The URL can be used to link to this page

Home My WebLink AboutLot Certifications - 01/15/2019 FORT COLLINS: 301 North Howes Street, Suite 100, 80521 | 970.221.4158 GREELEY: 820 8 th Street, 80631 | 970.395.9880 | W EB: www.northernengineering.com November 16, 2018 Mr. Shane Boyle City of Fort Collins 700 Wood Street Fort Collins, Colorado 80521 RE: 305 Circle Drive – Grading and Drainage Certification Fort Collins, Colorado Dear Shane: The owners of the residence at 305 Circle Drive reconstructed the home on their property over the past year. During the drainage certification for this project, the City of Fort Collins expressed concerns over the increased impervious area from the previously approved design. The patio and walkw ay areas within the rear yard are significantly different than the approved plans. This letter provides documentation of the drainage impacts of the as constructed conditions at 305 Circle Drive. Northern Engineering provided a grading plan and drainage analysis for the proposed design on March 28, 2017. During construction, the rear patio was reconfigured, and additional walkways were provided within the rear yard. Northern Engineering surveyed the rear yard on August 29, and we re-analyzed the site drainage conditions. Figure 1 shows the impervious areas between the pre-and post-development conditions. According to Figure 1, there was an increase of 1,191 ft2. The post-development conditions exceed the City’s allowable increase of 1,000 ft2. The total increase of impervious area from pre-development conditions results in an increase of 0.07 cfs to the 2-Year Storm and 0.27 cfs to the 100-Year Storm. The increased impervious area of 1,191 ft2 will result in a rise of 0.01 ft within Circle Drive in front of the residence; however, the City will allow an increase impervious area of 1,000 ft2. Therefore, the runoff was recalculated assuming an increase of 191 ft2 to determine the impact on Circle Drive from the impervious area that exceeds the City’s req uirements. The additional 191 ft2 of impervious area results in an increase of 0.01 cfs to the 2-Year Storm, 0.04 cfs to the 100-Year Storm, and no rise within Circle Drive. The additional impervious area will have no impact to the flowrates within Circle Drive. The total increase in the flowrate from the impervious area is split between the larger building footprint and the increased pathways in the rear yard. Sixty percent of this increase occurs from the building addition as discussed within Northern Engineering’s March 27, 2017 drainage analysis. The remaining 40% increase is a result of the reconfigured rear patio, additional walkways, and the concrete valley pan along the south property. Runoff from the building’s roof drains will flow in two dire ctions: • collected by a storm sewer along the north property line that discharges into the curb and gutter for Circle Drive, • or discharge directly into a valley pan constructed along the south property line and discharging into the curb and gutter for Circle Drive via a sidewalk culvert. The runoff from the remaining 40% will first flow towards the North-South Alley as part of the runoff