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Home My WebLink About637 COWAN STREET - PDP/FDP - FDP130030 - REPORTS - DRAINAGE REPORTDRAINAGE ADDENDUM COLORADO CIVIL GROUP, INC.  1413 W. 29th Street  Loveland, Colorado 80538  970-278-0029 January 3, 2014 Mr. Wes Lamarque Stormwater Engineering City of Fort Collins 700 Wood Street Fort Collins, CO 80521 Re: Nathan's Subdivision Drainage Addendum CCG Project No: 0036.0001.00 Dear Mr. Lamarque, This letter serves as the Nathan's Subdivision Drainage Addendum that was requested with your comments dated August 13, 2013. Nathan's Subdivision is located on the west side of Cowan Street, north of E Laurel Street. This subdivision will create two single family lots listed below:  Lot 1 ‐ 637 Cowan Street  Lot 2 ‐ 641 Cowan Street Nathan's Subdivision is bounded by a single family home (627 Cowan Street) and multi‐family development to the north, undeveloped properties to the west between Nathan's Subdivision and the alley, a single family home to the southwest (722 E Laurel Street), a church to the south (726 E Laurel Street), and the Cowan Street right‐ of‐way to the east. The drainage addendum included Lots 1 and 2 of Nathan's Subdivision and offsite areas that appear to drain onto these two lots. The Rational Method was used to calculate the runoff from the identified basins, which can be found in Tables A ‐ C included with this Addendum. A drainage analysis was performed for the current condition (EXISTING) which has a runoff of 0.2 cfs and 1.8 cfs for the 2‐year and 100‐year storms, respectively. The runoff from the planned developed lots (LOT 1 and LOT 2) has a combined runoff of 0.2 cfs and 1.9 cfs for the 2‐year and 100‐year storms, respectively. This drainage analysis included site improvements within the lots and excluded the Cowan Street right‐of‐way. Topographic survey points were obtained approximately 10 feet outside the Nathan's Subdivision boundary. Using these survey points, it was very difficult to determine offsite drainage patterns due to the existing flat slopes, structures, fences, etc. Visual observations, along with the survey points, were used to make the following assumptions:  LOT 1 OS (NORTH) ‐ The southerly 20 feet of 637 Cowan Street drains to the common lot line with LOT 1. This area is approximate. The multi‐family development along this boundary appears to drain to the north and not towards LOT 1. Based on the topographic survey points, there appears to be an existing sump along this common lot line that does not drain to Cowan Street. This will need to be corrected and may include some offsite grading to correct this issue. DRAINAGE ADDENDUM COLORADO CIVIL GROUP, INC.  1413 W. 29th Street  Loveland, Colorado 80538  970-278-0029  LOT 1 OS (WEST) ‐ The vacant property to the west of LOT 1 is very flat and does not appear to have a clear drainage path. For the purposes of this letter, we have assumed that this area drains onto LOT 1. This area is approximate.  LOT 2 OS (WEST) ‐ The vacant property to the west and southwest of LOT 2 is very flat and does not appear to have a clear drainage path. For the purposes of this letter, we have assumed that this area drains onto LOT 2. LOT 2 OS (WEST) also includes a portion of the 722 E Laurel Street property. This area is approximate. A small wall is located near the common lot line between LOT 2 and the church at 726 E Laurel Street. This wall appears to drain the church property directly to Cowan Street. Table C ‐ Basin Hydrology identifies the runoff 2‐year and 100‐year runoff from each of these Basins. For the purposes of this analysis, it was assumed that half of the runoff from each basin, excluding LOT 1 OS (NORTH), drained to each lot line. The runoff was combined to identify swale requirements at three Design Points. The Design Points and their tributary areas are listed below: ܲܦെ 1 ൌ ൬ 1 ܱܶܮ 2 ൰ ൅ ܱܵ 1 ܱܶܮሺܱܴܰܶܪሻ ൅ ൬ ܱܵ 1 ܱܶܮሺܹܶܵܧሻ 2 ൰ ܲܦെ1െ2ൌ ൬ 1 ܱܶܮ 2 ൰ ൅ ൬ 2 ܱܶܮ 2 ൰ ൅ ൬ ܱܵ 1 ܱܶܮሺܹܶܵܧሻ 2 ൰ ൅ ൬ ܱܵ 2 ܱܶܮሺܹܶܵܧሻ 2 ൰ ܲܦെ 2 ൌ ൬ 2 ܱܶܮ 2 ൰ ൅ ൬ ܱܵ 2 ܱܶܮሺܹܶܵܧሻ 2 ൰ The UD‐Channel v1.05 spreadsheet by UDFCD was used to calculate the swale capacity for these three Design Points. The spreadsheet results for all three swales are included with this letter. On single family lots “disconnection of impervious surfaces” is sufficient to meet Low Impact Development (LID) requirements. The roof downspouts will drain across the lawn or other impervious surfaces. This is equivalent to a disconnection of impervious area. If you have any questions or comments, please do not hesitate to contact me. Sincerely, COLORADO CIVIL GROUP, INC. Chris E. Messersmith, P.E. TABLE A ‐ BASIN INFORMATION (SF) 20% 95% 95% (%/100) (acres) EXISTING 10,990 9,420 400 1,170 31% D 0.25 LOT 1 5,630 2,835 500 2,295 57% D 0.13 LOT 2 5,360 3,121 501 1,738 51% D 0.12 LOT 1 OS (NORTH) 1,500 995 0 505 45% D 0.03 LOT 1 OS (WEST) 2,652 2,652 0 0 20% D 0.06 LOT 2 OS (WEST) 6,308 5,732 0 576 27% D 0.14 Landscaped Areas Concrete Building Basin Area Envelope/Roofs Basin I.D. Basin Categories and Areas (SF) Percent Imp. Soil Classification Basin Area TABLE B ‐ BASIN DATA C2 C5 C100 (%/100) 1.00 1.00 1.25 (minutes) EXISTING 31% D 0.23 0.31 0.57 0.23 0.31 0.71 5.0 LOT 1 57% D 0.39 0.44 0.62 0.39 0.44 0.78 5.0 LOT 2 51% D 0.35 0.40 0.61 0.35 0.40 0.76 5.0 LOT 1 OS (NORTH) 45% D 0.31 0.37 0.59 0.31 0.37 0.74 5.0 LOT 1 OS (WEST) 20% D 0.17 0.26 0.55 0.17 0.26 0.69 5.0 LOT 2 OS (WEST) 27% D 0.21 0.29 0.56 0.21 0.29 0.70 5.0 1Time of concentration tc ≥ 5 minutes for urban watershed per UDFCD RUNOFF 2.4.4. Basin I.D. Percent Imperviousness Soil Classification C5 C100 Time of Concentration1 Runoff Coefficient Runoff Coefficient Adjustment, Cf C2 TABLE C ‐ BASIN HYDROLOGY 100‐Year 2‐Year 100‐Year I100 Q 2 Q100 (acres) (minutes) (in./hour) (in./hour) (ft 3 /s) (ft 3 /s) EXISTING 0.25 5.0 0.23 0.71 2.85 9.95 0.2 1.8 LOT 1 0.13 5.0 0.39 0.78 2.85 9.95 0.1 1.0 LOT 2 0.12 5.0 0.35 0.76 2.85 9.95 0.1 0.9 LOT 1 OS (NORTH) 0.03 5.0 0.31 0.59 2.85 9.95 0.0 0.2 LOT 1 OS (WEST) 0.06 5.0 0.17 0.55 2.85 9.95 0.0 0.3 LOT 2 OS (WEST) 0.14 5.0 0.21 0.56 2.85 9.95 0.1 0.8 COMBINED RUNOFF‐SWALES 2‐Year 100‐Year Q2 Q 100 (ft 3 /s) (ft 3 /s) DP 1 0.2 1.2 DP 1‐2 0.3 2.0 DP 2 0.1 1.2 1Runoff coefficients include a 1.00 Cf factor per Table RO‐12 2Runoff coefficients include a 1.25 Cf factor per Table RO‐12 3Combined Runoff‐Swales includes a 1.33 Design Factor Basin I.D. Time of Concentration 2‐Year 1 C2 Basin Area Rainfall Intensity Runoff 3 Design Point Runoff 2‐Year I2 Runoff Coefficients, Cf 100‐Year 2 C100 Project: Channel ID: Design Information (Input) Channel Invert Slope So = 0.0050 ft/ft Manning's n n = 0.040 Bottom Width B = 0.00 ft Left Side Slope Z1 = 3.00 ft/ft Right Side Slope Z2 = 3.00 ft/ft Freeboard Height F = 0.00 ft Design Water Depth Y = 0.60 ft Normal Flow Condtion (Calculated) Discharge Q = 1.23 cfs Froude Number Fr = 0.37 Flow Velocity V = 1.14 fps Flow Area A = 1.08 sq ft Top Width T = 3.60 ft Wetted Perimeter P = 3.79 ft Hydraulic Radius R = 0.28 ft Hydraulic Depth D = 0.30 ft Specific Energy Es = 0.62 ft Centroid of Flow Area Yo = 0.20 ft Specific Force Fs = 0.02 kip Normal Flow Analysis - Trapezoidal Channel NATHAN'S SUBDIVISION SWALE DP 1 UD-Channels_v1.05, Basics 1/3/2014, 7:50 AM Project: Channel ID: Design Information (Input) Channel Invert Slope So = 0.0100 ft/ft Manning's n n = 0.040 Bottom Width B = 0.00 ft Left Side Slope Z1 = 3.00 ft/ft Right Side Slope Z2 = 3.00 ft/ft Freeboard Height F = 0.00 ft Design Water Depth Y = 0.65 ft Normal Flow Condtion (Calculated) Discharge Q = 2.15 cfs Froude Number Fr = 0.53 Flow Velocity V = 1.70 fps Flow Area A = 1.27 sq ft Top Width T = 3.90 ft Wetted Perimeter P = 4.11 ft Hydraulic Radius R = 0.31 ft Hydraulic Depth D = 0.33 ft Specific Energy Es = 0.69 ft Centroid of Flow Area Yo = 0.21 ft Specific Force Fs = 0.02 kip Normal Flow Analysis - Trapezoidal Channel NATHAN'S SUBDIVISION SWALE DP 1-2 UD-Channels_v1.05, Basics 1/3/2014, 7:48 AM Project: Channel ID: Design Information (Input) Channel Invert Slope So = 0.0100 ft/ft Manning's n n = 0.040 Bottom Width B = 0.00 ft Left Side Slope Z1 = 3.00 ft/ft Right Side Slope Z2 = 3.00 ft/ft Freeboard Height F = 0.00 ft Design Water Depth Y = 0.55 ft Normal Flow Condtion (Calculated) Discharge Q = 1.38 cfs Froude Number Fr = 0.51 Flow Velocity V = 1.52 fps Flow Area A = 0.91 sq ft Top Width T = 3.30 ft Wetted Perimeter P = 3.48 ft Hydraulic Radius R = 0.26 ft Hydraulic Depth D = 0.28 ft Specific Energy Es = 0.59 ft Centroid of Flow Area Yo = 0.18 ft Specific Force Fs = 0.01 kip Normal Flow Analysis - Trapezoidal Channel NATHAN'S SUBDIVISION SWALE DP 2 UD-Channels_v1.05, Basics 1/3/2014, 7:49 AM