Road to Stream Connectivity: Implications for Forest Water Quality in a Sub-Tropical Climate

Abstract

Unsealed roads and tracks are acknowledged as the major sources of sediment pollution in forested catchments. In particular, road to stream connectivity via gullied pathways as well as via diffuse overland flow can contribute to significant fine sediment inputs to forest streams. At present in the State forests of New South Wales (NSW), Australia, road drainage spacings are determined on the basis of road slope. In this study forest road surveys were conducted across seven coastal catchments near Coffs Harbour on the sub-tropical NSW mid north coast to determine connectivity between gravel roads and streams via channelised and diffuse pathways under a range of rainfall intensities. A total of 10.82 km of representative road sections was assessed, comprising 129 relief pipes and 22 mitre drains. Of the 151 drains surveyed, gullies were evident at the outlets of 26 relief pipes (20.2%) but at none of the mitre drains. Relationships previously derived between contributing road length and hillslope gradient, and between contributing area and hillslope gradient adequately predicted thresholds of gully formation at drain outlets. During lower intensity storms with average recurrence intervals of 10 years or less, less than 20% of drains are connected to streams via overland flowpaths. However, the degree of diffuse connectivity increases when contributing area takes account of table drains and cut batters, as well as with increasing rainfall intensity. We conclude that when constructing new roads or reassessing drainage on existing roads in forest environments, in addition to preventing erosion of the road surface, gully formation and connectivity with streams via diffuse overland flow should be avoided. This requires factoring in contributing area, hillslope gradient at drain outlets and distance to the nearest stream. Preventing or reducing road-to-stream connectivity is essential for reducing impacts on water quality across all land tenures.