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Diverting protest

New estimates of possible flooding have meant a major upgrade of Australia's largest concrete dam

Warragamba is Australia's largest concrete dam, holding about 80% of the city's water supply. Located about 70km south west of Sydney, the dam is owned by a state government entity Sydney Catchment Authority.

Completed in 1960, the dam was upgraded in 1989 after reassessment of peak outflow, to increase capacity from a 1:700 flood in one year safety level to 1:100,000. Work by previous owner, Sydney Water raised the crest by 5m from its original 137m and added post-tensioned steel anchors.

However recent estimates showed that probable maximum flood (PMF) levels could be higher than previously thought possible and plans were put forward to raise the crest height of the dam by 23m. This solution outraged environmentalists who claimed that it would threaten the upstream wilderness area and the scenic Kowmung river.

The alternative was an auxiliary spillway, which is now under construction on the east bank of the Warragamba Gorge. The work requires major earthworks, installation of the largest fuse plugs ever used on a dam in Australia, and extensive landscaping. The project is due to be completed by the end of 2001 at a cost of A$150M ($229M).

The spillway will prevent dam overtopping and possible collapse in extreme flooding which could cause an estimated A$4.5bn of damage downstream.

Construction work is managed by Australian Water Technologies. The main construction contract, including all the earthmoving, was awarded to Abigroup Contractors in late 1998.

The spillway has been designed as a curved concrete lined chute, 700m long. Starting upstream with a broad flat section adjacent to the right abutment of the dam, the chute falls away to 50m at its lowest point, narrowing in width from over 190m to 65m, with a maximum wall height of 50m.

Super-elevation is included from about 300m from the dropover point at the upstream end, and the longitudinal slope increases down the spillway in increments to a maximum of 16.7% leading to the flip bucket - the curved lip of the spillway which serves to disperse the energy of the water.

The New South Wales Department of Public Works and Services was responsible for the engineering studies, concept and specifications for the early work and the auxiliary spillway design, working with Snowy Mountains Engineering Corporation which completed detailed design work for the spillway.

Construction will require blasting and excavation of 4.3Mt of earth and rock (see box).

The spillway channel contains two main cuttings, the upper one through the dam abutment ridge and the lower through Waratah Ridge, with maximum excavation to depths of 45m. Where it crosses the Folly Creek gully, the right side of the spillway is being supported by a rockfill cement-stabilised embankment.

The lowest batter on either side of each ridge cut is vertical, and large sections will be faced with concrete. The upper batters are being sloped at 25% and will be unlined. This should minimise joint failures in the batters which are being formed, generally, 15m high and with 4m berms between each elevation.

The lining, which will take 65,000m of concrete, will have a maximum thickness of 300mm. Anchors on the floor of the spillway resist uplift pressure. Eductor holes provide underfloor drainage.

Fuse plugs installed at the entrance to the auxiliary spillway will prevent it going into use unless there is a major flood. The five 14m high embankments are separated by divider walls which include the piers for the access bridge to the dam crest. The first fuse plug embankment is designed to erode rapidly at a flood stage equivalent to about a 1 in 750 annual exceedence probability event.

All embankments are set to erode and fill material wash away in time to maintain the mean flood level below the target level for the critical PMF.

In addition, the walls' curved plan shape means they will act partially as hydrofoils during flood discharge conditions.

Spoil from the excavation will be deposited at a minor site on the right bank of the dam, but mainly to form a 20m high terraced embankmnet on the left bank, adjacent to the national park.

The primary excavation tool is a giant 110t Komatsu PC1100 face shovel - more usually found in mining operations. After drilling and blasting, the Komatsu loads material into the 18strong fleet of 50t Caterpillar 773D and Komatsu HD465 dumptrucks which take it to the fill areas. The landscaping fleet includes Komatsu D375D and D475 dozers and Caterpillar D10s, supported by a Cat 992 wheel loader.

The scale of the muckshift can be deduced from the capacity of the fleet which during the first three months of the contract was shifting 10,000m 3aday. That figure has now increased to 14,000m

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