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Home My WebLink AboutDrainage Reports - 03/12/2021 KEEFE CIVIL, INC 825 Union Street Golden, Colorado 80401 (970) 215-6808 meg@keefecivil.com October 19, 2020 Keefe Civil Project Number: 2020-017 Dan Mogen, EI, CFM Stormwater Engineering & Development Review Division City of Fort Collins Utilities 700 Wood Street Fort Collins, Colorado 80522 RE: 308 Cherry Street Final Drainage Memo – FDP190011 Dear Dan: The purpose of this letter is to describe the proposed site improvements and potential drainage impacts at 308 Cherry Street (Exhibit A). The 0.23-acre project site is a developed single-family residential lot, currently being used for business purposes, with established landscaping located northwest of the North Howes Street and Cherry Street intersection in Fort Collins, Colorado. Current conditions at the site include a single-story house, three small garden sheds, a concrete parking area north of the house, a wood deck, established trees, several garden areas, and a concrete driveway. The lot is bound by existing single-family residential properties to the east and west, railroad right-of-way to the north, and Cherry Street right-of-way to the south. According to NRCS Soil Survey of Larimer County, the site consists of Paoli fine sandy loam (Exhibit B). The entire site has been classified as being within hydrologic soil group A, which typically has a high infiltration rate. Per the flood insurance rate map (Exhibit C), the lot is located within Zone X. Zone X is defined as an area of minimal flood hazard and does not have a base flood elevation associated with it. The site is also located outside City of Fort Collins floodplains. Historically, existing site stormwater is split along the northern third of the house. Much of the existing house and driveway drains towards Cherry Street to the south either along the landscaped western lot line or following a raised curb along the eastern property line driveway. The remaining third of the house and parking lot drains to the north. There is an existing localized low point directly north of the parking lot, which naturally performs as an infiltration basin (identified in Exhibit A). Offsite flows from the west have historically drained across the property via overland flows. What historic drainage does not infiltrate the backyard, sheet flows either into the railroad right-of-way to the north or the neighbor’s property to the east. The proposed site grading improvements are limited to the area north of the existing concrete parking area and include a two-story carriage house with no basement, a wood deck, gravel walkways, and a flagstone patio with gravel fill. Two existing garden sheds are intended to be removed during construction. 308 Cherry Street Final Drainage Memo Keefe Civil Project No.: 2020-017 Page 2 of 2 October 19, 2020 All proposed building roof drains shall be directed to the north and outfall into railroad right-of- way. Offsite flows from the west shall be collected in the proposed drainage swale west of the carriage house and drain towards the north. A timber wall is proposed along the eastern property line to help divert potential developed drainage flows away from the eastern neighbor’s property. The lot’s total adjusted impervious area will increase by 754 square feet (sf), as indicated in calculations shown below. Hardscape or Hard Surface Historic Lot Area (sf) Adjusted Historic Area (sf) Developed Lot Area (sf) Adjusted Developed Area (sf) Asphalt, Concrete 2,153 2,153 2,169 2,169 Rooftop 1,490 1,490 1,860 1,860 Patio and Pavers (40%) 380 152 1,300 520 Total Impervious Area 4,023 3,795 5,329 4,549 Adjusted Basin Imperviousness 754 sf The composite runoff coefficient and drainage basin runoff calculations for the historic and developed site are limited to the basin draining to the rear of the lot and are included in the enclosed tables (Exhibit D). Using the Fort Collins Stormwater Criteria Manual, the proposed stormwater runoff increase will be almost unnoticeable at the site, as indicated in the summary table below. 