Development of a forest road erosion calculation GIS tool for forest road planning and design
Forestry plays an important role in New Zealand's economy. The industry is almost entirely based on planted forests covering 1.8 million hectares of New Zealand. Forestry operations, however, need a dense forest road network. On hilly forested terrain, these roads are often the main source of sediment which can cause degradation of streams and waterways. Appropriate road placement and design may help reduce soil erosion and subsequent environmental impacts. To facilitate forest road planning and reduce potential soil erosion, a Forest Road Erosion Calculation Tool (FORECALT) was developed for ArcGIS 9.1 using the Water Erosion Prediction Project (WEPP) model. The FORECALT tool uses a DEM, a GIS vector based road network map, and a series of road definition selection tools to parameterize and run the WEPP model for the combined set of all individual segments of a road network. The model is able to simulate erosion from cut slopes, road surfaces, and road drainage ditches. The model simulates both insloped and outsloped forest roads. Results of erosion and runoff per segment of forest road are displayed via he GIS interface. Outsloped forest road erosion can be displayed graphically as a map layer and total insloped forest road erosion is calculated for selected outlet points. The FORECALT model was applied to a forest road network in the Whangapoua forest in the Coromandel Peninsula of New Zealand. Erosion and runoff predictions were made for a variety of soil and road type configurations. This application illustrates the versatility of the tool and its ability to aid in reducing environmental impacts by improving the design and planning of road networks. Sensitivity analyses of the model showed that the model is sensitive to the definition of the road segment lengths as well as DEM resolution. Segments of up to 80 meters represent the original DEM profile with reasonable accuracy; however, even small variations in calculated slope profiles may impact WEPP model erosion predictions. Runoff predictions are not affected by DEM resolution or segment length selection. Although further research is needed to understand the complex relationship between input DEM resolution and segment length selection, modelling results suggest that the model would be a valuable asset for forest managers.