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��� <br />F+ � <br />N�� <br />�e o <br />R�� <br />��� <br />�� <br />�` <br />�y <br />M <br />� <br />M <br />, �N <br />d� <br />CT'r'v p- rT•'�rT^ 017-SiT�' DE�"_"�'!'T_ON REOUIR�'>!EVTS <br />The si�ing of on-site detenticns in t:1e City af £Jerett is <br />based on a 2s hour desig� sto� :..�ical o� long duration <br />ainte= stor�s ia Eve_ett. For sites larger t:an 5.o zc_es, <br />on-site detenticns sys.eys r+ill be sized by City o: Evetett <br />personnel vith t:�e use �f EPA's connuter based s:o �water <br />manage�ent aodel (Se"2�4!). A copy of the coanuter printout will <br />re provided to t'�e apolicant at his/her reques�. <br />Fo: sites s�aller than 5.0 ac�es, a series of tables shocrn <br />beloc+ allor+ the applicant to dete�ine his or her on-site <br />cetention reqairesen.. These tables �:ere develoned baszd upon <br />a series of c�:.icu�er siaulations of st�ra::at=r rvnof: f=cz a <br />hv�ot:�etical 20 ac_e drai�ace basin. This 20 ac=e d:ai�age <br />basin included ten parcels varyinc in shane and si�e f_o� 0.25 <br />ac�es to 5.0 ac�es. <br />�:o« '-o use ��e tzbles•_ <br />The first sten is to de`e�ine t`�e design stor� crit=ria �or <br />t:�e site in aues�ion. Table 1 shoe:s the desicn stor� c=ite=iz <br />�or eicht drzinace basins/arezs in £✓erett. The loca�`_ons of <br />.he zrainage basins are sho•�, in figure 1. <br />n�ter the desicn stora c�_ e=ia ase dete_�ized, `�e allowable <br />release rzte �or t`e on-site det=ntion syste� is deteri.aed by <br />using tables 1 through 4. To use tz.bles 1 t.'irouch 4, the <br />ex=sting slope and ground cover conditions o� :�e site in <br />cuestion need to be deter�ined.� <br />� The slane af t:�e site nust be dete�iae3 <br />� 1�'�' sur�ey or the site or by using . t:e <br />�� � t000granhicai aaps. The slope refe=s to <br />I � floc� rather thzn t,:ie sloee oE the channel <br />t:�e site. The existing yround c�ver <br />�'� � Woods or fores�e3; as lasms, grass or <br />�� coaeacted earth ar g_avel sur:aced; <br />surfzce. <br />� <br />f=oa a t�noy_anhical <br />City's 1986 aerial <br />the slooe of overland <br />or saale t'�a� drains <br />can be classi�ied as <br />landscaeing; zs bare <br />or as an �ner�ious <br />The 7iniaua size ori�ice z1loWed for on-site deten:ion systems <br />is 2". Assu�inc a b�rda orifice (c�ntraction coef�icieat of <br />0.51) znd a head of t::o feet, t'�e �ini�ua flow race t�rouah z <br />2" orifice is 0.13 c�s. Therefore, allowable releasz rates of <br />less than .13 c�s should not be used to calculate on-site <br />detention reauireaents (see examole sz belocr). <br />� <br />