Road Surface Erosion Potential

Step Wise Guide

Transportation/Energy

· Which roads in my watershed have the greatest potential to deliver fine sediment to streams, particularly fish bearing waters?

Unpaved roads are often a major source of fine sediment to streams and rivers. Road surface erosion depends on several key factors including the gradient of the road, length of hydrologic connectivity (or road drainage diversion potential), the distance of the road (drain point) to the nearest stream (or whether the road directly intersects the stream), road surface type (gravel, native soil etc.), traffic level (high, moderate, low), climate (storm characteristics), road geometry (width, inslope, outslope), and characteristics of the ditch (vegetated and unvegetated). NetMap's road erosion tools can be used over large areas to identify which roads have the characteristics conducive to high surface erosion and high sediment delivery potential (to streams). Field validation is highly recommended once predictions have been made.

Step 1: Go to NetMap Road (transportation/Energy) Module > Road Surface Erosion. There are three models available: (1) road erosion index, (2) road (WEPP), and GRAIP-Lite. Prior to the WEPP and GRAIP-Lite road erosion tools, a user must first calculate road hydrologic connectivity (or road drainage diversion). Once the road drainage diversion calculation in available, one or all three erosion tools can be applied.

Step 2: For the quickest and easiest calcuation, Go To the Road Erosion Index tool that only uses road gradient, road hydrologic length (RoadDrain layer, which is created automatically when loading a NetMap dataset is loaded. It can be used as a screening tool to identify which road segments may have the highest road erosion potential. It could be used to prioritize field surveys, road maintenance and where to be watchful following large storms or wildfires.

Step 3: For more detailed predictions of volumetric estimates of road surface erosion, Go To the WEPP road surface erosion tool. This tool requires estimates of many road factors, including the climate in the watershed or landscape your are interested in. This tool can require up to an hour to run in large watersheds, but it can also be applied to only a portion of the road network (specific road segments selected in ArcMap). Predictions include road surface erosion production (mapped to road lines) and road sediment delivery to streams (mapped to stream segments). This tool requires that analysts first apply the Road Drainage Diversion Tool.

Step 4: For another tool for calculating road surface erosion volume and sediment delivery to streams, Go To the GRAIP-Lite tool. Unlike the road WEPP model, GRAIP-Lite requires users to apply a known road erosion "base" rate. There are base rates available in several landscapes in the western US (western OR, eastern OR, ID, MT). Or users, can estimate base erosion rates for their particular landscape, or better yet, users can develop a study to measure erosion rates in their particular watershed.

Step 5: To search for where the highest predicted road surface erosion rates intersects the best fish habitats or the most sensitive channels, Go To Basic (analysis) Tool Module>Risk Analysis>Stream Segment Overlap tool. This tool requires that analysts first apply the Road Drainage Diversion Tool.