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Home My WebLink About504 S WHITCOMB STREET, CARRIAGE HOUSE - FDP210006 - SUBMITTAL DOCUMENTS - ROUND 3 - DRAINAGE REPORT Highland Development Services, Inc. 6355 Fairgrounds Avenue, Suite 100 | Windsor, CO 80550 | 970.674.7550 Memorandum TO: Robert Kirkpatrick FROM: Adele Bartel, E.I.T., Jason Claeys, P.E. DATE: 10/26/21 PROJECT: 504 S. Whitcomb St. HDS JOB NO: 20-1039-00 SUBJECT: Drainage Summary This memo has been prepared to describe the existing conditions at 504 S. Whitcomb St, the adjacent alley, and the drainage impacts of the proposed alley house and change in impervious area. Below is a vicinity map showing the general location of the property: The property is approximately 9,000 sqft (50’ wide by 180’ deep, 0.21 acres) and is located along the east side of S. Whitcomb St and south of W. Mulberry St. A shared alley is located along the east property line and single-family residences are adjacent to the north and south. The existing single-family residence is located toward the west side of the lot with a shed adjacent to the alley to the east and a small garden shed along the south property line. The shed adjacent to the alley is proposed to be removed, along with portions of the existing sidewalk, to accommodate the proposed addition of a detached alley house, and additional gravel parking area. 10/26/21 504 S. Whitcomb St. 10/26/21 Page 2 of 3 Based on aerial topography and a couple site visits, it appears there is a highpoint approximately midway along the east-west property lines of the existing single-family residence. Approximately 1/3rd of the site drains west to Whitcomb St and 2/3rd of the site drains east to the alley. The alley appears to be paved with asphalt, though the surface is degrading and there are a few potholes. A topographic survey was completed to confirm the existing alley slopes. The alley has slopes ranging from 1.5% to 2.0% draining to the north, and an approximate cross slope of 1.5% draining to the west side of alley. An existing residence is located adjacent to the alley along the east side of the alley. Based on the survey, drainage is directed away from the existing residence. In its current condition, during a 100-yr storm event, the runoff depth would inundate the residence’s existing crawlspace vent (see attached Appendices). Measures have been taken to ensure that the proposed improvements do not exacerbate the current alley drainage, described on the following page. Below are photos for reference describing the alley: 504 S. Whitcomb St. 10/26/21 Page 3 of 3 Upon the second site visit after measuring slopes with a digital level, it was discovered that there is a highpoint at the midpoint of the south side of the single-family residence, as well as a lowpoint between the sidewalk and the parkway at the front of the lot. The property owner will modify the parkway landscaping to provide positive drainage relief to S. Whitcomb St and add roof gutters along the south roofline and east porch cover to redirect roof drainage toward S. Whitcomb