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Home My WebLink About405 SCOTT AVENUE CARRIAGE HOUSE PDP AND FDP - FDP190019 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTKEEFE CIVIL, LLC 825 Union Street Golden, Colorado 80401 (970) 215-6808 September 18, 2019 Keefe Civil Project Number: 2019-020 Shane Boyle, P.E. Development Review Engineer Stormwater Engineering & Development Review Division City of Fort Collins Utilities 700 Wood Street Fort Collins, Colorado 80522 RE: 405 Scott Avenue Carriage House Drainage Memo Parcel #9710425005 Dear Shane: The purpose of this letter is to describe the proposed site improvements and potential drainage impacts at 405 Scott Avenue (Parcel #9710425005). The site is located on the west side of Scott Avenue in Fort Collins, Colorado between West Mulberry Street and West Magnolia Street. The lot is bound by an existing residence to the north, a church parking lot to the south, Scott Avenue right-of-way to the east, and a gravel alleyway to the west. The 0.37-acre project site is a developed residential lot with established landscaping, a single- story house, two gardening sheds, carport, and a detached garage with access from the alley (Exhibit A). The project site is located within the Old Town Drainage Basin area. The garage concrete approach on the western property line, historic Basin A, drains towards the alley. Historic Basin B drains to a low point in the backyard of the property and infiltrates into the existing landscaping. Historic Basin C drains towards Scott Avenue. Per the flood insurance rate map (Exhibit B), 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 proposed site improvements include a carriage house, concrete patio area, and gravel parking adjacent to the alley, updated landscaping, and an updated ADA-compliant concrete sidewalk adjacent to Scott Avenue (Exhibit C). The drainage patterns will remain consistent with the existing conditions; however, in an attempt to minimize any potential increased stormwater retention in the backyard, all proposed carriage house roof drains and the gravel parking area will drain towards the alley. Developed drainage Basin C will remain unchanged and was not included in the drainage calculations. The composite runoff coefficient and drainage basin runoff calculations for the historic and developed site are included in the enclosed tables (Exhibit D). Using the Fort Collins Stormwater Criteria Manual, the proposed site runoff will increase into the alley (Basin A) and decrease towards the backyard infiltration area (Basin B). 405 Scott Avenue Drainage Memo Keefe Civil Project No.: 2019-020 Page 2 of 2 September 18, 2019 2-year 10-year 100-year Basin A Historic Runoff 0.02 cfs 0.03 cfs 0.08 cfs Basin A Developed Runoff 0.09 cfs 0.15 cfs 0.38 cfs Basin B Historic Runoff 0.15 cfs 0.26 cfs 0.70 cfs Basin B Developed Runoff 0.14 cfs 0.25 cfs 0.69 cfs Based on a recent site visit and field survey (Exhibit C), stormwater in the alley will drain south to the Mulberry Street flowline (5035.40) prior to impacting either the existing church (finished floor 5038.07) or the existing housing structures on lots 12 through 16 adjacent to the alley west of the project site. Since Basin B is an existing infiltration basin and Basin C remains unchanged with proposed site improvements, only Basin A was factored into the total increase in impervious area calculations. Hardscape or Hard Surface Historic Area A (sf) Developed Area A (sf) Change in Impervious Area (sf) Asphalt, Concrete 280 280 Rooftop 0 854 Pavers 0 0 Gravel 0 96 Total Impervious Area 280 1,230 950 Based on the enclosed design, it is my professional opinion the additional runoff generated by this development will not cause adverse impacts to downstream private properties. I herby attest that this letter for the final drainage design for the 405 Scott Avenue Carriage House 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 Best regards, Megan L. Keefe, PE / Manager