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HomeMy WebLinkAboutGEORGE T SANDERS COMPANY PUD - PRELIMINARY & FINAL - 76-88H - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFINAL DRAINAGE AND EROSION CONTROL REPORT FOR THE GEORGE T. SANDERS COMPANY P.U.D. FORT COLLINS, COLORADO Prepared for: The George T. Sanders Company 10201 West 49th Avenue Denver, Colorado 80033 Prepared by: WESTEC 2629 Redwing Road, Suite 200 Fort Collins, Colorado 80526 N0. 95437 July 24, 1995 .4tti1n2111""t 16466 ' E •S �• \&I — — .... �. \1/VVC� ■ Cam.. WELSH ENGINEERING SCIENCE & TECHNOLOGY, INC. July 24, 1995 Mr.Glen D. Schlueter City of Fort Collins Utility Services Stormwater 235 Mathews Fort Collins, Colorado 80522 RE: THE GEORGE T. SANDERS COMPANY P.U.D. - 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. We have used the historic release rate from a 2 year storm to determine the developed detention pond volume. If the client is so inclined we may be able to obtain permission from property owners between the Sanders site and Fossil Creek to use the historic release rate from a 100 year storm. However, at this point it appears questionable that the savings will make the effort worthwhile. If you have any questions or require any modifications please call me. Thank you. Sincerely, W ESTEC Gwltt Maurice H. Lutkin, P.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 Nevada Arizona Elko Winnemucca Pleasanton Englewood Nevada Nevada California Colorado f FINAL DRAINAGE AND EROSION CONTROL REPORT FOR THE GEORGE T. SANDERS COMPANY P.U.D. FORT COLLINS, COLORADO DEVELOPMENT LOCATION AND DESCRIPTION Location The George T. Sanders Company of Denver plans to construct a warehouse and distribution center for its own use and a similar building for speculation on the same site in Fort Collins. The site contains 2.475 acres and will be subdivided, 1.264 acres for the Sanders building and 1 .211 acres for the second property. The site is bounded on the east by Weberg Furniture, on the west by the Burlington Northern Railroad, on the south by Mills Brothers Nursery and on the north by undeveloped property partially occupied by Fossil Creek Nursery. The site is situated in the Northeast One Quarter Section of Section 2, Township 6 North, Range 69 West of the 6th P.M. in Larimer County. The general location is shown on the attached figure. Property Description The property is presently covered with native grasses and slopes from the northwest to the southeast at an average of 4%. Some off -site drainage from the north and west moves across the site bringing the approximate drainable area to 2.88 acres. DRAINAGE BASIN IDENTIFICATION Description The proposed development lies in the Fossil 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. Development Constraints Runoff from the site presently drains across Mills Brothers property to ponding immediately to the south. From there, overflow eventually reaches Fossil Creek. There are two main flow paths, one through a natural depression in the southeast corner of the site (this will be utilized as the location of the new detention pond), and 95437200 07/21/95 4.5/FU1.UWG HB 7 RLP Lopez Elementary RL RP w 5 z w HB a HB v RP RLP hb w ���C7 o hb u rp RLP hb s�1 rp rmp 1\\ wii -rPBP by OUT rh bP HARMONY ROAD OUT \\ RLP McGraw Elementary m P d OUT o as 0 G.T. SANDERS, P.U.D. Fairway Lade by bl RL SCALE IN FEET 0 1000 2000 BL r hb by................ j \T/ YY C� ■ Ct.. GEORGE T. SANDERS Werner Elementary FIGURE No. .1 GEORGE T. SANDERS SITE LOCATION MAP the other at the southwest corner of the site along the railroad right of way. Pertinent criteria established for the Fossil Creek Drainage Basin allows runoff from a 100 year event under historic conditions to be routed directly to Fossil Creek without detention. However, owners of intervening property must grant permission for this to occur. Such is the case here and initially the more conservative approach has been followed. Runoff from the developed site from a 100 year event will be detained and released at the rate of runoff from a 2 year event under historic conditions. Hydrologic Criteria The Rational Method was used to design the drainage system for this project. Calculations All calculations for this report have been made in accordance with criteria established by the City of Fort Collins. Variances No variances are requested. The runoff from the west side of Fossil Creek Drive was accounted for when Weberg Furniture was developed and will not be taken into consideration here [3]. DRAINAGE DESIGN Concept The site has been graded to convey as much runoff as possible to the detention pond location in the southeast corner of the site. The roofs may drain to all sides of each building and the site has been graded accordingly. The detained stormwater will be released to the south across Mills Brothers property to the ponding to the south. Calculations are enclosed in the appendix. Prints of the Grading and Drainage Plan and the Drainage Study and Erosion Control Plan are in an envelope at the end of this report. Details As mentioned above, the drainable area totals 2.88 acres and when developed, will be made up of 1.16 acres of grassed or landscaped areas, 0.44 acres of gravel in the outdoor storage yard and 1.28 acres of roofs and paved surfaces. A time of concentration was calculated at the southeast corner of the site as being 24 minutes. The intensity of a 2 year event at this time of concentration equals 1.65 inches per hour as determined by rainfall intensity curves for this area. A discharge of 0.95 cfs was calculated using a runoff coefficient of 0.2 for historic conditions. This was used as the allowable release rate in determining the required detention pond volume. A composite runoff coefficient