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Home My WebLink AboutFOUR SEASONS PUD 9TH FILING (IRONGATE)FINAL DRAINAGE REPORT for FOUR SEASONS NINTH FILING P.U.D Fort Collins, Colorado April, 1994 prepared by Northern Engineering Services, Inc. 420 S. Howes, Suite 202 Fort Collins, Colorado 80521 (303)221-4158 Project Number: 9329.00 4 11 Northern Engineering Services, Inc. April 6, 1994 Mr. Basil Hamdan Stormwater Utility City of Fort Collins 235 Mathews P.O. BOX 580 Fort Collins, Colorado 80522-0580 RE: Four Seasons Ninth Filing P.U.D. Fort Collins, Colorado Project No. 9329.00 Dear Basil: Northern Engineering is pleased to submit this Drainage Study for your review. We have addressed your comments contained in the red -lined drainage report and Drainage and Erosion Control Plan dated March, 1994. If you should have any questions or comments as you review this report, please feel free to contact me at your earliest convenience. Sincerely, NORTHERN ENGINEERING SERVICES, INC. VvN'�:s i-t), vJ (- Mary Wohnrade, Michael F. Jones, P. ,��`.�'�yry� RED do of 25033 � �= 420 S. Howes • Suite 202 • Fort Collins. Colorado 80521 • (303) 221-4158 • Fax (303) 221-4159 I Falcon Dr , I Falcon No eI NOW Dr f•Id Frlmroaal p� LC 3 AO i y3 Dalton Dr i Poanll MI Da Don Or Iq Waaf/la I) it exrr PI g 4 nE 8 v,'.pnf Cr i 'erlxfon 1 pNlb'� Cf Nnu: Rd PAV Cr, Donnlo Cf e.dford Cf, a Northern Engineering Services 418 S. Howes. Suite D • Fort Collins. Colorado 80521 • (303) 221-4158 •7E r yt+� t1 E 00MoAA Craps D. M s, v of PAvnlon Ln FOUR SEASONS NINETH FILING P.U.D. Fort Collins, Colorado heet Final Drainage Report for FOUR SEASONS NINTH FILING P.U.D. Fort Collins, Colorado April, 1994 . I. GENERAL This report summarizes the results of a storm drainage . investigation for the Proposed Four Seasons Ninth Filing, a -- residential P.U.D.. The project occupies approximately 7.0 acres in the northwest quarter of Section 35, Township 7 North, Range 69 West of the 6th Principal Meridian in Fort Collins, Colorado. Methods outlined in the Fort Collins Storm Drainage Design Criteria were used, along with methods presented in the Storm Manual by the Urban Drainage and Flood Control - Drainage Criteria District. Previous drainage work referenced as part of this investigation included studies for "The Village at Four Seasons, Phase One", (September, 1984), "Four Seasons Fifth Filing", (July, _ 1985) and "Four Seasons Fourth Filing", (1981-1985), all of which were prepared by M&I Consulting Engineers, Fort Collins, Colorado. . Due to the relatively small basin size, and in keeping with previous work, the rational method was used to estimate peak stormwater runoff flows from the project site. Calculations made as part of this investigation along with other supporting material, - are contained in the appendix to this report. II. SITE DESCRIPTION . The project site is located in the McClellands/Mail Creek Drainage Basin, is currently undeveloped and is vegetated with natural grass and weeds. It has a portion of an old apple orchard at the eastern end, with two(2) mature cottonwood trees. An abandoned irrigation lateral divides the site into a northern and a southern section. The site slopes gently downward from southwest to northeast at approximately two (2) percent. The site is bounded on the north by Horsetooth Road, to the west by Century Drive and _ to the south by Arbor Avenue. The eastern portion of the site is bounded by Four Seasons Third Filing and by the John Michie property. An abandoned irrigation lateral runs west to east along S Horsetooth Road between the curb and the walk. The northern portion of the site is lower than Horsetooth Road. An attached Drainage and Erosion Control Plan shows the existing site 0 topography. ■ ■ ■ ■ ■ ■ Four Seasons Ninth Filing P.U.D. Final Drainage Report Page 2 III. HISTORICAL CONDITIONS The majority of the on -site stormwater flows to a depression in the northeast corner of the site where eventually it spills onto the Michie property to the east. This spill is estimated to be 1.29 cfs and 4.82 cfs for the 2-year and 100-year storms respectively, at Design Point H3 (See Table A). A smaller portion of land, approximately 1.1 acres, along Arbor Avenue drains into the street gutter and makes its way to the Four Seasons Tract A Detention Pond. This pond has been sized to accommodate runoff flows from the Ninth Filing site. During the development for the Fifth Filing of Four Seasons, a master drainage plan for the entire Four Seasons development was prepared (M&I, 1984). Portions of related reports are included in the appendix to this report. This plan included the area being proposed for the Four Seasons Ninth Filing (Referenced in the reports as the Fourth Filing). The majority of the Ninth Filing is included in Basin A which delivers its developed runoff flows to a series of inlets near the corner of Bonita Avenue and Benthaven Street. From this point the runoff is conveyed to the Tract A Detention Pond. The remainder of the Ninth Filing site, approximately 1.8 acres, was to drain into Horsetooth Road. A letter report (M&I, June, 1981, updated 1985) confirmed the capacity of the gutters and the Tract A Pond to accommodate developed runoff from the Ninth Filing. 