Quantifying Fine Sediment Erosion From Unsealed Roads Draining to the Great Barrier Reef Before and After Applying Best Management Practices

Unsealed roads and their construction and maintenance are a direct anthropogenic source of sediment in river catchments. Maintenance practices use graders to form the road crown, add gravel material, reshape table and diversion drains, and remove vegetation from batters. Repeat high-resolution terrestrial laser scanning (TLS) was used to quantify unsealed road erosion at six road segments (2.5 ha) over two years each with average rainfall to assess (1) baseline erosion from status quo maintenance, and (2) changes in erosion by applying Best Management Practices (BMPs) to reduce fine sediment delivered to the Great Barrier Reef (GBR). Baseline erosion rates were 132 t/ha/yr locally of all size classes and 38 t/ha/yr < 20 µm to GBR, higher than natural catchment rates (<2 t/ha/yr). Suspended sediment concentrations (<20 µm) were 10 times higher downstream of the road crossings compared to upstream. BMPs implemented in the second year included no grading disturbance of drains and batters for grass recovery, woody vegetation control with herbicide, drain rock lining or grade control structures, rock mulching steep batters, rock chutes at gully heads, and rock mattress floodways. Normalised by a control segment, vegetation recovery on batters and drains resulted in the lowest reduction in erosion (22%), compared to the addition of rock mulch and check dams (42–43%) and more frequent water diversion (69%). Wholistic management funding for road condition and safety; vegetation and weed spread; sheet, rill and gully erosion; and GBR pollution should be treated as a complete package by Federal, State, and Local governments.