A combined rainfall simulator and tracer approach to assess the role of Best Management Practices in minimising sediment redistribution and loss in forests after harvesting

Abstract

In this paper, we use the combined results from two contrasting methods to investigate the effectiveness of Best Management Practices (BMPs) on sediment erosion and redistribution in managed forestry environments in southeastern New South Wales (NSW), Australia. The first method employed a rainfall simulator at the skid track (snig track)—water bar (cross-bank)—general harvest area (GHA) spatial scale. This showed the majority of production and redistribution occurring from the skid tracks and coinciding with the period after initial disturbance. However, 5 years after harvesting, little redistribution between these elements was occurring. Furthermore, water bar structures were effective in trapping coarse sediment generated from skid tracks but markedly less so for fine <63 μm material. The second method involved constructing a tracer budget. This was based on measuring the 137Cs content in each of the landscape elements of an entire watershed, which also enclosed a 5-year forest coupe selected for the simulator experiments. The 137Cs budget effectively balanced with 61.1±4 and 59.2±4 MBq present prior to, and after, harvesting, respectively. Total recovery was 97±10%, which indicated no net loss of sediment from the study basin over the 6-year post-harvesting period within uncertainties. Of the total sediment redistribution, some 13±4% in losses occurred from the skid tracks and log landings, whilst net gains of 10±1% occurred in the GHA, filter strip and water bars. On a per unit area basis, the filter strip retained ∼8 times the 137Cs and associated sediment of the GHA; however, the greatest overall trapping of fine sediment occurred in the GHA. Combining these two approaches suggests that management of filter areas is paramount, although the GHA remains the primary sediment buffer for the system. Greater effort to disconnect flow paths, retain roughness and reduce overland flow volumes in order to increase sediment deposition should occur here.