PERMFILE134266 - Laserfiche WebLink

Home Browse Search

REFUSE PILE EXPANSION end area greater than 35 sf shall be designed to safely pass the 25-year, 24-hour precipitation event without a head of water at the entrance unless suitable ~'pro[ection against erosion or fill saturation is in place at the entrance and exit of the culvert." The CMP culvert was actually designed to carry the 100-year, 24-hour storm peak Flow without head of water at the inlet. The 100-yr, 24-hr precipitation even[ peak flow in Sylvester Gulch at the crossing location was estimated using the U.S. Army Corps of Engineers HEC-1 Flood Hydrograph Package computer program, developed by the Hydrologic Engineering Center in Davis. California. The version used forKhis study was a 1994 release by Dodson & Associates, Inc. Houston, Texas. The SCS dimensionless unitgraph method (SCS, 1972) was used to estimate runoff, lag times, and peak Flows. Eleven sub-pond drainages were delineated as shown in Appendix L, and surface areas and channel reach lengths were measured. V-notch channels with a Manning roughness coefficient of 0.06 and I : I sideslopes were used to simulate the drainage channels. An SCS runoff curve number (CN) of 60 was used for the watershed surface and is based upon the following ~~trevious studies of vegetation and soil cover: 1) Baseline Hvdroloev/Geoloev Report, Woodward- Clyde,July, 1978; 2) Refuse Pile Expansion Baseline Soils Study, Savage & Savage, Inc., January, 1996; and 3) Refuse Pile Expansion Baseline Veeetation Studv, Savage & Savage, Inc. January 1996. The hydrologic soil-cover complexes for the Sylvester Gulch watershed consist of grass-oak cover, in good condition, in a B to C hydrologic soil group. According to chapter 9, Figure 9.5 of the Department of Agriculture National Engineering Handbook, Section 4, Hydrology (March, 1985), the SCS CN should ranee from 43 to 65; which supports [he CN estimate of 60 as being sufficien[lyconservative. The 100-year, 24-hour, Type II precipitation event was se[ at 2.8 inches. The estimate for the peak Flo++' in Sylvester Gulch was kept conservative by assumine no loss of flow within the channel reach (i.e., no losin_ reaches). A kinematic ++~ave stream routine method was used to convey a hydro~raph from one drainage junction to the next before summing. The peak runoff for Sylvester Gulch at the wnction with Highway 133 was estimated to be 263- ~,. cfs. Refer to Appendix L for the complete HEC-1 output printout. A CMP culvert was sized [o convey the peak runoff and withstand a 25-ton wheel load. The following parameters were used for the culvert design: • Type (Flow -critical depth at inlet, control at inlet, barrel flow partial • culvert slope-7.5% • culvert length -minimum 100 ft • Manning roughness constant for CMP - 0.022 • flow velocity at inlet- 14.5 ft/sec • CMP culvert diameter- 7 ft. • CMP culvert corrugation- 6-in. x 2-in. • CMP- 12 gagemulti-pla[ecircularpipe • CMPfillcover-2.5ft The 7 ft. diameter culvert is designed allow the estimated 100-year, 24-hour precipitation event to pass without a head of water at the entrance. 6.8 Impacts to Wetland A wetland was identified in the northeast comer of the proposed RPE, and eliminated with the construction of the sediment pond. MCC has mitigated the filling of this wetland by constructinga wetland mitigation site on other (non-permit area) land owned by MCC. See Appendix C for correspondence with U.S. Army Corps of Engineers regarding approval of this activity and the mitigation plan. 7.0 ROADS 7.1 Haul Roads A new haul road was needed to deliver the refuse to the RPE. The location of the haul road for [he firs[ three phases is shown on Map ~3. The road width varies from 14 feet to 18 feet. Topsoil stripped from the road will be hauled to the NSSA. The average grade of the road is approximately 7.9% with no grades exceeding 12%. The plan and profile location of the initial haul road is shown in Figure N-I. A culvert has been placed in Sylvester Gulch to cross Harding Lawson Associates 15