Integrated modelling of water delivery options for the Coorong South Lagoon

Abstract

The Coorong, along with the Lower Lakes and Murray Mouth is one of the largest of the internationally significant wetlands recognised under the Ramsar Convention within the Murray Darling Basin. Severe drought in the Basin over recent years has resulted in record low inflows and has had a significant impact on the ecological health of the Coorong. However, there is still further volume that flows west and out to sea through the Lower South East constructed drainage network that could potentially be diverted north and delivered to support the Coorong South Lagoon (CSL). The Coorong South Lagoon Flow Restoration Project (CSLFRP), which is part of the South Australian Government's Murray Futures program and funded by the Australian Government's Water for the Future strategy, has to date investigated options for diverting significant volumes of water from the drainage network of the South East northwards to the Coorong using a combination of purpose-built floodways and existing flow paths. This paper presents the investigations, in particular the modelling work undertaken, and the outcomes of the CSLFRP study. The aim of this study is to provide greater confidence in estimates of water availability from the untapped resources in the South East drainage system available, to potentially be diverted to the CSL using two main flow paths. Accurate estimation of water availability is complicated by a lack of flow gaugings, extremely flat terrain, shallow water table, and highly modified landscape in the region. To do this, a range of modelling tools and techniques were used to replicate the processes of rainfall-runoff generation, losses and conveyance of water. The key modelling processes involved were: • Hydrologic modelling of the South East catchments to generate time-series of flows from runoff and diversions. • Hydraulic modelling of the flow paths to provide estimates of width, depth and volume of floodways/drains and also water surface and water elevation for input to the transmission loss and water balance analysis. • Transmission loss analysis to estimate losses to groundwater for various flow paths and flow rates. • Water balance modelling to estimate annual yield volumes to the CSL. Diversion and runoff volumes from the hydrologic model, drain's dimensions from hydraulic model and losses to groundwater from the transmission loss model were used as inputs to the water balance models. Based on the maximum diversion limits determined, the yield delivered to the CSL was estimated under different climate conditions. Analysis of the probability of occurrence of different yields expected for the different flow paths, which highlights the major difference between proposed flow paths, was also undertaken. Finally, the yield expected from each diversion point was calculated to illustrate what incremental flow diversion increase would be achieved by a staged approach to a potential civil works program.