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Home My WebLink AboutMENNING REPLAT - BDR210004 - SUBMITTAL DOCUMENTS - ROUND 4 - DRAINAGE REPORT KEEFE CIVIL, INC 3125 Crockett Street Fort Collins, Colorado 80526 (970) 215-6808 meg@keefecivil.com November 9, 2021 Keefe Civil Project Number: 2021-007 Wes Lamarque Development Review Engineer Water Utilities Engineering City of Fort Collins 700 Wood Street Fort Collins, Colorado 80522 RE: 418 and 420 North Grant Avenue Drainage and Erosion Control Memo Parcel # 9711219019 Dear Wes: The purpose of this letter is to describe the proposed site improvements and potential drainage impacts at 418 and 420 North Grant Avenue (Exhibit A). The 0.295-acre project site is a developed single-family residential lot with established landscaping located on the east side of North Grant Avenue between Cherry Street and Sycamore Street in Fort Collins, Colorado. The site is located within the Old Town drainage basin. Current conditions at the site include a single-story house, flagstone pathways, a wood patio, established trees, and lawn area. The lot is bound by existing single-family residential properties on the north and south lot lines, North Grant Avenue right-of-way to the west, and a dirt alley way to the east. Per the NRCS Soil Survey of Larimer County (Exhibit B), the site entirely consists of Paoli fine sandy loam and is classified as being within hydrologic soil group A. These soils are anticipated to have a high infiltration rate when thoroughly wet and a high rate of water transmission. Per the flood insurance rate map, 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. Also, the site is outside of all City of Fort Collins mapped floodplains. (Exhibit C) Historic and developed drainage basins are shown in Exhibit D. Historically, site stormwater in the front yard has drained with gentle slopes from the east to west into North Grant Avenue right-of- way (historic basin A). Stormwater in the backyard has historically flowed from the north to the south, onto adjacent private property, with gentle slopes (historic basin B). A small section of the backyard sheet flowed into the alley way (historic basin C). Based on field survey and visual inspections, the alley drains south from the project site and towards the eastern alley access point along Cherry Street. With the subdivision of the lot into two parcels (418 North Grant Avenue to the north and 420 North Grant Avenue to the south), proposed site improvements will include two two-story single- 418 and 420 North Grant Avenue Keefe Civil Project No.: 2021-007 Page 2 of 3 November 9, 2021 family residential homes with basements and two detached garages with alley access out to Cherry Street. The proposed driveways shall be constructed with concrete aprons and a small gravel parking area east of the 418 North Grant Avenue garage. All house roof drains are proposed to drain towards North Grant Avenue right-of-way, minimizing stormwater flows to the alley way (developed basin A). Aside from a very small landscape and patio area south of the 420 North Grant Avenue house, the remainder of the site will drain east, into the existing alley. Approximately 2,550 square feet (sf) of existing unpaved alley will require grading improvements to effectively drain stormwater flows from the site towards Cherry Street right-of-way (Exhibit A). Based on visual inspections of the alley and field survey by others, it is my professional opinion the proposed improvements presented in this drainage memo will not negatively impact downstream private properties adjacent to the alley before draining into Cherry Street right-of-way to the south. Where possible, impervious areas and downspouts shall be routed into pervious areas with landscaping to allow for project stormwater infiltration and increased water quality with site runoff. Since the lot’s total adjusted impervious area increase is below 5,000 sf, water quantity detention requirements are waived for this development. The lot’s total adjusted impervious area will increase by 4,645 sf, as indicated by calculations shown below. Hardscape or Hard Surface Historic Lot Area (sf) Adjusted Historic Area (sf) Developed Lot Area (sf) Adjusted Developed Area (sf) Concrete 80 80 890 890 Rooftop (90%) 1,350 1,215 5,255 4,730 Deck and Pavers (40%) 495 198 340 136 Gravel (40%) 0 0 200 80 Total Impervious Area 1,925 1,493 6,485 5,756 Adjusted Basin Imperviousness 4,263 sf The proposed site imperviousness is ~35%, which is less than the 60% allowed within the Old Town drainage basin master plan (ICON Engineering 2017). The composite runoff coefficient and drainage basin runoff calculations for the historic and developed site are included in the enclosed tables (Exhibit E). Using the Fort Collins Stormwater Criteria Manual (2018), the proposed stormwater runoff will increase towards both North Grant Avenue right-of-way and the alley way, as indicated in the summary table below. 