The major source of Sydney's water supply, Warragamba Dam, Australia's fourth largest dam, was raised to its present height of 142 metres in 1989. Its primary function is to provide a water supply for the Sydney Metropolitan area and when adequate quantities of water are stored to meet these needs, electricity is generated and fed into the NSW grid by the hydro-electric power station located at the base of the dam. Warragamba dam is classified a 'Major Dam' under Australian National Committee on Large Dams (ANCOLD) definitions and is in the High Incremental Flood Hazard category.

To prevent overtopping, an auxiliary spillway is being constructed adjacent to the dam to divert floodwaters in excess of the capacity of the existing spillway around the dam wall. The combined spillway capacity (the existing central spillway plus auxiliary spillway) will be sufficient to safely pass a Probable Maximum Flood (PMF).

The auxiliary spillway is a major civil engineering project due for completion in mid-2001. It has been undertaken by the Sydney Water Corporation (now known as the Sydney Catchment Authority) to improve the safety of the dam. The contract for the design and engineering of the spillway was awarded to SMEC, the Snowy Mountains Engineering Corporation.

The project involves major excavation - some 1.8 million cubic metres of sandstone is being removed and transported to a soil emplacement- and the concrete lining of the auxiliary chute spillway which will be approximately 650 metres long and drop 50 metres before discharging to the river.

The linings, and joints in linings within the spillway, have to be designed to withstand spillway flows at very high velocity and the components must be capable of handling considerable hydro-dynamic forces.

Five fuse plugs - embankments that wash away when overtopped - are being built downstream of the inlet to prevent the auxiliary spillway from operating in flood flows smaller than about a one in 750 chance of occurrence in any year. The fuse plugs are constructed of rockfill, gravel and sand, with a clay core. Generally, the floor and wall of the spillway chute are in the form of a reinforced concrete lining against the excavated rock surface, however, one section of the spillway over an existing creek line will be constructed on compacted fill with cantilever walls on either side.

A bridge across the spillway, and a new road, are designed to provide access to the crest of the dam, valve house - which channels the water flow, and hydro-electric power station at the base of the dam.

Following concept design carried out by the Department of Public Works and Services, SMEC is responsible for the detail design development - from concept to production of construction drawings.

4D Model was used for design and modelling of the auxiliary spillway and associated access structures such as roads and the bridge approach, along with calculation of spoil volumes. The spillway required comprehensive design work to achieve correct water flow and required a number of design modifications in the dimensions of the deflector walls and fuse plugs to achieve appropriate flow control.

The design had to be continually modified for volumes of spoil placement and slope analysis to fit placement within specified confines nominated by the client. The spoil would not fit into the area initially defined and required many modifications of the design.

Reports of cut and fill areas and volumes were given on a section-by-section basis plus colour coded depth volumes for validation and plots. Similarly the perspectives provided using 4D were an easy way to convey a design to people who find engineering plans and contours difficult to interpret.

SMEC is also using the 4D Drainage Module in the Warragamba project. This supports the display of drainage networks typically required for development projects and new land subdivisions.