2023-10-31_REVISION - M1973021 (2)

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2. Drainage Design Criteria The criteria used for the proposed drainage design is based on requirements listed in the Jefferson County Storm Drainage Design and Technical Criteria (Criteria, Jefferson County 2019) and MHFD's Urban Storm Drainage Criteria Manual (Manual, MHFD 2016 and 2018) and the Colorado DRMS Policies of The Mined Land Reclamation Board (MLRB Policies). 2.1 Development Criteria Reference and Constraints Multiple reports and drainage studies specific to the Site were referenced and influenced the proposed design. A Phase I Drainage Report was prepared for the Site by Chang Engineering (Chang Engineering) to support the first amendment to the PUD-1, which included sub-basin delineations and descriptions, as well as the proposed drainage facility design for the Site. A memorandum discussing the feasibility of the diversion of UD1 to UD2 above site was completed by GEI in 2018. The stormwater evaluation by Apex in 2020 assessed existing detention pond sizing and drainage patterns for the Site. As discussed in Section 2, the outfalls from the Site eventually flow to major drainageways, and along these flow paths there are existing structures beneath the nearby highways (CO-8 and US-285). The proposed drainage design will manage water within the mine footprint versus conveying a discharging UD1 to UD2, as proposed in the Phase 11 Drainage Report. This on-site stormwater management strategy will not impact downstream structures and will not exceed existing conditions downstream of the Site. 2.2 Hydrological Criteria Runoff for the proposed design uses the Colorado Urban Hydrograph Procedure (CUHP). Table 2-1, which is based on the one-hour rainfall depths for the Morrison— Red Rocks Amphitheatre location, shows the design rainfall used to size the proposed facilities. The design storm for peak flow is the 100-year storm. Table 2-1. Design Storm Runoff Recurrence Interval 1-hour Rainfall Depth (inches) 5-year 1.04 100-year 2.22 2.3 Hydraulic Criteria Table 2-2 shows the hydraulic criteria for the proposed design. The criteria were developed based on limitations associated with the proposed drainage design location and in accordance with Jefferson County design criteria. Table 2-2. Hydraulic Design Criteria Diversion Channel Maximum longitudinal bench slope 5% Channel side slope 2:1a Total capacity 100-year peak flow 1670 Broadway,Suite 3400,Denver,CO 80202-4824 hdrinc.com (303)764-1520