A Tool to Analyze Environmental Impacts of Forest Roads on Forest Watersheds

The construction and use of forest roads can have impacts on geomorphic processes and erosion patterns in forested basins. Analyzing these impacts will help the forest managers to effectively manage the road and road drainage system and hence minimize the negative impacts of forest roads. To manage forest roads effectively the USDA Forest Service (USFS) has developed a road inventory from GPS road surveys. The primary goal of the inventory is to document the condition of the roads, sources of sediment, and how sediments that originate from the road prism are ultimately routed to the hillslope and stream network. The Forest service road inventory is stored in Geographic Information System (GIS) shapefiles following a data structure that has been developed for this purpose. In this project a GIS tool was developed to use the road inventory information to analyze the effects of forest roads on forest basins. This tool is based on a formal database schema (database model) for the representation of forest road and road drainage information. A pre-processing tool was developed to ingest information from the USFS road inventory into the database, performing quality control and consistency checks. The functionality of the GIS tool is divided into three modules. The first module calculates the sediment production for each road segment from slope, length, road surface condition and road- side drain vegetation information in the road inventory. In this calculation slope is derived by overlaying the road path on a Digital Elevation Model (DEM). Sediment production is accumulated to drain points by adding the sediment production from contributing road segments. These drain point sediment loadings are then used in a DEM weighted flow accumulation function to calculate sediment load inputs to streams. The second module analyzes the impact of forest roads on terrain stability. Terrain stability is assessed by calculating the specific discharge due to road drainage and using this, together with slope, as inputs to an infinite plane slope stability model. The final module analyzes the contiguity and fragmentation of stream network fish habitat using road inventory information on the potential blockage of fish passage at stream crossings.