2022-02-23_PERMIT FILE - M2021059 (13)

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To: fir. Pcter Bays Date: Fcbrua►ti 2 1 20121 Page?0 Response 2 Pit inlet weir designs are shown on reclamation plan Exhibit F-3 (Attachment 6). Comment 3 Please explain why bridge decks and piers are not included in the model. It appears that these structures could impact the velocities offlows into Cell 3, which is relatively close to the railroad bridge (approximately 200 feet upstream). Response 3 Two hydraulic models were completed under this project: HEC-RAS 1D (1D) and HEC- RAS 2D(21)). The 1 D model calculated the anticipated back water affects associated with the project's proposed construction and mining activities, and to meet Weld County's no- rise condition during their dictated flow event(estimated by the Colorado Hazard Mapping Program, CHAMP). The 2D model was used primarily to predict where and at what flow rates water would enter the proposed pits,and to calculate location specific velocities, flow depths and shear stresses so that flows into and out of the pits could be managed. Before the release of HEC-RAS 6.0(December 2020)a bridge could not be modeled in 2D unless it was modeled as a culvert. In addition, HEC-RAS bridge scour calcs have historically been calculated in 1D. The use of 1D vs 2D is slowly evolving and we have not fully made the switch between 1 D and 2D(this analysis was started just after December 2020 and was completed in HEC-RAS 5.0.7). Therefore,bridge decks and piers were not included in the 2D model because it was not possible to do so at the time. The effects on flow velocities into mining Cell C3 calculated in the 2D model as a result of existing railroad and Highway 257 bridge decks and piers are inconsequential because: • The flood stage during the 100-year event is below the elevation of the bridge girders for both the railroad and highway bridges. Therefore, there would be no hydraulic effects caused by bridge decks • The railroad and highway bridge piers are hydraulically downgradient of Cell C3. Therefore, any upstream hydraulic effects caused by downstream obstructions during subcritical flow would only decrease upstream flow velocities making the predicted 2D cell C3 flow velocities and shear stress estimates conservative • The cross-sectional area of the bridge piers is small compared to the overall un- obstructed area between the bridge abutments. Therefore, any backwater effects that would occur due to the bridge piers are assumed to be relatively negligible 20220209_10.15_DRMS_Adequacy_Review_Response_W STE Rdocx TELESTo