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HomeMy WebLinkAboutHORSETOOTH EAST BUSINESS PARK (LOT 3) - IP SITE PLAN REVIEW - 43-94C - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTI Horsetooth Business Park, First Filing April 4, 1991 Page 3 There will be additional detention ponds constru Horsetooth ness ar op ese a itional Donds will be tj._ n the 35+ inch outfall pin ine. The allowable release ra a or all of the future parcels to be developed in the Horsetooth Business Park shall be 0.17 c.f.s. per gross acre. There is an existing 18 inch storm drain running east from the manhole at the intersection of Automation Way and Elk Road. This 18 inch line flows to the east and is part of a previously proposed system which will possibly be abandoned, therefore we are plugging the 18 inch pipe in the manhole and outflowing the manhole via a new 15 inch pipe draining to the south. The attached computation sheets show how we arrived at the allowable outflow, required detention volume and volume of the detention ponds. The attached grading and drainage plan shows the proposed grades and drainage facilities. Phillip Robinson, P.E. & L.S. Sheet 3 Horsetooth Business Park, First Filing April 4, 1991 Page 2 We prnpn5e to use 15 inch RCP storm drains for all the pipes in the Horsetooth Business Park, Sheet 12 of this report is a pro i e ot the proposed storm pipe system. Also shown on sheet 12 is the calculated hydraulic grade line, along with calculations of pipe flow velocity and capacity at critical points. We calculated all of the velocities and capacities in the pipe system based on the pipe outflow going into a completely filled Collindale Business Park detention pond. This is the most conservative situation, since the high water line elevation of the Collindale Business Park pond will be lower for all storms except the 100 year storm, and even during the 100 year storm, it will not reach the full level for 3 hours. The calculated ca a i y of the outfall n;oa as it ente the Collindale Busj-nes.sPa.rk—oncLis 5.3 c,fswhich is close enough to the 5.5 c.f.s. allowable release rate. The �draulic_grade line is a maximum_0.2.2 feet bye manhole _1_ids,, which is not enouch to pop the manholelids_. There is a safety valve built into the storm pipe system, w ich is that when there is more runoff entering the pipe system through street catch basins than the downstream pipes can carry, there will be backflow into the detention ponds. Detention ponds - constructed in the Horsetooth Business Park shall have an outflow restriction late lohjed, so that- it will allow buifflow to meter t e pond through a 15 inch opening. Sheet 13 of this report shows the detention ponds for this filing, along with the approximate location, size, and high water line of future ponds. The small detention pond located in the north parking lot will provide 14,594 cubic feet of the total required 96,433 cubic feet of detention volume. The outflow from the small pond is 0.57 c.f.s., of the allowed 1.76 c.f.s., therefore, the outflow from the large pond located at the south end of the site will be the difference, which is 1.19 c.f.s. The 6 proper volume will accumulate in the north detention pond due to the fact that we have used the 0.57 c.f.s. release rate and the basin area of 1.9 acres to calculate a detention requirement of 14608 cubic feet, which is substantially the same as the maximum pond volume of 14594 cubic feet. This release rate of 0.57 c.f.s. is larger than the allowable (1.9) (0.17) = 0.32 c.f.s., but we have compensated for this by decreasing the release rate of the south pond from (8.48) (0.17) = 1.44 c.f.s. to 1.19 c.f.s. The large detention_nonci located at the south end of the site —. a proposed capacity of and a rgguir d c�t defeiame of 81839 cubic feet, so there is 28724 cubic feet excess capacity. The bottom of this pond is to be left fTat --so it will become a wetlands, with cattails and wetlands grasses. The wetland will become an effective settling basin and will provide a better water quality into the storm drainage system. The restricted outflow of 1.19 c.f.s. will slow the flow through the pond and allow the wetland settling area to be more effective. Sheet 2 LOCATION: The Horsetooth Business Park is located on the southwest corner of I'y Horsetooth Road and Timberline Road. The property is bounded on the north by Horsetooth Road, on the east by Timberline Road, on the west by the Union Pacific Railroad, and on the south by Collindale Business