2016-09-02_HYDROLOGY - M1992069 - Laserfiche WebLink

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Mr. Roger Schmidt September 2, 2016 Page 10 Table 2. Flow Velocities at F Street Agricultural Field. Field Channel Flow Rate Flow Depth Velocity Condition geometry (cfs) (ft)' (ft/sec)' Pre-mining Slope: 0.004 ft/ft 50 0.49 1.0 Bottom width: 15 ft L. bank: 300:1 R. bank: 60:1 300 1.0 1.5 Post-mining Slope: 0.33 ft/ft 50 0.24 2.8 Bottom Width: 75 L. bank: 1:1 R. bank: 1:1 300 0.79 5. 7771 Note: 1) Flow depths and velocities are based on the normal flow depth for the pre-mining condition with flow across the field and for the post-mining condition for critical flow depth with flow down the 3:1 slope of the pit excavation. The lower velocities in the flows across the field in the pre-mining condition (e.g., approximately 1.5 feet per second for 300 cfs) would not be expected to erode the silt, sands and lean clay materials at the site identified in the Geotechnical Engineering Report for the Boyd/Freeman Ditch Repair(Olsson Associates, 2014), particularly when the river's flood flows are expected to be transporting colloidal silts. In contrast, the higher velocities in the post-mining, or headcut- forming condition, (e.g., roughly 5 feet per second at 300 cfs) could be expected, given a reasonable degree of engineering certainty, to exceed the permissible velocities for those materials (Chow, 1959)and thereby cause erosion, leading to formation of a headcut. An additional consideration regarding channel hydraulics in the area of interest is the bridge owned by the Great Western Railroad which spans the Cache La Poudre River immediately northwest of the F Street agricultural field. The bridge has numerous piers in the river and observation of the bridge on June 22, 2015 revealed a substantial volume of aggraded gravel and sediment upstream of the bridge (see Appendix A, Photo A.22). The deposited material upstream of the bridge indicates slowing of water velocity as the river passes the bridge piers, which would result in a backwater condition upstream from the bridge during high flows. The backwater upstream from the railroad bridge is shown in the most recent FEMA floodplain mapping(Figure 3a). GRAVEL PIT PERMITTING Review of documents shows that the Al Pit was developed with permits obtained through and approved by Weld County and the Colorado Division of Reclamation and Mining Safety (DRMS) and its predecessor agencies. Documents reviewed also show approved DRMS permitting for the Iverson/Martin Marietta Pit.