Application of HEC RAS to hydraulic modelling of an irrigation scheme to determine potential for capacity increase

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.