2021 – Mundaring Weir, Stilling Basin and Outlet Works Risk Assessment

Sonel Reynolds, Alex Gower, Bob Wark

During the outlet works upgrade in 2017 it was found that the valve pit and stilling basin at Mundaring Weir were not founded on rock. Based on these observations and the arrangement of the spillway and outlet works, it was considered that during significant spillway overflow events, a high velocity jet could displace the stilling basin slabs, erode the underlying material, and progress to failure of the outlet pipe and valve pit. A comprehensive risk assessment was conducted to estimate the likelihood of stilling basin slab uplift, erosion of the underlying material, and failure of the outlet works. A geotechnical investigation was undertaken comprising drilling nine boreholes and a program of geophysical downhole logging. Computational Fluid Dynamic (CFD) modelling was used to determine the pressure fluctuations and turbulence intensity over the spillway slab which could lead to uplift. The erodibility of the rock mass material below the stilling basin slabs was assessed using the outcomes of the geotechnical investigations and CFD output, with analyses based on the Kirsten Index and eGSI. A net benefit analysis was conducted to assess whether preventative remedial works were justified. Through this process it was demonstrated that the business risk was low and risk reduction measures were not justified.