2-year 10-year 100-year Basin A Historic Runoff 0.06 cfs 0.10 cfs 0.27 cfs Basin A Developed Runoff 0.26 cfs 0.45 cfs 1.14 cfs Basin B Historic Runoff 0.08 cfs 0.13 cfs 0.36 cfs Basin C Historic Runoff 0.00 cfs 0.01 cfs 0.02 cfs Basin B Developed Runoff 0.15 cfs 0.26 cfs 0.71 cfs 418 and 420 North Grant Avenue Keefe Civil Project No.: 2021-007 Page 3 of 3 November 9, 2021 Erosion Control Measures The structural practices for the site will consist mainly of silt fence, vehicle tracking control pad, and concrete washout area. The locations of these structural practices and the corresponding construction control measures fact sheets are included in Appendix F. After construction has been completed at the site, all disturbed areas shall be landscaped. The anticipated disturbed area is estimated at 0.35 acre. The City of Fort Collins erosion and sediment control escrow calculations for the site are included in Appendix G. Conclusions 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 418 and 420 North Grant Avenue 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. Best regards, Megan L. Keefe, PE / Manager Keefe Civil, Inc. Enclosures Cc: Greg Menning, Property Owner Torey Lenoch, Lenoch Construction Heidi Shuff, Studio S Architecture, LLC References Fort Collins Stormwater Criteria Manual. 2018. ICON Engineering. 2017. Old Town Drainage Basin EPA SWMM Conversion and Hydrologic Update. EXHIBIT A GARAGE TOF = 5005.90 GARAGE TOF = 5004.90 PROPOSED RESIDENCE FF=5007.38 TOF=5006.20 MAX. FG=5005.7 FORT COLLINS, COLORADOSHEET418 & 420 NORTH GRANT AVENUEKEEFE CIVIL, INC3125 CROCKETT STREETFORT COLLINS, CO 80526(970) 215-6808MEG@KEEFECIVIL.COM5 OF 8 PROPOSED RESIDENCE FF=5007.58 TOF=5006.40 MAX. FG=5005.9 ** ALL ROOF DRAINS MUST DRAIN TOWARDS NORTH GRANT AVENUE AND NOT ONTO ADJACENT PRIVATE PROPERTY ** _________________________________ SECTION A-A A A ** ALL ROOF DRAINS MUST DRAIN TOWARDS NORTH GRANT AVENUE AND NOT ONTO ADJACENT PRIVATE PROPERTY ** B B _________________________________ SECTION B-B GRADING PLANGRANT AVENUEALLEY EXHIBIT B Hydrologic Soil Group—Larimer County Area, Colorado (418 & 420 North Grant) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/9/2021 Page 1 of 444935594493567449357544935834493591449359944936074493559449356744935754493583449359144935994493607492438492446492454492462492470492478492486492494492502492510492518 492438 492446 492454 492462 492470 492478 492486 492494 492502 492510 492518 40° 35' 35'' N 105° 5' 21'' W40° 35' 35'' N105° 5' 18'' W40° 35' 33'' N 105° 5' 21'' W40° 35' 33'' N 105° 5' 18'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84 0 15 30 60 90 Feet 0 5 10 20 30 Meters Map Scale: 1:379 if printed on A landscape (11" x 8.5") 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 (418 & 420 North Grant) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/9/2021 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 0.4 100.0% Totals for Area of Interest 0.4 100.0% 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 418 & 420 North Grant Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/9/2021 Page 3 of 4 Tie-break Rule: Higher Hydrologic Soil Group—Larimer County Area, Colorado 418 & 420 North Grant Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/9/2021 Page 4 of 4 EXHIBIT C 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 4/9/2021 at 4:32 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°5'41"W 40°35'9"N 105°5'3"W 40°34'42"N Basemap: USGS National Map: Orthoimagery: Data refreshed October, 2020 2,257 376.2 420 North Grant Avenue 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 286.0 1: WGS_1984_Web_Mercator_Auxiliary_Sphere Feet286.00143.00 Notes Legend 1,715 Parcels 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 DRAINAGE EXHIBITFORT COLLINS, COLORADOSHEET418 & 420 NORTH GRANT AVENUEKEEFE CIVIL, INC3125 CROCKETT STREETFORT