TAS – Mossy Marsh Dam Upgrade – risk mitigation and asset augmentation – completion update

Hydro Tasmania has completed strengthening of 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. 

The two drivers for the upgrade are: 

1. To reduce the likelihood of failure due to flood overtopping and piping related mechanisms; and 
2. To construct an emergency spillway in preparation for scheme modifications that are underway as part of Hydro Tasmania’s Battery of the Nation program. 

Upgrade works included: 

• 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. Works were completed in February 2023.

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

Planning for a Tasmanian ANCOLD Young Professional’s’ site visit is underway. 

Oliver Giudici

Project Manager, Hydro Tasmania