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HomeMy WebLinkAboutDrainage Reports - 06/12/2017 (2)FORBES ENGINEERING Full Service Civil & Structural Engineering June 9, 2017 Dan Mogen City of Fort Collins Stormwater Engineering 700 Wood Street Fort Collins, CO 80521 City of F Collins Appr Itf Plans Approved Date: 0- zm RE: Grading & Drainage Memo for site improvements at; 412 North Whitcomb Street (Parcel #97111-89-001), 518 Cherry Street (Parcel #97111-19-017) and 520 Cherry Street (Parcel #97111-19-008) Fort Collins, CO 80521 Mr. Mogen, This letter and attached exhibits are to serve as the grading & drainage memo for the above referenced single family residential properties. All three properties are located in the block at the northeast corner of North Whitcomb Street and Cherry Street. The home at 412 North Whitcomb is in the Maldonado Subdivision and was originally built in 1910) it has one single family residence on the property. The property at 520 Cherry Street currently has a duplex built in 1940 and a concrete parking area. The home at 518 Cherry is a single family residence built in 1948. The surrounding neighborhood is all single family residential lots (see attached existing grading & drainage plan for block layout). Proposed site improvements are as follows; - The 412 Whitcomb property will be extended to the east property line of 520 Cherry Street connecting with the existing alley. Improvements include a revised 1972 square foot home with a 280 square foot guest room; connecting breezeway and a detached garage (see proposed grading & drainage plan). It will maintain a concrete walk in the location of the original walk and the garage will be built with a concrete apron & gravel drive. It is the intent that the new residence has a finish floor elevation approximately at the same elevation of the original building. The finish floor elevation for the detached garage has been set at approximately 0.5 feet above the existing grade. - The 520 Cherry Street property will have a new 1248 square foot single family residence with reconfigured lot lines and a revised concrete drive. The duplex will be converted into a garage that will serve both the new home as well as the existing residence at 518 Cherry Street (see proposed grading and drainage plan). The 518 Cherry property will remain relatively the same but for the reconfigured lot lines and an addition of the garage. The converted garage will have a concrete apron and gravel drive connecting to the existing gravel alley. Existing contours for the grading & drainage plans are based on survey data provided by Stewart & Associates Land Surveying, Inc. dated June 14, 2016. None of the properties lie within any flood plains and the historic storm water runoff presently is directed towards the street gutter system of North Whitcomb and Cherry Streets. These historic drainage patterns will be maintained. Drainage flows for the three properties were analyzed together and are broken up into two sub -basins. To help maintain existing flow patterns, the proposed improvements will use roof gutters and downspouts to direct the storm water runoff towards the west into the Whitcomb gutter system and towards the east and south into the alley and Cherry Street gutter system (see attached proposed grading and drainage plan). The grade at the face of the new buildings will be elevated approximately 6 inches and the ground will be graded to slope (at 1 % minimum) the drainage away from the homes. Proposed grading for the development is comprised of all grading necessary for the construction of the two buildings. This includes the built-up portion around the proposed northeast garage to allow for the west to east surface drainage to flow around the structure as well as a drainage swale along the common property line between lots 2 & 3 to force surface flows to travel to Cherry Street and not across lot 3. The grading improvements around the new house, garage, driveway and the addition to the house on lot 1 has been calculated at less than 10,000 sq. ft. so the project will not require erosion control measures during the development process. Because of the existing building's north setback, it is too narrow to provide an adequate drainage swale along the north side of the 421 Whitcomb property. Because of this and to assist the drainage along the north side, all of the roof drains along the north side of the home will be connected into a buried drainage pipe which will direct storm run-off west, daylighting at the west property line. The pipe will be a minimum of 6" in diameter and will be sloped a minimum of 0.5%. Along with the on -site grading, the alley will also be regraded along the east side of lot 3. This will help prevent flows from the development crossing the alley and into the properties to the east. The existing high point for the alley is located approximately due east of the north lot line of lots two & three. This will be maintained with the storm water run-off flowing to the north and south from that point. The existing 2-year storm water run-off traveling south crossing the sidewalk at the alley/Cherry Street connection has been calculated at 0.11 cubic feet per second (cfs). 