The Eildon Alliance is currently undertaking the upgrade of Eildon Dam as part of Goulburn Murray Water’s dam improvement program. The Alliance is responsible for all aspects of the project, including planning, approvals, detailed design and documentation, construction and commissioning. The dam is an 80 m high zoned earth and rock fill embankment with a concrete lined spillway excavated through the left abutment. The spillway control structure is a 33 m high mass concrete dam, with a 60 m wide gated overflow section. The spillway chute is a 435 m long, 60 m wide concrete lined channel through steeply dipping fresh to highly weathered rock. The dam and spillway were originally designed to pass a peak discharge of 3 400 m3/sec however recent hydrology indicates the current PMF discharge will be 6 900 m3/sec.
Work on the spillway upgrade included a physical model study that identified very poor flow conditions resulting in very large transient pressures. It was concluded that the existing concrete lined discharge channel will likely fail in relatively frequent flood events, with the potential for extensive erosion of the chute foundation. In a major flood event it is possible that the failure of the chute could ultimately lead to undercutting of the spillway structure.
This paper discusses the design approach selected to secure the spillway structure including lifting of the concrete slabs, erosive potential of the underlying rock strata and erosion mechanisms of down cutting and back cutting once the chute has failed. The paper also includes a discussion of the international work in this field used to quantify the measures required to protect the spillway structure at Eildon Dam.

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