Unsealed roads as suspended sediment sources in an agricultural catchment in south-eastern Australia

In this paper, we investigate the importance of unsealed roads as suspended sediment sources in a predominantly agricultural catchment within a temperate zone in south-eastern Australia. A selection of sediment tracers and an improved multivariate numerical mixing model are used to quantify the source composition of suspended sediment samples collected during six high-frequency rainfall events. Relative contributions from gravel-surfaced roads, grouped lands (un-gravelled roads, pasturelands and cultivated lands on basalt-derived soils), cultivated lands on granite-derived soils, and forest to sediments in the falling limbs of event hydrographs were 0.41±0.17, 0.18±0.13 0.13±0.11 and 0.14±0.07, respectively. The relative contributions during a peak discharge event were 0.52±0.12, 0.30±0.17, 0.15±0.11 and 0.17±0.08, respectively. Relative contributions per unit area from the gravel-surfaced roads, grouped lands and cultivated lands on granite soils were as high as 500-, 100- and 10-times, respectively, the areal contributions from the forest. These results indicate that unsealed roads are moe important suspended sediment sources than agricultural lands for the suspended sediment samples investigated in the study catchment. The contributions from the gravel-surfaced roads increased with increasing sediment particle size class. Consequently, we believe that considerable emphasis should be placed on unsealed roads, when sediment control measures are planned for agricultural catchments.