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CONTROL OF SEDIMENT EXPORT FROM
<br />THE FOREST ROAD PRISM
<br />J. M. Grace III
<br />ABSTRACT. The effectiveness of four road turn-0ut ditch treatments (vegetation, rip-rap, sediment fences, and settling basins)
<br />in reducing sediment export to the forest floor was evaluated. These four runoff control methods are commonly prescribed
<br />to control fomst road runo>q'and sediments. The study utilized runoff samplers, runoff diversion walls, sediment filter bags,
<br />and erosion stakes to evaluate runoff concemration reductions through the treatments and sediment export downslope of the
<br />treatments. Settling basin, sediment fence, and vegetation treatments had no significant differences in runoff concemration
<br />reductions, which averaged mote than 40%. The rip-rap was significantly less effective at reducing runoff concentration than
<br />were either the vegetation or sediment fence. The sediment basins were very effective during smaller storm events, but not
<br />during the largest storms, when they overflowed. The sediment basin exported the least amount of smaller-sized sediment
<br />that is likely to be delivered to stream systems.
<br />Keywords. Forest roads, Soil erosion, Conservation practices, Surface runoff.
<br />oil erosion and eventual stream sedimentation
<br />adversely affect the nation's water quality. Eroded
<br />sediment can carry nutrients and pesticides, which
<br />degrade water quality and affect aquatic life. The
<br />effects of human disturbances on soil erosion and
<br />sedimentation are major concerns inland management in the
<br />U.S. Special attenflon has been given to sediment transport
<br />from non-point sources resulting from management
<br />activities on forested lands.
<br />Forested lands have minimal erosion and sedimentaton
<br />when there is minimal soil disturbance from forest opera-
<br />tions. The increased surface cover from trees in combination
<br />with protection from raindrop impact by forest titter negates
<br />the damaging effects of rainstorm energy. A different
<br />condition is encountered once the vegetation and surface
<br />litter aze disturbed by forest operations, resulting in acceler-
<br />ated erosion and degradation of stream water quality (Grace,
<br />1998; Reinhart et al., 1963). Forest roads have been
<br />emphasized as major contributors of potentially dettitnental
<br />impacts (Hinkley and Brown, 1993; Reid and Dunne, 1984).
<br />They account for as much as 90% of all erosion losses on
<br />forested lands (Hoover, 1952; Megahan, 1972; Patric, 1976).
<br />Fligh levels of soil erosion can occur following road
<br />constmction and maintenance, which disturb forest cover
<br />and the forest floor (Grace et al., 1998; Grace, 1999). Forest
<br />floor disturbance can accelerate typically negligible forest
<br />erosion rates of 0.30 Uha by more than ahundred-fold. Forest
<br />roads have clearly been defined as [he major contributor of
<br />Article was submitted for review N October 2000; approved for
<br />publication by the Emerging Areas Division of ASAE in April 2002.
<br />Presented at dte 1999 ASAE Awual Meeting as Paper No. 995048.
<br />The author is John MrFem Glace III, ASAE Member, Research
<br />Engineer, USDA Forest Service, G.W. Andrews Forestry Sciences Lab,
<br />Soudrem Research Station, SRS~703, 520 Devall Drive, Auburn, AL
<br />36830: phone: 33426-8700; fax: 334-821-0037; a-mail:
<br />jmgiace~fs.fed.us.
<br />sediment transport to streams from forestlands (Anderson e[
<br />al., 1976; McClelland et al., 1999; Packer, 1967; Patric,
<br />1976). Several factors contribute to the increased erosion
<br />potential of forest roads: (1) removal of surface cover,
<br />(2) concentrated flow in ditches, (3) interception of subsur-
<br />face flow, (4) deshuction of the natural soli stmcture,
<br />(5) increased slopes, and (6) compaction. The combinafion
<br />of these factors greatly increases potential for soil detach-
<br />ment and transport and requires special design considerations
<br />to reduce environmental impacts on surface water quality.
<br />The environmental impacts of forest roads are largely
<br />dependent on site chazac[eristics, topography, and geograph-
<br />ical azea. More specifically, erosion and sedimentation from
<br />roads vary depending on soil characteristics, climatic factors,
<br />and watershed hydrology. Swift (1985) found that roadside
<br />slopes and ditches accounted for the majority of sediment
<br />produced from the road prism in a study in the southern
<br />Appalachians. Haze roadside slopes and ditches in Swift's
<br />study produced greater than 150 Uha during a 4-month winter
<br />period, Grace et al. (1997) found that baze roadside slopes
<br />alone produced slightly greater than 40 Uha over a 6-month
<br />period. The roadside ditch is considered a major azea of
<br />concern in mitigating erosion losses from the road prism in
<br />most geographical regions.
<br />Most state Best Management Practices (BMPs) recom-
<br />mend the installation of some form of erosion control
<br />techniques to reduce sediment exported from the road prism.
<br />Alabama's BMPs suggest the installation of water diversion
<br />techniques such as turn-out ditches, water bazs, and broad-
<br />based dips at the time of toad construction to minhnize
<br />sediment export (Alabama Forestry Commission, 1993).
<br />BMPs state that adequate drainage is one of the most
<br />important factors in controlling sediment movement from the
<br />road system. Some state BMPs aze more detailed and
<br />aggressive; Kentucky, for example, passed a law in 1998 to
<br />promote good s[ewazdship on forestland by requiring that
<br />loggers and operators use appropriate BMPs during timber
<br />harvest.
<br />Transactions of the ASAE
<br />Vol. 45(4): xxx-xxx 2002 American Society of Agricultural Engineers ISSN0001-2351
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