5301 COLBY AVE 2018-01-02 MF Import

Lln-ItE.TC OEShIN STnRMS Intensity - 1're�luency - Duration curve!: are available for the city of Everett. It: is desirable to consider the effects of tmo storms - one of high intensity bhort duration and one of lew intensity long duration. A s1prm of 2O minute duration for the former and of 6 hours duration for the latter would be suitable. Thy following tabulation gives the necessary information: Storm Rainfall Total Storm Frequency Duration Intensity Prp_cipitatinn 10 year 20 min 1.3 ins/hr 0.4.i ins 10 year 6 hr 0.2 i.ns/hr 1.2 ins 25 year 20 min 1.6 ins/hr 0.53 inq 25 year 6 hr 0.25 ins/hr 1.5 ins 100 year 20 min 2.0 ins/hr 0.63 ins 100 year G hr 0.3 ins:/hr 1.0 ins PUNOPF c01:Fr1C717NT:• A full range of values of runoff coefficients is given in Oho procedures manual. Generally, the following values will be acceptable: Virgin wooded site 0.15 to 0.20 Parks and lawns 0.35 itdsidential Properties 0.45 Paved areas 0.90 OUTLET 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 cis = (0.63) (Area of Orificv.) � head where "head" is the vertical distance flour the water surface to the horizontal centerline of the orifice. DETENTION PONDING, Detention storage may 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.