Rural Road Assessments – The following quoted excerpts are from a summary report to the California Department of Fish & Game, prepared by Pacific Watersheds Associates, for one of the road assessment projects on private land that LEGACY was involved in. These excerpts explain the process of identifying problem sites in a road system and the importance of improved rural road design for the prevention of sediment pollution to Class I streams. The property and creek names have been omitted due to privacy concerns.
“Perhaps the most important element needed for long term restoration of steelhead habitat productivity, and the eventual recovery of salmonid populations in [this creek], is the reduction of accelerated erosion and sediment delivery to the channel system from upland erosion.”
“In the first phase of [this] inventory project all roads within the study area were identified and age dated from historic aerial photography and a map was developed color coding the roads by age.”
“The second phase of the project involved a complete inventory of the road system and stream crossings. The base maps, updated through analysis of aerial photos by New Growth Forestry personnel and Linda Gray of LEGACY – The Landscape Connection, depict the primary road network in the watershed and shows the location of sites with future erosion and sediment delivery to the stream system Road assessment map identifying sites where there is a potential for future sediment delivery to the stream system that could impact fish bearing streams in the watershed. Inventoried roads, including both maintained and abandoned routes, were walked and inspected by trained personnel and all existing and potential erosion sites were identified. Sites, as defined in this assessment, include locations where there is direct evidence that future erosion or mass wasting could be expected to deliver sediment to a stream channel.”
” Virtually all future road-related erosion and sediment yield in [this] watershed is expected to come from three sources: 1) the failure of road fills (landsliding), 2) erosion at or associated with stream crossings (from several possible causes), and 3) road surface and ditch erosion.”
“Approximately 5,484 yds3 of future road-related sediment yield in [this] assessment area could originate from erosion at stream crossings. This amounts to nearly 18% of the total expected future sediment yield from the road system. The most common problems which lead to erosion at stream crossings include: 1) crossings with undersized culverts, 2) crossings with culverts that are likely to plug, 3) stream crossings with a diversion potential and 4) crossings with gully erosion at the culvert outlet. The sediment delivery from stream crossing sites is always classified as 100% because any sediment eroded at the crossing site is then delivered to the channel. Even sediment which is delivered to small ephemeral streams will eventually be delivered to downstream fish-bearing stream channels.”
“Three road design conditions indicate a high potential for future erosion at stream crossings. These include 1) undersized culverts (the culvert is too small for the 100 year design storm flow), 2) culverts that are prone to plugging with sediment or organic debris and 3) stream crossings with a diversion potential. The worst scenario is for the culvert to plug and the stream crossing to wash out or the stream to divert down the road in a major storm. These road and stream crossing conditions are easily recognizable in the field and have been inventoried in [this] watershed.”
“Preventative treatments include such measures as constructing critical dips (rolling dips) at stream crossings to prevent stream diversions, installing larger culverts wherever current pipes are under-designed for the 100 year storm flow (or where they are prone to plugging), installing culverts at the natural channel gradient to maximize the sediment transport efficiency of the pipe and ensure that the culvert outlet will discharge on the natural channel bed below the base of the road fill, installing debris barriers and/or downspouts to prevent culvert plugging and outlet erosion, respectively, and armoring the downstream fill face of the crossing to minimize or prevent future erosion.”