Title: Erosion at decommissioned road-stream crossings: case studies from three northern California watersheds
Author: Flanagan, Sam A.; Fuller, David; Job, Leonard; Morrison, Sam;
Source: In: Standiford, Richard B.; Weller, Theodore J.; Piirto, Douglas D.; Stuart, John D., tech. coords. Proceedings of coast redwood forests in a changing California: A symposium for scientists and managers. Gen. Tech. Rep. PSW-GTR-238. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. pp. 53-59
Publication Series: General Technical Report (GTR)
Post-treatment erosion was observed for 41 decommissioned road stream crossings in three northern California watersheds. Sites were purposefully selected in order to characterize the nature and range of post-treatment erosional responses. Sites with the highest visible erosion were selected in order to better understand the dominant process and incorporate any lessons learned into future projects. Sites were also intentionally selected where post-treatment erosion appeared to be negligible, or excavation techniques were judged be have been fully effective at removing erodible material. In these cases, our objectives and methods remained identical, but we wanted to examine the conditions that led to the apparent negligible erosion.
Results are consistent with other findings in the region. Erosion volumes ranged from 1.5 m3 to 60 m3, or, 0.1 percent to 4.5 percent of the initial volume excavated during treatment. Erosion averaged 11 m3 per site or 0.4 percent of excavated volume in the Headwaters Forest Reserve and 21 m3 per site or 2.4 percent of excavated volume in Lacks Creek. Repeat monitoring of a sub-set of sites in the Headwaters Forest Reserve over subsequent years, indicates that 99 percent of post-treatment erosion occurs in the first year following treatment. Channel incision is the dominant process of sediment production from treated sites, accounting for 80 percent of observed erosion. In response, woody debris has been incorporated into recently excavated crossings with the intent of providing armor and roughness elements to reduce channel incision. At those sites where post-treatment erosion is apparently minimal, incision remains the dominant process of sediment generation and can exceed 20 m3. In some cases, excavation techniques were judged to be largely effective at removing erodible material, however, interstitial material stored between larger, more immobile clasts can produce surprisingly large erosion volumes as this material is winnowed away during higher flows.
Keywords: erosion, road decommissioning, watershed restoration, sediment
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Flanagan, Sam A.; Fuller, David; Job, Leonard; Morrison, Sam. 2012. Erosion at decommissioned road-stream crossings: case studies from three northern California watersheds. In: Standiford, Richard B.; Weller, Theodore J.; Piirto, Douglas D.; Stuart, John D., tech. coords. Proceedings of coast redwood forests in a changing California: A symposium for scientists and managers. Gen. Tech. Rep. PSW-GTR-238. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. pp. 53-59.
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