Soil colour as a tracer of sediment dispersion from erosion of forest roads in Chichester State Forest, NSW, Australia

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A rainstorm on 27–28 May 2003 caused a debris slide in the fill batter of a recently constructed road crossing of a zero-order stream in Compartment 107, Chichester State Forest, Australia. The slide became more liquid downslope, changing to a debris flow. This flow completely blocked the inlet of the next downstream road crossing of the same stream, where it had changed to first order. Gullying of the downstream fill batter at the second crossing was caused by water flowing over the road. The estimated volume of sediment removed by the upslope debris slide was 350 m3, and the estimated erosion volume by gullying of the downstream fill batter at the second crossing was 100 m3. Mass movements have occurred in Compartment 107 before any forestry activities because of the presence of colluvium-filled bedrock depressions. Furthermore, the channel downstream of the second crossing has transported much larger quantities of coarser sediment than what was mobilized during and after the event of 27–28 May 2003. Channel bank exposures reveal up to 2.5-m-thick, poorly sorted gravels, which, on the basis of soil development in the overlying deposits, appear to be older than 200 years. Sediment was deposited in the channel downstream of the second crossing, as determined using Munsell Soil Colour as a fingerprinting technique. The volumes involved were relatively minor and were only deposited in slackwater areas as spatially disjunct, relatively thin patches. Sediment was restricted to the first 500 m downstream of the second crossing and was unlikely to have reached the main stream, Allyn River, in any measurable quantity. The channel downstream of the second road crossing is a high-energy feature because it contains a range of channel units (waterfalls, bedrock, boulder and log steps, cascades, etc.) formed by erosion of resistant materials. Similar and larger events to that of 27–28 May 2003 in the following 13 months did not cause further road erosion, indicating that the installed rehabilitation measures stabilized the road.

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