Kijlstra’s precast headwalls have helped ensure the timely completion of a major sea defence project near Selsey on the West Sussex coast.
The Medmerry Managed Realignment project (recently completed by Team Van Oord, a British-Dutch joint venture contractor specialising in coastal and marine engineering) is a managed realignment scheme where new defences are built inland from the coast and a new intertidal area is allowed to form on the seaward side.
The new defences comprise an earth embankment approximately 6.8km long enclosing the new intertidal area and protecting inland property and infrastructure from flooding.
Team Van Oord has constructed a trapezoidal drainage channel along the landward side of the 5.2m high embankment to collect the flow from existing channels – known locally as ‘rifes’ – which drain the surrounding farmland.
This flow is channelled to four outfall structures, incorporating Kijlstra’s precast headwalls, that pass through the embankment and allow discharge to the intertidal area.
Team Van Oord member contractor Mackley Construction, which was tasked with building these outfalls, chose the precast option mainly for speed and quality purposes.
The traditional method, using in-situ concrete and brick or block construction, would have required a great many man hours working on-site in four small but remote locations, says Kijlstra project manager Stephen Gainsley.
“Access to each location was difficult and the ground conditions were very soft,” he says. Minimising the amount of work required in-situ helped reduce plant and vehicle movements as well as cutting the number of operatives needed on site.
Brought in at an early stage in the design process, Kijlstra developed the design of the outflows so the headwalls are supported on in-situ concrete bases. “The structures are all similar, though not identical,” says Stephen Gainsley.
Mackley assembled the 3.5m high headwalls on site, simply lifting the units into place on the prepared bases. The modular design concept allows various types of flow control devices, such as flap valves, penstocks, weirs and hydrobrakes, to be fitted at the factory, thus speeding up site installation and ensuring a quality finish.
Mackley’s project manager Terry Gretton says that precast had clear benefits over traditional in-situ methods: “This was much quicker and less likely to encounter snags on site,” he says.
The four outfalls were completed in early autumn and now allow fresh water to drain through the embankment while keeping salt water out.
In early November, Team Van Oord opened a 110m long breach in the shingle bank, allowing the tide to sweep across the intertidal area enclosed within the new embankment and changing this part of the coastal landscape forever.