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Home My WebLink About613 S MELDRUM ST CARRIAGE HOUSE - FDP200028 - SUBMITTAL DOCUMENTS - ROUND 4 - DRAINAGE REPORT KEEFE CIVIL, INC 3125 Crockett Street Fort Collins, Colorado 80526 (970) 215-6808 meg@keefecivil.com June 17, 2021 Keefe Civil Project Number: 2021-006 Matt Simpson Development Review Engineer Water Utilities Engineering City of Fort Collins 700 Wood Street Fort Collins, Colorado 80522 RE: 613 South Meldrum Street Drainage Memo FDP200028 Dear Matt: The purpose of this letter is to describe the proposed site improvements and potential drainage impacts at 613 South Meldrum (Exhibit A). The 0.217-acre project site is a developed single- family residential lot with established landscaping located on South Meldrum Street between Myrtle Street and Laurel 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, a small garden shed, flagstone pathways, concrete and wood patio areas, established trees, several garden areas, and a brick driveway. The lot is bound by existing single-family residential properties on the north and south lot lines, South Meldrum Street right-of-way to the east, and a dirt alley way to the west. Per the NRCS Soil Survey of Larimer County (Exhibit B), the site entirely consists of Fort Collins loam and is classified as being within hydrologic soil group C. These soils are anticipated to have a slow infiltration rate when thoroughly wet and a slow 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) Historically, site stormwater has drained with gentle slopes from the west to east into South Meldrum Street right-of-way, when not infiltrated into established landscaping (historic basin A). Based on field survey and visual inspections, the alley drains north from a high point located adjacent to the southwest corner of the site. Historic and developed drainage basins are shown in Exhibit D. The proposed site improvements include a two-story carriage house with a basement and an attached garage with access off the alley. The proposed driveway shall be constructed with gravel and the existing garden shed will be removed prior to construction. All carriage house roof drains, aside from one, are proposed to drain towards South Meldrum Street right-of-way, minimizing stormwater flows to the alley way. A small drainage basin (<350 sf adjusted impervious area) west of the carriage house is proposed to drain into the existing alley. 613 South Meldrum Street Drainage Memo Keefe Civil Project No.: 2021-006 Page 2 of 3 June 17, 2021 Hardscape or Hard Surface Towards Alley Developed Site Area (sf) Adjusted Developed Area (sf) Concrete 100 100 Rooftop (90%) 80 72 Gravel (40%) 440 176 Total Impervious Area 620 348 Based on visual inspections of the alley and field survey by others, it is my professional opinion proposed improvements presented in this drainage memo will not negatively impact downstream private properties adjacent to the alley before draining into Myrtle Street right-of-way to the north. Figure 1: Adjacent alley way looking north towards Myrtle Street 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 square feet (sf), water quantity detention requirements are not required for this development. Overall Site Hardscape or Hard Surface Historic Site Area (sf) Adjusted Historic Area (sf) Developed Site Area (sf) Adjusted Developed Area (sf) Concrete 500 500 850 850 Rooftop (90%) 1,540 1,386 3,240 2,916 Gravel, Patio, and Pavers (40%) 1,150 460 1,320 528 Total Impervious Area 3,190 2,346 5,410 4,294 Adjusted Site Imperviousness 1,948 613 South Meldrum Street Drainage Memo Keefe Civil Project No.: 2021-006 Page 3 of 3 June 17, 2021 The total site imperviousness is 21%, which is less than the 55% 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 from the site will increase with development as indicated in the summary table below. 2-year 10-year 100-year Basin A Historic Runoff 0.17 cfs 0.29 cfs 0.80 cfs Basin A Developed Runoff 0.03 cfs 0.05 cfs 0.13 cfs Basin B Developed Runoff 0.01 cfs 0.01 cfs 0.03 cfs Basin C Developed Runoff 0.02 cfs 0.03 cfs 0.08 cfs Basin D Developed Runoff 0.11 cfs 0.19 cfs 0.60 cfs Basin E Developed Runoff 0.14 cfs 0.25 cfs 0.72 cfs Developed drainage basins B and C will allow overland drainage flows onto private properties to the north and south; however, there will be no concentrated flows from the proposed site improvements and the flows will be minimal, as indicated above. 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 hereby attest that this letter for the final drainage design for 613 South Meldrum 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. Best regards, Megan L. Keefe, PE / Manager Keefe Civil, Inc. Enclosures Cc: Jeff Palomo, Property Owner References Fort Collins Stormwater Criteria Manual. 2018. ICON Engineering. 