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• SSR188 - Bed-material grain size changes in the Ngarradj Creek catchment between 1998 and 2003 (PDF - 777 KB)

Executive summary

The catchment of Ngarradj Creek is located in the wet dry tropics of the Northern Territory, Australia, and is a major right-bank tributary of the Ramsar-listed Magela Creek wetlands. It contains the Energy Resources of Australia Jabiluka project area which is currently in a long-term care and maintenance phase. As part of a long-term study of rates of geomorphic processes within the catchment, a series of research projects were initiated in 1998 by the Environmental Research Institute of the Supervising Scientist (eriss). One of the projects involved determining baseline graphic grain size statistics of the bed material and other sediment stores within the catchment and evaluating any impacts of the Jabiluka project area on these statistics. The graphic grain size statistics investigated included graphic mean size, inclusive graphic standard deviation, inclusive graphic skewness, graphic kurtosis and normalised kurtosis.

Bulk sampling was used to collect mainly bed material but also bar, bench, floodout, splay and distributary channel sediments on the tributaries draining the Jabiluka project area as well as Ngarradj Creek at the Swift Creek gauge downstream of the junction with the Jabiluka project area tributaries. The gauging reaches on Ngarradj Creek at upper Swift Creek and on East Tributary, upstream of the Jabiluka project area, were also sampled at the same time and served as multiple reference sites that were not impacted by the Jabiluka project area. Bed-material was systematically sampled every dry season between 1998 and 2003 but other sediment storages were usually sampled only once between 1998 and 2000. The 56 permanently monumented cross sections used to assess channel changes served as the bed-material sample sites because they ensured that the sample sites could be accurately relocated each year. All samples were sieved and graphic grain statistics were calculated from the cumulative percentage grain size distributions.

The grain size statistics for the period 1998 to 2003 indicated that:

• Tributary Central had the coarsest graphic mean size and the greatest range in grain size statistics because three different types of river reaches were sampled. There was no change in graphic grain size statistics between 1998 and 2003.
• Tributary North main gully and Tributary North tributary gully have similar grain size statistics which are different from those for the Tributary Central and gauging station river reaches. There was no change in graphic grain size statistics between 1998 and 2003 on Tributary North main gully but inclusive graphic skewness and graphic and transformed kurtosis increased after 1998, most probably due to the effects of dry season fires.
• The East Tributary, upper Swift Creek and Swift Creek gauging station reaches have similar graphic grain size statistics. Graphic mean size increased at all three sites between 1998 and 1999 and the data analysis indicated that this increase was statistically significant at ρ = 0.05 level for East Tributary (ρ = 0.01) and Swift Creek (ρ = 0.014), although the increase was not significant (ρ = 0.053) at upper Swift Creek. Graphic and transformed kurtosis also changed significantly after 1998 at upper Swift Creek and inclusive graphic skewness changed significantly after 1998 at the Swift Creek gauge. After the increase between 1998 and 1999, graphic mean size remained relatively constant at all three main channel sites until 2003. The trend in graphic mean size recorded at the downstream Swift Creek gauge was also observed at the upstream East Tributary and upper Swift Creek sites indicating that it was an upper catchment-driven and not a project area-driven change. A coarse-grained phase of sand transport in the main channel network commenced after 1998 but was not generated by the Jabiluka project area tributaries, despite the construction of the Jabiluka project area during the 1998 dry season. The multiple upstream reference sites that were not impacted by the Jabiluka project area were important to determine that this increase was caused by a similar change at both upstream reference sites. A longer record (greater sample size) is still required before the change is significant at ρ = 0.05 in the upper Swift Creek gauge reach.

The grain size statistics constitute thorough baseline information for the Ngarradj catchment and can now be used to determine any subsequent changes due to future activities on the Jabiluka project area. Box plots for each site, when combined with appropriate statistical tests, can be used to assess future departures from these baseline data. This will be important if the Jabiluka project area is reopened in the future as well as to assess the efficacy of the Jabiluka project area rehabilitation.