Loading...
HomeMy WebLinkAboutDrainage Reports - 06/12/1985'DRAINAGE REPORT for. 1298 NORTH COLLEG AVENUE. June 12, 1985 Prepared for: LARRY MI.CKELSON Prepared by: D & W, Inc. Consulting Engineers & Land Surveyors & w INC. COLORADO REGISTERED PROFESSIONAL ENGINEERS & LAND SURVEYORS M. Worayeth, M.Sc. M. DeDecker, P.L.S. 2217 AIRWAY AVE. FORT COLLINS, COLORADO 80524 PHONE (303) 484-0533 June 12, 1985 City of Fort Collins Storm Drainage Division 300 West Laporte Avenue P.O. Box 580 Fort Collins, CO 80522 File No. 69-7N-1-54 Attn: Mr. Tom Gathman Re: Drainage Report for proposed Burger Inn, 1298 North College Ave. Dear Tom: Submitted herewith is the drainage report for 1298 North College Avenue. The hydrology and hydraulic analyses are in compliance with the City of Fort Collins, "Storm Drainage Design Criteria: date May, 1984. Based on the conclusion.of.:this_:report,,-:the:.applicant would__like,_:to request a variance from the City of.Fort Collins' requirement on site detention. If you have any questions, comments or need additional information please advise. Respectfully submitted, Mohamed S. Worayeth P.E. President MSW:hb cc: Mr Larry Mickelson Property & Improvement Surveys - Topographic Maps - Subdivisions - Accident & Construction Surveys TABLE OF CONTENTS Title Page Cover Letter Drainage Report 1. I. Estimated Runoff Quantity 1. for existing conditions. 2 year initial storm 1. 10 year initial storm 2. 100 year initial storm 2. II-. Estimated Runoff Quantity after proposed development.- 3. 2 year initial storm 3. 10 year initial storm 4. 100 year initial. storm 4. Conclusion 5. 11 DRAINAGE REPORT This drainage report is compiled from the following references. '1. Storm Drainage Criteria, City of Fort Collins, May 1984. 2. Dry Creek Major Drainage Way Planning, City of Fort Collins, April, 1980. 3. Federal Emergency Management Agency (FIMA) Maps. 4. D & W, Inc. Survey and Topo Map. 5. Reigles Design Group, Architectural Plan. The drainage report is for 1298 North College Avenue. The site exist along the east side of College Avenue, north of Conifer Street and south of Bristlecone Street. The size of the site is 0.913 acres. The site is zoned HB - Highway Business,.within the limits of Fort Collins city. I. Estimated runoff quantity "Q" in cfs under existing conditions. 1. Using Rational Method Runoff Storm Drainage criteria for 2 year initial storm, based on design storm frequencies for Highway Business. a - The existing description of zoning is classified as highway business with Runoff Coefficient equal to 0.95 (C = 0.95) b - The average slope with 1.09 foot drop within 185 feet 1ong.is 0.006 (S = 0.6%). c - Area of the proposed site is 0.913 acres = 39,775.00 square feet. d - To determine time of concentration in minutes T = 1.87 ( 1.1 - C_Cf) D1/2 c S1/3 Where T c = Time of concentration, minutes S = Slope of basin, C = Rational Method Runoff Coefficient D = Length of basin, feet Cf = Frequency adjustment factor S = 0.6% D = 185 feet 1. C = 0.95 C f = 1.0 for 2 year storm Then 1.87 1.1 - 0.95 x 1.0 )( 185)1/2 T (. 21.00 c ( 0.006 )1/3 minutes e - Determine precipitation .in inches per hour from figure - (3-1) Storm Drainage Design. I = 1.8 inches/hour f - Determine runoff quantity Q = CfCIA Where = Flow quantity, CFS A Total area of basin, acres Cf = Frequency adjustment factor C = Runoff Coefficient I Rainfall intensity, inches/hour Then Q = (1.0) x (0.95) x (1.8) x (0.913) = 1.561 CFS 2. Using Rational Method Runoff Storm Drainage criteria for 10 - year initial storm. Cf = 1.0 C = 0.95 D = 185 feet S = 0.006 Then Tc = 21 minutes From figure (3-1) I = 3.7 inches/hour Q = CfCIA = (1.0) x (0.95) x (3.7)(0.913) = 3.21 CFS 3. 