Rowallan Dam upgrade – flood forecasting
Client: Hydro Tasmania
Location: Tasmania, Australia
Date: 2014 – 2015
Forecasting lake levels and floods to enable safe construction works and mitigate flood risks during a major dam upgrade.
Rowallan Dam is a 43m-high earth and rockfill dam on the Mersey River in Tasmania’s north, constructed in 1968. Major work was undertaken in 2015 to bring the dam up to modern engineering standards and better manage dam safety risks.
To do the work safely, the level of Lake Rowallan had to be lowered significantly, and a forecast of lake levels was needed to guide the valve openings necessary to keep the lake at the target level. Flood forecasting was required to indicate when emergency backfill of the dam crest may be required to prevent inundation or breaching of the works with serious consequences for the storages and population at risk downstream.
The solution was to use a hydrological model to forecast lake levels and the time to reach critical levels that could jeopardise the dam, always ensuring sufficient time for emergency backfilling.
The flood forecasting model for Rowallan Dam was revised to include the latest rainfall forecasts, indications of uncertainty in forecasts, and valve operation. A plan was put in place to ensure that critical rainfall and level gauges were operational. All input data was automatically checked for major errors or inconsistencies. Redundancies in the modelling and a stand-alone back-up system at the dam site ensured robustness.
Plots were developed showing the best-estimate forecast of inflow and lake level, high and low uncertainty bounds on these estimates, forecast valve opening required, and rainfall over the catchment. The entire modelling process ran automatically and provided an updated forecast every 2 hours of the construction period, with SMS and SCADA alerts set to trigger if the high forecast exceeded the trigger level. If a flood alert was issued, the Dam Safety Emergency Plan would be enacted.
The Rowallan flood forecasting system allowed operators to confidently manage the flood risk during construction works. Operators and construction managers used the plots to guide valve operation to keep the lake levels low and to ensure emergency preparedness. Our collaborative approach with the dam owner enabled risks to be quickly evaluated and appropriate actions taken, ensuring safe construction and mitigating the risks of dam failure.