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Home My WebLink About1.14 Grading & Drainage Plans13. GRADING AND DRAINAGE PLANS Introduction The grading and drainage plan for the Cerise Gravel Pit is shown on the attached Grading and Drainage maps. Grading and Drainage Maps The following maps show the grading and drainage patterns of the Cerise Gravel Pit. They are attached to this section. • Existing Conditions • Phase 1B Mining • Phase IC Mining • Phase 2 Mining • Phase 3 Mining • Phase 4 Mining • Final Reclamation Conditions Site Map See the Mining Maps contained under the Site Plan section. Drainage Structures Grading and Drainage Map: Existing Conditions shows the irrigation ditches and other water control structures on site. The remaining Grading and Drainage Maps show the drainage structures that will be constructed over the course of the operation. Topography & Grading Plan The topography and grading of the site at each stage is shown on the Grading and Drainage maps. Soil and Snow Stockpiliing Specific topsoil stockpile locations are identified on the Mining Maps. Snow will be stored with the mined out areas of the site if needed. Drainage Plan This analysis of the drainage of the Cerise Gravel Pit relies on baseline data from the Drainage Report for Lafarge Cerise Mine Site by TetraTech. This report was part of Resolution 2011-44. it is incorporated into this application for reference purposes. 43 Stormwater Inputs Drainage and runoff for the site has been evaluated for both the 25 -year and 100 -year storm events (both 24-hour duration storms). Runoff leaving the site must not exceed the existing 25 - year runoff, and any detention facilities must contain the 100 -year runoff. Table 2 below shows the existing conditions runoff data for the area and the site. Figure ER -10 from the TetraTcch report shows the drainage basins Table 1 refers to. Table 2 — Drainage Srarrnrary for Existing Conditions Basin Drainage Area (acres) Discharge (cfs) 25 -Year 100 -Year A 2,029 81.3 150.4 B 136 7.8 15.2 C 307 33.2 63.4 D 142 10.1 23.3 Crystal Creek 2,614 108.4 198.2 The site access evaluated in the TetraTech report is being constructed per the requirements of Resolution 2011-44, and therefore is not being evaluated in this document. Only Basin D drains to the site, and therefore is thc only basin that will be evaluated in this document. Mining Impacts on Drainage The mining and processing activity shown on thc Site Plan maps will have to handle all Basin D runoff. The amount of runoff generated by storm events in Basin D can be increased by reduction in infiltration into the ground. This occurs when topsoil and vegetation are removed from an area. As mining activity occurs at Cerise, this arca will fluctuate based on how much arca is disturbed, how much is reclaimed, and how much is undisturbed. Therefore, the worst case will be when the maximum disturbance area is exposed. This is at the end of mining in Phase 3, when 31.4 acres will be disturbed. The 25 -year event is 1.95 inches, and the 100 -year event is 2.58 inches. Since the disturbed areas will all drain to the pit, and therefore not discharge, the 100 -year runoff for 31.4 acres is compared to the pit size to ensure sufficient storage capacity. The runoff volume is calculated using the SCS method, and is shown below ;t4 Runoff Curve Number and Runoff Project: Phase 3 Mining By: B Langenfeld Date: 05/23/14 Location: Cerise Gravel Pit Checked: Date: Developed 1. Runoff Curve Number (CN) Cover description CN Soil Group Area(Acre) Mined Land, Disturbed (Poor) 87 C 31.4 CN (weighted): 87.0 Total Area: 31.4 Acre 2. Runoff Return Period: 100 -Year Rainfall, P: 2.58 in Runoff, Q: 1,3783 in Runoff Volume: 3.6066 Acre -Ft The total capacity of the mining pit at this stage is over 1500 ac -ft, more than enough to contain all of the disturbed area's 100 -year event runoff. The reclaimed areas, including vegetated berms will drain off site in areas. These areas are evaluated along with the undisturbed areas to see if their total 25 -year runoff exceeds the existing conditions runoff for the site. This combined revegetated/undisturbed area is 34.08 acres. According to the calculations below: 45 Graphical Peak Discharge Project: Phase 2 - Vegetated Areas By: B Langenfeld Date: 04/30/14 Location: Cerise Gravel Pit Checked: Date: Developed 1. Data: Drainage area• A = 34.0800Acres Runoff Curve Number. CN = 74 Time of Concentration- Tc = 0.39 Storm Type• = II Pond and swamp areas spread throughout watershed = 0.00 percent of A 0.0000 Acres 2. Frequency yr = 25 3. Rainfall,P(24-hour) in = 1.95 4. Initial abstraction, Ia = 0.7027 5. Compute Ia/P = 0.3604 6. Unit peak discharge, qu csm/in = 446.79 7. Runoff,Q in = 0.3268 8. Pond & swap adjustment factor,...Fp = 1.00 9. Peak Discharge,qp cfs = 7.7747 The existing conditions have a discharge of 10.1 cfs from the site, greater than the 7.8 cfs that would be generated by a 25 -year storm during the worst case mining at the Cerise Gravel Pit. At all other times in the operations life either more area will drain into the pit, or the area discharging off site will be less. This demonstration shows that no additional runoff will be generated by mining activity at the Cerise Gravel Pit. Equipment Storage Equipment will be stored in the mining pit. Fuel will be kept near the scale structure. Portable toilets will also be kept near the scale building. Temporary Roads The only temporary roads on site will be in pit ramps that will change and move over the course of the operation. These can be seen on the Mining Maps. Areas of Steep Slope There are two main areas of greater than 20% slope at the Cerise Gravel Pit. The mining slopes, which will be a combination of 1 H:1 V slopes with periodic benches, and the final reclaimed slopes that will be 3H:1 V. All other slopes will be less than 20°`0 (5H:1 V). 46 Construction Schedule Description Years of Mining Build topsoil berm and overburden stockpile. Revegtatc bene and pile immediately upon completion. Mine out Phase IA and 1B. 1.7 Mine out Phase 1C phase after backfilling Phase 1 A. Topsoil and revegetate Phase 1A. 3.4 Reclaim most of Phase 1 B and some of Phase IC while mining Phase 2. 5.9 Reclaim Phase 2 while mining Phase 3. 4.7 Reclaim Phase 3 while mining Phase 4. 2.2 Reclaim remaining disturbed areas such as berms and the scale arca. 0.3 18.2 This schedule is based on a production rate of 437,000 tons per year. It can vary if production varies, which will vary according to market conditions. Permanent Stabilization and Erosion Control Revegetation of topsoil and overburden berms and piles will occur promptly following their construction, typically in the same calendar year. As mined out areas are backfilled and graded to final topography, they will also be topsoiled and seeded. Disturbed areas will drain to the mining pit. The Grading and Drainage maps show the drainage patterns for the site in each phase. Neighboring Areas To the east and west are the Powers Pit and the Blue Pit, two construction materials operations. Immediately north of the site is the Cerise family home and remaining portion of the property. Further north arc residential neighborhoods. 47 Certifications I, Ben Langenfetd, a Colorado registered Professional Engineer, prepared the attached grading and drainage plan. Date 0V I, A, representing United Companies, have reviewed and accept the attached ( gradi.ngynd d linage plan. `—� //f,Q t;. --/o /4 Date 48