9731 18TH AVE W 2018-01-02 MF Import

STOIt�I\VATER SIT� PI.AN Nrn rFR��nun r nNE IUNE ?2, 2005 � Thc flow path for the time of concentration calculatian consists of the follo��ing: � Flow Type Lenath (ft) _ Slo� (ti/ft) Time min SUect Flow (grass) 300 0.03 23.8 Sliallow Flow (grass) 700 0.10 3.-} - Channcl Flow (swalel 1 S00 0.005 ? 1? , Total [imc of conccntration = 53.4 minutcs. The upstremn runoff rates are: Retum Period Peak Rate(cf.$) Volume ct 2 Ycar 15.9 3R0,900 100Ycar 51.8 1,111,700 PAGE 7 CULVERT SI"LING: Thc proposed culvcrt is a 3S"x �7" CSP arch pipc widi 2-'_'i3"slr�" corrugations and will be depressed 20 percent into the strcam bed. Appcndix A includes a spreadsheet which calculates the hr,ad�vater elevation for the 2 and 100 yr events based upon Inlet Control and Oudet Control as rcpresented in thc DOE Manual. DO�VNSTREA�1 CAPACITY Thc City of Lverett rcquested a review of the 100 yr event in the area of dic proposed development to dctennine if therc is thc dangcr of tlooding any of the proposed units. Downstream of the site, a[ the tirst developmcnt, the channel is crosscd by an access road that prov�dcd 2 24" cmp culvcrts for a strcam crossing. No spccific invert elevations for these pipes �vere providcd by die survcy. To Uc conscrvative, we will assume that the culverts are at the lowcst channcl elc��ation from die site topographic survcy or 538.5. The 100 ycar runoff fi�om the upstream basin is 52 cfs. 1�Jith hvo culverts, each cuh�ert �vould be expected to carry 50% of the flow. At 26 cfs, a 24" cmp culvert requires 4 foot of Headwatcr using the nomograph for corrugated culverts with inlet control. This thcn gives a 100 year flow eleaation of 542.5 at the culverts. Thc lowest tinished floor ele�ation (PFE) on the proposed development is Si0.5. This is 8 fcct aUovc the expectcd headwatcr from 81e downstrcam culverts. Furthcr morc, die proposed cul��crt under die access (discussed helow) when taking into consideration the taihvatcr from the hvin culverts has a heachvater of 543.8. This is 6.7 feet below the (owest FFE. It �s typical to dcsign a FFB to be 2 foot above the 100 yr ilow elevations. [n this c�se, that condition is more th�n met. 'rhese quick calculations are plus or minus a ti�ot on the outside. Futher analysis could be perfonned to nan�ow the numbers down ro the tcntli, but 1 feel it is evident that thc proposed Uuilding gradcs will not be thmatcncd thc 100 year fluw in thc wetland channel as long as the cul��crt inlets are kept fee of debris by the do�timstream property owners. DO�VNSTREAII 111TIGAT[ON No downstrcum mitigatiun is proposed bcyond providing a storm dr�inage system that mects the rcquirements of the City of Gvcrett Stom�water Management Manual. J�5wcavucsim�:-c�,co, -.a in. - L'�,c7 rcno � a�,� n,�• os-oas�oo_�..T�e�:t�ssa wa�r.om ooc OWH/srm