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October 15, 2014 THE CIVIL ENGINEERING GROUP <br /> ,a;,e�ti�i�,� :'�� <br /> City of Everett Public Works �:�r= 9� �{,,,•�t�*�� <br /> ATTN: Jane Zimmerman ,:� ,,,�;:„q <br /> 3200 Cedar Street <br /> Everett, WA 98201 <br /> RE: Addendum to Drainage Report - Silver Lake Townhomes <br /> Dear Jane Zimmerman, <br /> The Silver Lake Center Townhomes project is being purchased by a builder and they have <br /> proposed minor chan�es to the unit footprints that have resulted in an increase in impervious <br /> surface. As discussed, since the soils on site are suitable for small areas of infiltration, this o <br /> additional impervious area will be mitigated through the use of pervious concrete on several 4 <br /> of the unit driveways. This letter is an addendum to the approved drainage report dated ° <br /> August 15, 2014. J <br /> 3 <br /> The proposed additional impervious surface is as follows: 3 <br /> Basin A (northern portion�: 1,320 SF � <br /> Basin B (southern portion): 5,000 SF � <br /> � <br /> � <br /> � <br /> Following are the areas that we are proposing pervious concrete in front of some of the units: N <br /> � <br /> X <br /> Basin A (northern portion): 2,467 SF (units 78-84) �` <br /> Basin B (southern portion): 6,110 SF (units 34-39, 51-52, and 59-73) �, <br /> � <br /> � <br /> Per the geotech report, the long term infiltration design rate is 2 InchesJhour. This will more o <br /> than adequately infiltrate all runoff from the pervious driveways. We also choose driveways ,� <br /> that are on the high side of the site as well as on the south side of the buildings to the � <br /> greatest extent possible per the Geotech recommendations. a <br /> This approach will result in the vault basin areas remainin� the same in the developed � <br /> condition with the pervious pavement areas behaving as pervious areas. Therefore no � <br /> revisions to the modeling of the vautts will be required. � <br /> Let us know if you have any questions or if you need any additional information. " <br /> > <br /> � <br /> � <br /> Sincerely, o <br /> ,_�. �, 3 <br /> 4�.,E.��°�-C+c,�`�•2 0 <br /> �.�' GY WASl�� �'.f� � <br /> Mark Villwock, PE �`� ' � � <br /> Vice President y '�` �� v <br /> � <br /> i`': L <br /> O <br /> 'e qp`c» wQ' o <br /> �P <br /> � �FO/S'TE�� �� ' w <br /> �SS��NALg�G � Z <br /> O11� N <br /> , " <br /> Engineering r� I3UV� the Standard <br />