720 80TH ST SW BASE FILE 2018-01-02 MF Import

E✓ERETT DESIGN STORMS Intensity -- Frequency - Duration curves are available for the City of Fvarett. �- It it is desirable to consider the effects of two storms - one of high intensity ..l' short duration and one of low intensity long duration. A storm of 20 minute duration for the former and of 6 hours duration for the latter would be suitable. The following tabulation gives the necessary information: r- ". :form Rainfall' Total Storm Frequency Duration intensity Precipitation 10 year 20 min 1.3 ins/hr 0.43 10 year 6 hr 0.2 ins/hr 1.2 ins 25 year 20 min 1.6 ins/hr 0.53 in, 25 year 6 hr 0.25 ins/hr 1.5 ins 100 year 20 min 2.0 ins/hr 100 year 6 1.8 63 ins hr 0.3 ins/hr 0.ins RUNOFF COEFFICIENTS ' A full range of values of runoff coefficients is givan in the procedures ' tcanual. Generally, the following values will be acceptable: Virgin :rooded site 0.15 to 0.20 Parks :,nd lawns 0.35 Resi4ential Properties 0.45 Paved areas 0.90 0UTLET CONTROL A widely used form of outlet control from a detention facility is an APWA Type II catch basin with an appropriately sized orifice positioned at outlet. Typical drawings for this type of control are attached. For practical purposes the diameter of the orifice opening (with low head) should generally be not less than 2-1/4 inches. The flow through an orifice may be determined from the following formula Discharge in cfs n (0.63) (Area of Orifice) 64.4 head where "head" is the vertical distance from the water surface to the h.rizontal centerline of the outlet. DETENTION PONDING Detention storage r.<y be provided by the following means: (1) In catch basins and underground Piping system - Adequate provision for maintenance of the underground piping should be included in the design.