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HomeMy WebLinkAboutBMC WEST PUD - PRELIMINARY - 1-96 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTREFERENCES 1. City of Fort Collins, Colorado, Storm Drainage Design Criteria and Construction Standards, May 1984, Revised January 1991.. 2. Urban Drainage and Flood Control District, Denver, Colorado, Urban Storm Drainage Criteria Manual, Volume 3 - Best Management Practices, September 1, 1992. during construction are shown on Standard Form C in Appendix IL As shown on Form C the silt fence will be constructed prior to overlot grading. Permanent landscaping will be placed after the completion of the project. EROSION CONTROL SECURITY The erosion control security requirement for the site was estimated based on the specification of Section 2.0 of the Manual. The cost of the erosion control measures at the site as detailed above were estimated to cost $5252 with a security requirement of $7878. The estimated cost to revegetate the disturbed areas with a dryland grass mix according to the city's 1993 mulch and revegetation bid was $4550 with a security requirement of $6825. The larger amount is required prior to construction. Both cost estimates are detailed in Appendix IL 1 it I Cover Factor Measures Cover factor measures at the site during construction will consist of the new building I pad, the new asphalt parking areas and storage yard and the existing paving, gravel surfaces and walks in the BMC West yard and along Timberline Road. Cover factor measures for the site after construction is completed will consist of implementation of the landscaping plan which calls for sod and landscaping. Cover factor values for the paved and sod areas were assigned values of 0.01. The calculations for the weighted C - Factor during and after construction are presented on Standard Form B in Appendix II. Seeded turf grass with dry mulch will be used in the swale areas and assigned a cover factor of 0.06. "During Construction" Effectiveness Evaluation The Net Effectiveness for the erosion control measures at the site during construction was calculated as 81 percent and exceeds the "During Construction" Performance Standard value of 77.6 percent. Calculations of the basin effectiveness value and the Net Effectiveness for the site are presented on Standard Form B in Appendix II. "After Construction" Effectiveness Evaluation The Net Effectiveness for the erosion control measures at the site after construction was calculated as 98.7 percent and exceeds the "After Construction" Performance Standard value of 91.3 percent. Calculations of the basin effectiveness value and the Net Effectiveness for the site are presented on Standard Form B in Appendix II. WIND EROSION ANALYSIS The surface soils at the site are rated as moderate relative to wind erodibility. Construction at the site will begin in May 1996 at the end of the windy season of November to May. Construction disturbance at the site as detailed above will occur across the entire site. The necessary access in this area will restrict the available area for placement of a wind barrier. For this reason the wind erosion features to be constructed at the site will consist primarily of natural features at the site, water erosion controls and the features to be constructed. The wind erosion control measures which will be implemented during construction will include the items listed below and are detailed on the Drainage and Erosion Control Grading Plan. The silt fencing The high ground to the west INSTALLATION SCHEDULE Construction at the site is proposed to start in May 1996 and continue during the summer. The approximate date of implementation of the erosion control measure RAINFALL EROSION ANALYSIS Site Soils The Larimer County, Soil Conservation Service Soils Report identifies the soils at the site as Loveland Clay Loam described as having slight rainfall erodibility. The rainfall erosion season as detailed in the City of Fort Collins, Erosion Control Reference Manual (the Manual) is between May 1 and October 31. The natural drainage of the site is from the west to the east. The site is currently covered with native grass. The Manual details the site as having moderate wind erodibility characteristics. The predominant wind direction is from the west-northwest in this area. The wind erosion season as detailed in the Manual is between November 1 st and May 31 st. Construction at the site is estimated to begin in May 1996 and continue through September of 1996. Performance Standard The estimated "During Construction" and "After Construction" Performance Standard for the site are 77.6 and 91.3, respectively. For the purpose of estimating the site's Performance Standard the entire site was evaluated based on final development contours. The limits of disturbance and sub -basins are shown on the Drainage Study and Erosion Control Plan. The Performance Standard for the site was estimated using the methods detailed in the Manual and the estimated, average slope length and slope for the site. Flow lengths and slopes developed for calculation of the "During Construction" Performance Standard is presented on Standard Form A in Appendix IL The Performance Standard was taken from Table 5.1 of the Manual. The "After Construction" Performance Standard is calculated by dividing the "During Construction" Performance Standard by 0.85. Practice Factor Measures Sediment control practice measures for the site will consist of the construction of a silt fence and straw bale dikes. The location and details of these practice measures are shown on the Drainage and Erosion