8620 HOLLY DR 2018-01-02 MF Import

i .i � r�� r(i!)l4 -� Ci � � -1 ir"nlrlfr� /JffScl'� ,; l INIRODUCTTON AND EXISTING CGNDITIONS: � ... fhis project will construct a faciliry for CCI�Analytical Labs, Inc. at 8520 Holly Dr., near the interseciion of Holly Drive and 3rd Ave SE. The property is in a heavily developed azea, and is virtually the only azea that has not been developed:...Vegetation on the site consists of grass and brush. in general, the site has a mild slope from the northeast_ to the southwest. There is somewhat of an existing man-made (soiry Jane—not to be chau—n s`tie, person-made) ditch/depression near the center of the property. There'is also_a spoil-pile (approximately four feet high) Reaz the southeast property comer. Soils on the site are classified by the Snohomish Counry Soil Survey as Alderwood Urban Land Complex (Hydrologic Group `C'). These soils have the chazacteristics of being moderately deep with moderate permeability above the hardpan and very slow through it. Runoff is slow and the hazard of water erosion is slight. DEVELOPED CONDITIONS: The project will construct a laboratory building, asphait parking lot, frontage improvements, and storm water controls. Access to the site will be from the north side of Holly Drive. The developed site wili slope towazds the southwest comer of the property, where storm runoff from the parking lot will be collected in a detention vault. Runoff from the roof of the proposed laboratory will be collected in a downspout at the southeast corner of the structure, and ti�htlined to tte detention structure. Holly Drive slopes from the northeast to the southwest, and the access into the future laboratory will slope down to the north away from Holly Drive. Hence, runoff from the project frontage will flov; towards the site access, then into the parking azea and towards the vault inlet. Runoff south of the access will not be collected and will flow to the southwest along the gutter line, and be collected by a catch basin alons the adjacent property's - frontage. The storm water management system is comprised of two basic components: a wet vaelt and a water quality pond. Runoff from the pazking will enter the vault via an access riser with an inlet ' rR'. _ grate in the northwest corner of the vault, directly into the first of two cells. The first cell of the vault has both live ar.d dead storage. The dead storage component provides pre- }�� +, treatmendsediment removal for the six month design storm event, ar.d has dimensions of 3 ft(d) �� ' x 15 ft(w)x 40 ft(f) (1,800 ft3). 1 he total volume of runoff generated during the sir, month storm '�� under developed conditions is 1,589 ft'. As the maximum allowable percentage of the six month storm that can be retained in the wet vault is 85% (or 1,351 ft' = SS% of 1,589 ft3), additional retention (>_ 240 ft') will be provided in a water quality pond downstream of the vault structure. Above the dead storage in the vault is 2.85 ft (including 0.5 ft of freeboard) o:live storage. Influent will pass from the first cell to the second cell via a knock-out in the baffle/support wall near the south end of the vault. A flow restrictor has been designed to release detained storm ' ,: .i � . water at 50% oF the two year and ]00% of thc 10 and ]00 year storm event under pre-developed � ?� conditions. :3 �- Downstream of the vault is a water quality pond that provides an additional 380 ft3 of retention. The combined volume retained in the vault and pond is 2,180 ft3, which exceeds the required � < retention volume of 1,589 ft3. The pond has side slopes of 3H:1 V, and a total depth of twe feet. �' ' C� �:uzc��c�a�s.--`oo�as amos-n.ao: 3 �� ; � x:� � "`.^�tta�'--p..tY�s� _ �n ✓��:. ��:.,,y ""?"'S� �_�� k� ��' -_ � ��' eM1+f6ipM eu„�'°"�i+"!.w+ . `;+.r�rtn�a}epi4,/y �f'��`'�,+b}�i�'��'��„p'�s �` � . � y.2�ts'�iN4�.ry9'�fl`K$' f.aLfiM„}� �� ",yI,Y�"'"�.,.w +��,.�M^�^' . ._._.._.. ..� _ _.. -��'��'-'-" Y �� -, �.�5�n`. r+z+� ..,��.�. 3i. _�- . �f��J'�'.b . e.�...._ . . _ `