Blue mountains sewer tunnel improvement project - geomorphology assessment

The Blue Mountains Sewer Tunnel is 39 km long and transports wastewater from Mount Victoria to Winmalee Wastewater Treatment Plant (WWTP). There are sections of the tunnel that are joined by vertical drop shafts so that it can follow the steep contours of the land. During heavy rain, rainwater enters the wastewater system, increasing the flows in the system by up to 20 times. Two drop shafts at Lugano Court and Lawson Road in Springwood are currently undersized and are at risk of structural damage during extreme high flows. Sydney Water explored six strategies to resolve the problem. The preferred solution is made up of building a new larger drop shaft at Lugano Court, modifying the structure at Lawson Road and minor hydraulic modifications at WWTP. One of the options considered involved releasing dilute wastewater from the system into an unnamed creek near the Lawson Road drop shaft. The profile of the project escalated following community consultation because local residents were deeply concerned about the potential environmental impacts on waterways. An objective assessment was then sought by SewerFix Wet Weather Alliance for Sydney Water for an expert Geomorphologist to describe the physical condition of the existing waterways and predict likely stability impacts as a result of the proposed options. In response Cardno undertook a geomorphology assessment using detailed site investigations and numerical modelling. Site investigations involved the use of Rapid Geomorphology Assessment proformas and intrusive soil testing for shear strength, particle size distribution and cohesion. Numerical modelling involved hydrology, hydraulic and bank stability estimation. A Bank Stability Estimation Model (BSTEM) was used for the first time in NSW to predict the waterway stability in existing conditions and under a range of design scenarios.