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Turbidity Off of Forest Roads in Oregon

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Pp. 191-197 in Total Maximum Daily Load (TMDL) Environmental Regulations–II Proceedings of the 8-12 November 2003 Conference (Albuquerque, New Mexico USA), Publication Date 8 November 2003.  .(doi:10.13031/2013.15557)
Authors:   E.F. Dent, K.A. Mills, J. Robben
Keywords:   Turbidity, wet-weather hauling, BMP effectiveness, forest roads

The use of gravel-surfaced forest roads by log trucks during wet periods has been well- documented as a major source of fine-grained sediment to streams and associated stream turbidity. This Oregon Department of Forestry (ODF) monitoring project was designed to quantify turbidity increases associated with wet-weather hauling and identify forest road and use factors related to turbidity increases at stream crossings.

ODF monitored roads and turbidity during the 2001 and 2002 winter seasons (November to April). Data collection included traffic levels, road drainage, road surfacing, and road design matrices. Water samples were collected upstream and downstream of crossings (crossing pairs) and analyzed for turbidity. Thirty-two haul routes and 174 stream crossings were surveyed and 438 crossing pairs analyzed for turbidity.

Eighty-nine to 90% (2002 and 2001 seasons respectively) of the sample pairs showed a change of 20 NTUs or less. The remaining 11 to10% ranged from an increase of 20 to 780 NTUs. In general turbidity increases were associated with longer drainage ditches, fines in the aggregate, heavy truck traffic, and shallower rock surfacing. Relationships between turbidity and precipitation suggest the rates of sediment delivery to streams were highest from forest roads when rainfall levels were 1.5 – 3.0 inches.

These findings demonstrate there are practical measures that can be applied to better comply with Department of Environmental Quality turbidity standards. Results were used to modify existing road drainage rules and develop new rules for wet season road use.

Liz Dent1 is the Monitoring Coordinator and Keith Mills2 is a geotechnical engineer for the Oregon Department of Forestry. 2600 State Street, Salem, Oregon. 97310 Joshua Robben3 works for the city of Portland in Oregon.

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