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Barrage make up

Cardiff Bay

Several projects have combined to make up the unique S-shaped tidal dam which now runs across the Cardiff Bay from Penarth cliffs to Queen Alexandra Head in the Cardiff Docks.

Embankment and breakwater, sluices and locks, the complicated fish pass and even the temporary sand bund built for the concrete works, were each major challenges, now come together to impound a new waterfront for the city.

Fresh water will eventually fill the lake which contains the estuaries of the River Taff and River Ely, It forms a 200ha enclosure, 4.5m deep with a potentially picturesque 12.8km waterfront.

The dam, designed by a team from Gibb, comprises an 800m long embankment running from the northern docks end of the bay in a curve following the main shipping channel. The 9.3m high structure has a seaward rockfill core with side armouring; behind is a wide and gently sloping sand embankment separated by graded filter layers, acting as an impermeable layer and a base for landscaping of the lakeside. The complete embankment is 100m wide at the base, tapering to 25m, and topped by a service road.

Abuting the embankment is the concrete wall of the 8m wide fish pass, one of the largest in Europe. It is a complex construction of sluices, channels and fishpools which also forms the north side training wall of the sluice channel. Here five double leaf vertical steel gates, each leaf 9m wide and 7.5m high, control flows of up to 2,300m3/s of water, both in and out of the bay if necessary. In final operation the gates will hold back spring tides which reach 7.6m above mean sea level, considerably higher than the 4.5m reservoir level. The fifth gate next to the fish pass has a seaward side enclosure wall around its stilling basin, retaining water in a pool where fish can gather.

The southern sluice side is a sand filled cofferdam island on which is sited an elevated steel frame control building. Then comes a complex of three boat locks. Varying in width between 8.2m and 10.5m, each is 40m long, capable of accommodating up to nine yachts moored three abreast around pontoons. Elegant bascule bridges connect the roadway across the shallower, inside gates.

A short double skin sheet piled cofferdam forms an abutment on to the southern Penarth shore and also acts as the northern part of a carpark area for operations staff and visitors.

Finally, the two breakwater arms curve seawards from the lock exits, forming an enclosed harbour shelter. These comprise 17m high concrete box caissons open on one side to let in waves and dissipate energy by reflection effects inside. The outer flank of the breakwater also doubles as the water flow training wall on the seaward side of the sluices.

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