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Home My WebLink About611 LAPORTE AVE - CARRIAGE HOUSE - FDP230018 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE MEMO: 611 LAPORTE AVENUE FORT COLLINS | GREELEY 1 | 2 July 28, 2023 Engineering Development Review Team City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 RE: DRAINAGE LETTER FOR 611 LAPORTE AVENUE FORT COLLINS, COLORADO NE PROJECT NUMBER: 1980-002 Dear EDR Staff: Northern Engineering Services is pleased to submit this Drainage Letter addressing the stormwater impacts of the proposed carriage house improvements at 611 Laporte Avenue, Fort Collins (Lot 4 and the west half of lot 3, Block 271 Loomis Addition). The site is bounded to the west and east by single-family homes, to the north by Laporte Avenue, and to the south by an alley. This Drainage letter has been prepared in accordance with the Fort Collins Stormwater Criteria Manual to demonstrate the proposed change in imperviousness and describe the proposed stormwater conveyance. EXISTING CONDITIONS: The lot consists of a single-family home, a storage shed, stone walking paths areas, landscaping, and a gravel parking area along the south side of the lot. Runoff from the front of the house sheet flows north to the existing curb and gutter along Laporte Avenue. Runoff from the back of the house sheet flows south and east to the stone patio and a low point in the backyard. Runoff from the southern portion of the lot sheet flows to another low point in the back yard and to the alley, then west to Whitcomb Street. Currently, there is 504 SF of impervious area draining to the alley from the lot. PROPOSED CONDITIONS: In the proposed condition, the existing storage shed will be demolished, a carriage house and a walking path will be constructed. The existing gravel parking area will be regraded and replaced in-kind. The carriage house will be located on the south end of the lot, with the walking path connecting the new building to the existing sidewalk in front of the home along Laporte Avenue. The lot is proposed to be regraded such that the runoff from the carriage house will be collected in a small, piped landscape drain system. The downspouts from the new roof will be collected in roof drain connections and landscape inlets will be installed such that all new runoff will be conveyed by this drain, to the front of the lot and discharged to the Laporte Avenue curb and gutter through a sidewalk chase. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE MEMO: 611 LAPORTE AVENUE FORT COLLINS | GREELEY 2 | 2 The imperviousness of the lot is increased by 765 SF with the proposed improvements per the attached calculations. The proposed grading and drainage design prevents an increase in runoff to the alley by capturing and conveying runoff from all impervious surfaces. The following table shows the total change in the impervious area of the site. % Impervious Comparison table Surface Area (SF) % Impervious Existing 10,071 27% Proposed 10,082 35% Difference of % Impervious 8% The proposed landscape drainage pipe must be installed at a very low slope of 0.20% in order to collect the runoff from all proposed impervious areas and discharge to Laporte Avenue due to the very low existing slope of the lot. However, despite the low slope, the 4” pipe is sized to adequately convey the 100-year storm runoff produced by the new impervious area from each half of the new carriage house. The 6” pipe is likewise adequately sized to convey the 100-year storm runoff produced by the total increase in the impervious area of the site. Capacity calculations are attached herewith. CONCLUSIONS: The drainage design proposed with this project will effectively comply with the City of Fort Collins Drainage Criteria. The calculated impervious area proposed for this