Supervising Scientist Report 172
Moliere DR, Boggs GS, Evans KG, Saynor MJ & Erskine WD
Environment Australia, 2002
ISBN 0 642 24378 6
- SSR172 - Baseline hydrology characteristics of the Ngarradj catchment, Northern Territory (PDF - 1,460 KB)
The catchment of Ngarradj1, located in the wet dry tropics of the Northern Territory, Australia, is a major right-bank tributary of the Ramsar-listed Magela Creek wetlands. The Ngarradj catchment will be the first to be affected should any impact occur as a result of mining operations at the Energy Resources of Australia Jabiluka Mine. As part of a long-term study of the impact of mining at Jabiluka on the Ngarradj catchment, an attempt has been made to estimate the baseline hydrological characteristics of the catchment.
General diurnal trends and analysis of high magnitude storm events at the Ngarradj catchment during the three year monitoring period (1998–2001) show that peak rainfall and runoff occur late in the afternoon to early in the morning. The average time taken from the start of low frequency, very intense rainfall periods to peak runoff at stream gauging stations within Ngarradj catchment ranged between 45 minutes to 5.2 hours. These lag-times were examined in relation to the hydrological and geomorphological characteristics of the catchment, which showed that stream length and mean channel slope were the most significant factors in predicting lag-time at each catchment outlet. The understanding of the diurnal cycle of rainfall over the Ngarradj catchment and the corresponding lag-time for runoff after a storm event has important implications for the design of an effective stream sediment monitoring regime within the Ngarradj catchment.
Hydrology model parameters for the Ngarradj catchment were fitted to the US Army Corps of Engineers Hydrologic Modelling System (HEC-HMS) using the three years of observed rainfall-runoff data collected at each gauging station. The calibrated model was then used to generate a long-term runoff record using 20 years of rainfall data collected at Jabiru airport and the Jabiluka Mine. Flood frequency analysis of these data indicate that during the 20 year period there were 2 very large peak discharge events (ARIs of 1:55 y and 1:18 y), while the rest had ARIs of less than 1:6 y. Further analysis of the large flood events showed that they were associated with the largest recorded storm event in Kakadu and two consecutive high-intensity rainfall events respectively. However, it is important that future work determines how well the model and the current parameters predict very large flood events (>1:10 y event) as they have high significance in the assessment of risk and geomorphological change.
1 Ngarradj: Aboriginal name for the stream system referred to as 'Swift Creek' in earlier studies. Ngarradj means sulphur crested cockatoo. The full term is Ngarradj Warde Djobkeng. Ngarradj is one of several dreaming (Djang) sites on or adjacent the Jabiluka mine lease (A Ralph, Gundjehmi Aboriginal Corporation 2000).