from the rear yards. Due to a high point within the north-south alley along the western edge of the property, the rear yard was divided into two drainage basins: SW Corner and NW Corner. Figures 2 through 4 provide the hydrologic calculations for both the SW Corner and NW Corner Basins. The below table provides a summary FORT COLLINS: 301 North Howes Street, Suite 100, 80521 | 970.221.4158 GREELEY: 820 8 th Street, 80631 | 970.395.9880 | W EB: www.northernengineering.com of the increased runoff from the rear yard improveme nts and the increase throughout the entire site from the additional impervious area. Table 1: Flowrates from Rear Yard DESIGN POINT BASIN ID TOTAL AREA (sq. ft) C2 C100 2-yr Tc (min) 100- yr Tc (min) Q2 (cfs) Q100 (cfs) A SW Corner (Post-Development) 3,812 0.46 0.58 7.0 5.9 0.10 0.48 B NW Corner (Post-Development) 2,712 0.53 0.66 6.7 5.6 0.09 0.40 B NW Corner (Pre-Development) 4,201 0.46 0.58 7.5 6.4 0.11 0.52 Site Impact from additional 1,191 sq ft 1,191 0.95 1.00 5.0 5.0 0.07 0.27 Drainage from the SW Corner Basin will sheet flow across the rear yard into the North-South Alley. The crown within the North-South Alley will redirect the flow towards the East-West Alley along the south property line. Drainage will then flow back towards 305 Circle Drive and into the valley pan constructed along the south property line. The valley pan will discharge into the Circle Drive curb and gutter via a sidewalk culvert. The calculated 2-Year and 100-Year Storm Events are 0.10 cfs and 0.48 cfs respectively. A survey of the alleys determined the critical location, with respect to neighboring properties, will occur at the intersection between the North-South and East-West Alley. According to Figure 5, the 100-Year Storm Event will remain within the alley cross-section and not impact any of the neighboring properties. Drainage from the NW Corner Basin will sheet flow across the rear yard into the North -South Alley. The drainage will collect within North-South Alley and flow north towards Lake Stre et. The flowrate for the 100- Year Storm Event will decrease from 0.52 cfs to 0.40 cfs. Therefore, the improvements within the NW Corner Basin will not impact any of the neighboring properties. Please feel free to contact me with any questions you may have. NORTHERN ENGINEERING SERVICES, INC. Frederick S. Wegert, PE Project Engineer Encl. 11/16/18 AL L E Y ALLEY X X X X X X X X X X X XXXXXX AL L E Y ALLEY ROOFTOP ASPHALT SURFACE AREA (SF)% IMPERV. 3,142 0 6,328 100% 100% 0%0 TOTALS PRE-DEVELOPMENT 3,142 1,197 305 CIRCLE DRIVE FORT COLLINS COLORADO 301 N. Howes Street, Suite 100 Fort Collins, Colorado 80521 E N G I N E E R N GI EHTRON R N PHONE: 970.221.4158 www.northernengineering.com DESCRIPTION EXISTING VS. AS-BUILT IMPERVIOUS AREA DRAWN BY F. WEGERT DATE NOVEMBER 13, 2018 PROJECT 1273-002 Fig.1 DRAWINGSCALE 1"=20' CONCRETE LANDSCAPE IMPERV. AREA (SF) 1,197 100% 10,667 4,339TOTAL= ROOFTOP CONCRETE ASPHALT PAVERS SURFACE AREA (SF)% IMPERV.IMPERV. AREA (SF) 1,675 0 0 100% 100% 100% 40% TOTALS 10,667 5,530TOTAL= 0 0 1,675 0 LANDSCAPE 5,530 0%0 POST-DEVELOPMENT 3,855 3,855 305 Circle Drive CHARACTER OF SURFACE: