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Home My WebLink About504 S WHITCOMB STREET, CARRIAGE HOUSE - FDP210006 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORT Highland Development Services, Inc. 6355 Fairgrounds Avenue, Suite 100 | Windsor, CO 80550 | 970.674.7550 Memorandum TO: Robert Kirkpatrick FROM: Jason Claeys, P.E. DATE: 3/29/21 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 300 sqft shed adjacent to the alley to the east and a small garden shed along the south property line. The 300 sqft shed is proposed to be removed, along with portions of the existing sidewalk, to accommodate the proposed addition of a detached 600 sqft alley house, and 300 sqft of additional gravel parking area. Attached is the provided Site Development Plan for reference. 504 S. Whitcomb St. 3/29/21 Page 2 of 3 The existing property has approximately 2,870 sqft of impervious area, comprising of roof, concrete sidewalk/patio, and gravel parking areas. The weighted percent impervious of the existing improvements is approximately 26%. Based on aerial topography and a site visit, it appears there is a highpoint approximately midway of the existing single-family residence, with approximately 1/3rd of the site draining west to Whitcomb St and 2/3rd of the site drainage east to the alley. The alley within the area of 504 S. Whitcomb St appears to be paved with asphalt, though the surface is degrading and there are a few potholes. The alley is estimated to have an approximate slope of 1.5% 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, and it appears the alley is sloped away from the building foundation. Below is are photos for reference and you will notice the direction of the snow melt away from the building: Accounting for the existing improvements to remain, removal of the existing 300 sqft shed, and the addition of the 600 sqft alley house and 300 sqft gravel parking area, the site has 504 S. Whitcomb St. 3/29/21 Page 3 of 3 approximately 3,902 sqft of impervious area, with a weighted percent impervious of 35%. This is a 1,032 sqft impervious area increase and weighted percent impervious increase of 8%. (Please note that the impervious areas include the gravel parking area which is considered to be 40% impervious.) Attached is an impervious area summary for reference. Based off of aerial topography, it appears that the basin draining to the north end of the alley is approximately 0.59 acres and estimated to be approximately 30% impervious. With the addition of the alley house and gravel parking, it is estimated that the basin would be approximately 31% impervious. The runoff rates were determined using general assumptions of the basin based on aerial photos/topography and the rational method. The 100-yr runoff rate within the alley is estimated to be 2.36 cfs for the existing condition and 2.42 cfs for the proposed condition, a 0.06 cfs increase. Referencing these flows and general assumptions about the alley, the hydraulics of the alley were analyzed using Hydraflow. The existing flow depth within the alley during a 100-yr storm is estimated to be 0.17 ft deep. Under the proposed conditions, it is estimated that the 100-yr storm flow depth is 0.18 ft deep, a 0.01 ft (or 1/8”) increase. Based on this analysis, the runoff from proposed improvements at 504 S. Whitcomb St will remain within the alley and not inundate the downstream residences adjacent to the shared alley. It is recommended with the construction of additional gravel area, the depression/flowline along the west side of the alley be maintained, if not enhanced while maintaining positive drainage to the north, to ensure that runoff remains along the west side of the alley. Please do not hesitate to call (970.674.7550) or email (jclaeys@highland-ds.com) if there are any additional questions. 