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Three Tributaries Dams Dambreak Assessment

Molino Stewart

Dambreak modelling and consequence assessment for urban detention basins.


The catchments of Orphan School Creek, Green Valley Creek and Clear Paddock Creek have become increasingly urbanised. The construction of detention basins has historically been a major component of the floodplain risk management approach adopted to mitigate the increased runoff and floodplain risk resulting from intensification of development.  As a result of this approach, the Three Tributaries catchment contains fourteen detention basins.

Due to the number of these basins and their integral role in floodplain risk management within the catchment, it is necessary to quantify the risk of failure of the basins structures, and to estimate the potential impacts on flood risk resulting from failure. Some of the detention basins are of a large size and are in reasonably close proximity to residential areas, education facilities or commercial centres. Therefore any failure of the structures could potentially have significant consequences. This dam-break assessment considered when each of the dams could potentially fail and the consequences of such dam failure.


We undertook an initial review into previous modelling. As a result of the review, it was found that the existing models used to assess flood behaviour were not suitable for the requirements of the Three Tributaries Floodplain Risk Management Study (FRMS). These TUFLOW models were updated, with overview from WMAwater. We then completed additional revisions to the model to reflect construction of waterway infrastructure in the study area that occurred after the last flood study, and prepared revised risk precinct mapping, and maps of design flood depths and velocities estimated using the revised models. We then undertook dam-break modelling and consequence assessments for 13 dams. This included cascade failure assessments.


The Consequence Category of dam failure at each of the basins assessed as well as the incremental consequence. For basins where the PMF is not the trigger event for dam failure there was very little incremental consequence between PMF natural flood and dam-break scenario, thus confirming the highest incremental consequence is generally likely to occur at the trigger dam failure event, just large enough to overtop the basin, rather than in larger flood events. It was found that although cascade failure does not significantly increase the dam-break consequences, it does cause some dams to overtop earlier thereby increasing the risk of such consequences occurring.

The dam-break and consequence assessments were used to prioritised remedial works to mitigate dam failure risk.