The study has been used to analyze the transfer system’s seasonal flow capacity at both current and future demand levels.
East Gippsland Regional Water Authority supplies water and sewerage services to sixteen towns in Victoria. Its largest water supply network is the 65km (40 mile) Mitchell River system, which serves a population of approximately 20,000 in nine towns including Bairnsdale, Lakes Entrance and Metung. Water is drawn from the Mitchell River at the western end.
Tourist influx puts high seasonal demand on the system between December and March, especially in the seaside towns, explains Earth Tech project engineer Heath Miles. There is insufficient transfer capacity to supply the all the peak summer demands, particularly those of the town of Lakes Entrance at the far end of the system.
East Gippsland Water caters for this by adapting the system’s operation. A 450Ml (120 million gallon) reservoir near Lakes Entrance is filled during the nine months of low season and the storage is drawn down once demand rises.
Improving the system
Earth Tech had already developed a water quality improvement plan for the Mitchell River water supply network. The plan involves conversion of the system from large, open water storage into smaller, enclosed balancing tanks. Water quality will improve, as water detention time will be reduced and the new storage tanks will be sealed. Filtration plants are also planned as part of this project at each of the source tanks.
An InfoWorks WS model was created to plan the system operation for daily and weekly simulation periods. It is also used for specific flow events, such as the peak and average days. Further analysis has been carried out to examine patterns over a prolonged period - years and decades -with a particular focus on the section where capacity problems are experienced.
The sensitivity study focused on the section of the network between Sarsfield Reservoir and the town of Lakes Entrance. The 160Ml (42 million gallon) Sarsfield Reservoir is filled by pumping further upstream. Flow gravitates from Sarsfield to a balancing storage tank at Sunlakes, supplying several small towns along the way. However, the Sarsfield to Sunlakes pipeline has insufficient capacity to meet the peak demands into the town of Lakes Entrance. To overcome this, a reservoir at Toorloo on the other side of the town is filled during the low season.
“Then during high season, we split the town of Lakes Entrance into two parts by closing valves,” explains Mr Miles. Part is supplied from the main Sarsfield reservoir via the balancing tank at Sunlakes, with the remainder coming from Toorloo.
EnlargeExploring the options
Several previous studies had investigated the options for supplying Lakes Entrance demand directly from the Mitchell River system, eliminating use of the Toorloo Reservoir. “We looked at either increasing the capacity to run by gravity or putting in pump stations,” says Mr Miles. “But we found that the pipe augmentations were too massive and not financially viable.”
The conclusion was that there should be a continuation of the current approach of transferring water into Toorloo during low season and drawing it out in the high season. It was decided to model the system’s operation over a long period, to ensure that the flows are sufficient to balance annual demand.
The InfoWorks model was first used to model short time periods. It gave good results, confirming that there was insufficient transfer capacity during the peak months to supply Lakes Entrance from the pipeline, but that there was excess flow available during low season.
“We then needed to run a long-period model to make sure that the low season surplus in flow exceeded the peak season’s deficit so that flows could balance over the year,” says Mr Miles. A sensitivity study was carried out to determine the relationship between transfer flows and demand across the year. The key inputs for this were the InfoWorks modeling results for demand and transfer flows, annual growth projections for the next 25 years and the annual demand from bulk meters and other flow gauging.
The InfoWorks modeling results were prepared for the peak, average and minimum days as well as three intermediate days. The relationship between transfer flow and demand was plotted for a series of demand events over a 25 year period, using values from the InfoWorks WS model. A spreadsheet was then used to calculate daily demand and transfer, taking account of annual growth and the filling and depletion of the storage.
Periods when the system has insufficient capacity were apparent from the resulting graphs, as the line showing demand would rise above that for transfer flows.
More flexible management
“Our modeling showed that the current system operation of physically splitting the town into two segments is inflexible because the transfer flow does not necessarily match the demand in the town,” says Mr Miles. “The solution is to allow the natural boundary to move within system. This maximizes the usage of transfer flow, while making sure the system balances over the year.”
Under the plan, flows into Lakes Entrance will be optimized by balancing the use of the two reservoirs according to the level of demand.
Earth Tech has recommended that there should be a control on the Toorloo water treatment plant. This would allow a natural boundary to be maintained at whatever point is appropriate within the system, rather than partitioning it rigidly into two sections.
The boundary of the areas supplied by each reservoir will change during the day, under the control of the Toorloo water treatment plant. During low demand, excess flow will continue to be transferred into the Toorloo reservoir but none will be supplied from there. Under moderate demand, the Toorloo treatment plant acts to maintain a natural boundary so that some of the water for Lakes Entrance is supplied from Toorloo and some from the Sunlakes tank. As demand increases, the Toorloo plant will increase the amount of demand that it services by shifting the natural boundary closer to Sunlakes. This will ensure that the 45Ml (12 million gallon) Sunlakes tank is not excessively depleted.
As well as bringing better optimization of the flows, the new system will also be easier to operate than the current system. Operators will no longer have to adjust valves in the field to suit changing conditions.
The modeling has shown that the system can supply the required demand during the year, says Mr Miles, and that flows can be optimized without using the current approach of physically partitioning the system. “The analysis has given us greater confidence, as we have tested the sensitivity of transfer flow to the demand in the system,” says Mr Miles.
This article is based on a presentation given by Earth Tech project engineer Heath Miles at the InfoWorks WS Australian user group meeting held in Sydney in November 2007.