Relative effectiveness of fillslope treatments in reducing surface erosion, Horse Creek Road, Nezperce National Forest, Idaho

Burroughs, E.R., Jr.; Watts, F.J.; King, J.G.; Hansen, D. 1984. Relative effectiveness of fillslope treatments in reducing surface erosion, Horse Creek Road, Nezperce National Forest, Idaho. Intermountain Forest and Range Experiment Station, Moscow, ID. Unpublished report. January 1984.

Keywords: fillslope, filter windrow, wood fiber mulch, erosion, roads

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Abstract: Two 70-foot long segments of a 1-1/2:1 fillslope, constructed on "border zone" soils, were subjected to repeated simulated rainfall events. Water and sediment yield were monitored for a total of 20 runs. For each run, about 0.8 inch of precipitation was applied in 23-30 minutes with a kinetic energy estimated at about 60% of the energy associated with a natural event of this intensity.

Both fillslope plots had an existing 6-year old stand of planted vegetation consisting primarily of orchard grass, smooth brome, timothy, and Idaho fescue. Aerial cover on plot 1 was 97.1% and the weight per unit area of vegetal material was 2.67 tons per acre, with litter comprising 63% of the dry weight biomass. On plot 2 aerial cover was 76.7% and the weight per unit area of vegetal material was 1.47 tons per acre, with litter comprising 51% of the biomass.

Four rainfall applications were made on plot 1 in a vegetates condition (three runs on plot 2). THe vegetation and sod were stripped from the plot, followed by four rainfall applications on the bare soil plot (only one run on plot 2). An additional four rainfall application were made on plot 1, protected with a wood fiber mulch which provided 80.6% aerial cover, and again with a slash filter windrow. The windrow provided essentially complete cover on the lower half of the fillslope, while the upper half of the slope was in a bare soil condition.

In their vegetated condition, plots 1 and 2 respectively reduced sediment yields by 99.55% and 99.46% compared to the sediment yields from the bare plots. The wood fiber mulch reduced sediment yield by 91% and the slash filter windrow reduced sediment yield by 87%.

Both the wood fiber mulch and the filter windrow provide good immediate protection of the fillslopes following construction until planted grass can become established. Once fillslopes become well vegetated, sediment yields are reduced to about 0.5% of thoose from bare slopes.

THis study does not reflect the type of detachment and transport processes occurring during natural rainstorms or snowmelt. Therefore, these data should not be used to extrapolate to seasonal or annual sediment production. The estimates of relative effectiveness of the fillslope treatments in reducing sediment yields are probably realistic for the granitic "border zone" materials in the Northern Rocky Mountains.

Moscow FSL publication no. 1984c