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Full report

• SSR158 - Geomorphic research to determine the off-site impacts of the Jabiluka Mine on Swift (Ngarradj) Creek, Northern Territory (PDF - 1,700 KB)

Separate sections

• Title and publishing information (PDF - 106 KB)
• Preliminary pages (PDF - 152 KB)
• Table of contents (PDF - 122 KB)
• Chapters 1-4 - Introduction, aims & background information (PDF - 244 KB)
• Chapter 5 - Swift Creek catchment characteristics (PDF - 598 KB)
• Chapter 6 - Details of essential catchment baseline and impact assessment studies (PDF - 1,301 KB)
• Chapter 7 - Conclusions and recommendations (PDF - 107 KB)
• References (PDF - 153 KB)

Executive summary

A literature review has found that there is limited background information on the environmental characteristics of the Swift (Ngarradj) Creek catchment, which contains the portal, retention pond and other head works for the Jabiluka uranium mine. To determine the baseline geomorphic characteristics of catchments in the Jabiluka Mineral Lease as well as the physical impacts of uranium mining, 13 sub-projects were proposed within the framework of a sediment budget.

1. Geomorphic mapping. The location and characteristics of the channel network and catchment geomorphology should be mapped by differential GPS and air photograph interpretation to produce a detailed geomorphic map of the Swift Creek catchment that can be used for environmental impact assessment of the mine.
2. River reach definition and mapping. Morphologically homogeneous channel reaches should be mapped and described throughout the catchment to provide a spatial framework for monitoring and impact assessment and to measure future changes in reach boundaries and/or characteristics. A preliminary classification based on air photograph interpretation and limited field inspections has been produced for further testing and refinement.
3. Historical channel stability/instability. The historical stability/instability of each channel reach should be assessed from all available vertical air photographs to provide an understanding of pre-mining channel behaviour and to provide baseline conditions for the assessment of post-mining channel changes.
4. Fluvial sediments. Grain size characteristics of channel boundary and floodplain sediments and the volume of the alluvial store should be determined for each channel reach to provide baseline conditions for the assessment of post-mining impacts on sediment movement.
5. Digital elevation model. A digital elevation model of the whole catchment should be constructed for landform evolution modelling by SIBERIA for environmental impact assessment, mine management, design of a stable rehabilitated mine site and prediction of mine-derived sediment deposition sites downstream of the mine site.
6. River gauging stations. At least four river gauging stations (two on control rivers and two on mine-impacted rivers) with pluviometers should be installed and operated to obtain hydrological information on natural catchment conditions and on the impact of mining. The control rivers are Swift Creek upstream of the mine site and Tributary East. Energy Resources of Australia (ERA) also operate two gauging stations on 7J and North Magela Creeks, which are appropriate controls, if they are not impacted by road construction. The mine-impacted stations are Swift Creek downstream of the mine site and Tributary Central.
7. Suspended sediment transport and turbidity. Detailed suspended sediment and turbidity measurements should be undertaken at each eriss river gauging station to calculate natural and mine-induced suspended sediment loads during the Wet season.
8. Bedload transport. Detailed bedload measurements should be undertaken at each eriss river gauging station to calculate natural and mine-induced bed-material loads. Bedload constitutes a large proportion of the total sediment load in the Alligator Rivers Region and is essential for an assessment of the impact of mining on total sediment yield.
9. Bed scour depths. Scour chains should be installed and measured annually at each gauging station and on mine site tributaries to determine the maximum depth of bed scour (active bedload) during each Wet season.
10. Contemporary channel erosion rates. Bank erosion and knickpoint migration rates should be selectively measured to determine the significance of in-channel sediment sources in comparison to the sediment yields generated on the mine site and from the undisturbed catchment.
11. Contemporary channel stability. Permanently marked cross sections should be installed and used to monitor the amount of bed sediment storage and/or large scale channel erosion throughout the river network.
12. Sediment storages. The volume of sediment in discrete sediment storages downstream of the mine site should be mapped and measured.
13. Riparian vegetation and large woody debris. The significance of riparian forests and large woody debris for stabilising sinuous sandy channels and storing bed material on Swift Creek and East Tributary should be evaluated.

These projects were proposed, described and initiated at short notice before the commencement of the 1998/1999 Wet season. This was necessary to ensure that some catchment characteristics had been measured before the first flush of the Wet season after initial disturbance for mine establishment. No program of hydrologic and geomorphic research had been commenced in the Swift Creek catchment by eriss before June 1998. Furthermore, initial geomorphic measurements of the mine site tributaries and Swift Creek were also required before the first flush of the 1998/1999 Wet season to help determine the environmental impacts of mining.

The above projects target channel stability/instability, sediment sources, sediment storages, sediment pathways and sediment fluxes within the channel network of Swift Creek. Sites impacted by uranium mining will be compared with similar natural sites upstream of the influence of mining. A standard or modified BACI (Before After Control Impact) design was not possible because mining had started before any of the projects could be implemented.