PRELIMINARY DRAINAGE REPORT LATHAM 200 MMSCFD GAS PROCESSING PLANT LOTS B, RECORDED EXEMPTION 1211-2-1, RECX13-0096 LOCATED IN THE NORTH 1/2 OF SECTION 2, TOWNSHIP 3 NORTH, RANGE 66 WEST, 6 TH PRINCIPAL MERIDIAN, COUNTY OF WELD, STATE OF COLORADO PREPARED FOR: ANADARKO PETROLEUM CO 1099 18 th STREET, SUITE 16000 DENVER, COLORADO 80202 PREPARED BY: SAMUEL ENGINEERING, INC. 8450 EAST CRESCENT PARKWAY, SUITE 200 GREENWOOD VILLAGE, CO 80111 ISSUED FOR USR PERMIT USR18-00XX PRE-17-0185 REVISION A DECEMBER 2017
LATHAM GAS PLANT PRELIMINARY DRAINAGE REPORT CERTIFICATION I hereby certify that this report for the preliminary drainage design of the Latham Gas Plant was prepared under my direct supervision in accordance with the provisions of the Weld County Storm Drainage Criteria for the owners thereof. Mark A. Skelskey Registered Professional Engineer State of Colorado No. 39396
TABLE OF CONTENTS I. GENERAL LOCATION AND DESCRIPTION... 1 III. DRAINAGE DESIGN CRITERIA... 4 IV. DRAINAGE FACILITY DESIGN... 6 V. CONCLUSION... 7 VI. REFERENCES... 8 VII. APPENDICES... 10
I. GENERAL LOCATION AND DESCRIPTION A. Location The subject project is located in part of the North 1/2 of Section 02, Township 03 North, Range 66 West of the 6 th Principal Meridian, County of Weld, State of Colorado. The property address is 16226 Weld County Road 38, Fort Lupton Colorado. The property is located south of County Road 38 and east of County Road 33. No public streets are proposed within the property. There are no open channels, lakes, streams or irrigation ditches in the area of the proposed development. The subject property is not located in a floodplain. The surrounding area development consists of a several abandoned buildings previously used for poultry farming, several gas wells, gas piping, a large gas processing plant to the northeast and open ground adjacent to the site. Further to the northwest are four single family residences.
B. Description of Property Exhibit 1 Vicinity Map The property is owned by Anadarko Petroleum and is a 239.91-acre lot of which 70-acres will be used for the proposed gas processing plant and 3.45-acres will be used for an electrical substation. The remaining 166.46-acres will remain mostly undisturbed except for a 2500 linear foot access road to the site. The access road will be along the alignment of an existing access used for access to existing well pads. The road will be widened to accommodate plant construction and operations traffic. The undeveloped portions of the property are primarily vacant, un-irrigated pasture land with no trees. The ground cover consists mainly of prairie grasses and weeds. There are no drainage ways or open channels on or adjacent to the property.
Topographic relief over the property ranges from 4908 to 4852 in elevation and is characterized by mostly flat grades that typically range from 0.5% to 3%. Some isolated areas on the western side of the site can approach 9% slopes. Surrounding properties are all zoned agricultural and include residential to the north, a gas processing plant to the northeast. Agricultural land with oil & gas facilities surrounds the site. Historic stormwater flows are generally in an east and northeast direction across the parcel. The flows then follow roadside ditches before heading north into open fields. Ultimately the receiving waters are the Platte Valley Ditch and eventually the South Platte River. There are three (3) soil types present, Valent sand (USC rating SP-SM) as the most predominant, Vona loamy sand (Also USC rating SM) and Olney loamy sand (USC rating SM), Valent sand (USC rating SP-SM) being the smallest area. Soils in the project area have a hydrologic rating of A. There are no irrigation facilities on or adjacent to the property. Initial construction will include site grading for an oil and gas support facility (cryogenic facility) including amine units, utility buildings, maintenance buildings, PDC buildings, dehydration units, cryogenic units, refrigeration units, residue compressors, vessels, tanks, towers, and miscellaneous support facilities for the 200MMSCFD processing facility. Grading for stormwater control and a 4.5-acre-foot capacity detention pond scheduled to be constructed northeast of the plant on lot B. the stormwater detention pond is also part of the initial site grading. Final site surfacing will consist mainly of road base and gravel inside of the site fencing and native grasses following Weld County requirements for the remainder of the property. Terracon Consultants performed a subsurface exploration dated August 31, 2017. Groundwater was encountered 20 to 32 below the existing surface, far below any proposed roads or foundations.
