Dames & Moore, 1999 - USDA Forest Service

The results of an analysis of topographic map data (e.g., average slope, reach length, reach height) for each
slope reach are summarized in Table 4.2-3. This data, in particular average slope,.was used in combination
with the field characterization and trench log data to determine the criteria for assessing erosion potential.
~verage slope was defined as the slope from the top of the tailings (at the tailings edge) to the base of the
riprap (as best as could be determined from the map contours and field knowledge of the riprap location).
The criteria for determining erosion potential of each slope reach was based on the field mapping, .
observations of erosion processes occurring on the slopes, topographic map data, and the estimated high
water lines. Specific criteria were developed that included:
t
Average slope of reach from the top of the slope to the base of the riprap
Range of slopes occurring across the reach
Slope setback From creek channel (the average distance from the toe of the slope to the
bank of the main channel)
Slope setback from crest of riprap (the average distance from the toe of the slope to the
crest of the riprap)
Composition of slope materials (e.g., sand talus, gravel cap, cemented sands)
Condition or existence of vegetation, andlor grass mats I
Extent and magnitude of current erosion processes I
Table 4.2-2 summarizes the criteria for grading the slopes erosion potential. I
Based on the developed criteria, an erosion potential for each slope reach was assigned (Table 4.2-3)
(Figure 4.2-21). In general, slopes along tailings pile 1 have a moderate to moderately low erosion potential
due to the low average slopes, the distance that the pile is setback from the creek, the amount of vegetation
present, and the condition of the gravel cap. Slopes along tailings pile 2 have a high to moderately high
erosion potential due to the steep slopes, high amount of exposed tailings, little or no setback to the creek,
and the lack of a gravel cap or vegetation. Slopes along tailings pile 3 have a moderate to low erosion
potential due to the lower height, relatively sufficient setback (i.e., sufficient space to build talus to the angle
of repose), the amount of vegetation present, and the good condition of the gravel cap.
There is some level of uncertainty associated with the findings of the erosion potential evaluation due to the
difficulty in quantifying all of the variables associated with surface erosion. However, the findings are
consistent with our expectations based on past experience on other site with similar geologic and hydrologic
conditions.
4.2.5 Mine Subsidence Potential
As mentioned in Section 4.1.3.1, the uppermost stopes within the mine above the 1500-level are mapped
to be within approximately 50 feet of the ground surface. Figures 4.2-14 and 4.2-1 5 display the lateral and
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17693-00S-OI9Vuly 19, 1999;4:51 PM:DRAn FINAL RI REPORT