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i <br /> . <br /> i <br /> I <br /> PUBL/C WORKS � ' LE � <br /> EV ETT <br /> �EC�t�� (o -�a• Zoel <br /> It�Vll'sN' MrMO1tANDUM D � � � � � � <br /> � D �} <br /> TO: Gordon Wi�chcr JUL 112001 <br /> i <br /> ....................._.............. . . . <br /> . .. .. ............_ <br /> I�IiOM: Janc"Limnurmm� CITY OF EVERETT <br /> Permit Servlces <br /> UA1'L: June 6, 2001 +� <br /> I <br /> SU[SJIs'CP: llrainage Calculations/Requiremenls, JanSport 13u'sl�iug, Mounlain <br /> View Qusiness Centre <br /> I have the following comments on thc proposed stormwater conirol facilities and <br /> calculations for the Jansport building: <br /> Thc infilu�ation system hns been designed in accordancc with the Ciry's rooflop <br /> infiltration system slanJards, which is undcrstandable. Thc manual does not make it. <br /> cicar, bul lhose slandards werc dcvclopcd fur residcntial rooftop systems serving • <br /> individual rooftops and associated residenlial impervious arcas of less ticm 5,000 squarc <br /> Jfcci. '1'hc pruposcd roollop systcni will rcccivc ihc runafl'from morc ihan 4 acrct of roof. <br /> '1'herefore, ihe system must be dcsigned to fully comply with the Ciry's stre,un bank � <br /> crosion conuol standarJs, and any ovcrllows from Ihe systcm for U�c ]0 and 100-ycar I <br /> storms may not exceed the "exisling (pre mining) Ilows from the site. In addition, thc <br /> vol:nne correction factor musl be applicd :�s describeJ in Section 1-3.2.2.2 of Uu Ciry's � <br /> stonnwater manual. � <br /> �ri,� s�o�,:�m,���i �������»�„« s��nn�<< a different infihration design than is currenUy I <br /> \ proposed. Given the location and clevation of tl�e proposed infilu�ation system rclative to <br /> � Uic adjacent strcet and sidewalk, the City will require speciGc approval of the final design <br /> by thc gcotechnical cngincer and spccilic analysis of thc cxpected hydraulic graJc line <br /> (saturatcd soil elcvation) from Ihc facility and relativc to lhr, su'cct/sidcwalk. <br /> 7'hc use of five-foot diametcr, perforated CMP as the basic infiltration stniclure does not <br /> sccm a witic idca. In Ihe Gvereu area, CMP has bccn obscrvcd lo corrode rclatively <br /> � yuickly. And perforating thc CMP wiUi onc-inch Jiamctcr holcs wouldn't help mucli! <br /> The prcvious dcsign was an open-i�ottomcil concrcle vault, which would be preferaUle <br /> flom a longe��ity st:mdpoint. Also, in the evcnt that thc faciliry begins to fail due io <br /> clogging of ihc soils, Ilic opcn-hottomcd vaull would providc acccss lo thc cloggcd I:iycr ' <br /> whcreas thc perfaraicd pipc woulJ not. Duc to ihc size of thc arca bcing trcatcd, somc i <br /> � <br /> '�2 I <br />