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Home My WebLink About1605 PETERSON PLACE CARRIAGE HOUSE - FDP200023 - - DRAINAGE REPORT 4025 AUTOMATION WAY, STE B2 | FORT COLLINS, CO 80525 | 970-698-6046 | www.CivilWorxEng.com December 1, 2020 Dan Mogen Development Review Division City of Fort Collins Utilities 281 N. College Ave. Fort Collins, Colorado 80524 Re: 1605 Peterson Place, Carriage House – Drainage Narrative Dan, I am writing this letter to discuss the drainage impacts associated with the addition of a ‘carriage house’ to the existing duplex located at 1605 Peterson Pl. (the property). The intent of this letter is to satisfy the City of Fort Collins requirements on drainage analysis associated with the proposed improvements to the property. The owner of the property is proposing a 912 square foot carriage home located at the NW corner of the lot, the drainage from this building will be conveyed eastward to the extent practical. The existing drainage patterns of the site can be summarized by 2 basins, an eastern and a western existing basin. The western basin is a small 0.01 sliver of the property area which abuts and drains to the Peterson St. alley to the west. Most of the property generally drains to the east where flows would enter the curb and gutter at Peterson Pl. The east basin is 0.25 acres in size with slopes varying between 0 and 4%. For a summary of basin areas and surface coverages see Table 1 below. Surface Type Area (Sq.Ft.) Percent Imperv. (%) I * A Existing Drainage Total Area 11,175 23.4% 2617.6 Asphalt, Concrete 676 100% 675.7 Rooftop 1,044 90% 939.4 Gravel 1,744 40% 697.6 Pavers 396 40% 158.6 Lawn/ Landscaping 7,315 2% 146.3 Proposed Condition Drainage Total Area 11,175 31.6% 3534.4 Asphalt, Concrete 676 100% 675.7 Rooftop 1,956 90% 1760.2 Gravel 1,977 40% 790.8 Pavers 464 40% 185.6 Lawn/ Landscaping 6,194 2% 123.9 Table 1) Proposed and Existing Basin characteristics and area cover 4025 AUTOMATION WAY, STE B2 | FORT COLLINS, CO 80525 | 970-698-6046 | www.Civilworxeng.com Based on initial design scoping meetings with the client, it was requested that this lot improvement would not impact the tenant side of her existing home. To respect this design constraint, the limits of this construction and grading for the proposed project will not address any of the existing drainage on the tenant side of her property. See the construction plans for the proposed limits of construction, but this is to say that we cannot address any existing drainage issues outside of the construction limits. With the added impervious area, the increase to the anticipated stormwater flows for the 2-year and 100-year design storm were calculated. A summary of the existing and proposed runoff rates can be found below in Table 2 (calculations are available in attachments). This additional runoff generated by the proposed condition will be conveyed eastward via a proposed grass swale as shown on the plans. This swale was positioned as close to the northern property line as possible while maintaining positive drainage away from the existing duplex as well as the property to the north. A Tc of 5 minutes will be assumed based on the December 2018 COFC Stormwater Criteria manual. Existing drainage conditions will be improved by employing this new swale as there were some very flat areas at the west and north side of the existing home, which should be graded to drain to the proposed swale. In an effort to slow down the runoff as it runs through the swale, I will encourage the client to plant a few rows of taller decorate grasses (buffalo grass, or equivalent) positioned perpendicular to the swale flowline just before the swale reaches the outfall near the sidewalk at Peterson Pl. Basin Proposed Existing Total Flow 2- year (cfs) Total Flow 100-year (cfs) Total Flow 2- year (cfs) Total Flow 100-year (cfs) East Basin 0.33 1.78 0.29 1.26 West Basin 0.02 0.09 0.01 0.04 Table 2) Flow comparison for existing drainage versus proposed conditions 4025 AUTOMATION WAY, STE B2 | FORT