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Home My WebLink AboutDrainge Report 08.29.2023 1 DRAINAGE REPORT FOR GRAND HOGBACK RANCHES NEW CASTLE, COLORADO DRAINAGE PLAN SUBMITTAL Applicant: West Divide Ranch, LLC Prepared by: High Country Engineering, Inc. 1517 Blake Avenue, Suite 101 Glenwood Springs, CO 81601 (970) 945-8676 HCE JOB NO. 2201740 August 29, 2023 2 TABLE OF CONTENTS SECTION PAGE I. PURPOSE .......................................................................................................................... 3 II. LOCATION ....................................................................................................................... 3 III. EXISTING CONDITIONS .............................................................................................. 3 IV. PROPOSED CONDITIONS ............................................................................................ 4 V. SUMMARY ....................................................................................................................... 5 VI. REFERENCES .................................................................................................................. 6 APPENDIX A: Hydrologic Calculations APPENDIX B: Hydraulic Calculations APPENDIX C: Culvert Drainage Basins @ Soil Maps/ Descriptions 3 I. PURPOSE The purpose of this report is to address the storm water drainage for the proposed roadway infrastructure modifications for the property located in Garfield County Colorado, near the Town of New Castle. The road is mostly complete and proposed modifications are being recommended to ensure the existing road is built to Garfield County criteria and that the existing culverts are sized adequately to convey the large offsite basins to the south. II. LOCATION Roadway infrastructure and improvements are proposed south of Interstate 70 and south of CR 335. The proposed roadways include the middle access road for the development named Vulcan Mine Road and is approximately 850 feet long, and the eastern access named Old Tipple Road, which is approximately 750’ long. Both are part of Grand Hogback Ranches Plat. The proposed roads will provide access to lots 5-13 within the Grand Hogback Ranches shown in Figure 1 below. Our understanding is that lots 14-18 will be responsible for their own access requirements and improvements onto Colorado River Road. Figure 1. Grand Hogback Ranches Site III. EXISTING CONDITIONS The soil types, as designated by the Natural Resources Conservation Service are primarily Camborthids-Rock within the Grand Hogback Ranches site. This is hydrologic group D. The existing vegetation on the site is historically shrub and undeveloped. Hydrologic reports from NRCS are located in Appendix A. 4 The road as it currently exists provides access into the property at several locations. Stormwater flows are conveyed through existing infrastructure to existing offsite culverts under CR 335 (Colorado River Road). The proposed road will be improved and storm infrastructure with the added roads will be updated by providing conveyance of storm water flows for the 25 year storm event. Downstream culverts on Colorado River Road are undersized and will not be modified with these improvements. IV. PROPOSED CONDITIONS In the proposed condition the site is divided into basins tributary to a total of four culverts under the existing and proposed road. There are no proposed inlets or curb and gutter to convey flow. All flow will sheet flow off the road and be captured in an improved roadside ditch where it will be captured by the proposed new culverts and released towards existing culverts under CR 335. Each culvert is sized to convey the 25 year storm event. There are two proposed intersections with CR 335. The existing ditch conditions are very shallow and at minimum grade. Based on that condition, it is proposed to have a valley pan on the eastern intersection and a pipe buried under the western intersection to work with existing conditions and to convey flows across the proposed access points. These crossings will have minimum flows due to the existing culverts that cross CR 335 upstream of the access points. The maximum distance upstream to existing culverts is at the crossing at Tract 5 that has a culvert approximately 600 feet upstream. The peak runoff rates for each basin has been calculated using SCS Curve Number Method. A summary table for each basin has been provided below in Table 1. The two access roads have also been analyzed to provide detention for the impervious roadway surfaces. The basins evaluated for detention were limited on the lower bench to provide a more accurate detention calculation based on the small added impervious roadway areas, which provides acceptable hydrographs to calculate detention. A map of the basins is located in the appendix. Detention ponds were designed to provided 6” of settling at the bottom of the pond and a historic release at the 6” elevation and an overflow pipe at the top of the required detention volume. A 6” freeboard is provided above the overflow to allow for the overflow in case of plugging. Table 1. Basin Summary Culvert/Basin ID Area (ac) Composite CN Q25 (cfs) Q100 (cfs) 6 35.06 69 12.51 29.74 8 37.84 69 13.51 15.69 East Rd. Det. 5.29 68(ex.)69(pr) 2.89 495 cf 11 34.96 69 11.61 27.91 13 55.56 69 19.83 47.14 Mid Rd. Det. 5.32 65(ex.)66(pr) 1.14 318 cf West Rd. Det. 14.02 67(ex.)68(pr) 3.65 1227 cf 16 58.62 70 6.43 26.15 Tract 13 Access point 0.67 80 2.67 3.61 5 Proposed and existing culverts are summarized below in Table 2. Table 2. Culvert Summary Culvert ID Existing Culvert Proposed Culvert Size (in) Upstream Elev (ft) Downstream Elev (ft) Length (ft) # of Barrels Size (in) Upstream Elev (ft) Downstream Elev (ft) Length (ft) 6 12 5607.59 5606.20 47.68 1 24 See plan See plan 48 8 12 5614.94 5615.22 30.04 1 24 See plan See plan 34 11 12 5618.64 5618.90 29.92 1 24 See plan See plan 30 13 18 5599.33 5597.26 39.98 2 18 See plan See plan 47 V. SUMMARY The Drainage Plan for Grand Hogback Ranches includes mostly existing ditches and additions to existing culverts as well as three detention ponds to provided detention for the proposed roadways. Both existing and proposed infrastructure will convey offsite flows across the existing road and release into historic culverts in CR 335. The sizing of the culverts is based on the minor (25 year) events and culvert locations will not be changed from existing locations. 6 VI. REFERENCES 1. Soil Survey of New Castle Area, Colorado, U.S. Department of Agriculture, Soil Conservation Service, May 1985. 2. Precipitation –Frequency Atlas of the Western United States, Volume III-Colorado, National Oceanic and Atmospheric Administration, 1973. 7 HYDROLOGIC CALCULATIONS 8 9 10 11 12 13 EAST DETENTION TRACT 6 – 495 CF 14 MID DETENTION TRACT 11 – 318 cf 15 WEST DETENTION TRACT 13 – 1227 cf 16 17 Tract 13 access point 18 HYDRAULIC CALCULATIONS 19 PROPOSED 24” Tract 6 12.51cfs 20 Existing 12” Tract 8 2cfs PROPOSED 24” Tract 8 11.51cfs TOTAL TRACT 8 2 CULVERTS 13.51CFS 21 PROPOSED 24” Tract 11 11.61cfs 22 2 New and 1 Existing 18” Tract 13 13.83 cfs 23 Tract 13 ACCESS POINT 3.61 cfs 24 EAST DETENTION RELEASE 24” pipe and 11” orifice Tract 6 MID DETENTION RELEASE 18” pipe and 6” orifice Tract 11 25 WEST DETENTION RELEASE 24” pipe and 10.8” orifice Tract 13 18” Borrow Ditch 26 DRAINAGE BASINS & SOIL MAPS AND DESCRIPTIONS 27 Drainage Basins with Soil Types 28 29 30 31