COLLINS, CO 80526(970) 215-6808MEG@KEEFECIVIL.COM1 OF 1 FOR DRAINAGE REVIEW ONLY NOT FOR CONSTRUCTION HISTORIC DRAINAGE BASINS DEVELOPED DRAINAGE BASINS PROPOSED RESIDENCE NOTE: ALL ROOF DRAINS MUST DRAIN TOWARDS GRANT PROPOSED GARAGE PROPOSED GARAGEPROPOSED RESIDENCE NOTE: ALL ROOF DRAINS MUST DRAIN TOWARDS GRANT ALLEYALLEYHOUSE DECK EXHIBIT E Composite Runoff Coefficient Calculations418 and 420 North Grant Avenue, 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 C3Area (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Area (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 SurfaceConcrete0.958016007300Rooftop0.957353,9006151,3550Deck0.407502103400Pavers0.5075013500Gravel Parking0.500002000Total Impervious Area9654,0609602,6250Landscape or Pervious SurfaceLawns, Sandy Soil, Flat Slope < 2% 0.10 1,122 1,440 9,428 4,2500Lawns, Sandy Soil, Avg Slope 2-7% 0.15 0000362Total2,087 0.46 0.57 5,500 0.73 0.91 10,388 0.16 0.20 6,875 0.38 0.48 362 0.15 0.19Notes:Historic Drainage Basin CDeveloped Drainage Basin ADeveloped Drainage Basin BHistoric Drainage Basin BSurface TypeRunoff Coefficients1Historic Drainage Basin A3. 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 5-1.418 & 420 N Grant Ave - drainage.xlsx Drainage Basin Runoff Calculations 418 and 420 North Grant Avenue, 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 0.048 0.46 0.57 35 2 5.6 4.6 NA 5.6 5.0 2.74 4.68 9.95 0.06 0.100.27B 0.238 0.16 0.20 80 2 12.5 11.9 NA 12.5 11.9 2.05 3.50 7.42 0.080.13 0.36C 0.008 0.15 0.19 25 4 5.6 5.4 NA 5.6 5.4 2.76 4.72 9.76 0.00 0.010.02A 0.126 0.73 0.91 10 5 3.3 0.7 85 2% swale 2.1 0.7 5.0 5.0 2.85 4.87 9.95 0.26 0.45 1.14B 0.158 0.38 0.48 35 2 6.3 5.4 75 2% swale 2.1 0.6 6.9 6.0 2.55 4.36 9.31 0.15 0.26 0.71Notes: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 5-1.418 & 420 N Grant Ave - drainage.xlsx EXHIBIT F N. GRANT AVENUEALLEY WAYALLEY WAYALLEY WAYCHERRY STREET GARAGE GARAGE PROPOSED RESIDENCE FORT COLLINS, COLORADOSHEET418 & 420 NORTH GRANT AVENUEKEEFE CIVIL, INC3125 CROCKETT STREETFORT COLLINS, CO 80526(970) 215-6808MEG@KEEFECIVIL.COM6 OF 8 SP CWA SF VTC EROSION CONTROL PLANPROPOSED RESIDENCE CWA VTC SP SF SF Concrete Washout Area (CWA) MM-1 November 2010 Urban Drainage and Flood Control District CWA-3 Urban Storm Drainage Criteria Manual Volume 3 MM-1 Concrete Washout Area (CWA) CWA-4 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Silt Fence (SF) SC-1 November 2010 Urban Drainage and Flood Control District SF-3 Urban Storm Drainage Criteria Manual Volume 3 SC-1 Silt Fence (SF) SF-4 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Stockpile Management (SP) MM-2 November 2010 Urban Drainage and Flood Control District SP-3 Urban Storm Drainage Criteria Manual Volume 3 MM-2 Stockpile Management (SM) SP-4 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Vehicle Tracking Control (VTC) SM-4 November 2010 Urban Drainage and Flood Control District VTC-3 Urban Storm Drainage Criteria Manual Volume 3 SM-4 Vehicle Tracking Control (VTC) VTC-6 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 EXHIBIT G Project:Disturbed Acres:0.35 EROSION CONTROL BMPs Estimated Quantity Units Unit Price Total Price 290 LF $2.00 $580.00 1 EA $500.00 $500.00 1 EA $500.00 $500.00 Sub-Total:$1,580.00 1.5 x Sub-Total:$2,370.00 Amount of security:$2,370.00 Total Acres x Price/acre:$252.00 $720.00 Sub-Total:$252.00 1.5 x Sub-Total:$378.00 Amount to Re-seed:$378.00 Minimum escrow amount:$1,500.00 Erosion Control Escrow:$2,370.00 Miniumum Escrow Amount 418 and 420 North Grant Avenue Unit Price of Seeding per acre: “The amount of the security must be based on one and one-half times the estimate of the cost to install the approved measures, or one and one-half times the cost to re-vegetate the disturbed land to dry land grasses based upon unit cost determined by the City's Annual Revegetation and Stabilization Bid, whichever is greater. In no instance, will the amount of security be less than one thousand five hundred dollars ($1,500) for residential development or three thousand dollars ($3,000) for commercial development” Concrete Washout Area (CWA) Final Escrow Amount Erosion and Sediment Control Escrow/Security Calculation for The City of Fort Collins BMP Amount Silt Fence (SF) Vehicle Tracking Control (VTC) Reseeding Amount 5/4/2021 8:15 AM C:\Users\megst\OneDrive\Documents\Keefe Civil\2021-07_Lenoch\Erosion Control Report\Grant - Escrow.xls