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Home My WebLink AboutDrainage Reports - 12/29/1977DRAINAGE REPORT for EVERGREEN PARK (4th FILING) FORT CCLLINS; COLORADO Prepared For Evergreen ---2 1635 Blue Spruce Dr. Fort Collins, Colorado 29 December 1977 File No. 19-2-23 Prepared by Northeast Engineering 817 E. Douglas Rd. Fort Collins, Colorado TABLE OF CONTENTS Introduction Purpose Design Criteria Procedure Conclusions Calculations Watershed Location 1 1 1 2 3 4 - 7 W Rainfall Intensity Duration Curve 9 DRAINAGE REPORT for EVERGREEN PARK (4th Filing) Introduction Evergreen Park is located North of the downtown portion of Fort Collins and East of College Avenue. The 4th Filing is located in the East J of Section 1, Township 7 North, Range 69 West of the 6th P.M. The 4th Filing drains in a south- easterly direction towards the Lake Canal. The centerline of the Lake Canal is the southeast borderline of the 4th Filing. The 4th Filing contains 33.18 acres and is comprised of SOS single family lots. The lots are typically 70' x 120' in size. Purpose This drainage report has been prepared to develop an overall drainage plan for the subdivision, to determine the need for a storm sewer system within the subdivision, to determine and size necessary retention ponds. In addition, the 100 year design storm runoff was determined to verify that high water levels would not reach foundation elevations. The report will serve as criteria for final design of the necessary drainage facilities. Design Criteria The Rational Method was used as the basic analysis tool to determine rates of runoff for various design storms throughout the subdivision. The Rational Method is based on the formula Q=CIA.Cf. The various parameters are explained as follows: I. "Q" is the rate of storm water runoff in cubic feet per second. 2. "I" is the rainfall intensity in inches per hour based on a predetermined time of concentration. Intensities for this report were taken from "Rainfall - Intensity -Duration" curves published by Fort Collins and dated November, 1975. 3. "C" is a runoff coefficient based on land use and soil characteristics. 4. "A" is the area in acres contributing runoff to the study point. 5. "Cf" is an adjustment coefficient which is applied to the Rational Formula to take into account antecedent moisture conditions for major storms. Values are -1- indicated as follows: Recurrence Cf Interval (years, to 10 1.G 25 1.1 50 1.2 100 1.25 The Rational Method is sufficiently accurate for non-complex areas up to 200 acres in size. Manning's Equation was used to determine flow capacities in streets, swales, ditches and conduits. The modified Manning's Equation was used to determine low flows within the curb and gutter of streets. The general form of the equation is Q=(0.56) (Z/n) (s)#(y)8/3. For high flows above the top of the curb the normal Manning's Equation was used. This equation is written as Q=(A) (1.486) (r)2/3 (s)z/n. The "n" values used in both equations were 0.016 for paved street surfaces, 0.035 for grassed surfaces, and 0.013 for concrete surfaces. The 2 year, 10 year, and 100 year design storms were used for the analysis. The 2 year storm was used to determine the need for storm sewers within the subdivision. The allowable inundation level used to determine the point at which the storm sewer system should begin was the top of curb. The 10 year storm was used to size facilities discharging into the retention ponds. The 100 year storm was used to determine the probable maximum inundation levels and insure no flooding of foundations and to size the retention ponds. Procedure Based on the existing ground contours and on the street layout indicated, street drainage patterns were set such that the subdivision is divided into two watersheds with two separate outfall points. Watershed No. 1 drains into the South retention pond. Watershed No. 2 drains into the east retention pond. The 50' easement along the southeast border containing the Lake Canal was not considered as a contributing area. The west I of Redwood Street along the west border was included in area 1. It is assumed that the area west of Redwood will not contribute runoff to the proposed retention area in Tract B. The two year storm peak rate of runoff was analyzed at each major intersection. Street cross -sections were then analyzed to determine the maximum allowable flow quantities based on allowable inundation depths which in turn determined whether or not storm sewers were necessary. The ten year storm peak rate of runoff was then determined at each outfall point to -2- size the facilities discharging into the retention ponds. The two retention ponds were sized based on the 100 year runoff. The 100 year storm peak rate of runoff' was determined at each outfall point to calculate the high water levels and insure that no foundations would be at or below the 100 year high water elevations. Conclusions Results of the analysis of each area will be discussed as follows% 1. Area No. 1 Area No. 1 contains 7.4 acres and the average street grade is 0.4�. The peak rate of runoff for the two year storm is 2.7 c.f.s. and the maximum allowable street capacity is 14 c.f.s. Therefore, the two year storm can be carried by the street. The peak rate of runoff at the outfall point from the ten year storm is 5.6 c.f.s. The peak rate of runoff from the 100 Year design storm is 10.0 c.f.s. The street will carry the 100 year design flow without overtopping the curb. 2. Area No. 2 Area No. 2 contains 24.7 acres and the minimum street grade is 0.30%. The peak rate of runoff from the two year storm is 10.9 c.f.s. and the allowable street capacity is 12 c.f.s. Therefore the two year storm can be carried by the street. The peak rate of runoff from the ten year storm is 21.4 c.f.s. The peak rate of runoff from the 100 year storm is 39.2 c.f.s. The maximum depth of storm runoff in the streets due to the 10C year storm is 1.0 feet. The reauired storage for Tract "A" is 2.9 arse -feet. 3. Area No. 3 Area No. 3 contains 2.7 acres and consists of the north half of the Lake Canal including the access road. No changes or improvements are proposed for this area. The detailed calculations for Evergreen Park (4th Filing) are attached to this report for review. -3- _(��Od7�e 2 Year ./J�s�sr-, Siai^.yr �vs�o� Area 74 a �s Ru l0h ' c � P{/r, c.,� o, 95' Sfree7� s laPe D. 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