2017. Old Town Drainage Basin EPA SWMM Conversion and Hydrologic Update. EXHIBIT A PROPOSED BUILDING TOF=5006.50 MIN GFF=5005.07 GARAGE TOF=5005.67 ROOF DRAINS MUST NOT DRAIN ONTO PRIVATE PROPERTY TO NORTH OR SOUTH GRADING PLANFORT COLLINS, COLORADOSHEET613 SOUTH MELDRUM STREET UTILITY PLANSKEEFE CIVIL, INC3125 CROCKETT STREETFORT COLLINS, CO 80526(970) 215-6808MEG@KEEFECIVIL.COM3 OF 6SOUTHMELDRUMSTREETALLEYA A _________________________________ SECTION A-A A A EXHIBIT B Hydrologic Soil Group—Larimer County Area, Colorado (613 South Meldrum St) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/8/2021 Page 1 of 444920624492070449207844920864492094449210244921104492062449207044920784492086449209444921024492110492936492944492952492960492968492976492984492992493000493008 492936 492944 492952 492960 492968 492976 492984 492992 493000 493008 40° 34' 47'' N 105° 5' 0'' W40° 34' 47'' N105° 4' 57'' W40° 34' 45'' N 105° 5' 0'' W40° 34' 45'' N 105° 4' 57'' 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: 1:355 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 (613 South Meldrum St) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/8/2021 Page 2 of 4 Hydrologic Soil Group Map unit symbol Map unit name Rating Acres in AOI Percent of AOI 35 Fort Collins loam, 0 to 3 percent slopes C 0.3 100.0% Totals for Area of Interest 0.3 100.0% 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 Hydrologic Soil Group—Larimer County Area, Colorado 613 South Meldrum St Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/8/2021 Page 3 of 4 Aggregation is the process by which a set of component attribute values is reduced to a single value that represents the map unit as a whole. A map unit is typically composed of one or more "components". A component is either some type of soil or some nonsoil entity, e.g., rock outcrop. For the attribute being aggregated, the first step of the aggregation process is to derive one attribute value for each of a map unit's components. From this set of component attributes, the next step of the aggregation process derives a single value that represents the map unit as a whole. Once a single value for each map unit is derived, a thematic map for soil map units can be rendered. Aggregation must be done because, on any soil map, map units are delineated but components are not. For each of a map unit's components, a corresponding percent composition is recorded. A percent composition of 60 indicates that the corresponding component typically makes up approximately 60% of the map unit. Percent composition is a critical factor in some, but not all, aggregation methods. The aggregation method "Dominant Condition" first groups like attribute values for the components in a map unit. For each group, percent composition is set to the sum of the percent composition of all components participating in that group. These groups now represent "conditions" rather than components. The attribute value associated with the group with the highest cumulative percent composition is returned. If more than one group shares the highest cumulative percent composition, the corresponding "tie-break" rule determines which value should be returned. The "tie-break" rule indicates whether the lower or higher group value should be returned in the case of a percent composition tie. The result returned by this aggregation method represents the dominant condition throughout the map unit only when no tie has occurred. Component Percent Cutoff: None Specified Components whose percent composition is below the cutoff value will not be considered. If no cutoff value is specified, all components in the database will be considered. The data for some contrasting soils of minor extent may not be in the database, and therefore are not considered. Tie-break Rule: Higher The tie-break rule indicates which value should be selected from a set of multiple candidate values, or which value should be selected in the event of a percent composition tie. Hydrologic Soil Group—Larimer County Area, Colorado 613 South Meldrum St Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/8/2021 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 3/10/2021 at 11:54 AM 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°5'17"W 40°35'N 105°4'40"W 40°34'33"N Basemap: USGS National Map: Orthoimagery: Data refreshed October, 2020 2,257 376.2 FCMaps 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 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 DRAINAGE EXHIBITFORT COLLINS, COLORADOSHEET613 SOUTH MELDRUM STREET UTILITY PLANSKEEFE CIVIL, INC3125 CROCKETT STREETFORT COLLINS, CO 80526(970) 215-6808MEG@KEEFECIVIL.COM6 OF 6 PROPOSED RESIDENCE FOR DRAINAGE REVIEW ONLY NOT FOR CONSTRUCTION HISTORIC DRAINAGE BASINS DEVELOPED DRAINAGE BASINS EXISTING RESIDENCE EXISTING RESIDENCE SOUTH MELDRUM STREETALLEYSOUTH MELDRUM STREETALLEY EXHIBIT E Composite Runoff Coefficient Calculations639 Smith 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 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 SurfaceConcrete0.955001000115160500Rooftop0.951,54080001,4651,540Gravel0.5004400000Patio and Pavers0.501,150020130565720Total Impervious Area3,190 620202452,1902,760Landscape or Pervious SurfaceLawns, Clayey Soil, Flat Slope < 2% 0.20 6,2520445001,500Lawns, Clayey Soil, Average Slope 2-7% 0.25 0280001,1150Lawns, Clayey Soil, Steep Slope >7% 0.35 00027000Total9,442 0.40 0.50 900 0.51 0.64 465 0.21 0.27 515 0.52 0.65 3,3050.64 0.80 4,260 0.61 0.76Notes:Developed Drainage Basin EDeveloped Drainage Basin A Developed Drainage Basin B Developed Drainage Basin C Developed Drainage Basin DSurface 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 5-1.613 Meldrum - drainage.xlsx Drainage Basin Runoff Calculations 639 Smith 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 0.217 0.40 0.50 180 2 14.0 12.0 NA 14.0 12.0 1.93 3.30 7.42 0.17 0.29 0.80A 0.021 0.51 0.64 20 4 3.1 2.4 NA 5.0 5.0 2.85 4.87 9.95 0.03 0.050.13B 0.011 0.21 0.27 5 10 1.7 1.6 NA 5.0 5.0 2.85 4.87 9.95 0.01 0.010.03C 0.012 0.52 0.65 10 33 1.1 0.8 NA 5.0 5.0 2.85 4.87 9.95 0.02 0.03 0.08D 0.076 0.64 0.80 12 5 7.5 1.2 155 2% swale 2.1 1.2 8.7 5.0 2.33 3.98 9.95 0.11 0.19 0.60E 0.098 0.61 0.76 120 2 8.0 5.5 NA 8.0 5.5 2.41 4.12 9.69 0.14 0.25 0.72Notes: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 5-1.613 Meldrum - drainage.xlsx