Park. I The total Hor tooth Business Park contains 32.2 acres. The Horsetooth Business Park First Fill located at the northwest corner of the site, adjacent to the pion Pacific Railroad, and contains a gross area of 10.38 Iacres to the center of adjoining streets. Horsetooth Business Park is {l 'T'oMed in the Foothills Drainage Basin. Portions of the site have been previously platted as South Collins Tech Center First, Second, and Third Filings. EXISTING CONDITIONS: The pavement, curbs, and sidewalks 'on Horsetooth Road are completed, and portions of the pavement, curb, and sidewalk on Timberline Road are constructed. Some of the utilities, pavement, curbs, and walks shown on the South Collins Tech Center First Filing plans have been installed, although a part of these improvements do not fit the present plan, and will be removed. Other than the previously installstreets,_the site is a Dasture covered with vegetal ion There are no offsite flows onto the site. There is an irrigation ditch which flows from west to east across the south edge of the site. This ditch belongs to Mr. Bob Shields, and delivers water from Warren Lake to sites east of Timberline Road. This ditch is open from a point on the west side of the Union Pacific Railroad to the west side of Timberline Road, and picks up historic surface flow from the railroad right-of-way, along with some run-off from the Horsetooth Business Park site. We anticipate that in the future this ditch will be put in a pipe from Timberline Road to the east side of the Union Pacific Railroad, and that it will still carry historic surface run-off from the railroad right-of-way. PROPOSED IMPROVEMENTS: I' The Horsetooth Business Park First Filing consists of 10.38 acres, 7.93 acres of which are to be impervious. We plan to provide stormwater e ed ention for the difference bee—twen te' 2 year historic runoff and the I 100 year developed runoff. This calculates to be 2.21 acre_ feet of detention. There will be a small detention pon ocated in the parkinac, I o e-no.rtheas.t_-co.roer.of.._thesite, and a large detention pond located along the south sid of the site. The maximum allowable outflow from the total 32.2 acres ofthg Horsetoot usiness Pa.rk..site_is... .5..c.f.s,. The 5.5 c.f.s. outflow is determined 6y the available caRacity of the existing 24 inch pine under imber ine Road. he 24 inch pipe was irisiaTTed�in T989 as apart o the Foothil is Drainage Basin major improvements, its purpose being to carry storm flows from the Collindale Business Park and from the Horsetooth Business Park. The allowable outflow per gross,acre of the Horsetooth Business Park is ivided---- by- ... ..equals0.17c.s. der acre_ therefore I; the otal outflow from the 10.38 acre site is 1.76 c.f.s. Sheet 1 ATTACHMENT A SUPPORTING DOCUMENTS REFERENCES 1. James H. Stewart & Associates, Drainage Report, Horsetooth Business Park, First Filin , April 1991. 2. City of Fort Collins, Colorado, Storm Drainage Design Criteria and Construction Standards, May 1984, Revised January 1991. 3. Denver Regional Council of Governments, Urban Storm Drainage Criteria Manual, Volumes I and ll, March, 1969, 6th Printing Vol I September 1978. • The railroad embankment west of the site. • The walls and structural fill of the building. �' • The paved parking and walk areas. Wind erosion control measures which will be implemented after construction will consist of 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 I the wind erosion control features will remain intact to counter soil loss during vegetative establishment. INSTALLATION SCHEDULE 1, Construction of the new office building is proposed to start in Mid -May 1995 and continue during the summer with the floors completed by June of 1995. The approximate date of implementation of the erosion control measure during construction are shown on Standard Form C in Appendix 11. 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 late -summer prior to the next wind erosion season. EROSION CONTROL SECURITY The erosion control security requirement for the new office building 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 $1,020 with a security requirement of $1,770. The estimated cost to revegetate the disturbed areas with a .dryland grass mix according to the city's 1993 mulch and revegetation bid was $2,860 with a security requirement of $4,290. The larger amount is required prior to construction. Both cost estimates are detailed in Appendix II. The building area was assigned a C - Factor value of 0.01. 