Laserfiche WebLink
Slormwater Convevance <br /> Design Criteria for Stormwater Conveyance System <br /> . All Existing and Proposed conveyance systems shall be design ro convcy <br /> �vnoff from the 25-year storm. (COESMM 6-I) <br /> . Tight-lined conveyance systems receiving runoff from relatively small and/or <br /> highly impervious drainage areas shall be sized using ilows determined using <br /> thc rational method. (COESMM 5-l) <br /> • A backwater analysis may be required for a proposed or existing pipe system <br /> if the abiliry of the pipe system to convey the peak rate of runoff from the 25- <br /> year design storm event may be affected by tailwater conditions anywhere in <br /> the pipe system. <br /> • Structures such as catch basins and manholes for proposed pipe systems, must <br /> provide a minimum of 0.5 fect of frceboard between the headwater surfacc <br /> (hydraulic grade line) and the top of the structure for t7ow from the 25-year <br /> storm. However, structures may overtop for flow resulting from a 100-year <br /> storm. <br /> Calculations for Stormwater Conveyance System <br /> • Rational method and the local City of Everett equations for rainfall intensity <br /> were used as a basis for calcidating the 25-year design srorm flow. <br /> • Backwater analysis was run using the 25-year design storm tlow using the <br /> stormwater treatment systems by-pass overFlow elevation as tailwater control. <br /> • A summary of this information is provided in Table 1. Backup calculations <br /> and detailcd backwater analysis arc proviJed in Appendix B. <br /> 7'able 1- Conveyance System Summary <br /> System Arev (ac) Peak 25-yr Flow Minimum <br /> (cf's) Frecboard (f1) <br /> Phnsc 1 System 5.25 8.47 .63 <br /> Phasc 2 System L93 3.1 I .50 <br /> Combincd Tntal 7.18 I L58 .50 <br /> Stormwater/Detention Trealment <br /> 13outyurd Detention and Sewer Outfull <br /> . Pcr thc Dcp�u�tmcnt of Gcolo�y rcquireuicnts for indusu�ial boaryard dischargc <br /> thc undcrground dctcntion tanks havc becn analyzcd to attcnuatc a minimum <br /> of thc 2-ycar 24 hour d�sign Ilow for dischargc to thc Ciry ot'Evcrctt's <br /> tianitury sewer system. <br /> . �� sununary of�his information is provided in Table 2. Backup calculations <br /> and dctailcd I'low anulysis arc providcd in Appendix C. <br /> - � - <br />