Using GIS, Terrain Attributes and Hydrologic Models to Predict the Risk of Soil Erosion and Stream Water Deterioration Caused by Forest Roads

Abstract

Most forestry systems use extensive unsealed road networks for timber harvesting and other forest management activities. These forest roads are significant sources of runoff and sediment to streams, causing impacts on the quantity and quality of stream water. The impact of unsealed forest road networks on stream water quality has long been recognised and documented by many researchers (e.g. Takken et al., 2005; Flanagan et al., 2003; Croke and Mockler, 2001; Megahan et al. 2001; Croke et al., 1999; Montgomery, D.R., 1994 and Anderson et al. 1976). However, the importance of the degree of road-to-stream connectivity has only generated a limited amount of focused research and documentation (e.g. Takken et al., 2005;Farabi, 2005; Hairsine et al., 2002 and Croke and Mockler, 2001). To predict, control and mitigate the negative impacts to stream water quality arising from forest roads, it is necessary to understand the hydrological connection between the sediment source and the stream. Thus, knowing where flow pathways from roads will reach and affect stream water quality is important for both managers and researchers. This mostly depends on the characteristics of the road layout and of the road-to-stream hydrologic connectivity. . The problems occur when this connectivity exists and the roadderived runoff with associated sediments are delivered to the adjacent streams. The overall approach in forest road planning and maintenance is to be able to simulate, predict, and remedy or mitigate the impacts of the forest road on the elements at risk; in an efficient way. In this study, the feasibility of predicting the likelihood of sheet erosion occurrence along the road systems and risk of this to the stream water quality were explored using selected terrain attributes as indicators. The study investigated how the risk to soil erosion and stream water deterioration arising from unsealed forest roads can be predicted, mapped and highlighted using different GIS techniques, terrain attributes and hydrologic models. Several methods were used to carry out and evaluate the study. A 10,477 ha forested catchment in the Australian Capital Territory (ACT) was used as a case study area for collecting the necessary field data. The results showed that a small number of variables, such as hillslope gradient, Road Contribution Length (RCL), Road Contribution Area (RCA), Compound Topographic Index (CTI), Stream Power Index (SPI), drainage area and distance were the variables most highly correlated to the probability of the erosion occurring on the surface of the roads and at the outlet of drainage structures. The results also demonstrated the usefulness of GIS in combination with mathematical (algorithm) and hydrological models as a method for determining the level of road-to-stream connectivity by calculating the flow distance between the outlet of drains and streams. A combination of derived and independent variables was used to map the final risk assessment. The occurrence of problems in the elements at risk (soil and water) was presented as a set of grid layers using GIS overlay applications. The final result of this study is an integrated methodology for "Forest Road Impact Assessment" (FRIA), which uses GIS techniques, terrain attributes and hydrologic models to identify likely sources of stream sediment from forest roads.