II. DRAINAGE BASINS AND SUB-BASINS A. Major Basin Description The subject property is not part of the Weld County Master Drainage Plan. For drainage basin delineation and surrounding areas, refer to the drainage plans that will be prepared with the construction drawings. Available information shows the property at the top of a basin with very little offsite flows able to cross the proposed plant site. All drainage from the proposed gas plant will be routed into the proposed detention pond. III. DRAINAGE DESIGN CRITERIA A. The drainage concepts and designs have been completed in accordance with the guidelines Weld County Storm Drainage Criteria. No deviations from the Criteria were used in development of this drainage study. Other references used in developing this report include the Urban Drainage Criteria Manuals. B. Development Criteria Reference and Constraints Stormwater discharge from the plant will also generally drain across the site by sheet flows, then into ditches or swales that will discharge into the forebay section of the detention pond. On-site V-Ditches will be utilized in some areas to channel flow to the detention facilities. The ditches will have shallow slopes and low velocity flows. All onsite ditches are designed to accommodate the 10-year design flows at a minimum. Existing roadside ditches adjacent to the property are not well defined and will be improved with the construction of the proposed Phase II site improvements. Erosion control best management practices including rip-rap, silt fence, erosion control logs, erosion blankets, terracing etc. will be installed throughout the construction areas. C. Hydrological Criteria
The design Rainfall for the site was compared between the NOAA Atlas 2, volume III and the IDF tables published by UDFCD. The IDF tables have a higher design value and will be used for this site. The 10 and 100 year values used for this project are 1.38 and 2.63 inches per hour respectively. The design reoccurrence rainfall intensities for this project have been established as the 10-year and 100-year one-hour events. The rational method was used to calculate peak runoff for the property. Excel spreadsheets from UDFCD were used to calculate peak runoff, WQCV, Detention pond 10-yr and 100-yr storage volumes and WQCV 40 hour release quantities. Bentley Flow Master was used to calculate culvert sizes, ditch capacities, orifice openings and weir sizing. Excel spreadsheets were used to crosscheck detention pond capacities, riprap sizing and Impervious values. Detention pond volumes, maximum discharge rates and WQCV were calculated using UDFCD Detention Basin Volume Estimating Workbook, Version 2.35, released January 2015 using the modified FAA method. Refer to Appendix A D. Hydraulic Criteria The project site is located within a Non-Urbanizing Drainage Area. The detention pond will be designed using the 1-hour, 100-year storm event and release of the detained water will be at the historic runoff rate of the 1-hour, 10-year storm falling on the undeveloped site Detention pond capacity calculations were derived using UDFCD spreadsheets and end area method. The detention pond outlet structure will be a CDOT type D inlet with a XX-inch reinforced concrete outlet pipe that has full flow capacity equal to the 100-year allowable unit release rate. The outlet structure controls the WQCV for the 40-hour drain time with the use of a perforated orifice plate and circular holes. The 10-year release rate is controlled with a rectangular opening orifice as part of the Type D structure at the 10- year water surface elevation.
No check or drop structures were used in the drainage design. IV. DRAINAGE FACILITY DESIGN A. General Concept Wherever possible offsite flows are directed around the proposed development and returned into the natural flow patterns. These flows will be channeled around the site and into the existing drainage ditches and culverts. Information included in this report will include an overall site plan, pre and post development drainage basin delineation, construction drawings for the detention pond, tables for rainfall intensity, area impervious values and various calculation sheets for culverts, ditches, ponds and appurtenances. Refer to Exhibit 5 Drainage Plans Pre-Development, Post Development B. Specific Details All roads on the site will be gravel surfaced and designed with a 2% ± cross slope. The asphalt paving has a 2% crown with no curb/gutter. All culverts with the exception of the detention pond discharge pipes are designed for the 10-yr storm. All on-site culverts have less than 85% of the full-flow capacity. All ditches are deep or wide enough to handle the 10-yr storm event with 1-ft of freeboard. The detention pond storage capacity was determined using the UDFCD spreadsheet, Detention Basin Volume Estimating Workbook, Version 2.3 by the modified FAA method. The pond size and shapes were determined from the required volume..
Exhibit 2 SITE PLAN V. CONCLUSION. A. Compliance with Standards This drainage study has been prepared in accordance with the Weld County Storm Drainage Criteria and Urban Drainage Criteria Manuals and accepted Professional Engineering Practices.
B. Drainage Concept This drainage study has been designed to convey developed flows through two extended detention basin detention ponds designed with controlled discharge rates for the minor and major storms. The flows than travel through established drainage ways in a manner consistent with the predevelopment drainage patterns. With properly constructed and maintained ponds along with the proposed erosion control measures this development will not adversely impact the existing drainage or existing downstream developments. No existing Weld County Master drainage plan was available for this project site. There are no irrigation ditches or facilities on or near the subject property. VI. REFERENCES A. Weld County Storm Drainage Criteria, Weld County Department of Public Works Weld County Colorado, October 2006. B. Urban Storm Drainage Criteria Manuals, Urban Drainage and Flood Control District. June 2001