2016-09-02_HYDROLOGY - M1992069 - Laserfiche WebLink

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Mr.Roger Schmidt September 2, 2016 Page 9 lower flows. When the water surface is above the top of the wasteway, water flows across the wasteway along the lowest ground surface to the low point on the west side of the concrete bike path and, when the water surface elevation is sufficiently high (as it clearly was when the pipeline breach occurred on June 2, 2014), water spills across the bike path and on to the F Street agricultural field (see Figure 8). • A longitudinal cross section of the concrete bike path located on the west side of the F Street agricultural property indicates a low point in the path (approximately 4 inches lower than the adjacent portions of the trail) that would direct the first flow that overtops the bike path (see Figure 9.0 and Figure 9.1, cross-section A-A for the bike path from topographic data collected in 2014). It is noted that while the bike path is raised above the adjacent grade elevation, the path generally follows the contours of the native grade, so the location where flows overtop the path are in similar proportions to the flow pattern prior to the bike path being constructed and the overall effect of the bike path, with respect to directing flows in the area, was relatively minor. However, the slight elevation of the bike path probably resulted in lesser sheet flows to the east towards the pit than would have otherwise occurred and more flows directed to the railroad bridge. HYDRAULIC ANALYSIS A preliminary hydraulic evaluation was conducted to evaluate flow velocities across the F Street agricultural field for two conditions: 1. Pre-mining condition — This describes flows across the agricultural field through the natural swale (longitudinal slope approximately 0.4 percent) that existed in the area prior to the pit construction and prior to the headcut being formed (based on topographic data from 2013 [Figure 8]). 2. Post-mining condition — This describes flows down the east side of the embankment (slope 3H:IV) into the Al Pit to replicate conditions when the breach was formed in June 2014 (based on topographic data from 2014 [Figure 9.0]). Flow velocities for both conditions were estimated using Urban Drainage and Flood Control District (UDFCD) UD Channels, v. 1.04 for both 50 cfs and 300 cfs to understand the variability in velocity for a range of flow rates. Results of the analysis are summarized in Table 2 and show the differences in flows and velocities in the pre-mining and post-mining conditions: Results presented in Table 2 show that the estimated flow velocities in the post-mining condition are nearly three times higher, or more, than the pre-mining condition, depending on the flow rate. The increased velocities in the post-mining condition are caused by the steep slope of the pit's western bank(33 percent) directing the flows into the gravel pit. This is in contrast with the mild slope across the agricultural field(0.4 percent) in the pre-mining condition.