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• SSR176 - Bank erosion in the Ngarradj catchment: Results of erosion pin measurements between 1998 and 2001 (PDF 1,040 KB)

Executive summary

A sediment budget has been adopted to assess the physical impacts, if any, of the Jabiluka uranium mine on the Ngarradj catchment in the seasonally wet tropics of northern Australia. Permanently marked cross sections are used to measure large-scale bank erosion and sediment storage, and erosion pins are used to measure slower rates of bank retreat (Erskine et al 2001). A total of 193 erosion pins were installed at 49 sites in seven formally defined channel reaches on four streams in the Ngarradj catchment and were measured at the end of both the wet and dry seasons for up to three years between 1998 and 2001. The four streams were Tributary North and Tributary Central, which drain the Jabiluka mine site, and Ngarradj and East Tributary. The seven channel reaches of Erskine et al (2001) that were investigated included:

• The Gullied Reach on lower Tributary North;
• The Sinuous Reach, Large Capacity Reach and Small Capacity Reach on lower Tributary Central;
• The Forested Meandering Reach (upper Swift Creek or upmain gauge - UM) and Sinuous Reach (Swift Creek gauge - SC) on Ngarradj; and
• The Forested Meandering Reach on East Tributary (East Tributary gauge - ET).

The bank erosion measurements occurred during a period of above average rainfall and streamflow (1998-2001) and established that:

• Substantial bank erosion occurred during the wet season on the mine site tributaries by rapid lateral migration (Tributary Central) and by erosion of gully sidewalls by a combination of within-gully flows and overland flow plunging over the sidewalls (Tributary North);
• Bank erosion also occurred during the dry season by desiccation and loss of cohesion of the sandy sediments, by faunal activity and by dry flow processes;
• Channels with dense riparian vegetation (forested meandering reaches) did not generate significant amounts of sediment by bank erosion;
• Deposition (ie negative pin values were commonly recorded) was also locally significant, despite the sandy bank sediments. This indicates that bank erosion is an episodic process that is not characterised by quasi-continuous bank retreat; and
• Bank profile form and channel planform exert a strong control on erosion rates during the wet but not during the dry season.

Bed scour was greater at the gauging stations than in the mine site tributaries over the same time period that the erosion pin measurements were made (Saynor et al 2002b). Therefore, bed scour and consequent bank undermining were not significant causes of bank erosion in the Ngarradj catchment.

The present erosion pin program exhibited a number of minor shortcomings that should be redressed in future. It is recommended that:

• The erosion pin program in the Ngarradj catchment should be reduced by concentrating solely on the mine site tributaries.
• The erosion pin program should cease at the upper Swift Creek and East Tributary gauging station sites due to the stable channel boundary. The forested banks and high large woody debris loading contribute to this stability.?
• Pins are not effective at the Swift Creek gauging station because of the sandy, gently sloping banks. The current program should be terminated.
• Pins are not needed at the cross sections on Tributary Central where rapid lateral migration is currently occurring and permanently marked cross sections have been installed (TC06, TC07, TC11).
• A recovery peg and/or base line should be installed at the top of the bank to aid in pin recovery.
• Stainless steel pins should be used to stop rusting. Rusted pins cause the binding of sediment to the pin and cause problems with pin measurement.
• Pins should never be replaced when they have been buried.
• Erosion pin sample sites should representatively cover the full range of channel planforms and bank types to enable meaningful spatial extrapolation of the results.
• The reduced but improved erosion pin program should be implemented to target the most active bank erosion sites and to enable a more cost-effective and management-focussed program to be maintained.