Analysis And Management Of Unseasonal Flooding In The Barmahmillewa Forest Australia

John Wiley and Sons
Publication Type:
Journal Article
Chong Joanne and Ladson Ar 2003, 'Analysis And Management Of Unseasonal Flooding In The Barmah-millewa Forest, Australia', John Wiley & Sons Ltd, vol. 19, no. 2, pp. 161-180.
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The Barmah-Millewa Forest is the largest red gum forest in the world and lies adjacent to the middle reaches of Australia’s River Murray. Regulation of the River Murray, to supply water for irrigation, has changed the watering regime of the forest and thus is degrading its environmental values. The watering regime has been changed in two ways: (1) there are now fewer large winter/spring events that inundate extensive areas because these floods are mitigated by irrigation storages; and (2) there are more small summer/autumn events that flood low-lying areas and are caused by the way the river is operated to supply irrigation demand. The increased frequency of these small unseasonal floods is the subject of this paper. During the irrigation season, water to meet irrigation requirements must be released four days in advance to allow for travel time from storages to irrigation areas upstream of the Barmah-Millewa Forest. If there is heavy summer rainfall, irrigators cancel their orders so the flow that would have been diverted, remains in the river and causes a small ‘rain rejection’ flood. At the same time, river freshets from unregulated tributaries can also increase river flows. The River Murray channel in this area has low capacity and these high flows result in water spilling into the forest. Based on analysis of pre-regulation conditions (1908–1929) and current conditions (1980–2000), forest flooding has increased from 15.5% of days to 36.5% of days between December and April. In particular, small, localized floods, which cover less than 10% of the forest, occur at least eight times more frequently now, than before regulation. Work by others has related these hydrologic changes to tree death and changes in floristic structure in wetland systems. There are also economic costs because much of the water that spills into the forest is not available for irrigation. Two solutions to unseasonal flooding are described in this paper. One is to limit the maximum flow in the river during the irrigation season so there is capacity to convey at least some of the rain rejection flows without spilling water into the forest. The other is to maintain airspace in a diversion weir (Lake Mulwala) upstream of the forest to store the surplus water when orders are cancelled. Preliminary economic analysis shows the preferred option is to increase airspace in Lake Mulwala which provides net benefits of at least Aus$1.4 million per year along with unquantified environmental benefits from decreased unseasonal forest flooding.
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