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 mulched/revegetated 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 areas and the undisturbed grassed areas were not changed from "During Construction" conditions. The calculations for the weighted C - Factor for each basin during and after construction are presented on Standard Form B in Appendix II. "During Construction" Effectiveness Evaluation The Net Effectiveness for the erosion control measures at the site during construction was calculated as 79.1 percent and exceeds the "During Construction" Performance Standard value of 77.8 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 97.3 percent and exceeds the "After Construction" Performance Standard value of 91.6 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 new office building site are rated as moderately wind erodible. Construction at the site will begin by May, 1995 at the end of the wind 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. IThe wind erosion control measures which will be implemented during construction will 1. include the items listed below and are detailed on the Erosion Control and Grading and Drainage Plans. • The south drainage channel along the south margin of the development. • The silt fence along the east margin of the property. • The stormwater detention pond berm to the north of the proposed building. i RAINFALL EROSION ANALYSIS Site Soils The Larimer County, Soil Conservation Service Soils Report identifies the soils at the site as belonging the Nunn Clay Loam described as having slight 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 Manual states the predominant wind direction is from the west-northwest in this area. �C 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 Mid -May 1995 and continue through September,1995. Performance Standard The estimated "During Construction" and "After Construction" Performance Standard { for the new office building site are 77.8 and 91.6, 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 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 office building development will consist of the construction of a silt fence and a sediment trap in the existing detention pond to the north. The location and details of these practice measures are shown on the „ Drainage Study and Erosion Control drawing and the Details drawing. I, 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 Cover factor measures for the site during construction will consist of the building floor. C The mass balance analysis, using a composite runoff coefficient of 0.85 and a 100 year event, indicates the development of the Horsetooth East Professional Office Building will require a detention volume of 43,000 cubic feet (the volume at 2 hours was used per the Denver Regional COG regulations for a balance that is still increasing at that time [31). This, added to the requirements for Public Service of 81839 cubic feet, brings the total requirement to 124,839 cubic feet. The reconstructed pond i provides 121,302 cubic feet and the ponds in the parking lot, less than 18 inches deep, provides an additional 7,300 cubic feet, a total of 128,602 cubic feet. The release rate from the original pond established by Stewart [11 for Public Service was 1.19 cfs. The release rate for the proposed office building is 0.75 cfs bringing the new release rate from the reconstructed pond to 1.94 cfs. This will be accomplished by enlarging the opening in the outlet works from 21 square inches, as constructed under Horsetooth East Business Park, First Filing, to 27 square inches. Development Criteria Constraints The constraints to developing this site are described by Stewart and Associate [1]. They include a maximum permitted release rate of 0.17 cfs per acre for all development in Horsetooth East Business Park. The release rate from 4.4 acres, the area within the lot lines of the site, equals 0.74 cubic feet per second. Hydrologic Criteria The Rational Method and the runoff from a 100-year storm were used to design the drainage system for the Horsetooth East Professional Office Building development . Calculations are included in Appendix I. 