Control Plan in the envelope in the back of this report. Practice factor measures for the site after construction will be dismantled and thus a weighted P - Factor of 1.0 was used to calculate the effectiveness. The calculations for the weighted P - Factor for each basin during and after construction are presented on Standard Form B in Appendix II. I1 Calculations All calculations for this report have been made in accordance with criteria established by the City of Fort Collins and the Denver Urban Drainage and Flood Control District. Variances No variances are requested. DRAINAGE DESIGN Concept While no detention is required to regulate flows to Spring Creek from this site, an effort is expected and will be made to improve the quality of the runoff over what would be the case if it was collected in inlets and piped directly to Spring Creek. This will be achieved through the use of grass lined swales and grass buffer strips which will act as sedimentation filters. The drainage from the south swale will be discharged to an existing wetland area along Spring Creek. The drainage from the north swale will be discharged as before. Sheet runoff from the eastern portion of the proposed storage yard will be collected in concrete pans and will discharge to a stone filled gallery. Flow from this gallery will continue as sheet flow over the existing Spring Creek drainage way for about 100 feet before passing through a narrow band of existing wetland. This combination of drainage paths and their surfaces should produce a moderate potential for overall pollutant removal capacity [2]. Calculations are enclosed in the appendix. Prints of the Grading Plan and the Drainage and Erosion Control Plan are in an envelope at the end of this report. Details Basin areas were determined. Composite runoff coefficients were calculated using 0.25 for grass areas and 0.95 for impervious areas. Design points were established within the basins and flows for the 2 year, 10 year and 100 year storms were calculated. Flows in the swales were analyzed. It was found that the 2 year storm will develop depths of approximately 6 inches or less at velocities of approximately 2 feet per second or less. This is well within the suggested parameters as is the length of sheet flow for the eastern portion of the storage yard described above [2]. It was also found that the swales could carry both the 10 year and the 100 year storms without incurring excessive velocities. The two 15 inch pipes in the south swale at the yard drive and the shallow box culvert under the Spring Creek walkway will also handle the 100 year flow. I FINAL DRAINAGE AND EROSION CONTROL REPORT FOR THE BMC WEST P.U.D. FORT COLLINS, COLORADO DEVELOPMENT LOCATION AND DESCRIPTION i� Location �I BMC West plans to construct a cabinet manufacturing building and storage shed on land immediately north of their present facility at the northeast corner of Prospect Avenue and Timberline Road in Fort Collins. The site, fronting on Timberline Road contains approximately 6.55 acres and is bounded on the north by a Western Mobile yard and on the east by the Spring Creek drainage way. The majority of the site lies within the A4 Flood Zone. This property is situated in the Southwest Quarter Section of Section 17, Township 7 North, Range 68 West of the 6th P.M. in Larimer County. Property Description The property is presently vacant and is covered with native grasses. The surface drains from Timberline Road about 800 feet to the east to Spring Creek at about a 0.7% grade. IDRAINAGE BASIN IDENTIFICATION ' Description The proposed development lies in the Spring Creek Drainage Basin. DRAINAGE DESIGN CRITERIA ' Regulations The City of Fort Collins Storm Drainage Design Criteria [1] is being used as the basis ' for this report supplemented and modified by the Urban Storm Drainage Criteria Manual Volume 3 [2]. Development Constraints The highest projected elevation of the 100 year flood on this site is approximately ' 4905. Offsite drainage from about 0.5 acres comprised of half of Timberline Road and a portion of BMC West's existing development must be handled by the new site's drainage system. Hydrologic Criteria The Rational Method was used to design the drainage system for this project. \•I� � "0S�§ a ® \�/ V V C L. WELSH ENGINEERING SCIENCE & TECHNOLOGY, INC. January 22, 1996 Mr.Glen D. Schlueter City of Fort Collins ' Utility Services Stormwater 235 Mathews Fort Collins, Colorado 80522 ' RE: THE BMC WEST P.U.D., FORT COLLINS FINAL DRAINAGE AND EROSION CONTROL REPORT Dear Mr. Schlueter: Transmitted herewith is a copy of the subject report for your review and approval. All ' calculations for this report have been made in accordance with criteria established by the City of Fort Collins with some modifications suggested by the Denver Urban Drainage and Flood Control District. If you have any questions or require any modifications please call me. Thank you. Sincerely, WESTEC �ir 6� .l 1 M urice H. Lutkin, .E. Manager, Civil Engineering Division 2629 Redwing Road, Suite 200, Fort Collins, Colorado 80526-2879, Ph. 970 226 3535 Fax 970 226 6475 Reno Tucson Elko Winnemucca Pleasanton Englewood Nevada Arizona Nevada Nevada California Colorado FINAL DRAINAGE AND EROSION CONTROL REPORT FOR THE BMC WEST P.U.D. FORT COLLINS, COLORADO Prepared for: BMC WEST INCORPORATED Prepared by: WESTEC 2629 Redwing Road, Suite 200 Fort Collins, Colorado 80526 N0. 95455 January 22, 1996 FINAL DRAINAGE AND EROSION CONTROL REPORT FOR THE BMC WEST P.U.D. FORT COLLINS, COLORADO \I&I'm � Ar� � � � �. '11W VV C a-=p ■ C % . WELSH ENGINEERING SCIENCE & TECHNOLOGY, INC.