Melbourne Water is owned by the government of the state of Victoria, in the south east of Australia. It manages Melbourne’s water supply catchments, removes and treats most of Melbourne’s sewage, and manages rivers and creeks and major drainage systems in and around the city.
The city of Melbourne is serviced by separate sewerage and stormwater drainage systems rather than one combined system. Approximately 92% of the city’s wastewater is delivered to two treatment plants, sited in the east and west of the city.
In 1995 the integrated water authority was split into four separate businesses – Melbourne Water, which provides bulk supply functions, and three retail companies: City West Water, South East Water and Yarra Valley Water.
Melbourne Water is responsible for 385km of large sewer mains - mostly a gravity system, with four major pumping stations to lift flows into shallower mains. The retail companies are responsible for around 20,000km of branch and reticulation mains. The company’s original hydraulic model was built in the mid-1990s using HydroWorks. It was subsequently migrated to InfoWorks CS in January 2001.
The Melbourne Water Macro Model: a history
Melbourne Water (MWC) began to develop the hydraulic macro model of its sewerage network in 1994, in order to provide a better understanding of the performance of the sewerage system, particularly hydraulic deficiencies and asset condition.
The company’s environmental compliance standard is set by the Environmental Protection Authority Victoria (EPAV), which requires containment of sewer flows for rainfall events of up to one in five year Average Recurrence Interval (ARI) without overflow.
Work started in the mid-1990s, and is ongoing. For the purpose of building the model, the whole of the Melbourne sewerage system was partitioned into 19 separate natural drainage basins. Data from MWC’s electronic mapping system was verified using STC25, an industry standard sewer record collection database format,and surveys were undertaken where no data was available.
Basin models were initially calibrated with data acquired from short-term flow monitoring programs, typically of six weeks’ duration. Advances in both software and hardware allowed the separate basin files to be merged in 1997 to form a single set of files for the whole of the Melbourne sewerage system.
There was initially limited confidence in the simulated wet weather response, mostly because of a lack of concurrent sewage flow monitoring across the whole of the sewerage system, an opinion that was shared by an external auditor. Refinement of the wet weather calibration was not possible until an upgraded sewerage flow-monitoring network was in place, and significant rainfall events had occurred.
Melbourne Water established a permanent sewerage flow-monitoring network in the late 1990s, and data collected from over 80 flow monitors is now used to refine both the dry and wet weather calibration. The dry weather flow within the model was revised in 2004/05, and current simulated flows are representative of data from the two treatment plants and 80 flow monitors.
The latest version of the software is providing useful benefits, according to Tony Corbett, Team Leader, Sewerage Investigations, at Melbourne Water. He says: ‘The previous version of InfoWorks only allowed up to 99 wastewater profiles. The residential flow generation was restructured in 2004/05, and the latest model utilises 206 wastewater profiles.’
The real time control capability of the software is used to simulate the operation of the nine penstocks that control the flow diversions to the treatment plants. Also important is the ERS (Emergency Relief Structure), an overflow point that allows the discharge of wastewater during extreme rainfall events and emergency conditions such as sewer collapses. These structures provide protection on properties against sewage flooding, prevent uncontrolled spills, and they usually connect directly to the drainage system.
A complete assessment of sill level (the level at which an ERS will operate and spill) and structure of all MWC and its retailers’ ERSs are also included in the model. This has also been extended to include more retailers’ ERSs and pipes upstream of Melbourne’s sewerage system.
The ongoing project aims to review current and future system performance to meet the required level of service, and evaluate options and timing for planned upgrade works to meet environmental standards.
The model is used to develop operational strategies for asset maintenance, construction and contingency planning and to develop more efficient operating procedures, such as using existing system capacity to reduce the energy costs associated with pumping and treatment.
It helps to improve management of flows in the transfer system, in order to reduce spills and impacts on the environment. It is also used to evaluate asset management strategies, such as the effect on capacity if an aging sewer is slip-lined.
The project helps with maintenance works - for instance, by calculating storage time for shutdowns and inspections - and assess the hydraulic effects of sewer improvements further upstream by the retail water companies.
The model is used to inform asset investment decisions, both in terms of upgrading existing assets and in the timing and scale of new assets, and is a key tool to determine future treatment plant capacity requirements for wet weather events.
Improvements to the macro model are ongoing. As wet weather events occur, sewer wet weather response data are being added to the model to refine its wet weather calibration.
Tony Corbett says: ‘The model is currently achieving good results for peak wet weather flows, but it is clear that the wet weather volume has been underestimated. MWC is investigating the use of a more complex runoff routing model or use of a groundwater infiltration module.’