Runoff Coefficient1 Percentage Impervious1 Project: Streets, Parking Lots, Roofs, Alleys, and Drives:Calculations By: Asphalt …………………............................................................................0.95 95%Date: Concrete ……………….............................................................................0.95 95% Gravel (packed) ……….............................................................................0.50 50% Roofs…………………..............................................................................0.95 95% Pavers………………….............................................................................0.40 40% Lawns and Landscaping Sandy Soil Flat <2% …………..............................................................................0.10 10% Average 2% to 7%............................................................................0.15 15% Steep >7% …………............................................................................0.20 20% Clayey Soil Flat <2% …………..............................................................................0.20 20% Average 2% to 7%............................................................................0.25 25% Steep >7% …………............................................................................0.35 35%2-year Cf = 1.00 10-year Cf = 1.00 USDA SOIL TYPE: C Sub-Basin ID Sub-Basin Area (sq. ft.) Sub-Basin Area (ac.) Area of Asphalt & Concrete (ac.) Area of Roofs (ac.) Area of Gravel (ac.) Area of Landscaping 2% to 7% (ac) 2-year Composite Runoff Coefficient 10-year Composite Runoff Coefficient 100-year Composite Runoff Coefficient Composite % Imperv. SW Corner (Post-Development)3,812 0.09 0.46 0.46 0.58 46% NW Corner (Post-Development)2,712 0.06 0.53 0.53 0.66 53% NW Corner (Pre-Development)4,201 0.10 0.46 0.46 0.58 46% Impact from additional 1,191 sq ft 1,191 0.03 0.95 0.95 1.00 95% 1) Runoff coefficients are taken from the Fort Collins Stormwater Criteria Manaual, Table 3.2.1 and Table 3.2.2. 100-year Cf = 1.25 Impervious Area = 707 sq. ft., C = 0.46 Composite Runoff Coefficient with Adjustment FIGURE 2: COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS 305 Circle Drive F. Wegert November 13, 2018 Impervious Area = 656, C = 0.53 Impervious Area = 919, C = 0.46 Impervious Area = 191, C = 0.95 11/13/2018 10:45 AM D:\Projects\1273-002\Drainage\Hydrology\1273-002_Asbuilt Rational Fort Collins.xlsx\Composite C 305 Circle Drive Is Length >500' ? C*Cf (2-yr Cf=1.00) C*Cf (10-yr Cf=1.00) C*Cf (100-yr Cf=1.25) Length, L (ft) Slope, S (%) Ti 2-yr (min) Ti 10-yr (min) Ti 100-yr (min) Length, L (ft) Slope, S (%) Velocity, V (ft/s) Tt (min) 2-yr Tc (min) 10-yr Tc (min) 100-yr Tc (min) A SW Corner (Post-Development)No 0.46 0.46 0.58 65 4.00%6.1 6.1 5.0 75 0.50%1.41 0.9 7.0 7.0 5.9 B NW Corner (Post-Development)No 0.53 0.53 0.66 50 3.40%5.0 5.0 3.8 65 0.10%0.63 1.7 6.7 6.7 5.6 B NW Corner (Pre-Development)No 0.46 0.46 0.58 34 1.40%6.2 6.2 5.1 67 0.20%0.89 1.2 7.5 7.5 6.4 A Impact from additional 1,191 No 0.95 0.95 1.00 15 2.00%0.9 0.9 0.6 0 0.50%N/A N/A 5.0 5.0 5.0 Calculations By: Date: 305 Circle Drive F. Wegert November 13, 2018 FIGURE 3: TIME OF CONCENTRATION