COLLINS, CO 80525 | 970-698-6046 | www.Civilworxeng.com The addition of the carriage house to this lot is being designed with as little impact to the existing stormwater patterns as reasonably achievable. With the minimal additions to the gravel driveway and maximizing landscaped or lawn area, the resulting runoff increases are modest. Care has been taken not to increase flows to the Peterson St. alley and the Carriage House will have its gutters sloped to convey stormwater to the east, where the downspouts will be outlet towards the proposed swale. With the overall distance that stormwater flows must travel through the swale and the addition of grass buffers to reduce velocities and drop out any sediments within the storm flows. This project’s impact on the greater storm sewer in the area should be quite small, and with the efforts taken to help slow the stormwater velocities and reduce sediment transport; the existing downstream infrastructure should handle these flows without issue. Sincerely, Scott Nesbitt, PE Project Engineer CivilWorx, LLC 4025 AUTOMATION WAY, STE B2 | FORT COLLINS, CO 80525 | 970-698-6046 | www.Civilworxeng.com ATTACHMENTS Runoff Coefficients and % Impervious1605 Peterson Place Fort Collins, CO Roof (1)Asphalt Walks(1)Gravel(1)Lawns(1) %I = 90% %I = 100% %I = 90% %I =40% %I=2% acres sf sf sf sf sf sf C2 C100 CF EX-E EX-E 0.25 10751 1044 676 0 2028.8 7002 23.9% C/D 0.409 0.511 EX-W EX-W 0.01 419.3 0 0 0 111.2 308 12.1% C/D 0.316 0.395 0.00 0 - - - 0.3 11170 1044 676 0 2140 7310 23%0.406 0.507 PR-E PR-E 0.24 10468 1956 676 0 2126 5710 32.5% C/D 0.477 0.596 0.745 PR-W PR-W 0.02 702 0 0 0 291 411 17.8% C/D 0.354 0.442 0.553 0.00 0 - - - Total 0.3 11170 1956 676 0 2417 6121 32%0.469 0.596 Notes: (1) Recommended % Imperviousness Values from USDCM Vol 1 - Ch 5 - Table RO-3. Increased Lawns from 0% to 2% Impervious. (2) Runoff C is based on %I, UDFCD Eq RO-7 and correction factors in Table RO-4. (3) Impervious area assumed to be 2500 s.f. of roof and 500 s.f. of driveway per single family and twin home lot. Proposed Basins Existing Basins Onsite Total Basin Areas Composite Imperviousness (%I) Total TotalDesign Pt.NRCS Soil Type Composite Runoff Coefficients (2) D:\Civilworx\CivilWorx Dropbox\CivilWorx\Projects\2020\C20025 - Peterson Place Carriage House\06_Design\Reports\Drainage\1605 Peterson Pl - Hydrol_Std-updated.xlsm Calculations by: STN Date: 10/19/2020 Time of Concentration1605 Peterson Place Fort Collins, CO Area Length Slope ti (2)Length Slope Velocity (3)tt (4)Check tc?Total Length tc (5) acres ft % min ft % fps min min Urban? ft min min EX-E EX-E 0.2 Roof 0.73 ----Yes 0 5 5 EX-W EX-W 0.0 Lawn 0.09 ----Yes 0 5 5 0.0 - - - - - - - - PR-E PR-E 0.2 Roof 0.73 ----Yes 0 5 5 PR-W PR-W 0.0 Lawn 0.09 ----Yes 0 5 5 0.0 - - - - - - - - Notes: (1) C5 based initial ground type and Table RO-5 (4)tt=L/(V*60 sec/min) (2)ti = [1.8(1.1-C5)L1/2]/S1/3, S= slope in %, L=length of overland flow (400' max)(5)tc check (for urban or developed areas only) = total length/180 + 10 (3)V=CvS0.5, S=watercourse slope in ft/ft, UDFCD Equation RO-4 (6)min tc = 5 min Proposed Basins Existing Basins Initial Ground Type C5 (1)Cv ti+tt Final tc (6) tc Check for Urbanized Basin Initial Overland Flow Time (ti)Travel/Channelized Time of Flow (tt) Design Pt. D:\CivilWorx Dropbox\CivilWorx Dropbox\CivilWorx\Projects\2020\C20025 - Peterson Place Carriage House\06_Design\Reports\Drainage\1605 Peterson Pl - Hydrol_Std-updated Calculations by: STN Date: 10/19/2020 Rational Method Peak Runoff 1605 Peterson Place Fort Collins, CO Final tc (6)I2 I100 Q2 Q100 acre min in/hr in/hr cfs cfs EX-E EX-E 0.2 5 0.409 0.511 2.85 9.95 0.29 1.26 EX-W EX-W 0.0 5 0.316 0.395 2.85 9.95 0.01 0.04 0.0 - - - - - - - 0.30 1.29 PR-E PR-E 0.2 5 0.475 0.742 2.85 9.95 0.33 1.78 PR-W PR-W 0.0 5 0.354 0.553 2.85 9.95 0.02 0.09 0.0 - - - - - - - 0.34 1.86 Totals Totals Peak Discharge Proposed Basins Existing Basins Design Pt.AreaBasin C100 Rainfall IntensityRunoff Coefficients C2 Calculations by: STN Date: 10/19/2020