2-year 10-year 100-year Basin A Historic Runoff 0.11 cfs 0.18 cfs 0.49 cfs Basin A Developed Runoff 0.11 cfs 0.20 cfs 0.52 cfs Based on the enclosed design, it is my professional opinion the additional runoff generated by these site improvements will not cause adverse impacts to downstream private properties. I herby attest that this letter for the final drainage design for 308 Cherry Street was prepared by me or under my direct supervision, in accordance with the provisions of the Fort Collins Stormwater Criteria Manual. I understand that the City of Fort Collins does not and will not assume liability for drainage facilities designed by others. Thank you for the opportunity to submit this drainage memo. Please do not hesitate to reach out to me if you have any questions or comments regarding the above items. (970) 215-6808 or meg@keefecivil.com Best regards, Megan L. Keefe, PE / Manager Keefe Civil, Inc. Enclosures Cc: Kimberly Baker Medina EXHIBIT A WCSGWCFND #4 REBARW/BPC, LS 38470FND #4 REBARW/BPC, LS 38470BROKEN, IN CLEANOUTFND #4 REBARW/BPC, LS 38470G GCTVCTV CTVCTVCTVCTV XXXOHU OHU OHU OHU OHU OHU FORT COLLINS, COLORADOSHEET308 CHERRY STREET UTILITY PLANSKEEFE CIVIL, INC825 UNION STREETGOLDEN, COLORADO 80401(970) 215-6808MEG@KEEFECIVIL.COM3 OF 4GRADING AND DRAINAGE PLANCHERRY STREET EXHIBIT B Hydrologic Soil Group—Larimer County Area, Colorado (308 Chery Street) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/9/2020 Page 1 of 44493500449351044935204493530449354044935504493560449357044935004493510449352044935304493540449355044935604493570493090493100493110493120493130493140493150 493090 493100 493110 493120 493130 493140 493150 40° 35' 34'' N 105° 4' 54'' W40° 35' 34'' N105° 4' 51'' W40° 35' 31'' N 105° 4' 54'' W40° 35' 31'' N 105° 4' 51'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84 0 20 40 80 120 Feet 0 5 10 20 30 Meters Map Scale: 1:427 if printed on A portrait (8.5" x 11") sheet. Soil Map may not be valid at this scale. MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Rating Polygons A A/D B B/D C C/D D Not rated or not available Soil Rating Lines A A/D B B/D C C/D D Not rated or not available Soil Rating Points A A/D B B/D C C/D D Not rated or not available Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Larimer County Area, Colorado Survey Area Data: Version 15, Jun 9, 2020 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Aug 11, 2018—Aug 12, 2018 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Hydrologic Soil Group—Larimer County Area, Colorado (308 Chery Street) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/9/2020 Page 2 of 4 Hydrologic Soil Group Map unit symbol Map unit name Rating Acres in AOI Percent of AOI 81 Paoli fine sandy loam, 0 to 1 percent slopes A 0.2 100.0% Totals for Area of Interest 0.2 100.0% Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Rating Options Aggregation Method: Dominant Condition Component Percent Cutoff: None Specified Hydrologic Soil Group—Larimer County Area, Colorado 308 Chery Street Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/9/2020 Page 3 of 4 Tie-break Rule: Higher Hydrologic Soil Group—Larimer County Area, Colorado 308 Chery Street Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/9/2020 Page 4 of 4 EXHIBIT C USGS The National Map: Orthoimagery. Data refreshed April 2020 National Flood Hazard Layer FIRMette 0 500 1,000 1,500 2,000250 Feet Ü SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT SPECIAL FLOOD HAZARD AREAS Without Base Flood Elevation (BFE) Zone A, V, A99 With BFE or DepthZone AE, AO, AH, VE, AR Regulatory Floodway 0.2% Annual Chance Flood Hazard, Areas of 1% annual chance flood with average depth less than one foot or with drainage areas of less than one square mileZone X Future Conditions 1% Annual Chance Flood HazardZone X Area with Reduced Flood Risk