2003 – Lake Bellfield Flood Upgrade Design

Stephen Newman, Kelly Maslin

Lake Bellfield is a reserve storage for the Wimmera Mallee Water (WMW) Stock and Domestic System in North Western Victoria, constructed between 1963 and 1967. The dam is located on Fyans Creek approximately 3 km upstream from Halls Gap in an area of high tourist value and is rated in the Extreme category under ANCOLD guidelines. The dam consists of an earth and rockfill embankment 745 m long with a maximum height of 57 metres and retains a reservoir with a storage capacity of 78,500 ML.

Previous studies and a subsequent physical model study confirmed that the existing spillway does not meet the requirements of the current ANCOLD guidelines. The current flood capacity is approximately 40% of the Probable Maximum Flood. A range of potential upgrade options to pass the PMF were evaluated with a 1.9 metre composite earthfill and downstream concrete parapet wall raise in combination with spillway lowering of 3.4 metres selected. Construction of this option was completed in early 2003.

This paper describes the key features of the investigation and design including:

• a physical model study,
• evaluation of various options based on technical, financial, environmental and social criteria,
• design of the earthfill-parapet raise in the limited area available on the crest to provide full filter protection, acceptable short and long term deformations and adequate long term access to the dam and its associated structures, and
• design of the spillway cut including excavation and stabilisation in the very strong and abrasive Grampians Sandstone.