2017-01-03_HYDROLOGY - M1980244 - Laserfiche WebLink

Home Browse Search

DRMS Comment 3(note that this comment references Figure 4 of the CC&V July 20,2016 memorandum) Figure 4. The red line (July 7, 2016) appears to be about a 10 feet lower than the blue line(June 6,2016)at the middle bench (elevation—130),yet the caption indicates"No movement noted". Please provide an explanation for this change. CC&V Response to Comment 3 The process for generating the topography profiles involves collection of raw LIDAR data, processing the data into a triangulation,and then cutting the sections from the triangulation. The raw LIDAR data is a "point cloud", i.e.,the scan area is populated by a high density of points, each with northing,easting,and elevation coordinates(N, E,elevation). The scan is geo-referenced,so each point is in its correct location in space. However, a point cloud does not consist of an evenly-spaced grid of points,and each scanning event generates a different point cloud. Point clouds from separate scans generate unique triangulations,so discrepancies between sections cut from different triangulations can occur,even in a stable slope. In a slope that is deforming,distinct differences between topographic surfaces are usually captured in detail in scan data because of the densities of the point clouds. For a slope that is not displacing, discrepancies between sections cut from different scans should be minor, unless there are different"shadows" in the point clouds that affect the triangulation. Shadows are regions of the point clouds with less detailed data because of irregular surfaces,or missing data because of a blocked line-of-sight from the scanner). The area noted by DRMS has been inspected regularly during field inspections,with no indications of displacement related to the seepage observed. Given these observations,CC&V concludes that the differences between the profiles were produced by data processing, rather than deformation of the slope. Profiles cut along Section A-A'for scans collected on 6 June 2016(blue) and 19 September 2016(red)are shown in Figure 2. As noted in DRMS comments,there are discrepancies between the two sections, particularly at± elevation 130 feet. There is also good agreement between the segments of the profiles that represent angle of repose slopes, and the lower benches of the facility. The set-up location of the scanner was such that it was "looking down"on the lower benches,and the perspective for the angle of repose slopes also provided good return of the LIDAR signal. The instrument was "looking up"at the upper benches of the facility, and these benches created shadows in the point cloud. The triangulation software (I-Site Studio) used points around the shadow to generate topography through the shadow region,and the surface through the shadows was different in the two scans. Final Observations and Stability Analysis No obvious indications of instability related to the seepage were noted during the continual bi-weekly visual inspections of the ECOSA dump from June 2, 2016 to September 15, 2016. No further seepage was noted exiting the base of the ECOSA dump face after the waterline was turned off on August 18, 2016. The final scan of the ECOSA dump was conducted on September 19, 2016,approximately 30 days after the waterline causing the seepage was turned off. The first scan taken on June,2 2016 was overlaid and compared with the final scan. There is no material difference between these profiles, indicating no displacement in the waste dump surface between these dates,as shown in Figure 2. This exercise was repeated for sections every 50 feet across the entire scan area shown in Figure 2 for completeness. Results for these sections also indicated no material differences between the profiles. Differences in the cross-sections that would indicate slope instabilities in relation to the waterline seepage would be rounded bulging of the surface at toe or on the slope, which were not observed.