COMPUTATIONS Design Point Sub-Basin Overland Flow Gutter Flow Time of Concentration Velocity (Gutter Flow), V = 20·S½ Tt = L / 60V Tc = Ti + Tt (Equation RO-2) Velocity (Swale Flow), V = 15·S½ NOTE: C-value for overland flows over grassy surfaces; C = 0.25 Overland Flow, Time of Concentration: Gutter/Swale Flow, Time of Concentration: Project: (Equation RO-4) () 31 *1.187.1 S LCfCTi −= 11/13/2018 10:51 AM D:\Projects\1273-002\Drainage\Hydrology\1273-002_Asbuilt Rational Fort Collins.xlsx\Tc 305 Circle Drive A SW Corner (Post-Development)0.09 7 7 6 0.46 0.46 0.58 2.60 4.44 9.63 0.10 0.18 0.48 B NW Corner (Post-Development)0.06 7 7 6 0.53 0.53 0.66 2.60 4.44 9.63 0.09 0.15 0.40 B NW Corner (Pre-Development)0.10 7 7 6 0.46 0.46 0.58 2.52 4.31 9.31 0.11 0.19 0.52 A Impact from additional 1,191 sq ft 0.03 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 0.07 0.13 0.27 Project: Calculations By: Date: F. Wegert 305 Circle Drive FIGURE 4: RUNOFF COMPUTATIONS C100 Design Point Flow, Q100 (cfs) Flow, Q2 (cfs) 10-yr Tc (min) 2-yr Tc (min) C2 Flow, Q10 (cfs) Intensity, i100 (in/hr) Sub-Basin(s) Rational Method Equation: Rainfall Intensity: Rainfall Intensity taken from the Fort Collins Stormwater Criteria Manual (FCSCM), Tables RA-7 and RA-8 November 13, 2018 Intensity, i10 (in/hr) C10 Area, A (acres) Intensity, i2 (in/hr) 100-yr Tc (min) ()()()AiCCQf= 11/13/2018 10:53 AM D:\Projects\1273-002\Drainage\Hydrology\1273-002_Asbuilt Rational Fort Collins.xlsx\Runoff Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Nov 13 2018 Figure 5 - 100-Year Flow Depth Around Corner of Alley Triangular Side Slopes (z:1) = 50.00, 17.00 Total Depth (ft) = 0.20 Invert Elev (ft) = 4986.00 Slope (%) = 0.50 N-Value = 0.025 Calculations Compute by: Known Q Known Q (cfs) = 0.48 Highlighted Depth (ft) = 0.15 Q (cfs) = 0.480 Area (sqft) = 0.75 Velocity (ft/s) = 0.64 Wetted Perim (ft) = 10.06 Crit Depth, Yc (ft) = 0.11 Top Width (ft) = 10.05 EGL (ft) = 0.16 0 2 4 6 8 10 12 14 16 18 Elev (ft)Depth (ft)Section 4985.75 -0.25 4986.00 0.00 4986.25 0.25 4986.50 0.50 4986.75 0.75 4987.00 1.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Thursday, Nov 15 2018 Pre-Developed 100-Year Circle Drive Gutter Cross Sl, Sx (ft/ft) = 0.020 Cross Sl, Sw (ft/ft) = 0.083 Gutter Width (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 0.50 N-Value = 0.016 Calculations Compute by: Known Q Known Q (cfs) = 7.31 Highlighted Depth (ft) = 0.46 Q (cfs) = 7.310 Area (sqft) = 2.92 Velocity (ft/s) = 2.50 Wetted Perim (ft) = 17.19 Crit Depth, Yc (ft) = 0.47 Spread Width (ft) = 16.72 EGL (ft) = 0.56 0 2 4 6 8 10 12 14 16 18 20 22 24 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 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Thursday, Nov 15 2018 Post-Developed 100-Year Circle Drive Gutter Cross Sl, Sx (ft/ft) = 0.020 Cross Sl, Sw (ft/ft) = 0.083 Gutter Width (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 0.50 N-Value = 0.016 Calculations Compute by: Known Q Known Q (cfs) = 7.58 Highlighted Depth (ft) = 0.47 Q (cfs) = 7.580 Area (sqft) = 3.01 Velocity (ft/s) = 2.52 Wetted Perim (ft) = 17.45 Crit Depth, Yc (ft) = 0.48 Spread Width (ft) = 16.97 EGL (ft) = 0.56 0 2 4 6 8 10 12 14 16 18 20 22 24 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 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Thursday, Nov 15 2018 Post-Developed 100-Year Circle Drive Gutter Cross Sl, Sx (ft/ft) = 0.020 Cross Sl, Sw (ft/ft) = 0.083 Gutter Width (ft) = 2.00 Invert Elev (ft) = 100.00 Slope (%) = 0.50 N-Value = 0.016 Calculations Compute by: Known Q Known Q (cfs) = 7.35 Highlighted Depth (ft) = 0.46 Q (cfs) = 7.350 Area (sqft) = 2.92 Velocity (ft/s) = 2.52 Wetted Perim (ft) = 17.19 Crit Depth, Yc (ft) = 0.47 Spread Width (ft) = 16.72 EGL (ft) = 0.56 0 2 4 6 8 10 12 14 16 18 20 22 24 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 Reach (ft) CI R C L E D R I V E AL L E Y ALLEY (85.4 8 ) (85 . 3 3 ) (85 . 1 9 ) (85.0 9 ) (85 . 5 8 ) (85 . 5 3 ) (85 . 5 2 ) (85 . 7 7 ) (85 . 9 5 ) DS DS DS DS DS DS DS DS 3 - 6" RISER TF=87.7785.96 86.31 86.1 4 86.7 5 86.81 86.50 86.44 86.5 8 86.0 6 86.25 86.25 85.95 85.7 3 85.9 6 85.9 3 85.8 5 86.0 5 86.0 6 86.0 7 85.8 6 86.00 84.69 85.89 85.3 7 86.3 0 87.77 87.0 8 86.6 3 87.1 0 86.8 5 85.4 5 85.7 2 85.8 8 85.13 85.0 7 86.6 7 86.1 5 MIN 5" DRAIN BASIN 1-5 INV. IN=4984.82 (W) INV. OUT=4984.82 (E) FG=4985.96 N: 1450517.2 E: 3120783.7 MIN 5" DRAIN BASIN 1-4 INV. IN=4984.79 (W) INV. OUT=4984.79 (E) FG=4985.93 N: 1450517.2 E: 3120794.8 MIN 5" DRAIN BASIN 1-3 INV. IN=4984.75 (W) INV. OUT=4984.75 (E) FG=4985.85 N: 1450517.3 E: 3120810.8 MIN 5" DRAIN BASIN 1-8 INV. OUT=4984.90 (E) FG=4985.95 N: 1450517.1 E: 3120751.8 MIN 5" DRAIN BASIN 1-7 INV. IN=4984.87 (W) INV. OUT=4984.87 (E) FG=4985.73 N: 1450517.2 E: 3120763.1 8''X6'' REDUCER 1-6 INV. IN=4984.84 (W) INV. OUT=4984.84 (E) FG=4986.01 N: 1450517.2 E: 3120774.7 BEND 1-2 INV. IN=4984.72 (W) INV. OUT=4984.72 (E) FG=4985.89 N: 1450517.3 E: 3120823.2 ROOF DRAIN 1 w/ CONCRETE COLLAR INV. IN=4984.68 (W) FG=4984.68 N: 1450515.1 E: 3120839.3 16.27 LF 8" H D P E @ 0 . 2 5 % 12.46 LF 8" HDPE @ 0.25% 16.00 LF 8" HDPE @ 0.25% 11.08 LF 8" HDPE @ 0.25% 8.97 LF 8" HDPE @ 0.25% 2 - 7" RISER INSTALL TRENCH DRAIN DISCHARGE TO SUBDRAIN PER GEOTECH MAINTAIN 6" MIN SEPARATION FF TO GRATE SWALE 1' PAN SIDEWALK CHASE (SEE DETAIL 709 LCUASS) 0.6% 0.6% 2.0%2.0% 11.58 LF 6" HDPE @ 0.25% 11.38 LF 6" HDPE @ 0.25% CONNECT TO ROOF LEADER (TYP.) 2.0% 6.4% 3.0% 3.3% 3.9 % 5.9% 8.2% 2.3% 2.0% 2.0% 2.3% 4.1% 3.6% (84.9 3 ) (84.6 0 ) 1' PAN 86.8 0 86.5 8 CONNECT TO ROOF LEADER (TYP.) 2.6% 86.8 0 SIDEWALK CHASE (SEE DETAIL 709 LCUASS) (85 . 8 5 ) (85 . 9 2 ) (85 . 9 9 ) (8 6 . 0 6 ) (86 . 0 8 ) (86 . 0 8 ) (86 . 0 3 ) (85 . 7 8 ) 3.3 % 85.63 86.2 7 (85 . 9 0 ) (85 . 7 9 ) (85 . 7 2 ) (85 . 7 1 ) (85 . 6 6 ) 85.2 6 20.00 5.08 15.00 8.92 Sheet of 1 RU S S E L L R E S I D E N C E 30 1 N o r t h H o w e s S t r e e t , S u i t e 1 0 0 Fo r t C o l l i n s , C o l o r a d o 8 0 5 2 1 E N G I N E E R N G I E H T R O N R N PH O N E : 9 7 0 . 2 2 1 . 4 1 5 8 ww w . n o r t h e r n e n g i n e e r i n g . c o m 30 5 C I R C L E D R I V E FO R T C O L L I N S , C O L O R A D O G1 GR A D I N G P L A N NORTH 1 LEGEND: NOTES: CALL UTILITY NOTIFICATION CENTER OF COLORADO Know what's below. before you dig.Call R 77.71 (77.90) 1.8% (1.8%) A-A