504 S. Whitcomb St Impervious Area Summary Design Engineer: Design Firm: Project Number: Date: Summary Total Site Area (50'Wx180'D)9,000 sqft Description Area (sqft) % Impervious Ex House & Sheds 1,834 90% Ex Conc Sidewalk 336 90% Ex Gravel Parking 700 40% Remaining Turf/Landscape 6,130 2% Total Impervious Area & Weigthed %2,870 26% Description Area (sqft) % Impervious Ex House & Sheds 1,534 90% Ex Conc Sidewalk 176 90% Ex Gravel Parking 700 40% Prop Alley House 784 90% Prop Sidewalk/Patio 408 90% Prop Add Gravel Parking 300 40% Remaining Turf/Landscape 5,098 2% Total Impervious Area & Weigthed %3,902 35% Description Impervious Area (sqft) Weighted % Impervious Existing 2,870 26% Proposed 3,902 35% Difference 1,032 8% Proposed Impervious Area Comparison Summary J.Claeys Highland Development Services 21-1039-00 March 22, 2021 Existing Impervious Area 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 EXHIBIT3/29/21 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) 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 5% 0.25 * Lawn area assumed to be 5% to account for misc walks C2 to C10 C100 Ex Alley 25,603 0.588 2,502 3,963 0 1,618 17,520 29.7% 0.44 0.55 Prop Alley 25,603 0.588 2,502 4,263 0 1,918 16,920 31.1% 0.45 0.57 504 S. Whitcomb Street DEVELOPED IMPERVIOUS AREA CALCULATION DESIGN CRITERIA: Fort Collins Stormwater Criteria Manual BASINS: J.Claeys Highland Development Services 21-1039-00 March 29, 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* (Heavy, 2-7% 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 29.7% 0.44 0.588 140 0.0150 12.71 171 0.0100 0.14 0.025 1.61 1.77 14.49 311 11.73 11.73 Prop Alley Prop Alley 31.1% 0.45 0.588 140 0.0150 12.50 171 0.0100 0.14 0.025 1.61 1.77 14.27 311 11.73 11.73 March 29, 2021 504 S. Whitcomb Street DEVELOPED TIME OF CONCENTRATION J.Claeys 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 29.7% 0.55 0.588 140 0.0150 10.57 171 0.0100 0.14 0.025 1.61 1.77 12.35 311 11.73 11.73 Prop Alley Prop Alley 31.1% 0.57 0.588 140 0.0150 10.30 171 0.0100 0.14 0.025 1.61 1.77 12.08 311 11.73 11.73 March 29, 2021 504 S. Whitcomb Street DEVELOPED TIME OF CONCENTRATION J.Claeys 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 0.588 29.7% 11.73 0.44 0.26 2.07 0.54 11.73 0.44 0.26 3.54 0.92 11.73 0.55 0.33 7.23 2.36 Prop Alley Prop Alley 0.588 31.1% 11.73 0.45 0.27 2.07 0.55 11.73 0.45 0.27 3.54 0.94 11.73 0.57 0.33 7.23 2.42 Q2 Delta:0.01 Q10 Delta:0.02 Q100 Delta:0.06 10-yr Peak Runoff 100-yr Peak Runoff 504 S. Whitcomb Street DEVELOPED PEAK RUNOFF DESIGN CRITERIA: Design Point Sub-basin Area (acres) J.Claeys Highland Development Services 21-1039-00 February 19, 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 Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Monday, Mar 29 2021 Existing Alley User-defined Invert Elev (ft) = 100.00 Slope (%) = 1.50 N-Value = 0.025 Calculations Compute by: Known Q Known Q (cfs) = 2.36 (Sta, El, n)-(Sta, El, n)... ( -10.00, 100.20)-(20.00, 100.30, 0.025) Highlighted Depth (ft) = 0.17 Q (cfs) = 2.360 Area (sqft) = 1.69 Velocity (ft/s) = 1.40 Wetted Perim (ft) = 19.84 Crit Depth, Yc (ft) = 0.16 Top Width (ft) = 19.83 EGL (ft) = 0.20 -5 0 5 10 15 20 25 Elev (ft)Depth (ft)Section 99.75 -0.25 100.00 0.00 100.25 0.25 100.50 0.50 100.75 0.75 101.00 1.00 Sta (ft) Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Monday, Mar 29 2021 Proposed Alley User-defined Invert Elev (ft) = 100.00 Slope (%) = 1.50 N-Value = 0.025 Calculations Compute by: Known Q Known Q (cfs) = 2.42 (Sta, El, n)-(Sta, El, n)... ( -10.00, 100.20)-(20.00, 100.30, 0.025) Highlighted Depth (ft) = 0.18 Q (cfs) = 2.420 Area (sqft) = 1.89 Velocity (ft/s) = 1.28 Wetted Perim (ft) = 21.00 Crit Depth, Yc (ft) = 0.17 Top Width (ft) = 21.00 EGL (ft) = 0.21 -5 0 5 10 15 20 25 Elev (ft)Depth (ft)Section 99.75 -0.25 100.00 0.00 100.25 0.25 100.50 0.50 100.75 0.75 101.00 1.00 Sta (ft)