Supervising Scientist Division

Supervising Scientist Annual Report 2002 - 2003: Hydrological and Ecological Processes

Supervising Scientist, Darwin, 2003
ISBN 0 642 24383 2
ISSN 0 158-4030

3 - Environmental research and monitoring (continued)

3.4 Hydrological and Ecological Processes

The Hydrological and Ecological Processes programme provides advice on landscape processes to detect impacts that could arise during and after mining activities in the Alligator Rivers Region and on the conservation and management of tropical wetlands.

Activities in 2002-03 included:

3.4.1 Development of improved techniques to measure hydrology and sediment movement around Jabiluka

As part of a long-term study of the impact of initial mine construction at Jabiluka in the Ngarradj (Swift Creek) catchment, an attempt has been made to characterise the hydrological response of streams within the catchment.

A stream gauging network was established in 1998 in the Ngarradj catchment (Figure 3.8) to collect rainfall and baseline runoff information and sediment concentration data. A tipping bucket rain gauge was installed at each gauging station and rainfall data were also collected at the Jabiluka minesite. Runoff at each gauging station was measured by a pressure transducer. A stage activated pump sampler - which responds to changes in water level - was also installed at each gauging station to obtain 'continuous' suspended sediment data throughout the wet season. Sediment concentration was also measured through weekly grab samples taken during the period of flow.

Mean hourly rainfall and runoff data were collected during a three year monitoring period from 1998 to 2001. These were spatially averaged using the Thiessen Polygon method to determine the diurnal rainfall cycle over the Ngarradj catchment. Figure 3.9 shows that Ngarradj catchment rainfall exhibits a strong diurnal cycle with a peak in the late afternoon. It also shows that Ngarradj catchment runoff exhibits a strong diurnal cycle with a peak late in the evening to early in the morning.

An analysis of the largest flood events observed during the three year monitoring period at each gauging station showed that the average start of rainfall time and time of peak discharge of these events corresponded well to the overall diurnal cycle of rainfall and runoff.

Figure 3.8: Jabiluka site and the stream gauging network within the Ngarradj catchment

Figure 3.8: Jabiluka site and the stream gauging network within the Ngarradj catchment

Figure 3.9: Diurnal variation of rainfall and runoff within the Ngarradj catchment

Figure 3.9: Diurnal variation of rainfall and runoff within the Ngarradj catchment

Sediment concentration data for these events show that the majority of the suspended sediment transport occurs during the rising stage of rainfall and runoff. Given the strong diurnal cycle for runoff that peaks late evening to early morning, a typical weekly midmorning to mid-afternoon grab-sampling regime in the Ngarradj catchment would generally miss these periods of maximum sediment transport during flood events. These hydrological responses need to be considered when establishing a monitoring regime to assess the impact of catchment disturbance on suspended sediment loads in streams within Ngarradj catchment. To date, no mine-disturbance related sediment elevation has been observed.

3.4.2 Influence of riparian vegetation on bank erosion rates in the Ngarradj catchment around Jabiluka mine

Within the Ngarradj (Swift Creek) catchment, a range of different stream types has been developed over time by the movement of water and sediment from the Arnhem Land plateau to the wetlands of Magela Creek. The sandstone valley here progressively widens downstream with sand deposition becoming increasingly active. Another important factor is the prevalence and nature of riparian (stream side) vegetation, which has a significant influence on channel stability in the valley tract between the upper sandstone gorges and the lower floodplain wetlands.

Vegetation exerts an important control on stream dynamics and bank erosion rates which have been measured in Ngarradj over three years by the erosion pin and scour chain techniques. Dense riparian monsoonal forest protects the banks of the larger streams from erosion. Forested meandering reaches have been mapped on extensive lengths of Ngarradj and East Tributary. These are dominated by Allosyncarpia ternata (Figure 3.10). Several small streams originate on the lowlands and the sandstone outlier downstream of the headwaters and two of these, Tributaries Central and North, flow close to the Jabiluka minesite. These lowland channels are flanked by open woodland with no distinctive riparian vegetation (Figure 3.11). Seasonal grasses provide the dominant wet season ground cover.

Figure 3.10: Riparian vegetation along the Ngarradj channel, showing the dense canopy and the large trees (dominantly Allosyncarpia ternata) growing on the stable river bank.

Figure 3.10: Riparian vegetation along the Ngarradj channel, showing the dense canopy and the large trees (dominantly Allosyncarpia ternata) growing on the stable river bank

Photo: M Saynor

Figure 3.11: The channel of Tributary Central during the wet season showing actively eroding exposed banks with no trees.

Figure 3.11: The channel of Tributary Central during the wet season showing actively eroding exposed banks with no trees.

Note: The bank on the left has retreated by 1 m in 3 years and the sandy point bar on the right has advanced by 4 m over the same time.

Photo: M Saynor

The aim of this research is to determine the significance of mining-related impacts on stream dynamics in the Ngarradj catchment and to identify factors that should be taken into account in the management of the area.

Up to three years of erosion pin measurements in the Ngarradj catchment have established that:

In addition, the use of scour chains demonstrated that flood scour of stream beds during the wet season was matched by fill on the recessional streamflows during the late wet/early dry season. This natural pattern of scour and fill was not found to have caused bank erosion.

These outcomes highlight the importance of riparian vegetation in reducing erosion in the Ngarradj catchment, including in the impact zone of the mine. Further, it is clear that ongoing management in the area - of mining-related activity and fire - should take account of the importance of the monsoonal forest areas and the higher susceptibility to erosion of the woodland streams where riparian vegetation is not as prevalent.