100 -,Year Major Storm for existing conditions. Cf = 1.25 C = 0.95 C f C (1.25) x (0.95) = 1.19 which should not exceed 1.0, Then C f C = 1.0 T c = 14.minutes From figure (3-1) I = 6.2 inches/hour 2. Q = (1.0) x (6.2) (0.913) = 5.66 CFS II. Estimated Runoff Quantity "Q" in CFS after proposed development. 1. Determinerunoff quantity for 2 year Design storm returned period as initial storm. Under HB - Highway Business Zoning, the Rational Method Runoff.Coefficient is 0.95 (.0 = 0.95 ). If we calculate the Composite Runoff Coefficient it will be as follows: Run off Coefficient Area Square Feet Asphalt and Roof, C = 0.95 31,422.25 Lawns with slope 2%, C = 0.10 8,352.75 The Composite Runoff C N C - ( ClAl Where C Composite:.Runoff..Coe£ficient C.1 = Runoff Coefficient for specific area A A. = Area of surface with Runoff Coefficient C. 1 N = Number of different surfaces to be considered At = Total area over which C is applicable; The sum of all Ai's is equal to At C = (0.95) x (31,422.25) + (0.10) x (8,352.75)= 0.77 39,775.00 Time of Concentration in minutes for 2 year storm T = 46.18 minutes c Precipitation in inches/hour from figure (3-1) I = 1.1 inch/hour Runoff quantity for 2 year storm Q = 0.773 CFS The difference in runoff quantity for 2 year initial storm 3. before and after proposed development is Before development Q = 1.561 CFS After development Q = 0.773 CFS Difference = 0.788 CFS les runoff Since the amount of runoff is reduced after development than before, as a result of developing new landscaped area to the proposed site. 2. Determine runoff quantity for 10 year Design storm returned period as initial storm. Time of concentration T = 46.18 minutes c Precipitation in inches/hour from figure (3-1) I = 1.7 inch/hour Runoff quantity for 10 year initial storm Q = 1.2 CFS Also, the difference in runoff quantity for 10 year initial storm before and after proposed development is Before development Q = 3.21 CFS After development Q = 1.20 CFS Difference = 2.01 CFS less runoff 3. Determine runoff quantity for 100 year major Design storm return period. Time of concentration T = 19.24 minutes c Precipitation in inches/hour from figure (3-1) I = 5.3 inches/hour Runoff quantity for 100 year major.storm Q = (0.963) x (5.3) x (0.913) = 4.66 CFS Finally, the difference in runoff quantity for 100 year major storm before and after proposed development is . Before development After development Difference 4. Q = 5.66 CFS Q = 4.66 CFS = 1.00 CFS less runoff Conclusion The proposed development at 1298 North College Avenue will cause a reduction in the amount of runoff quantity by 0.788 CFS for,2 year storm 2,01 CFS.for 10 year storm 1.00 CFS for 100 year storm Since the existing condition has .no detention pond and because the proposed development will reduce the runoff that is generated by precipitation, the applicant would like to request a variance from the.City of Fort Collins' requirement for detention pond. However, the site is located within the 100 year flood plain for Dry Creek and a small part of the property on the southwest corner is in the flood way of Dry Creek. P.S. You will find enclosed a plat by D & W, Inc. which shows the existing structure, contours and existing elevations. Also, from major drainageway planning for Dry Creek,.April 1980, the following information concerning the flood elevation was gathered. 100 year flood elevation @ section 26 is 4974.20 feet. 10 year flood elevation @ section 26 is 4973.50 feet. 5.