Sediment generation from forest roads: bed and eroded sediment size distributions, and runoff management strategies

A rainfall simulator and overland flow study was conducted to determine in situ and eroded sediment size distributions for a range of forest road surfaces at 2 important commercial plantation centres in subtropical south-east Queensland, Australia; and parameters necessary for running the CREAMS model to assess erosion and sediment transport from road/table drain systems. Results revealed very low concentrations of fine particles in the surface of gravel roads, and somewhat higher proportions in ungravelled (dirt) road surfaces. However, there was considerable enrichment of fine particles in sediment eroded under simulated rain, with concentrations of particles <0.02 mm in diameter being up to 8 g/L. Table drains were generally resistant to scour by overland flows, with the only exception being a drain bordering a newly gravelled road. This drain was bare of vegetation and contained significant quantities of loose gravel from which the fine component was easily eroded. It demonstrated the need to construct both roads and table drains at the end of the wet season when consolidation and re-vegetation can occur under lighter rains during the dry season. CREAMS model runs for a 'standard' road and drain configuration predicted considerable enrichment of fine particles in sediment from all road surfaces. The major factor controlling predicted concentrations of fine particles was the rate of erosion from the road surface, with gravelled surfaces showing considerably less erosion than ungravelled surfaces. Because road surfaces will be significant sources of fine sediment during erosive rains, a second part of this study was designed to model whether hillslopes could be used to infiltrate runoff, thereby controlling sediment movement. For the modelled hillslopes-typical of those used to support commercial forest plantations in south-east Queensland-design runoffs from forest road turn-out drains could be infiltrated. It is suggested that forest managers use hillslope infiltration as the primary tool for managing flows and sediments from road turn-out drains, and that vegetative filter strips be used only as a secondary support tool.