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~~~ <br />• Mr. Brent Anderson <br />Page Two <br />October 1, 1990 <br />There is no evidence of recent movement in the scarp areas, and the= <br />drainage ditch at the toe is open. Comments from Rockcastle personnel <br />indicate that the ditch has not been cleaned for at least two years: The <br />slide shows no evidence of ongoing downslope movement.' ~" <br />While the slide is currently stable, there is always the possibility of <br />renewed movement if saturated soil conditions recur. This possibility is <br />inherent to the steep terrain which dominates the site. A number of <br />slides unrelated to road construction or other disturbance have been <br />observed in northwest Colorado on similar steep slope areas. While it is <br />not passible to completely prevent potential future movement as the <br />natural process of mass wasting is inevitable, measures can be taken to <br />minimize the potential for future movement and address concerns about long <br />term maintenance of the road. Four measures are examined conceptually. <br />These are: <br />• Diverting water around the head scarp <br />• Improving drainage at the toe of the slide <br />• Installing a toe buttress <br />• Annual inspections <br />• DIVERTING WATER AROUND THE HEAD SCARP <br />There are several areas around the head scarp where water could pool, <br />increasing infiltration to the base of the slide material. It would be <br />possible to divert water around the head scarp, thereby minimizing run-on <br />to the head scarp from upgradient areas. The field inspection <br />specifically addressed the question of installation of a drainage ditch <br />above the head scarp. <br />The vegetation above the slide is relatively dense and mature. In order <br />to install a diversion ditch above the slide it would be necessary to <br />first clear the heavy vegetation to provide for equipment access. Removal <br />of this vegetation would have the potential to significantly increase <br />runoff and erosion in the cleared area and could conceivably destabilize <br />the hillside, shifting the head scarp to the diversion ditch location. <br />In order to be effective, the ditch would need to flow to the east. <br />Examination of the hillside above the road between the curve and the <br />switchback at the bottom of the hill indicates a potential for surficial <br />instability along the entire stretch of road. Therefore, in order to <br />construct an upslope diversion ditch, the ditch would have to be at least <br />2,000 feet long discharging directly to a major drainage to the east. <br />There is no place along the haul road where this drainage could be routed <br />down the hillside into the haul road drainage ditch without significant <br />erosion. Even though there is considerable appeal in trying to divert <br />run-on from upslope areas, physical constraints of the"site, and the ~` <br />• condition of the hillside make this alternative unattractive._ " <br />