9 The proposed 2-year storm water run-off traveling south crossing the sidewalk has been calculated at 0.20 cfs, an increase of only 0.09 cfs. (see attached drainage run-off calculations). As previously stated, none of the homes are in a floodplain and the finish floor elevations have been raised so that the risk of flooding is very minimal. It is my conclusion that with proper construction practice and observance to the set finish floor elevation, no adverse affects to the site or to the surrounding properties are anticipated from the proposed improvement of this site. xevm r omes, r c FORBES ENGINEERING STORM WATER RUNOFF CALCULATIONS PROJECT: a2D�t�0 Sccr3Ai✓�Sta�1 Table RO-11 Rational Method Runoff Coefficients for Composite Analysis Character Run -Off of Surface Coefficient Streets, Parking Lots & Drives Asphalt 0.95 Concrete 0.95 Gravel 0.50 Roofs 0.95 Recycled Asphalt 0.80 Lawns, Sandy Soil: Flat, <2% 0.10 Average, 2% to 7% 0.15 Steep, >7% 0.20 Lawns, Heavy Soil: Flat, <2% 0.20 Average, 2% to 7% 0.25 Steep, >7% 0.35 Job # 27li0 Client- !S- W,4 Ve' Date: q1q 1 ZJl.-7 Revised: By: KEF Composite Runoff Coefficient Using Formula RO-8 C= iE1 Ci Ai /At Where: C = Composite Runoff Coefficient C i = Runoff Coefficient for Specific Area (Ai) Ai = Area of Surface with Runoff Coefficient of Ci, (acres or feet2) n = Number of different surfaces to be considered At= Total Area over which C is applicable, (acres or feet2) Existing Conditions for i,Ci 13.4Sid �A«%eS Watershed Notes: it -A TAKeS 41 VL Or Zo-rL.(S AILC-7' QuN -Dr-r. .4 Qok-jDjf ®r Ti(tc "• Bail" 5#w- -r /zoWs oNTe Tk�� P/t�P�*TZ-tY Tv G6T. �Nti iFde(1 tTY �',cIrj flNn Cl�c�'�R-Y S7; Basin A Areas AL(Landscape) Ac(Conc) AR(Roofs) Ac(Gravel) AG(Pavement) At(Total) Sq Ft Sq Ft Sq Ft Sq Ft Sq Ft Sq Ft �i66 z8z 7g� z3or o 65-3> Using the Infrequent Storm Adjustment Factor (Table RO-12) Storm Return Period (years) Frequency Factor Cf 2 to 10 1.00 11 to 25 1.10 26 to 50 1.20 51 to 100 1.25 c =. zs-v Adjusted Composite Runoff Coefficients CZ to CIO Cloo 01zs�y 0.3/g Page 1 / 3 . Storm Water Runoff Calculations Continued Calculated Using Rational Method Notes: 1. Runoff Coefficients (C) are from the City of Fort Collins Stormwater Criteria Manual, Table RO-11 and the Composite Runoff Coefficients are from the previous calculations 2. Rainfall Intensity (I) is from the Developed Intensity —Duration —Frequency Curve for the Fort Collins area Conditions, Basin Ste. Area (A) = �jS3j SF = t9,Zo Acres Time of Concentration Tc = T1 + TT s I Adjusted Composite Runoff Coefficients C2 Ctoo O.tr 0,3z. - Initial Flow Time Tl = [(1.87)(1.1-CzCf )(D)t/2]/(S)t/3 C2= 1211' & Ctoo =,3Z ,Dist = 3q ft, Drop =Q,r ft, S =1�t�) % Ti = $, Ir Min & 7, �g Min - Swale Travel Time ICACsT L`6 L At-c,EY L = i zy ft, Drop = X90 ft, S = 3,z % From Fig. RO-1 V = Z fps TT = 6'Z_ sec = j min. - Additional Travel Time 'I XAVt.2 10,�tc: r9FF eapF T z i N.rrJ. - Total Time of Concentration (/N 4Ire(L 4 N* spov-r) 2yr, T c = A0, % -5- Min From the PC Rainfall Intensity Curve; 100yr, Tc = 9.y8 Min I2 = Z, Z & I100 = %. 6 Rational Equation (Q=CIA) (A) = O,Zo Acres C2 = 017-f I2 = Z. Z Q2 = (C2)(I2)(A ) = l% CPS Ctoo = 0, 3Z Itoo = 7.16 Qtoo = (Ctoo )(Itoo )(A) _ ©,.5-0 CPS Page Z' J 3 -Proposed Conditions, Basin S,C " e0o"A[�Ar-Y) Watershed Note: ��- A/,c v6 o4ea,,1 Cxof57144 fvNDT-IDAiS, 4�u'T / Axtc F- � h4ls c b ca we'x Developed Sub —Basin A Areas AL(Landscape) Ac(Conc) AR(Roofs) 4(Gravel) Ap(Pavement) At(Total) Sq Ft Sq Ft Sq Ft Sq Ft Sq Ft Sq Ft, (9063 06y 165V Z7Z1 0 /a/ G98 Adjusted Composite Runoff Coefficients Area (A) = QLS Acres C2= d, q Cloo= Q S Time of Concentration Tc = TI + TT — Initial Flow Time TI = [(1.87)(1.1—CzCf )(D)'/2]/(S)'19 C2= I{1 & C1oo = 1rJ ,Dist = 9 9 ft, Drop = 1,y7 ft, S = /,y$ % TI= '(,*a Min & 9.63 Min — Swale Travel Time C,Q"94 Wc<&' ' Cho er Df 4'2kV(5L) L = I _%9 ft, Drop = 3,-� ft, S = Z.3 % From Fig. RO-1 V = Z fps TT = 79. S sec = 1,2 min. — Additional Travel Time Tyk e- T&.tC TIFF PV of �- Total Time of Concentration 2yr, T c - 13 1 Ce Min From. the FC Rainfall Intensity Curve; 100yr, To = /!, 9 Min 12 = Z, 0 & Iloo 7 Z Rational Equation (Q=CIA) (A } _ 0, ZS Acres C2 = D• if l 12 = Z.'O Q2 = (C2)(I2)(A ) _ ©, ZO CFS Cloo = 0.51 hoo = ?,,Z Qtoo = (Cloo )(Iloo )(A ) _ . 0. 92, CFS Page 3/ 3 NOT TO DRAINAGE EXHIBIT CHERRY STREET SOUTH BASIN (ALLEY) EXISTING CONDITIONS DRAINAGE EXHIBIT CHERRY STREET SOUTH BASIN (ALLEY) PROPOSED CONDITIONS