Channel morphodynamics and the impacts of mining on downstream sand-bed streams in the Ngarradj Creek catchment, Northern Territory, Australia / Michael John Saynor

2010

[Truncated abstract] Channel morphodynamics of Ngarradj Creek in the Northern Territory were investigated from 1998 to 2003. The aim of the investigation was to obtain an understanding of stream hydrology and channel dynamics as well as sediment movement and storage in the catchment. A number of baseline parameters were assessed, including stability of channel dimensions, scour and fill, bank erosion rates, bed material particle size statistics and hydrological characteristics. Gully initiation, bank erosion, changes in bed material grain size statistics and channel changes were established from the data collected. The Ngarradj Creek catchment contains the Jabiluka uranium mine and any contaminants and/or sediment potentially transported off the mine site could have environmental implications for the lower catchment including people using the wetlands it feeds. Construction of the Jabiluka uranium mine started in June 1998 with some ore being stockpiled on the surface in late 1999. An environmental management and standby phase occurred between late 1999 and 2003 when there was no mining or other activity. In December 2003 the mine site was placed in long term care and maintenance. Gully erosion associated with a track across a burnt grass swale was investigated. The track, near the Jabiluka mine site, was used intensively during the 1998 dry season. Repeated vehicle passes over the burnt grass increased soil bulk density and locally disrupted the root and algal mat, lowering the critical shear stress for sediment transport. Overland flow during the following wet season (1998/1999) was above average and eroded eleven discontinuous, flow-aligned scour holes in wheel ruts where the track crossed grassed sandy swales. Although the site was burnt again during the 1999 dry season, the scour holes did not coalesce during the 1999/2000 wet season, which was wetter than the

previous one.

The lack of change occurred because less traffic used the tracks and the eroded section was bypassed by a track detour. This allowed grass to re-establish. Scour holes on vehicle tracks in the region are an intermediate but reversible stage in the development of gullies in grassed swales. Bank erosion rates and processes across a range of spatial scales are poorly understood in most environments, especially in the seasonally wet tropics of northern Australia. Erosion pins were installed at 45 sites on five sand-bed stream reaches of the Ngarradj Creek catchment. Bank erosion was measured for up to 3.5 years between 1998 and 2002 at three spatial scales, namely a discontinuous gully, a small continuous channel and three medium-sized, continuous channels with riparian vegetation. The bank erosion measurements, spanning a period of average to above average rainfall, established that substantial bank erosion occurred during the wet season on the two smaller channels by rapid lateral migration and by erosion of gully sidewalls. The larger channels with riparian vegetation did not generate significant amounts of sediment by bank erosion...

Thesis (Ph.D.)--University of Western Australia, 2010

http://repository.uwa.edu.au:80/R/-?func=dbin-jump-full&object_id=30107&silo_library=GEN01