Assessing environmental flow requirements for a river-dominated tidal inlet

There are already numerous instances around the world where river mouths are classified as seasonally open. In the case of tidal inlets where river flow provides the erosive power necessary to maintain the opening increasing irrigation and water usage may lead to situations where previously stable inlets will also tend towards seasonal closure. Engineers and managers therefore, will need predictive tools to determine appropriate strategies to reduce or prevent total closure. This paper gives details of a statistically based model that has been developed to deal with a complex river-dominated tidal inlet. The model has been applied to a study of the River Murray mouth in Australia with a view to assessing the likely causes of closure and the management options that are available to prevent closure. The study used recorded water level data collected at sites outside and inside the mouth to derive a model of the mouth behavior and to relate this to river flow. The model was then used to assess two management options, on the assumption that additional flow could be made available. One option delivered a flushing flow over a month each year while the second provided a more constant increase in flow over the whole year. The key issue was to determine which option led to a better mouth opening. The main conclusion of the modeling work is that a previously suggested strategy of a large flushing flow over a month is likely to be less effective than providing a more constant additional outflow. This has significant implications for the river managers.