St. The existing property at 504 S. Whitcomb that drains to the alley is approximately 27.9% impervious. Accounting for the existing improvements to remain, redirection of the existing single-family residence roof drainage, removal of the existing shed, and the addition of the alley house and gravel parking area, the site is approximately 27.7% impervious, an approximate decrease of 0.2% imperviousness. See the attached calculations for reference. Runoff rates were determined utilizing the rational method. The existing peak runoff rate is 0.56 cfs and the proposed peak runoff rate is unaffected at 0.56 cfs. With the redirection of the existing single-family residence roof drainage toward S. Whitcomb St, the proposed improvements should not adversely impact the existing drainage conditions within the shared alley. Please do not hesitate to call (970.674.7550) or email (jclaeys@highland- ds.com) if there are any additional questions. W. MULBERRY ST.S. WHITCOMB ST.504 S. WHITCOMB ST.ALLEYHIGHLANDDEVELOPMENT SERVICES6355 FAIRGROUNDS AVENUE, SUITE 100 | WINDSOR, CO 80550PHONE: 970.674.7550 | EMAIL: Info@Highland-DS.com | www.Highland-DS.comOFSHEET DRAWN BY CHECKED BY DATE SCALE (H) HDS PROJ # SCALE (V)504 S. WHITCOMB ST.ALLEY DRAINAGE EXHIBIT10/21/21 1" = 30' N/A JTC JTC 21-1039-00 DR 1 1 N W E S 0 SCALE: 1" = 30' 3015 BASIN COEFFICIENT (100-YR) BASIN DESIGNATION BASIN AREA DESIGN POINT DRAINAGE FLOW ARROW DRAINAGE BASIN BOUNDARY LEGEND EX CONTOUR (AERIAL TOPOGRAPHY) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Thursday, Oct 21 2021 Existing Alley @ Crawlspace Vent User-defined Invert Elev (ft) = 5.16 Slope (%) = 1.50 N-Value = 0.025 Calculations Compute by: Known Q Known Q (cfs) = 2.24 (Sta, El, n)-(Sta, El, n)... ( 0.00, 6.11)-(5.91, 5.19, 0.025)-(10.95, 5.16, 0.025)-(21.09, 5.25, 0.025)-(27.73, 5.22, 0.025)-(29.02, 5.17, 0.025)-(29.12, 6.17, 0.025) Highlighted Depth (ft) = 0.13 Q (cfs) = 2.240 Area (sqft) = 1.96 Velocity (ft/s) = 1.14 Wetted Perim (ft) = 23.88 Crit Depth, Yc (ft) = 0.12 Top Width (ft) = 23.76 EGL (ft) = 0.15 -5 0 5 10 15 20 25 30 35 Elev (ft)Depth (ft)Section 4.50 -0.66 5.00 -0.16 5.50 0.34 6.00 0.84 6.50 1.34 7.00 1.84 Sta (ft) Crawl space vent flooded in existing 100-yr event. EXISTING ALLEYLOT 9 LOT 6 LOT 8 EXISTING ALLEYWHITCOMB STREETLOT 9 LOT 6 LOT 8 UNIT C 600 SF 24'x24' FG=5006.72 TF=5007.22 FFE=5008.40 MULBERRY STREET REVISIONNO.BYDATEHIGHLANDDEVELOPMENT SERVICES6355 FAIRGROUNDS AVENUE, SUITE 100 | WINDSOR, CO 80550PHONE: 970.674.7550 | EMAIL: Info@Highland-DS.com | www.Highland-DS.comPREPARED BY OR UNDER THE DIRECT SUPERVISION OF: FOR AND ON BEHALF OF HIGHLAND DEVELOPMENT SERVICES REVIEW SET NOT FOR CONSTRUCTION OFSHEET DRAWN BY CHECKED BY DATE SCALE (H) HDS PROJECT # SCALE (V)504 S WHITCOMB STREET - UNIT CGRADING & DRAINAGE PLAN10/21/21 1" = 10' N/A ACM JTC 21-1039-00 GR&DR 1 1 CALL UTILITY NOTIFICATION CENTER OF COLORADO CALL 2-BUSINESS DAYS IN ADVANCE BEFORE YOU DIG, GRADE OR EXCAVATE FOR THE MARKING OF UNDERGROUND MEMBER UTILITIES. before you dig.Call Know what's below. R 0 SCALE: 1" = 10' 2010510 N W E S LEGEND NUMBER OF STEPS HEIGHT OF RISER EXISTING MAJOR CONTOUR EXISTING MINOR CONTOUR PROPOSED MAJOR CONTOUR PROPOSED MINOR CONTOUR NOTES: 1.CONTRACTOR TO PROVIDE AND MAINTAIN ALL NECESSARY EROSION CONTROL MEASURES PRIOR TO AND THROUGHOUT CONSTRUCTION. 