Keefe Civil, LLC Enclosures Cc: Carolyn Haddad Brian A. Majeski, Assoc AIA, urban|rural design Barry Schram, Lamar Valley Craftsman EXHIBIT A ALLEY FORT COLLINS, COLORADO SHEET 405 SCOTT AVENUE UTILITY PLANS KEEFE CIVIL, LLC 825 UNION STREET GOLDEN, CO 80401 (970) 215-6808 MEG@KEEFECIVIL.COM HISTORIC DRAINAGE PLAN 4 OF 6 SCOTT AVENUE EXHIBIT B USGS The National Map: Orthoimagery. Data refreshed April, 2019. National Flood Hazard Layer FIRMette 0 250 500 1,000 1,500 2,000 Feet Ü 105°6'10.19"W 40°35'10.71"N 105°5'32.73"W 40°34'43.38"N SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT HAZARD SPECIAL FLOOD AREAS Without Zone A, V, Base A99 Flood Elevation (BFE) With BFE or Depth Zone AE, AO, AH, VE, AR Regulatory Floodway 0.of 2% 1% Annual annual Chance chance Flood flood with Hazard, average Areas depth areasdrainage of less less than than one one foot square or with mile Zone X Future ChanceAnnual Conditions Flood Hazard 1% Zone X Area Levee.to with See Reduced Notes. Flood Risk due Zone X Area with Flood Risk due to LeveeZone D NO SCREEN Area of Minimal Flood Hazard Zone X Area of Undetermined Flood Hazard Zone 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 digital map flood complies maps if with it is FEMA's not void standards as described for the below. use of The accuracy basemap standards shown complies with FEMA's basemap The authoritative flood hazard NFHL information web services is derived provided directly by FEMA. from This the map was reflectnot exported changes on or 9/amendments 3/2019 at 6:29:subsequent 54 PM and to this does date and time. becomeor The superseded NFHL and effective by new data information over time. may change This elementsmap map image do not is appear: void if basemap the one or imagery, more of flood the following zone labels, legend, FIRM panel scale number, bar, map and creation FIRM effective date, community date. Map identifiers,images for unmapped regulatoryfor purposes. and unmodernized areas cannot be used Legend OTHER FLOOD AREAS HAZARD OF OTHER AREAS STRGUECNTUERREASL FEATURES OTHER MAP PANELS 8 1:6,000 B 20.2 The point pin selected displayed by the on the user map and is does an approximate not represent an authoritative property location. EXHIBIT C 401 SCOTT AVENU LOT 1, 2, 3 AND LOT 4, LES BLOCK 10 SCOTT-SHER ALLEY FORT COLLINS, COLORADO SHEET 405 SCOTT AVENUE UTILITY PLANS KEEFE CIVIL, LLC 825 UNION STREET GOLDEN, CO 80401 (970) 215-6808 MEG@KEEFECIVIL.COM DEVELOPED DRAINAGE PLAN 5 OF 6 MATCHLINE SEE DETAIL BELOW MATCHLINE SEE DETAIL ABOVE SCOTT AVENUE MULBERRY STREET ALLEY EXHIBIT D Composite Runoff Coefficient Calculations 405 Scott Avenue Area (sf) Minor (2- and 10-yr) Composite C 2 Major (100-yr) Composite C 3 Area (sf) Minor (2- and 10-yr) Composite C 2 Major (100-yr) Composite C 3 Area (sf) Minor (2- and 10-yr) Composite C 2 Major (100-yr) Composite C 3 Area (sf) Minor (2- and 10-yr) Composite C 2 Major (100-yr) Composite C 3 Hardscape or Hard Surface Asphalt, Concrete 0.95 280 615 280 800 Rooftop 0.95 0 1,080 854 1,080 Pavers 0.50 0 120 0 120 Gravel 0.50 0 0 240 0 Total Impervious Area 280 1,815 1,374 2,000 Landscape or Pervious Surface Lawns, Clayey Soil, Flat Slope < 2% 0.20 110 6,825 621 5,015 Total Drainage Basin A 390 0.74 0.92 8,640 0.35 0.44 1,995 0.66 0.83 7,015 0.41 0.51 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. Surface Type Runoff Coefficients 1 Historic Drainage Basin A Historic Drainage Basin B Developed Drainage Basin A Developed Drainage Basin B Scott - drainage.xlsx Drainage Basin Runoff Calculations 405 Scott Avenue 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 0.009 0.74 0.92 15 2 2.1 1.0 NA 5.0 5.0 2.85 4.87 9.95 0.02 0.03 0.08 B 0.198 0.35 0.44 90 2 10.5 9.3 NA 10.5 9.3 2.21 3.78 8.03 0.15 0.26 0.70 A 0.046 0.66 0.83 30 3 3.1 1.9 NA 5.0 5.0 2.85 4.87 9.95 0.09 0.15 0.38 B 0.161 0.41 0.51 96 2 10.1 8.6 NA 10.1 8.6 2.21 3.78 8.38 0.14 0.25 0.69 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. Scott - drainage.xlsx