A network of high capacity rainwater run-off lagoons has been installed by geotechnical specialists Maccaferri at Lincolnshire’s new, state-of-the-art Energy from Waste [EfW], recycling plant at North Hykeham, Lincoln.
To manage building run-off and surface water drainage at Lincoln’s new Energy from Waste recycling plant, Consulting Engineers, Arup devised a two stage system of ground level “wet & dry” holding lagoons, constructed by Maccaferri.
The Lincoln EfW plant, which is designed to treat 150,000 tonnes of domestic waste per year and generate enough electricity to power 15,000 homes is built on a low lying brown-field site with a high local water table.
At planning stage, Environment Agency requirements stipulated that the development should have a maximum surface water discharge rate no greater than that of a comparable green-field site.
Limited availability of space within the site ruled out the use of a conventional SUDS–style system of large ground-level attenuation ponds. These would normally be used to allow return of surface water run-off from the buildings and surrounding hard-standing to the sub-soil through natural percolation.
To resolve the drainage issues, Consulting Engineers, Arup proposed a two stage system of ground level “wet & dry” holding lagoons – the smaller wet feeding a larger dry lagoon. Commenting on their design, Rhodri Lea, Arup Senior Civil Engineering Technician said: “The surrounding countryside is criss-crossed with minor streams and waterways and we needed to slow the surface-water discharge rates to around 10.0 litres per second to prevent them being overloaded. The two-stage plan gave us the required rainwater attenuation, even scaled up to a once in a 100 year event.”
To help with the detailed design Arup turned to Maccaferri whose experience in watercourse management and erosion protection proved invaluable in the construction of the lagoons.
The Maccaferri solution centred on impermeably lining the two ponds using the Company’s Bentomat, reinforced geosynthetic clay liner and enclosing them within 2.0m high stone filled, twisted-wire Gabion retaining walls. These near vertical walls help maximise the holding capacities of the ponds within a tight overall footprint.
The connected lagoons take advantage of slight falls in levels; the higher and smaller “wet” lagoon, roughly 35.0m long x 5.0m wide and 2.0m deep, accepting initial storm surge from roofs and surrounding hard pavement before gravity discharge through pipe-work to the much larger and lower “dry” lagoon.
The higher capacity dry lagoon, measuring approximately 100.0m x 10.0m x 2.0m, then allows managed return of captured surface water to nearby water courses. Because of the impermeability of the Bentomat liner, virtually no retained water is lost through percolation.
Maccaferri Bentomat is a geosynthetic clay liner specifically designed for landfill and lagoon lining. It consists of a layer of natural Wyoming Granular Sodium Bentonite sandwiched between a woven and non-woven geotextile. It is self-sealing if punctured, has excellent slope stability and high chemical and climatic resistance.
The Bentomat was supplied to on 5.0m wide rolls and installed by Maccaferri Construction, the Company’s specialist subsidiary. The liner was overlain with a 300mm layer of sharp sand followed by a 200mm layer for compacted site won fill. These layers give added protection to the liner and eliminate any risk of it “floating” due to back pressure of ground-water.
The lagoon walls were constructed using Maccaferri twisted wire Gabions which were hand filled with locally sourced stone. The walls provide earth retention and slope reinforcement and give added protection to the liner.
Maccaferri is the originator of the Gabion system which has become a familiar sight on construction projects throughout the country where their simplicity, durability and speed of installation have made them a default material for retaining wall construction.
Their physical mass and open, free draining composition gives them enormous structural strength and resistance to hydrostatic [water] pressures.
From an aesthetic perspective, their use of usually locally sourced fill material – typically off-cuts of quarried stone or slate, make Gabions a natural choice in areas of environmental sensitivity.
Maccaferri has held the original patent since the 1890s and is the World’s biggest producer.
Today's Maccaferri Gabion is a rectangular wire mesh box, nominally 2.0mx1.0mx1.0m, made from double twisted hexagonal mesh of steel wire. It is a simple design which has stood the test of time and which has changed relatively little over the years. Gabions are also BBA Roads and Bridges Certified with a recommended design life of at least 120 years.
The Lincoln Energy from Waste plant is seen as a landmark project which will dramatically reduce the amount of waste Lincolnshire sends to landfill.
Constructed by Civil Engineers Clugston for waste recycling specialists FCC, the site is expected to be working at full capacity in early 2014.
With the devastating effects of winter flooding in the south and west of England still fresh in the public mind, engineers continue to address the problem of extreme rainfall intensity and devise ways to improve the ability of rivers and watercourses to handle higher flow rates.
To help them assess the effects of changing channel profile and surface characteristics, Maccaferri has developed a straightforward and effective software package.
Their “Macra 1” software allows users to calculate flow depths in rivers and watercourses and see how altering the channel profile or modifying surface characteristics such as adding vegetation or applying protective measures can affect watercourse performance, instantaneously.
Readers can request the free software through the Maccaferri website.