of 0.6 was computed for the drainable area of 2.88 acres using runoff coefficients of 0.25, 0.50 and 0.95 for grassed areas, gravel area and impervious areas, respectively. The required detention pond volume for runoff from a 100 year event released at 0.95 cfs was determined using a Mass Balance spread sheet that automatically incorporates the distribution factor of 1.25. The volume required was 16,972 cubic feet. The detention pond as designed provides 17,253 cubic feet as computed by the trapezoid method. The release rate of 0.95 cfs will be achieved by an orifice of 4 inches in diameter in a plate fastened over the inflow end of the 15" pond drain. Positive piped drainage lines will be used to drain the low points at each truck loading dock and the roof leader runoff from the south side of the south building and convey the flow to the detention pond. RAINFALL EROSION ANALYSIS Site Soils The Larimer County, Soil Conservation Service Soils Report identifies the soils at the site as belonging the Kim Loam and Nunn Clay Loam described as having moderate rainfall erodibility. The rainfall erosion season as detailed in the City of Fort Collins, Erosion Control Reference Manual [21 (the Manual) is between May 1 and October 31. The natural drainage of the site is from the west to the east. A mixture of dryland grass and weed cover currently provides an estimated 50 percent cover over the entire site. 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 December 1995 and continue through June of 1996. Performance Standard The estimated "During Construction" and "After Construction" Performance Standard for the site are 78.9 and 92.9, 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 II. 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, a sediment trap, and a straw bale dike. The location and details of these practice measures are shown on the Drainage Study and Erosion Control Drawing attached to 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. Cover Factor Measures No cover factor measures were implemented for the site during construction due to the restricting size of the site. Cover factor measures for the site after construction is completed will consist of implementation of the landscaping plan which calls for landscaping or establishment of the grass cover of at least 60 percent cover over the disturbed areas of the site. Grass will be established with the use of hay or straw mulch applied and secured at a rate of 2 tons per acre. The landscaping and mulchedlrevegetated areas were assigned C - Factor values of 0.06 as selected from Table 5.2 and Figure 5.1. Cover factor values for the paved and gravel areas were assigned values of 0.01 and 0.05, respectively. The calculations for the weighted C - Factor for each basin during and after construction are presented on Standard Form B in Appendix Il. "During Construction" Effectiveness Evaluation The Net Effectiveness for the erosion control measures at the site during construction was calculated as 80.0 percent and exceeds the "During Construction" Performance Standard value of 78.9 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 93.1 percent and exceeds the "After Construction" Performance Standard value of 92.9 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 moderately wind erodible. Construction at the site will begin by December, 1995 at the beginning of the windy season of November to May and the site does not fall under any of the exclusions available in the Manual. Construction disturbance at the site as detailed above will occur across the entire site. The limited size of the construction area and 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 Erosion Control and Grading and Drainage Plans. The silt fencing along the south margin of the development. The existing curb and gutter along the east margin of the property. The stormwater detention pond berm to the south. The railroad embankment west of the site. The walls and structural fill of the building. The existing trees and road fill to the north of the site. Wind erosion control measures which will be implemented after construction will consist of landscaping or mulching and seeding the disturbed areas. Hay or straw mulch will be applied and crimped into the soil surface at a rate of 2 tons per acre after seeding of the site. The silt fence will be removed when the mulch is applied. The remainder of the wind erosion control features will remain intact to counter soil loss during vegetative establishment. INSTALLATION SCHEDULE Construction at the site is proposed to start December 1995 and continue during the Winter. The approximate date of implementation of the erosion control measure during construction are shown on Standard Form C in Appendix II. As shown on Form C the sediment trap and silt fence will be constructed prior to overlot grading. Permanent vegetation will be seeded and mulched after the completion of the project during the spring prior to the rainfall season. 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 $1,690 with a security requirement of $2,535. The estimated cost to revegetate the disturbed areas with a dryland grass mix according to the city's 1993 mulch and revegetation bid was $1,600 with a security requirement of $2,400. The larger amount is required prior to construction. Both cost estimates are detailed in Appendix II. REFERENCES City of Fort Collins, Colorado, Storm Drainage Design Criteria and Construction Standards, May 1984, Revised January 1991. 2. Denver Regional Council of Governments, Urban Storm Drainage Criteria Manual, Volumes I and II, March, 1969, 6th Printing Vol I September 1978. 3. RBD Inc., Engineering Consultants, Final Drainage Report for the Weberg P.U.D.. June 5, 1989, Revised July 20,1989.