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Home My WebLink About308 CHERRY STREET CARRIAGE HOUSE - FDP190011 - SUBMITTAL DOCUMENTS - ROUND 3 - DRAINAGE REPORTKEEFE CIVIL, LLC 825 Union Street Golden, Colorado 80401 (970) 215-6808 meg@keefecivil.com September 16, 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 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. Historically, the 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. The remaining third of the house and the parking lot drains to the north. When stormwater north of the parking lot does not infiltrate the established yard, it eventually flows either into the railroad right-of-way or across the neighbor’s lot to the east. Offsite flows from the west have historically drained across the property with gentle slopes from the southwest to northeast via overland flows. There is an existing localized low point directly north of the parking lot, which naturally performs as an infiltration basin (identified in Exhibit A). 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. 308 Cherry Street Drainage Memo Keefe Civil Project No.: 2020-017 Page 2 of 3 September 16, 2020 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. To allow for proper grading adjacent to the carriage house, the proposed top of foundation is 4989.60 ft and the highest finished grade against the foundation is 4989.1 ft. A drainage swale with a flowline at least six inches below the finished grade adjacent to the foundation is proposed along the west side of the carriage house. The drainage swale will capture offsite stormwater flows from the east and drain the flow north, away from the proposed carriage house. The remainder of the backyard will either flow south to the existing infiltration area or towards the northeast, following historic drainage patterns. A minimum 8-inch high timber wall or berm shall be installed along the eastern lot line, which will limit the potential stormwater flows onto the adjacent 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. 308 Cherry Street Drainage Memo Page 3 of 3 Keefe Civil Project No.: 2020-017 September 16, 2020 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, LLC Enclosures Cc: Kimberly Baker Medina & * re m & o <5a & EXHIBIT A W C S G W C FND #4 REBAR W/BPC, LS 38470 FND #4 REBAR W/BPC, LS 38470 BROKEN, IN CLEANOUT FND #4 REBAR W/BPC, LS 38470 G G CTV CTV CTV CTV CTV CTV X X X OHU OHU OHU OHU OHU OHU FORT COLLINS, COLORADO SHEET 308 CHERRY STREET UTILITY PLANS KEEFE CIVIL, LLC 825 UNION STREET GOLDEN, CO 80401 (970) 215-6808 MEG@KEEFECIVIL.COM 3 OF 4 GRADING AND DRAINAGE PLAN CHERRY 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 4 4493500 4493510 4493520 4493530 4493540 4493550 4493560 4493570 4493500 4493510 4493520 4493530 4493540 4493550 4493560 4493570 493090 493100 493110 493120 493130 493140 493150 493090 493100 493110 493120 493130 493140 493150 40° 35' 34'' N 105° 4' 54'' W 40° 35' 34'' N 105° 4' 51'' W 40° 35' 31'' N 105° 4' 54'' W 40° 35' 31'' N 105° 4' 51'' W N 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 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 0 250 500 1,000 1,500 2,000 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 SCREENArea 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 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 0 286.00 572.0 Feet 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, CO Area (sf) Minor (2- and 10-yr) Composite C2 Major (100-yr) Composite C3 Area (sf) Minor (2- and 10-yr) Composite C2 Major (100-yr) Composite C3 Hardscape or Hard Surface Asphalt, Concrete 0.95 2,153 2,169 Rooftop 0.95 1,490 1,860 Pavers, Flagstone 0.50 140 340 Gravel 0.50 0 310 Deck 0.40 240 650 Total Impervious Area 4,023 5,329 Landscape or Pervious Surface Lawns, Clayey Soil, Flat Slope < 2% 0.10 4,032 2,726 Total Drainage Basin A 8,055 0.32 0.41 8,055 0.34 0.43 Notes: 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 A Surface Type Runoff Coefficients1 Historic Drainage Basin A Cherry - drainage.xlsx Drainage Basin Runoff Calculations 308 Cherry Street, Fort Collins, CO Length (ft) Slope (%) Minor to (min) Major to (min) Length (ft) Slope (%) Channel Type Velocity (fps) tt (min) Minor Tc (min) Major Tc (min) 2-yr 10-yr 100-yr 2-yr 10-yr 100-yr A 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 0.49 A 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 0.52 Notes: 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 Time1 Travel Time2 Historic Developed Final Tc 3 Intensity (in/hr)4 Basin Area (acres) Minor Composite C Major Composite C Basin Flow (cfs)5 5. Per Fort Collins Stormwater Criteria Manual Equation 5-1. Cherry - drainage.xlsx 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 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