due to Levee. See Notes.Zone X Area with Flood Risk due to LeveeZone D NO SCREEN Area of Minimal Flood Hazard Zone X Area of Undetermined Flood HazardZone D Channel, Culvert, or Storm Sewer Levee, Dike, or Floodwall Cross Sections with 1% Annual Chance 17.5 Water Surface Elevation Coastal Transect Coastal Transect Baseline Profile Baseline Hydrographic Feature Base Flood Elevation Line (BFE) Effective LOMRs Limit of Study Jurisdiction Boundary Digital Data Available No Digital Data Available Unmapped This map complies with FEMA's standards for the use of digital flood maps if it is not void as described below. The basemap shown complies with FEMA's basemap accuracy standards The flood hazard information is derived directly from the authoritative NFHL web services provided by FEMA. This map was exported on 9/9/2020 at 4:22 PM and does not reflect changes or amendments subsequent to this date and time. The NFHL and effective information may change or become superseded by new data over time. This map image is void if the one or more of the following map elements do not appear: basemap imagery, flood zone labels, legend, scale bar, map creation date, community identifiers, FIRM panel number, and FIRM effective date. Map images for unmapped and unmodernized areas cannot be used for regulatory purposes. Legend OTHER AREAS OF FLOOD HAZARD OTHER AREAS GENERAL STRUCTURES OTHER FEATURES MAP PANELS 8 B 20.2 The pin displayed on the map is an approximate point selected by the user and does not represent an authoritative property location. 1:6,000 105°5'11"W 40°35'46"N 105°4'34"W 40°35'19"N 4,514 752.3 FCMaps - 308 Cherry St This map is a user generated static output from the City of Fort Collins FCMaps Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. City of Fort Collins - GIS 572.0 1: WGS_1984_Web_Mercator_Auxiliary_Sphere Feet572.00286.00 Notes Legend 3,430 Parcels FEMA Floodplain FEMA High Risk - Floodway FEMA High Risk - 100 Year FEMA Moderate Risk - 100 / 500 Year FEMA Map Panel City Floodplains City High Risk - Floodway City High Risk - 100 Year City Moderate Risk - 100 Year Growth Management Area Parks Schools Natural Areas City Limits EXHIBIT D Composite Runoff Coefficient Calculations 308 Cherry Street, Fort Collins, COArea (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Area (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Hardscape or Hard SurfaceAsphalt, Concrete 0.95 2,1532,169Rooftop0.95 1,4901,860Pavers, Flagstone 0.50 140340Gravel0.50 0310Deck0.40 240650Total Impervious Area4,0235,329Landscape or Pervious SurfaceLawns, Clayey Soil, Flat Slope < 2% 0.10 4,0322,726Total Drainage Basin A8,055 0.32 0.41 8,055 0.34 0.43Notes:3. Frequency adjustment factor per Fort Collins Stormwater Criteria Manual Table 3.2-3.1. Runoff coefficients per Fort Collins Stormwater Criteria Manual Table 3.2-2.2. Composite runoff coefficient per Fort Collins Stormwater Criteria Manual Equation 5-1.Developed Drainage Basin ASurface TypeRunoff Coefficients1Historic Drainage Basin ACherry - drainage.xlsx Drainage Basin Runoff Calculations 308 Cherry Street, Fort Collins, COLength (ft)Slope (%)Minor to (min)Major to (min)Length (ft)Slope (%)Channel TypeVelocity (fps)tt (min)Minor Tc (min)Major Tc (min)2-yr 10-yr 100-yr 2-yr 10-yr 100-yrA 0.185 0.32 0.41 140 1 17.2 15.4 NA 17.2 15.4 1.75 2.99 6.52 0.11 0.18 0.49A 0.185 0.34 0.43 140 1 16.7 14.9 NA 16.7 14.9 1.81 3.08 6.56 0.11 0.20 0.52Notes:3. Per Fort Collins Stormwater Criteria Manual Section 3.3.4. A minimum Tc of 5 minutes is required.4. Per IDF Table for Rational Method, Table 3.4-1 Fort Collins Stormwater Criteria Manual.1. Per Fort Collins Stormwater Criteria Manual Section 3.3.2.2. Per Fort Collins Stormwater Criteria Manual Section 3.3.3.Initial/Overland Time1Travel Time2HistoricDevelopedFinal Tc3Intensity (in/hr)4BasinArea (acres)Minor Composite CMajor Composite CBasin Flow (cfs)55. Per Fort Collins Stormwater Criteria Manual Equation 5-1.Cherry - drainage.xlsx