grid pattern of 30m deep grout injection holes was made across the dam’s footprint, though the take was not great.
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.
Downstream of the park a new curved embankment, currently two-thirds complete, 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.
Once again, spoil is being taken from the parkland upstream, allowing for a sequence of new ponds and wetland to be landscaped as part of the scheme.
A local volunteer group, the “Wrens”, which looks after the park, has been consulted on this programme.
Upstream of the park, at the small village of Wrenthorpe channel widening and deepening works have been undertaken.
“The difficulty is that the council insists that three of four lanes must remain open on the main road at all times,”
To do the finickity excavations, “in people’s gardens and tight spaces”, says Hamer, the contractor has been making use of a vacuum excavator, a device that works like a giant vacuum cleaner, which Hamer says can allow very delicate work to be carried out around tree roots and service lines without damage.
The most complex work was just beginning when GE visited the site in January.
This is 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.
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.”
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.
“That allows us to form a kind of bridge along the channel and we can divert two lanes of traffic that way into a side road. We then have four lanes of traffic to play with at all times, leaving just enough space,” says Hamer.
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 this spring and summer.