Supervising Scientist, Darwin, 2005
ISBN 0 642 24395 6
ISSN 0 158-4030
The Jabiluka uranium deposit is located in the Ngarradj catchment within Kakadu National Park (Figure 3.9). The mine site was constructed during the 1998 dry season, but no mining has yet occurred at Jabiluka. To reliably assess any future impact of mine-related erosion on stream sediment loads in the Ngarradj catchment, it is essential to determine the pre-mining sediment transport and geomorphological characteristics of the catchment.
In 1998, eriss installed stream gauging stations upstream (Upper Main – UM; East Tributary – ET) and downstream (Swift Creek – SC) (Figure 3.9) of the site to monitor suspended sediment movement within the catchment under pre-mining conditions. At each station rainfall, streamflow and suspended sediment concentration data are collected throughout the wet season. In addition, numerous cross-sections, scour chains and erosion pins were installed along the main channel and several tributaries within the Ngarradj catchment to determine the channel stability and geomorphological characteristics of the catchment under pre-mining conditions.
The rainfall and streamflow data collected at each station were used to calibrate a hydrology model for the Ngarradj catchment. The calibrated model was then used to generate a long-term runoff record using 20 y of rainfall data collected within the region. The predicted flow data were used to establish the flood frequency curve for the catchment, which has important implications in the assessment of risk and geomorphological change.
A total of 56 cross-sections have been surveyed during each dry season between 1998 and 2004. In nearly all the reaches the channels are reasonably stable apart from the active knickpoint retreat of Tributary North and also channel erosion by lateral migration, bed degradation and channel widening on Tributary Central. Aerial photograph interpretation showed that these geomorphic processes were occurring before the construction of the Jabiluka site and their rates of activity have not accelerated since it was developed.
A total of 30 scour chains were installed on some of the cross-sections to measure scour and fill between the wet seasons. Scour and fill are active geomorphic processes in the Ngarradj catchment that result in the reworking of the sandy bed sediment. Mean scour and fill rates were determined for each reach for each of the wet seasons. Allowing for plus or minus twice the standard error of estimate of the mean, average scour and fill for the scour chains during each wet season in each measurement reach usually overlap with each other. This indicates that mean annual scour and fill are not significantly different between reaches.
Bed material samples were also collected from each of the 56 cross-sections during each dry season between 1998 and 2004. Particle size analysis was completed on all of these samples and showed that any annual changes at the sites downstream of the project site were also present in the sites upstream of the project. The grain size statistics data constitute thorough baseline information for the Ngarradj catchment and can now be used to determine any subsequent changes due to future activities on the site.
Figure 3.9 The Ngarradj catchment showing the Jabiluka Mineral Lease, eriss ’s gauging stations and local creek names. SC refers to Swift Creek gauging station, TN Tributary North, ET East Tributary gauging station, TC Tributary Central, TS Tributary South, TW Tributary West and UM upper Swift Creek gauging station.
Up to four years of erosion pin measurements in the Ngarradj catchment have established that substantial bank erosion (up to 285 t/a) has occurred during the wet season on the site tributaries by rapid lateral migration (Tributary Central) and by erosion of gully sidewalls due to a combination of within-gully flows and overland flow plunging over the sidewalls (Tributary North). Bank erosion also occurred during the dry season by faunal activity, by desiccation and loss of cohesion of the sandy sediments, and by dry flow processes but at very low rates. Aerial photograph interpretation showed that these geomorphic processes were occurring before the construction of the Jabiluka site and their rates of activity have not accelerated since the site was developed. Channels with dense riparian vegetation did not generate significant amounts of sediment by bank erosion. Deposition was also locally significant, despite the sandy bank sediments. Bank profile and channel planform exert a strong control on erosion rates during the wet but not during the dry season.Sufficient information has now been collected on the geomorphic channel stability characteristics of the Ngarradj catchment to reliably determine whether they change in the future. The results of this study show that the geomorphic processes operating in the Ngarradj catchment have not been impacted on or accelerated as a result of the construction at Jabiluka.