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. �, � . Erich O. Tietze & qssac. 2067750236 P. 02 <br /> 1 <br /> -� �� �t�0�� fi� ��_ "�v�, ��- � <br /> MEMO Bramer. Chirt & o� Imo. <br /> ,, <br /> i <br /> Date: 2/8/93 ; <br /> To: Erich O, Tietze and Associates, Don Abbott � <br /> , <br /> Eram: RB Aldrich , <br /> � <br /> Proj. Number, 23405-01 ' <br /> { <br /> c Steve Wagner, Jane Zimmerman, Gordon W{chter, Dorothy� <br /> VanDevanter ; <br /> , <br /> � <br /> , <br /> � <br /> ; . <br /> Thank you for the opportunity to review the proposed 42 unit apartment complex n Silver <br /> L,ake. I have several comments that pertain to the project, par2icularly �yith r pect to <br /> erosion control and biofiltration facilities. All comments induded are intended t address <br /> the concerns of the Silver Lake Action Committee (SLAC). � <br /> $ig�JkraHo mFon ta {' <br /> I <br /> I. 8ecause phosphorus is consldered a limiting nutrient in Silver Lake, biof�ItzaH efforts <br /> should be designed to meet the goals of phosphorus reduction. Tt is unlikely that the <br /> bioHltration facilities shown on the proposed plan rrill meet these ;goals without <br /> extensive maintenance. For euample, the small size of the open water cofipon nt of the <br /> detention system will create some problems. Hecause the pond ia small, an there is <br /> little shading, elevated temperatures possible anoxic condidons will occ}lr in e pond <br /> during warmer months. These conditions will lead to release of phos�ho 'vnto the ;� <br /> w:,ter wlumn and impaets to the water quality of Silver Lake. ' <br /> , 2, With regard to the out#aU pipe hom the detention system on the NW co�ner o the site, <br /> ,� it is ]ikely that velocities hom the pond during high flow conditiqns (i ., those <br /> exceeding the design detention volumes) wi.11 scour the system and remove v getatton. <br /> This wiA create a condition where the biofiltration system will not funcHon as esigmed, <br /> nor wlll replacement of lost soit and vegetation be effecHve during ivinte montlu. <br /> •• Some method of inetering flows should be considered � <br /> 3. The biofiltraHon swale located on the northernmost portion of the site may no fuziction <br /> i J well as a biofilter. Because the swale system is located in what am�un#s to five foot <br /> � hole, and will be shaded by buildings and automobiles, vegetation 'grow robust <br /> enough to enhance biofiltration and withsWnd inundation for long per�o will not <br /> occur. Ironically, the typical response to poor growth of grass is to fer�ilize, practicv. <br /> which will further contribute available phosphorus to the lake. � � <br /> � It appears that swale desigri parameters are the minimum required by the;Cxty f fiveretc. <br /> Given the sensitivity of the site, and Silver i.ake to the input of phdspho , more <br /> aggressive biofiltratioz� measures should be considered for the treatment of st rmwater <br /> IeavIng the property. Typically, on a site as tlghtly designed as this on� is, b' filhaHon <br /> ' � facllities, particularly swales, tend to shrink or disappear altogether d ring the <br /> � construction phase. Some method of preventing this occurr�nce should b devised. <br /> , � <br /> , <br /> i <br /> { sca� • xs��rr�c e�,� • Seattle WA 9d101-1027 • czas�aaa-a.400 • �'taos) �z <br /> i ! <br /> I <br />