8220 5TH AVE W 2016-01-01 MF Import

- CIT'! Oe rVE�ETT ON-SIT?' DET�NTTON REOUIRr`!EVTS � The sizing af an-site detentions in t'�e City of E7erett is based oa a 24 hour design stora typical of long du:at�on xinLer stor�s in Everett. Fa: sites larger t:�an 5.0 acres, an-site detentians systems will be sized by City of Everett personnel with the use of EP�'s coanuter based storwater management model (5'r7P43) . A copy of the comnuter printout will be provided to t'�e apolicant at his/her request. For sites smaller than 5.0 acres, a series of tables shown below a11ow the anolicant to dete�ine his or her on-sit.e detention requirement. These tables Gere develoned base:: upon a series of cot�nu�er siaulations of stora�ater runoi' frcn a hypothetical 20 acre drainage basin. This 20 acre drainage basin included ten parcels var1ing in shape and sixe �=oa 0.25 acres to 5.o ac�es. How to use the t=_b1es: The first sten is ta dete�ine t'�e design stor� Ca].t2:�2 for the site in question. T�le 1 shows t`�e design stora cri�sria for eieht drainace hasins/areas xn E4reret�. The loca't'_ons of t:e drainage basins are sho•,�rn in figur: 1. e�Pter the des�cn stor� c�teria are de_e�ir.ed, t�e alleGable release rate for the on-site detention systzm is dete-�iaed by using tables 1 .through 4. To use t�les 1 t'�rouch 4, the existing slooe and ground cever conditions o� t�e site in cuestion need ta be deter.�ined. � The sloae of the site must be dete�iae3 from a t000�-anhical survey of t'�e site or by using t':e City's 1986 aerial t000grznnical maps. The slone refers to the slone of overland flow rather than the slcoe ot the chanr.el or swale that drains the site. The existing ground cover can be classiPied as woods or fores�e3; as la�+ns, grass or landscaping; as bare comoacted ear':h or gravel surPaced; or as an �ner�ious • suriace. The �inimum size orifice allowe3 for cr.-site dete�tio:i systems is 2" . Assuming a borda orifice (cor.traction coef�icieat of 0.51) and a head of t;:o feet, the mini=u� flow rac.e t�rougn a 2" ori£ice is 0. 13 rfs. Therefore, allowable release rates of less than . 13 c�s should not be uszc to calculate on-site detention xeauire�ents (se.e �:camole ;2 below) . ;