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Keywords:

  • gully initiation;
  • scour holes;
  • root and algal mat;
  • grass cover;
  • fire;
  • bulk density

Abstract

A track across a burnt grass swale was used intensively on the Jabiluka Mineral Lease (located adjacent to Kakadu National Park in the seasonally wet tropics of the Northern Territory, Australia) for a short time period 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 next wet season was above average and eroded eleven discontinuous, flow-aligned scour holes in the wheel ruts where the track crossed grassed sandy swales. Although the site was burnt again during the next dry season, the scour holes did not coalesce during the second wet season, which was wetter than the previous one, because infrequent traffic bypassed the eroded section allowing grass to re-establish. Scour holes on vehicle tracks in the Kakadu region are an intermediate but reversible stage in the development of gullies in grassed swales. Treatment of scour holes by soil conservation works may prevent gully formation.