project remains below the typical Fort Collins Stormwater Criteria Manual, Table 4.1.2 imperviousness of 50% for Low Density and will have negligible impact on the historic drainage basin. The grading and storm conveyance design for this site will effectively prevent additional impervious areas from discharging runoff into the alley. For additional information please see the attached Appendix. If you have any questions, please feel free to contact me at your earliest convenience. Sincerely, NORTHERN ENGINEERING SERVICES, INC. Carlos Ortiz García Ryan Banning, P.E. Project Engineer Project Manager Attachments · Attachment A - Hydrologic calculations · Attachment B – Roof Runoff and Pipe Capacity Calculations Runoff Coefficient1 Percent Impervious1 Project: Location: 0.95 90%Calc. By: 0.80 80%Date: 0.95 100% 0.50 40% 0.20 2% 0.20 2% Basin ID Basin Area (sq.ft.)Rooftop (sq.ft.) Flagstone (sq.ft.) Asphalt, Concrete (sq.ft.)Gravel (sq.ft.) Lawns, Clayey Soil, Flat Slope < 2% (sq.ft.) Percent Impervious C2*Cf Cf = 1.00 C5*Cf Cf = 1.00 C10*Cf Cf = 1.00 C100*Cf Cf = 1.25 Ex. 10,071 1415 1340 0 504 6812 27% 0.40 0.40 0.40 0.50 Ex. + Impr.10,082 2397 496 694 436 6059 35% 0.47 0.47 0.47 0.59 Lawns and Landscaping: 2) Composite Runoff Coefficient adjusted per Table 3.2-3 of the Fort Collins Stormwater Manual (FCSM). Lawns, Clayey Soil, Flat Slope < 2% USDA SOIL TYPE: C Undeveloped: Greenbelts, Agriculture Composite Runoff Coefficient2 1) Runoff coefficients per Tables 3.2-1 & 3.2 of the FCSM. Percent impervious per Tables 4.1-2 & 4.1-3 of the FCSM. DEVELOPED RUNOFF COEFFICIENT CALCULATIONS Rooftop Flagstone Asphalt, Concrete Gravel Streets, Parking Lots, Roofs, Alleys, and Drives: Character of Surface:611 Laporte Fort Collins C. Ortiz July 28, 2023 Notes: P:\1980-002\Drainage\Hydrology\1980-002 Rational (FC) Fort Collins - Lot Only Page 1 of 1 Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Jul 17 2023 Downspout Leader 0.20% (Capacity Proof, Steeper Acceptable) Circular Diameter (ft) = 0.33 Invert Elev (ft) = 100.00 Slope (%) = 0.20 N-Value = 0.013 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 0.23 Q (cfs) = 0.069 Area (sqft) = 0.06 Velocity (ft/s) = 1.08 Wetted Perim (ft) = 0.66 Crit Depth, Yc (ft) = 0.15 Top Width (ft) = 0.30 EGL (ft) = 0.25 0 1 Elev (ft)Section 99.75 100.00 100.25 100.50 100.75 101.00 Reach (ft) Conveyance from 1/4 carriage house roof Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Jul 17 2023 Double Downspout Leader 0.20% (Capacity Proof, Steeper Acceptable) Circular Diameter (ft) = 0.50 Invert Elev (ft) = 100.00 Slope (%) = 0.20 N-Value = 0.013 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 0.25 Q (cfs) = 0.126 Area (sqft) = 0.10 Velocity (ft/s) = 1.28 Wetted Perim (ft) = 0.79 Crit Depth, Yc (ft) = 0.18 Top Width (ft) = 0.50 EGL (ft) = 0.28 0 1 Elev (ft)Section 99.75 100.00 100.25 100.50 100.75 101.00 Reach (ft) Conveyance from 1/2 carriage house roof Channel Report Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Monday, Jul 17 2023 Whole Roof Downspout Leader 0.20% (Capacity Proof, Steeper Acceptable) Circular Diameter (ft) = 0.50 Invert Elev (ft) = 100.00 Slope (%) = 0.20 N-Value = 0.013 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 0.45 Q (cfs) = 0.267 Area (sqft) = 0.19 Velocity (ft/s) = 1.44 Wetted Perim (ft) = 1.25 Crit Depth, Yc (ft) = 0.26 Top Width (ft) = 0.30 EGL (ft) = 0.48 0 1 Elev (ft)Section 99.75 100.00 100.25 100.50 100.75 101.00 Reach (ft) Conveyance from entire carriage house roof 611 Laporte Carriage House BY: ROB 07.28.2023 **100-year storm - 5-minute time of concentration - 90% impervious C I Roof Area Q = (CIA) 0.95 9.8 1,240 SF 0.0285 ac 0.265 CFS * Assume 4 Downspouts Q per downspout 0.066256 CFS See Storm Pipe Proof Calculations Required 6" at 0.20% to pass 100 year from carriage house roof