2.SPOT ELEVATIONS ARE SHOWN ARE ON FLOWLINE OR BACK OF WALK UNLESS OTHERWISE LABELED. 3.WALKWAYS SLOPE 2.0% AWAY FROM BUILDINGS AND/OR TOWARD PARKING/DRIVE AREAS. WALK LABELS ARE SHOWN ON THE HIGH SIDE OF THE WALK. 4.ALL SUBGRADE TO BE PLACED/COMPACTED PER GEOTECHNICAL ENGINEER RECOMMENDATIONS. 5.FFE IS BASED ON THE TJI FLOORING SYSTEM (14.125"). 6.THE DRAINAGE IS DESIGNED TO MATCH EXISTING FLOWS FROM THE PROPERTY TO THE ALLEY. EXISTING CONDITIONS & DEMOLITION PLAN PROPOSED GRADING PLAN CONCRETE PAN DETAIL Design Engineer: Design Firm: Project Number: Date: % Impervious Runoff Coefficient C Return Period Frequency Adjustment Factor (Cf) 100% 0.95 2-year to 10-year 1.00 90% 0.95 100-year 1.25 90% 0.95 40% 0.50 15% 0.35 * Lawn area assumed to be 15% imperv and 0.35 coeff to account for misc walks C2 to C10 C100 Ex Alley 22,098 0.507 718 3,458 0 1,958 15,964 31.7% 0.48 0.60 Ex 504 5,818 0.134 0 761 0 723 4,334 27.9% 0.45 0.56 Prop 504 5,818 0.134 0 648 0 1,006 4,164 27.7% 0.44 0.55 504 S. Whitcomb Street DEVELOPED IMPERVIOUS AREA CALCULATION DESIGN CRITERIA: Fort Collins Stormwater Criteria Manual BASINS: A.McKenna Highland Development Services 21-1039-00 October 22, 2021 % Impervious values from Table 3.2.2 in the Fort Collins Stormwater Criteria Manual Land Use Paved Roof Walks Runoff Coefficients and Frequency Adjustment Factors for City of Fort Collins - Storm Water Criteria Manual Gravel/Pavers Lawns* (Clayey, Flat < 2% Slope) Sub-basin Designation Atotal (sq feet) Atotal (acres) Weighted % Impervious COMPOSITEApaved (sq feet) Aroof (sq feet) Awalk (sq feet) Agravel/pavers (sq feet) Alawn (sq feet) 504 S. Whitcomb St - Rational Calcs.xlsx Page 1 of 4 Highland Development Services Design Engineer: Design Firm: Project Number: Date: EQUATIONS: -Equation 5-3 -Equation 5-4 -Urbanized Check Equation 3.3-5 - CoFC Overland Flow CONSTRAINTS: 300 ft - Overland flow shall not exceed for developed condition 500 ft - Overland flow shall not exceed for undeveloped condition Final t c = minimum of t i + t t and urbanized basin check recommended minimum t c = 5 min for urbanized basins Time of Concentration (2-yr to 10-yr) Ex Alley Ex Alley 31.7% 0.48 0.507 191 0.0063 18.79 24 0.0078 0.14 0.025 1.42 0.28 19.07 215 11.19 11.19 Ex 504 Ex 504 27.9% 0.45 0.134 150 0.0047 19.24 13 0.0174 0.14 0.025 2.12 0.10 19.34 163 10.90 10.90 Prop 504 Prop 504 27.7% 0.44 0.134 70 0.0076 11.24 101 0.0040 0.14 0.025 1.02 1.65 12.89 170 10.95 10.95 October 22, 2021 504 S. Whitcomb Street DEVELOPED TIME OF CONCENTRATION A.McKenna 21-1039-00 Highland Development Services LENGTH (ft) DESIGN CRITERIA: City of Fort Collins Stormwater Criteria Manual Overland Flow Equations for City of Fort Collins Stormwater Criteria Manual SUB-BASIN DATA INITIAL/OVERLAND TIME (ti)TRAVEL TIME (tt) tc=ti+tt (min) Urban Check Final tc (min)DESIGN