The Cressy-Longford Irrigation Scheme delivers irrigation water supply from the outfall of Poatina Tailrace on Tasmania's Central Highlands Hydroelectric Power Scheme, to the agricultural region surrounding the towns of Cressy and Longford in North Eastern Tasmania. The scheme delivers approximately 10 000 ML of water annually to a land district of 10 000 ha. The scheme consists of mostly open, earth lined channels, with sections of concrete lined channel, pipeline, culverts, drop pipes, and siphons or drop siphons where required for hydraulic design purposes or to provide road access and crossings. The system consists of over 100 km of channel and pipe, in a network of 17 distinct channel sections. The full system and its subcomponents have been modelled, using the generally available package, HEC-RAS, to model open channel sections, approximate pipe performance and to model the network operation under various scenarios. The performance of pipe and culvert sections was independently checked using spreadsheet analysis. The objective of the analysis was to determine the maximum capacity of the system in its current state; determine restrictions to the flow of water through the network; and to identify strategies to increase the capacity to deliver water by at least 30 %. The key restraints on the analysis were to ensure that any increased volume of water carried into the network could be safely discharged out of the network to adjoining natural watercourses in the event that irrigation was suddenly stopped and/or water input to the system increased, for example in the event of a sudden rainstorm. It was found that the system was currently underutilised, and that in most cases the culverts were capable of running full, which greatly increased the possible flow rates allowable in the system. Acoustic Doppler Velocimeter measurements in sections of the channel were used to verify modelled flow rates.
There are many improvements to operation that can be made to a water distribution system once it has been constructed and placed in ground. Pipes and associated storages and pumps are typically designed to meet average peak daily demands, offer some capacity for growth, and also allow for some deterioration of performance over time. However, the 'as constructed' performance of the pipeline is invariably different to what was designed on paper, and this is particularly so for anything other than design flows, such as during times of water restrictions when there are significantly reduced flows. Because of this, there remain significant benefits to owners and operators for the adaptive and global optimisation of such systems. The present paper uses the Ouyen subsystem of the Northern Mallee Pipeline, in Victoria, as a case study for the development of an optimisation model. This has been done with the intent of using this model to reduce costs and provide better service to customers on this system. The Ouyen subsystem consists of 1600 km of trunk and distribution pipeline servicing an area of 456,000 Ha. The system includes 2 fixed speed pumps diverting water from the Murray River at Liparoo into two 150 ML balancing storages at Ouyen, 4 variable speed pumps feeding water from the balancing storages into the pipeline system, 2 variable speed pressure booster pumps and 5 town balancing storages. When considering all these components of the system, power consumption becomes an important part of the overall operation. The present paper considers a global optimisation model to minimise power consumption while maintaining reasonable performance of the system. The main components of the model are described including the network structure and the costs functions associated with the system. The final model presents the cost functions associated with the pump scheduling, including the penalties descriptions associated with maintaining appropriate storages levels and pressure bounds within the water distribution network.