0 IV. DEVELOPED CONDITIONS Plans for the development of the Ninth Filing call for extensive site grading to raise the site above the surrounding streets and to allow the majority of the developed drainage to flow to Arbor Avenue. A small area along the northern portion of the site (Basin D2) will drain into Horsetooth Road. Runoff leaving the developed site at Arbor Avenue is 7.22 cfs and 29.73 cfs for the 2-year and 100-year storms respectively as shown in Table A for Design Point D1. The anticipated 100-year flow calculated in the letter report is 13.82 cfs (see calculations, page 25 of the appendix). The increase in developed flow is due to an increase in both site imperviousness (from 0.50 to 0.65) and area (from 4.55 to 6.91 acres), from what was previously estimated in the letter report. The peak runoff at Horsetooth Road from Basin D2 (a drainage area of 1.59 acres), at Design Point D2, is 2.29 cfs and 8.11 cfs for the 2-year and 100- year storms respectively. This is compared to the aforementioned estimated acreage of 1.8 acres developed in the master plan. ■ ■ 1 Four Seasons Ninth Filing P.U.D. Final Drainage Report Page 3 IV. DEVELOPED CONDITIONS cont. A very small portion of the developed site (0.24 acres) adjacent to the Michie property receives some developed flows. These flows total 0.34 cfs and 0.92 cfs for the 2-year and 100-year storms respectively at Design Point D3, compared to historic flows of 1.29 cfs and.4.82 cfs (Design Point H3). There are no proposed stormwater inlets or piping on the developed site. Drainage flows to Arbor Avenue will be conveyed in inflow concrete curb and gutter in addition to four (4)-foot drainage -pans. All stormwater runoff from the Ninth Filing site has been routed as outlined in the master drainage plan (see letter report, June 19, 1981). Calculations of Arbor Avenue and Horsetooth Road street capacities have been included in the appendix to this report. A revised SWMM model is currently being generated in order to accurately represent existing conditions of the Tract 'A' and Seventh Filing detention ponds. Modifications to the existing SWMM will include new areas and flow widths for subcatchment's 165 and 168, and a revised weighted imperviousness factor for subcatchment 165. In order to permit overland flow to the north and into the south gutter of Horsetooth Road, the existing irrigation ditch must be filled tin. The is necessaryyof for thert Collins developmenthof theas red site that its abandonment VII. EROSION CONTROL Temporary erosion control will consist of a soil sealant applied to bare ground immediately following overlot grading. Six weeks following the commencement of overlot grading, any areas yet to be covered by impervious surfaces a using caatedlby the Utility Plan must be temporarily protected approved soil sealant for erosion control. Permanent vegetative erosion control will be used in conjunction with landscaping. Permanent revegetation of areas disturbed by construction will be the responsibility of the contractor. Sod grass and established native grass is to be installed when final grading is completed. If final grading has not taken place within 60 days after the completion of overlot grading, temporary vegetative erosion control measures must be taken until grasses can be installed. Four Seasons Ninth Filing P.U.D. Final Drainage Report Page 4 VII. EROSION CONTROL, con't The recommended species of temporary grass is Sorghum with an application rate of 10 drilled pounds/acre followed by straw mulch applied at a rate of 2 tons/acre and anchored by crimping. Refer to the Erosion Control Reference Manual for Construction Sites (City of Fort Collins, Storm Drainage Design Criteria) for details on approved methods. 4 4 LI 4 4 LI 4 4 4 ■ Runoff Coef Historical. DevelopedD1--a1S11I1Vwa .............................. -- TableA ............................................. 12 Iron Gate Drive Street Capacity ..................... 13 - 14 Arbor Avenue and Horsetooth Road Street Capacity .... 15 - 16 Detention Pond Calculations ......................... 17 - 19 Erosion Control Calculations ........................ 20 - 23 Letter Report, M&I Consulting Engineers ............. 24 - 28