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. DRAINAGE DESIGN Concept All drainage from the office building development will be conveyed to the existing detention pond on the north edge of the site. This pond was found to be smaller than was originally contemplated in the Horsetooth Business Park, First Filing although it is more than adequate to handle the detention requirements of Public Service's development of 81839 cubic feet [1]• The additional needs of the office building development was determined from a mass balance analysis and the pond will be reconstructed and ponding will be provided in the new parking area to handle the increased needs. Details The reconstruction of the pond will consist of deepening it to a bottom elevation of 4945, which is the original design elevation for the bottom of the pond and is the existing invert elevation of the outlet works. The pond spillway area will be regraded to actually allow high water ponding to the original design elevation of 4950.5 and the pond will be extended at the west end to create more capacity. The west side of the office building and the area between the west lot line and the railroad tracks will be drained to this extended pond via a concrete pan on the office building property. The office building parking lot runoff will be collected in two area inlets and piped to the pond. The grading involved will provide additional capacity in the parking lot at times of high water ponding to elevation 4950.5. All significant runoff from the site of the proposed office building will therefore be directed to the detention pond. RP RP RLP W rren Park NP ' ran Lake Reservoir,' Shepardson Elementary RLP HORSETOOTH ROAD Collindale Golf Course HORSETOOTH EAST PROFESSIONAL OFFICES 1 RH N.C.R. IP P.R.P.A. Lbp O Public Service k - o - IP 6 RMP r '; Bie Rorn Dt. sti2 CBW rlp -- -' `- P Ft Collins High School fp .. RIP � rip Kruse RE RP Elementary IP RIP bp EP SCALE IN FEET 0 1000 2000 WATER FIGURE 1 Date: MAR 1995 WASTE HORSETOOTH EAST PROFESSIONAL OFFICES & LAND Project: SITE LOCATION MAP INC. RLP I FINAL DRAINAGE AND EROSION CONTROL REPORT FOR HORSETOOTH EAST PROFESSIONAL BUILDING FORT COLLINS, COLORADO DEVELOPMENT LOCATION AND DESCRIPTION Location A new, two-story office building is planned for Lot 3 of the Horsetooth East Business Park as shown on the enclosed Site Map. A site layout at a larger scale is shown on a print of the Grading and Drainage Plan appended at the end of this report. The site, containing approximately 4.4 acres within the lot lines and 4.66 drainable acres, is bounded by the Robert Shields Ditch on the south, Collindale Golf Course on the west, Public Service Facilities on the north and Automation Way on the east. The site lies in the Northeast Quarter of Section 31, Township 7 North, Range 68 West, 6th P.M., City of Fort Collins, County of Larimer, State of Colorado. Prooerty Description The property is undeveloped. It is presently partially covered with native grasses and a stockpile of earth from recent construction in the area. The site slopes from the west to the east at a gradient of approximately 1 %. There is an existing detention pond along the north edge of the site that is utilized mainly by Public Service development although there is excess capacity that will be used for the proposed office building runoff. DRAINAGE BASIN IDENTIFICATION Description The proposed development lies in the Fox Meadows Drainage Basin. The Horsetooth Business Park, First Filing, Drainage Report [11, a portion of which is included in Attachment A, indicates that all development within the Horsetooth Business Park will be limited to a release rate of 0.17 cubic feet per second per acre and provides rationale for this restriction. DRAINAGE DESIGN CRITERIA Regulations The City of Fort Collins Storm Drainage Design Criteria [21 is being used as the basis for this report. WATER WASTE & LAND INC. March 1, 1995 Mr.Glen D. Schlueter City of Fort Collins Utility Services Stormwater 235 Mathews Fort Collins, Colorado 80522 2629 Redwing Rd. Suite 200, Fort Collins, Colorado 80526-2879 (303)226-3535 fax (303) 226-6475 RE: HORSETOOTH EAST PROFESSIONAL OFFICE BUILDING - 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. If you have any questions or require any modifications please call me. Thank you. Sincerely, WATER, WASTE *LANDC. M rice H. Lutkin, P.E. Ma ager, Civil Engineering Division Consulting Engineers and Scientists FINAL DRAINAGE AND EROSION CONTROL REPORT FOR HORSETOOTH EAST PROFESSIONAL OFFICE BUILDING FORT COLLINS, COLORADO Prepared for: The Neenan Company 2290 East Prospect P.O. Box 2127 Fort Collins, CO 80522 Prepared by: Water, Waste & Land, Inc. 2629 Redwing Road, Suite 200 Fort Collins, Colorado 80526 March 1, 1995 GC,�STEH o•• ,r��, C� Q 16466 ••S�ONgI F i•• Q r� <qTF OF CO 4,r<rru rn � rsD I �.. '