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Publications
Supervising Scientist Report 183
DR Moliere, 2005
Department of the Environment and Heritage 2005
ISSN 1325-1554
ISBN 0 642 24394 8
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Gulungul Creek is a small left bank tributary of Magela Creek and is one of the tributaries that will be the first to receive sediment generated as a result of rehabilitation at Energy Resources of Australia's Ranger mine. Over the next few years, a sampling regime will be established to develop an understanding of the sediment and associated contaminant transport characteristics in Gulungul Creek before rehabilitation at the mine site occurs. To guide sampling program design, an attempt has been made to estimate the hydrological characteristics of the Gulungul Creek catchment based on historical data collected within the creek between 1971 and 1993, a time period of 22 years.
Six of the 22 years of record had significant gaps in the flow data. The remaining 16 years of runoff record were used to determine an average annual runoff at the station of approximately 25 500 ML. A flood frequency curve was fitted using 19 of the 22 years of historical observed data. The fitted curve indicates that a 1:20 y, 1:50 y and 1:100 y peak discharge corresponds to flows of 168 m3 s-1, 260 m3 s-1 and 357 m3 s-1 respectively. In the historical record the 1:50 y peak discharge was exceeded only once - on 4 February 1980 (277 m3 s-1) - as a result of the most severe storm recorded in the Kakadu region.
General diurnal trends in the catchment show that peak rainfall and runoff occur late in the afternoon (~1800 h) and early in the morning (~0330 h) respectively. Analysis of some of the largest flood events that occurred showed that the average lag-time from the start of rainfall to peak discharge was 9.6 h ( SD = 1.8 h), which corresponds well to the general trend in mean rainfall and runoff. The predicted lag-times, based on the geomorphological characteristics of the catchment, for two current sampling site locations along Gulungul Creek are 7.6 h and 11.7 h respectively. In general, the majority of sediment movement in streams within the region occurs during a runoff event and, therefore, it could be assumed that suspended sediment and associated contaminant movement in Gulungul Creek would primarily occur late in the evening to early in the morning. These hydrological responses must be considered when designing a monitoring regime to establish baseline characteristics against which the impact of rehabilitation earthworks at Ranger mine on suspended sediment and contaminant loads in Gulungul Creek can be assessed. For example, it is clear that a typical mid-morning grab sampling regime in Gulungul Creek may not be adequate to characterise suspended sediment (and any associated contaminant) movement in the stream.
An alternative to suspended sediment sample collection is the continuous monitoring of turbidity as an indirect measure of suspended sediment concentration. It is recommended that the relationship between suspended sediment concentration and turbidity in Gulungul Creek be investigated. A significant relationship could be applied to the continuous turbidity data to better characterise suspended sediment movement in the catchment and to estimate suspended sediment loads. It would also be important to determine if stream contaminant concentration and suspended sediment concentration are closely correlated in Gulungul Creek. A target-sampling regime, which requires an understanding of the lag-time of the stream at the sampling location, could initially be adopted over a series of rainfall-runoff events to collect metal and suspended sediment concentration data at a range of flows. Should a significant relationship exist, a continuous record of metal flux could be estimated from continuous monitoring of streamflow and suspended sediment concentration (using turbidimeters).
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