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Haupt (1959), in a study of sediment flow distances
<br />downslope of non-point sources, found flow distances
<br />ranged from 1 to 113 m. The investigator concluded that
<br />sediment transport distances from a road embatilQnent into a
<br />buffer zone were affected by four factors: the slope
<br />obstruction index, slope length, road gradient, and cross-
<br />ditch interval squazed. This work resulted in an equation for
<br />determining safe filter strip widths based on these four
<br />factors. An alternative filter strip width equation was
<br />proposed by Swift (1986) based on his reseazch relating to
<br />brash barriers and type of drainage structure. The sediment
<br />transport distances without vegetation ranged from 11 [o
<br />96 m, with shorter distances below vegetated slopes. The
<br />front equation presented by Swift's reseazch was:
<br />slope distance (ft) = 43 + 1.39 X (slope percent) (1)
<br />Road designs that consider environmental impacts have a
<br />common principle: directing water away from the roadway
<br />with a non~rosive velocity. Many authors have suggested
<br />criteria for such designs, emphasizing water control and
<br />drainage structures, which aze two major influences on soli
<br />erosion and sedimentafion (Cook and Hewlett, 1979; Gaz-
<br />dner, 1978; Hewlett and Douglass, 1968; Murphy, 1985;
<br />Packer, 1967; Swifr, 1985; Weitzman and Trimble, 1952).
<br />Diversion of surface water into filter strips or other dispersion
<br />azeas reduces runoff volume and velocity and thereby
<br />directly influences soil detachment and transport by reducing
<br />the erosive energy of runoff water (Brinker, 1993; Trimble
<br />and Sartz, 1957). Typical design features utilized to remove
<br />water from the road prism include broad based dips,
<br />tutu-0uts, culverts, earthen berms, and outsloping. However,
<br />there is little research on the erosion control efficacy of many
<br />of the above-mentioned design features.
<br />Historically, most forest roads were designed with the
<br />primary goal of providing access [o azeas for management
<br />prescriptions, with little regard to environmental impacts.
<br />Currently, the concern in forest road reseazch is the design of
<br />road systems that consider three major objectives: environ-
<br />mental sensitivity, aesthetics, and sustainability. Removing
<br />water from the road system with minimal soil movement and
<br />preventing sediment from being exported to stream channels
<br />aze two goals for improved road designs. The most important
<br />method to accomplish these goals is to disperse flow, thus
<br />reducing erosive energy of the runoff water.
<br />The roadside ditch is a critical azea in forest road
<br />management because it serves as a zone of flow concentra-
<br />tion from the traveled way, cutslope, and upland forest. The
<br />roadside ditch has increased erosion potential due to large
<br />volumes of water at high velocities. Reducing sediment
<br />export from the road prism will likely reduce the amount of
<br />sediment that reaches streams, which causes detrimental
<br />effects [o aquatic life. I[ is hypothesized that the roadside
<br />ditch can be utilized to mitigate sediment export from the
<br />forest road system, thereby decreasing sediment transport
<br />distances downslope. Based on the above hypothesis, ditch
<br />erosion control techniques will be a major element in the
<br />reduction of environmental impacts of forest roads on forest
<br />lands.
<br />Oslecrrves
<br />This investigation evaluated [he effectiveness of four
<br />ditch turn-0u[ treatments in reducing sediment export to dte
<br />forest floor, in order to supplement or improve current road
<br />BMPs. The treatments were compazed based on the sediment
<br />filtered from each associated treatrnent. The purpose of this
<br />experiment was to test the hypothesis Utat there are no
<br />differences in the filtering capacity of the following four
<br />roadside erosion control techniques: (1) vegetafion, (2) rip-
<br />rap, (3) sediment fences, and (4) settling basins.
<br />MATERIALS AND METHODS
<br />The study site was none-mile section of access road on the
<br />Tuskegce National Forest in Macon County near Tuskegee,
<br />Alabama. Long-term average annual precipitation for the
<br />Tuskegee azea is 1300 turn, most of which is rainfall. The
<br />16-yeaz, 24-hour storm for the area is given as 165 mm.
<br />Rainfall amount and intensity, ambient temperature, and soil
<br />moisture were recorded by a weather station located on site.
<br />Runoff volumes in tort[-out sections were estimated based on
<br />mnoff emanating from road sections, taking into account
<br />infiltration rates for each associated road section and
<br />roadside ditch.
<br />Soi] on the study site is a Norfolk loamy sand, ranging
<br />from 6% to 12% slope. Norfolk soils are characterized by a
<br />1S-cm deep, gray, loose, sand surface layer. The upper
<br />subsoil layer is light brownish-yellow loamy sand, and dte
<br />lower soil layer, at a depth of 1.1 m, is yellow friable sandy
<br />clay. The forest road evaluated in this study was re-
<br />constructed, during atwo-week period in August 1997, to
<br />incorporate proper turn-0ut design and spacing. The
<br />crowned road with ditching had originally been constructed
<br />20 years earlier for forest management activities.
<br />Road turn-out ditches with similaz topography, road
<br />design, soils, and drainage were selected for this investiga-
<br />fion. Three replicates of Four sediment control techniques
<br />were used in this experiment: (1) vegetation, (2) rip-rap,
<br />(3) sediment fences, and (4) settling basins (fig. 1). Tutu-out
<br />ditch spacing and constmctionhe-construction followed
<br />specifications currently recommended by Alabama's Best
<br />Management Practices and used by the National Forests of
<br />Alabama (table 1). Tum~ut ditches were V-shaped with a
<br />minimum cross-sectional width of 1.2 m and depth varying
<br />according to surrounding topography. Turn-0ut ditch slopes
<br />were limited to 23'o to 4'Yo gades.
<br />Ditch treatments were hand-seeded with a mixture of
<br />Pensacola bahiagmss (Paspalum notatum) at 22.5 kg/ha,
<br />annual lespedza (Lespedza cttneata) at 5.6 kg/ha, white
<br />clover (Trifolium repens) at 11.2 kg/ha, and Kentucky
<br />31 fescue (Festuca arundinacea) at 28.1 kg/ha. Treatment
<br />azeas were mulched by hand with fescue hay at a rate of 4.5
<br />i/fta and fertilized with 13-13-13 fertilizer at a rate of 1.0
<br />t/ha. Treatments were applied within the treatment area, 3 m
<br />x 3.S m, with the 3-m length between inlet and outlet
<br />sampling points (fig. 2). Vege[adon treatments consisted
<br />solely of the seeding and mulching scheme mentioned above
<br />applied within dre treatment azea. Settling basins were
<br />designed on the basis of expected runoff from the road prism
<br />drainage azea supporting the tom-out ditch. Settling basins
<br />were designed to hold 38 mm (-25% of 10-yeaz, 24-hour
<br />storm) of runoff from drainage areas using the equation:
<br />V = A x d (2)
<br />ZRANSACRONS OP THa ASAE
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