Mossy Marsh Dam Upgrade

Client: Hydro Tasmania
Location: Tasmania, Australia
Date: 2021-23 

Upgrading a dam for a long, safe and effective life in an evolving hydropower scheme

Mossy Marsh Dam is a 70+-year-old earthfill embankment dam in the central highlands of Tasmania. It is part of the Tarraleah hydropower scheme, impounding water in the Mossy Marsh Pond and diverting flows from Clark Dam and the Wentworth Diversion to Tarraleah Power Station. The original dam was 6 m high, had a crest length of 230 m and was designed with no filter zones or emergency spillway.

In 2020, Hydro Tasmania engaged Entura to prepare a concept and subsequent detailed design for a major upgrade of the dam. The upgrade aimed to reduce the likelihood of failure due to flood overtopping or piping, as well as to construct an emergency spillway in preparation for modifications to the Tarraleah scheme as part of Hydro Tasmania’s Battery of the Nation vision.

Solution

For the upgrade, Entura undertook geotechnical investigations, provided environmental assessment and planning advice, and completed the preliminary and detailed design of the embankment, spillway and instrumentation. We also conducted a hydrological review including flood modelling and a hydraulic study of structures and roads downstream of the proposed spillway.

The design included a new emergency spillway and an upgrade of the embankment including full-height filter buttress and cut-off trench, downstream erosion protection and crest level raising using precast concrete panels. Our work also included design of seepage and reservoir level monitoring.

We provided support in the construction phase including site and quality assurance inspections and review, construction advice, reporting and as-constructed drawings.

Works commenced in August 2022, at a challenging time of year for civil construction in Tasmania’s central highlands, when it is common to experience very wet and cold conditions. However, effective collaboration between Hydro Tasmania, Entura and the civil contractor (Hazell Bros) saw the works successfully completed within 7 months in early 2023.

Services provided

• Concept design
• Detailed design
• Construction support

Outcome

Following completion of the civil works in March 2023, Mossy Marsh Dam reopened for recreational use in November 2023 after site rehabilitation. It will now serve Tasmanians for many more years of safe operation, and is ready to play its part in the future redevelopment of the Tarraleah hydropower scheme.