Engineering the Auxiliary Spillway at Warragamba - SMEC

ClientSydney Catchment AuthorityLocationAustralia, NSW
ConsultantSMECContractorJohn Gray

Project Summary

The major source of Sydney’s water supply is Warragamba Dam. Australia’s fourth largest dam, its primary function is to provide a water supply for the Sydney Metropolitan area. 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 in the High Incremental Flood Hazard category and to prevent overtopping of the dam in extreme flood events, an auxiliary spillway is being constructed adjacent to the dam to divert floodwaters in excess of the capacity of the existing spillway.

The auxiliary spillway is a major civil engineering project due for completion in mid-2001. The works are being undertaken on a design and construct basis for the Sydney Catchment Authority (previously known as the Sydney Water Corporation) to improve the safety of the dam. In December 1998, the contract for the design and engineering of the spillway was awarded to SMEC, the Snowy Mountains Engineering Corporation.

The Challenge

The project involves major excavation - some 1.8 million cubic metres of sandstone and overburden is being removed and transported to a spoil 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.

Five fuse plugs - embankments that are designed to wash away when overtopped - are being built at the upstream end of the spillway chute to prevent the auxiliary spillway from operating in flood flows smaller than about a one in 750 chance of occurrence in any year.

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 sides. A bridge across the spillway, and a new road, are designed to provide access to the crest of the dam, the Valve House and the hydro-electric power station at the base of the dam.

The Solution

Following concept design carried out by the Department of Public Works and Services, SMEC is responsible for the development of the detailed design - from concept to production of construction drawings. A vital component in SMEC’s engineering design work is 4D Model (later renamed 12d Model) software. Design draftsman on the project, Michael Kurtz, says it contains all the options necessary to produce a digital terrain model (DTM), including fast triangulation, contouring and sectioning routines, and to calculate spill volumes for accurate placement and subsequent costing of the work carried out.

Warragamba Project Manager, John Gray, said 4D Model software was primarily used for design and modelling of the auxiliary spillway and associated access structures such as roads and the bridge approach, along with calculation of spill volumes.

"Using 4D allows us to continually modify designs," Michael said, "and, most importantly, to do it quickly. 4D allowed us to complete re-designed versions very fast for presentation to the client and to show the designs interactively in plan and sectional views. We also had to continually modify the design to fit the spoil emplacement within specified confines nominated by the client. The spoil would not fit into the area initially defined and required many modifications of the design.

"The perspectives we can provide using 4D are an easy way to convey a design to people who find engineering plans and contours difficult to interpret," Michael said. SMEC uses the 4D alignment module to construct strings consisting of horizontal and vertical geometry. These horizontal and vertical components are created and edited interactively using the IP method on plan and section views.

SMEC is also using the 4D Drainage Module in the Warragamba project. This module supports the display of drainage networks typically required for development projects and new land subdivisions. An added bonus SMEC encountered using 4D was the ease with which the model output could be checked at any stage of the design process.

The Result

Following development of a design component, a copy of the model was produced for checking and further development by design engineer, Tim Loffler. Tim, who had no previous training in the software, said he found it "a most intuitive package" and that he could view and interrogate the model after getting a feel for the way in which the program operates.

"The program is very robust and will faithfully do what you ask it to do," he said.

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