Mossy Marsh Dam Upgrade – risk mitigation and asset augmentation

Hydro Tasmania are nearing completion of an upgrade to Mossy Marsh Dam, a small embankment in the Tarraleah Power Scheme in central Tasmania. The 1950s era 200m long, 6m high homogenous clay embankment impounds a pond in one of the scheme’s two water conveyance systems.

There are two drivers for the upgrade. The first is to reduce the likelihood of failure due to flood overtopping and piping related mechanisms. The second is to construct an emergency spillway in preparation for scheme modifications that are underway as part of Hydro Tasmania’s Battery of the Nation program.

Elements of the upgrade include:

• Embankment crest level raise to increase freeboard
• Downstream filter buttress and grouting of the outlet conduit to protect against internal erosion failure modes,
• Construction of a service spillway to pass flood inflows,
• Construction of an emergency spillway to pass capacity of the new conveyance from an operational error and system malfunction
• Downstream channel improvement works

Works commenced in August 2022, at a less than ideal time of year for civil construction in Tasmania’s central highlands. Rain, snow, mud, soup, slop. However, by late September good progress had been made and the works are on track for completion in late January 2023.

Good collaboration throughout the works between all parties: Project Management, Hydro Tasmania; Design support, Entura; civil contractor, Hazell Bros.

A Tasmanian ANCOLD Young Professionals site inspection is planned for early 2023.

Oliver Giudici

Project Manager, Hydro Tasmania