Slope Gouging/Pitting for Erosion Control
To test, and disseminate to the highway community in the arid West, a method of erosion control used in mine reclamation. Rill & gully erosion is a pervasive problem on roadways in the arid West, occurring with high frequency and great severity on both cut slopes and fill slopes. Due to both engineering requirements and R/W constraints, both cut and fill slopes may need to be steeper than is ideal for erosion control. The result is extensive rill & gully erosion . This erosion: 1) hinders revegetation establishment; 2) contributes to a public perception of industrial ugliness; and 3) can threaten slope stability. More critically, rill & gully erosion (RGE) produces considerable loads of sediment. This excess sediment itself causes serious problems: 1) increased cost for maintenance of cut ditches and culverts; greatly increased environmental impact of roads; and can result in water quality violations. For existing RGE on existing slopes, repair methods are limited and usually involve regrading the affected slope. Installation of terraces or minibenches requires precise elevation control. For future roadways, common erosion controls have proven inadequate (which is why the RGE problem exists). Erosion control blankets are usually installed incorrectly. Sediment wattles and logs are often not embedded deeply enough and are not always installed parallel to the contour. Terraces and minibenches are expensive, require precise surveying control, and usually are directed to downdrains which themselves cause problems. There is an alternative method that has been successfully used on the reclamation of abandoned mines in Utah, and is recommended by authors for other reclamation/revegetation applications. It is a form of extreme surface roughening, which I call "slope pitting." In slope pitting, a backhoe or track hoe is used to create a random, overlapping series of microbasins. The bucket is used to scoop out a basin as wide as the bucket and about a foot deep; the material scooped up is left on the downslope edge of the microbasin as a small embankment. Additional basins are created in a random, overlapping pattern. Immediately after the slope is pitted, it should be hydroseeded. The advantage of such slope pitting is that surface flow is caught by a microbasin; when that microbasin overflows, the runoff only runs to the next microbasin downhill, where it is again trapped. An advantage for both initial installation and for repair applications is that precise elevation control is not necessary. Unlike a terrace or minibench, runoff will not flow along a row of microbasins. Following seeding, fine materials including fertilizer and seed tend to accumulate in the microbasins along with runoff. Thus each microbasin forms a small ideal microclimate that aids in revegetation establishment. Over time, the microbasins will tend to fill in. However, each will still represent a little mini-terrace that will continue to slow runoff. And the additional revegetation success will help prevent further erosion. This proposal would test this erosion-control method on representative slopes in various regions of the arid West. If deemed successful, the method would be disseminated to state DOTs.
A pervasive environmental problem on Western highways could be addressed much more effectively, cheaper, and faster than with currently employed remedial measures. Secondary benefit: The value of looking outside our own industry and our own concepts would be demonstrated.
Randal Pair, AZ Department of Transportation
March 26, 2009
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