2004 – Hydraulic Model Study For Eildon Dam Improvement Project

Ian Hampton, Dr Mohand Amghar and James Willey

The Eildon Dam Improvement Project is being undertaken by Goulburn-Murray Water as part of its dam improvement program that includes an upgrade of the existing Lake Eildon spillway that passes through the left abutment of the dam. The main components of the spillway are a gated concrete gravity overflow section that is 33 m high and 60 m wide, a 435 m long low gradient spillway chute and a hydraulic jump stilling basin.

The spillway was originally designed, including a physical model, in the 1950s to pass a maximum discharge of 3,400 m3/s with a maximum reservoir head of 9.0 m above the spillway crest. This can be compared with the 2003 flood hydrology and flood routing studies that result in a PMF discharge of 6,900 m3/s and a maximum reservoir head of 14.1 m above the spillway crest.
A new physical hydraulic model study was carried out over 2003-2004 as part of the investigations by the Eildon Alliance for the Project. The model was tested with discharges up to and exceeding the upgraded PMF. Very turbulent conditions were observed at discharges exceeding the original design discharge including the formation, build-up and collapse of large diameter vortices in flow over the spillway crest and overflow section. The vortex phenomena resulted in the intermittent formation of high waves and very high transient pressure loadings at the downstream toe of the overflow section and extending to the upstream section of the spillway chute. The paper discusses some scaling issues, presents some of the salient results of the study and discusses their application to the 2003-2004 design of structural modifications for the spillway.

The paper includes a discussion and comparison of the 1950’s model study with the 2003-2004 study. The magnitude of the vortex phenomena could not be predicted from the previous studies, and it is recommended that investigations for upgrades of similar works that involve large increases in design discharges include detail examination of vortex phenomena.

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