POINT Sub-basin % Impervious C2-10 AREA (acres) tt (min) OVERALL LENGTH (ft/ft) tc (min) SLOPE (ft/ft) ti (min) LENGTH (ft) SLOPE (ft/ft) VELOCITY (ft/s) ROUGHNESS COEFFICIENT HYDRAULIC RADIUS (ft) ticttt+= V Ltt60 =1.871.1 10 180 +=LtcV.R2/3 S 1/2 504 S. Whitcomb St - Rational Calcs.xlsx Page 2 of 4 Highland Development Services Design Engineer: Design Firm: Project Number: Date: EQUATIONS: -Equation 5-3 -Equation 5-4 -Urbanized Check Equation 3.3-5 - CoFC Overland Flow CONSTRAINTS: 300 ft - Overland flow shall not exceed for developed condition 500 ft - Overland flow shall not exceed for undeveloped condition Final t c = minimum of t i + t t and urbanized basin check recommended minimum t c = 5 min for urbanized basins Time of Concentration (100-yr) Ex Alley Ex Alley 31.7% 0.60 0.507 191 0.0063 15.20 24 0.0078 0.14 0.025 1.42 0.28 15.48 215 11.19 11.19 Ex 504 Ex 504 27.9% 0.56 0.134 150 0.0047 15.95 13 0.0174 0.14 0.025 2.12 0.10 16.04 163 10.90 10.90 Prop 504 Prop 504 27.7% 0.55 0.134 70 0.0076 9.35 101 0.0040 0.14 0.025 1.02 1.65 11.00 170 10.95 10.95 October 22, 2021 504 S. Whitcomb Street DEVELOPED TIME OF CONCENTRATION A.McKenna 21-1039-00 Highland Development Services LENGTH (ft) DESIGN CRITERIA: City of Fort Collins Stormwater Criteria Manual Overland Flow Equations for City of Fort Collins Stormwater Criteria Manual SUB-BASIN DATA INITIAL/OVERLAND TIME (ti)TRAVEL TIME (tt) tc=ti+tt (min) Urban Check Final tc (min)DESIGN POINT Sub-basin % Impervious C100 AREA (acres) tt (min) OVERALL LENGTH (ft/ft) tc (min) SLOPE (ft/ft) ti (min) LENGTH (ft) SLOPE (ft/ft) VELOCITY (ft/s) HYDRAULIC RADIUS (ft) ROUGHNESS COEFFICIENT ticttt+= V Ltt60 =1.871.1 10 180 +=LtcV.R2/3 S 1/2 504 S. Whitcomb St - Rational Calcs.xlsx Page 3 of 4 Highland Development Services Design Engineer: Design Firm: Project Number: Date: EQUATIONS:I = rainfall intensity (in/hr) Q n = n -yr peak discharge (cfs)P 1 = one-hour point rainfall depth (in) C n = n -yr runoff coefficient t c = time of concentration (min) I n = n -yr rainfall intensity (in/hr) P 1-2yr =0.82 in A n = Basin drainage area (ac)P 1-10yr =1.40 in P 1-100yr 2.86 in BASIN SUMMARY: tc (min) Runoff Coeff (C2-10) C(A) (acres) Intensity (in/hr) Q2 (ft3/s) tc (min) Runoff Coeff (C2-10) C(A) (acres) Intensity (in/hr) Q10 (ft3/s) tc (min) Runoff Coeff (C100) C(A) (acres)Intensity (in/hr) Q100 (ft3/s) Ex Alley Ex Alley 0.507 31.7% 11.19 0.48 0.24 2.12 0.51 11.19 0.48 0.24 3.61 0.87 11.19 0.60 0.30 7.38 2.24 Ex 504 Ex 504 0.134 27.9% 10.90 0.45 0.06 2.14 0.13 10.90 0.45 0.06 3.65 0.22 10.90 0.56 0.07 7.46 0.56 Prop 504 Prop 504 0.134 27.7% 10.95 0.44 0.06 2.14 0.13 10.95 0.44 0.06 3.65 0.22 10.95 0.55 0.07 7.45 0.56 10-yr Peak Runoff 100-yr Peak Runoff 504 S. Whitcomb Street DEVELOPED PEAK RUNOFF DESIGN CRITERIA: Design Point Sub-basin Area (acres) A.McKenna Highland Development Services 21-1039-00 July 1, 2021 2-yr Peak Runoff City of Fort Collins Stormwater Criteria Manual % Impervious nnnnAICQ= (0.786651) 1 )10( 5.28 ct PI + = 504 S. Whitcomb St - Rational Calcs.xlsx Page 4 of 4 Highland Development Services