3900 BROADWAY BASE FILE 2016-01-01 MF Import

CITY OF EVERETT STORMWATER MANAGEMENT MANUAI spill-way is not feazible,the principal spillway must pass the entire peak ninaff expected from the � 100-year recumence interval storm. However,an attempt to provide a separate emergency spillway should always be made. Thc runoff calculedons sha►I be based on the site conditions duting consWction. The flow through the dewatering orifice cannot be utiliud when calculating the 100-year recurrence intcrval stotm elevation becausc of its potential to became clogged; therefore,available spillway storage must begin at the principal spillway riser crest. 2. T'he principal spillway desigoed by the pracedures contained in t6is standard will result in some reduction in the peak rate of runofE However,the riser outlet 6esign will not adequately control the pond discharge to the predevelopment discharge limitations as stated in Minimum Requirement tt7: Flow Control. However,if the basin for a permanent stocmwater deteution pond is used for a temporary sedwentatioo pond,the control swchue for the pemianent pond cen be used to maintain predevelopmcnt discharge limitations. The size of the pond,the ezpected lifc o[the construction projec;,the anNcipated downstream effects and the aoticipated wcather conditions during coushvcrion,should be considered to detertnine the need of additional discharge wntrol. a. Principal Spillway: Detemtine the roquired diameter!or the Frincipel spillway(riscr pipe). The diameter shall be the minimum necessary to pass the predcvelopcd, 10-ycar recurrence interval peak flow. Use Figure 3.5 to detemtine this diameter(h= I foot). Note: A permanen�control sfructure may be used instead oja tempnrary ruer. b. Emergency Overflow Spillway: Determine the required size and design of the emergency overflow spillway for the developcd 100-year peak ftow. c. DewateringOrifce(s): Deterniinc the requimd surface azea of the dewatering ori6ce(s)using the following equatioas: - Ao = 1.44(h)°'S per cfs of in[low Whcre: � Ao = orifice area(in square inches) h = head of water above orifice(height of riser in feet) After the ortfice surCace azea is found, canvert the required surface area of the orifice to the required diametcr D, in inchcs: D = 1.13(Ao)o.s The minimum orificc sizc shall bc I inch. The vertical,perforated tubing connectcd ro thc dewatering orifice must be at least 2 inches larger in diamcter than the ori5ce to improve(low chazacteristics. The size and number of pedorations in the tubing should be large enough so tha[the tubing does not restrict tlow. The orifice should control the flow rate. I Volume I!—Construction Sformwater Pollution Prevention 3-63