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Lagoon link

Middle East

A canal in the United Arab Emirate of Sharjah not only provides an ingenious marine solution but has also given the city a tourist attraction. Steve Turner reports.

The landscape in the centre of Sharjah enchants the visitor, with a beautiful inland lagoon surrounded by a spectacular high rise skyline.

But until recently all was not so well in the Khalid lagoon, and wildlife and the recreational events held on the water were beginning to be affected.

The head of the lagoon lies 6km from the sea, and tidal movements were not strong enough to renew the water. The result was stagnation and the water becoming very dark in colour.

However just a kilometre away, separated by land built up with roads, offices and shops is the Al Khan lagoon, kept fresh by frequently changed water.

Halcrow was commissioned by His Highness Dr Sheikh Sultan bin Mohammed Al Quassimi, the ruler of Sharjah to design and oversee the construction of a canal to link the two lagoons providing a source of fresh water to Khalid. A computer model of the coastline predicted that the tidal flow alone would be enough to flush sufficient water volumes into Khalid lagoon.

A set of tidal control gates at the Khalid end of the 30m wide canal are the key to the operation's success.

After high tide the Khalid empties first, creating a head difference on either side of the gates.

When this difference reaches 25mm, automatic sensors cause the gates to open allowing water to flow through. They remain open for five to six hours.

At low tide water filling Khalid back flows into the canal and on to Al Khan. When this flow reaches 15m/s the gates close, retaining fresh water in the now replenished lagoon. This sequence repeats twice a day with the tides.

The 300mm thick canal walls comprise 3,386 mass concrete blocks and 752 coping blocks, all cast on site. A Freyssisol support system of reinforced earth anchors quay walls into the fill behind using paraweb straps.

Canal excavation of the canal took place within sheet pile cofferdams. On completion water was overpumped and siphoned into the canal Included in Halcrow's design and supervision package were a footbridge, three 42m road bridges across the canal, and 10km of roads.

But the addition by His Highness, well into the contract, of public buildings alongside the canal left Halcrow just five months to the completion date to comply, a challenging task as Halcrow resident engineer Martin Johnson explained.

'Five contractors took a building each, and another did the footbridge. At the time we were still building the canal walls, so had to backfill urgently to allow the building piles to commence.

It was hectic, but it was completed on time'.

The gates are being handed over to the client this month, but the system is already in operation. And police divers who check the Khalid lagoon for obstruction before the start of regular Formula 1 powerboat races report much improved visibility. Fish now abound and there have even been reports of a turtle basking by the waterside.

The canal and footpaths provide a scenic route linking the two lagoons.

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