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The Big Disconnect – Stormwater Runoff and Forest Roads

By: Gregory Bassler, Northwest Management, Inc.

Stormwater runoff from impervious surfaces such as forest roads is often conveyed to waterways by ditches and culverts.  Runoff can carry pollutants (sediments) to streams and other waterbodies.  The issue of whether or not a forest road is a point source for discharge of a pollutant into a waterbody has been an ongoing debate in the Ninth Circuit Court of Appeals since 2006. Provisions of the 2014 Farm Bill which was recently signed into law, settled the issue by enshrining the “Silvicultural Rule” which exempts most forestry activities from Clean Water Act permitting requirements.  This was a huge win for forest landowners however; landowners are still subject to state-derived Best Management Practices (BMP’s) under the Clean Water Act.

The most effective strategy to keep water clean is to disconnect forest roads from the stream network.  The basic goal is to prevent direct sediment delivery into streams by dispersing runoff onto stable forest areas.  This will filter and settle suspended sediment out of dirty water prior to entering a waterway.  Collecting runoff through inside ditches that drain directly into a stream or wetland must be avoided.

Diverting road water away from streams and wetlands onto stable forest floor can be accomplished in several ways. secure server . One is by using road drainage structures to protect road surfacing and fill slopes, return water to its natural course, reduce velocity and protect discharge points from erosion.  All roads should have surface shape that drains water off the road surface, either crowned, outsloped or insloped.  Where surface water is diverted to the upslope side of the road with crowned and insloped roads, ditches and cross-drains are used.

Use of surface water bars, grade breaks and rolling dips will drain surface water and reduce velocity.  Surface water bars can be installed but are primarily used when a road won’t be used for hauling for an extended period of time.   Rolling dips can be the primary drainage system on forest roads and may be mixed with both outsloped or insloped systems.  Rolling drain dips are normally built into roads where grades do not exceed eight percent.  Construction of rolling dips varies depending on the road surface type, vehicle use, road grade, and surface erosion potential.

The ditch is an important part of the drainage system, and cross-drains (culverts) are needed to allow water to pass below the road profile, from the uphill side to the downhill side, as quickly as possible. Ditches should be at least one foot in depth and should be free of debris such as downed logs or large rocks.  Armoring of ditches with rock and installing rock check dams will reduce ditch-line erosion on steeper grades.  Installation of catch basins and sediment traps are used to collect water and sediment.

Ditch relief culverts are used on crowned or insloped ditched roads. Ditch relief culverts transfer water from a ditch on the uphill side of a road, under the grade, and release it onto a stable area below the road preventing water from crossing the road surface and softening the roadbed. Ditch relief culverts and roadside ditches should be used whenever reliance upon natural drainage will not protect the running surface, cut slopes, or fill slopes.  Rock armoring and placement of slash can be used as energy dissipaters.

Eliminating the connectivity between road surface runoff and stream/waterways is the best approach to maintain water quality.  Forest landowners must be vigilant in their efforts to integrate use of some or all road best management practices and to continually monitor runoff from forest roads.

Gregory Bassler is a forester with Northwest Management, Inc. with over 25 years of expertise in road design and layout.

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