Gully initiation and road-to-stream linkage in a forested catchment, southeastern Australia

This study reports the nature of sediment delivery pathways and road-to-stream linkage in a forested catchment in southeastern Australia, and evaluates the causal factors associated with this linkage. Detailed field surveying of approximately 20 per cent of the 75 km road network reveals that 18 per cent of road drains show complete channel linkage via gully development from a road outlet to a stream. An additional 11 per cent of road drains show evidence of partial channel linkage where the gully does not extend the full hillslope length. Inclusion of the full range of road-to-stream linkage categories, including direct linkage at stream crossings, road bridges and fords, results in a 6 per cent increase in drainage density since initial road construction in 1964. The majority of this linkage is associated with relief culverts draining cut and-fill roads in mid-valley positions. These drainage structures have contributing road lengths that are on average three times longer than those draining mitre drains on ridgetop roads. Runoff from these roads is also discharged onto hillslopes that are at least twice as steep as those used on ridgetop roads. Contributing road length (m) and the gradient of the discharge hillslope (tan theta) are successfully used here in a linear discriminant analysis to separate channelled and non-channelled flow pathways within the catchment. The successful delineation of these pathways using two easily measured variables suggests that this approach has potential in the planning and rehabilitation of forest roads. Copyright (C) 2001 John Wiley & Sons, Ltd.