8620 HOLLY DR 2018-01-02 MF Import

Per the C.O.L. 5tormwater Management Manual, Section 3-3.5.1, the water quality pond will be planted with wetland vegetation at a density of one plant per 2.25 ftZ. The following table presents the plant species, number to be planted, and the elevation range over which each species is to be planted: Plant Densi : 1 lant er 2.25 ft Species No. of Elevation Plants Ran e Hardstem Bulrush 6 551.40 Cattails 6 551.40 Yellow Iris 73 551.40 - 552.40 Soft Kush 36 552.40—553.40 Small Fruited Bulrush 36 552.40— 553.40 Runoff rates and volume calculations were perfonned using SBLJH and SCS Curve Number methodoloc�ies in "StormSHED" by Engenious Systems. The proposed detention system incorporates a 41% volume safety factor (C.O.E. Stormwater Management Manual Figure 1-3.1 with 76% impervious area under developed conditions). The release rates, detained volumes, and associated stages for the two, 10, and 100 year events, as well as the flow restrictor specifications, are in Section II of this report. UPSTREAM/DOWNSTREAM CONDITIONS: The proposed development is suRounded by developed properties which prevents little, if any, storm runoff from passing onto the subject property. AZditionally, even though Holly Drive slopes down towards the proposed development, most of the upstream area is diverted south through 3rd Dr SE, and away from the project site (see Appendix). Runoff from the remaining upstream area tributary to the project &ontage is collected in an existing storm system along the north side of Holly Drive. There is a catch basin approximately 50 ft from the east property line of the subject property. This catch basin conveys storm runoff to the sout}�east and towards the project frontage via a 12" culvert. The City as-builts show this culvert as discharging to an existing ditch �ear the southwest comer of the subject property (per field visits, this outlet was unable to be located and was probably filled over by the spoil piles). A second 12" culvert, at the downstream end of the ditch, conveys storm runoff approximately 70 R to an existing catch basin on the north side of Holly Drive. From here, storm runoff is conveyed south under Holiy Drive and to a conveyance system along the west side of 3rd Ave SE. As part of the frontage improvements, the ditch in the southwest comer of the subject property will need to be filled in and landscaped over. A catch basin will be placed at approximately the same location as the inlet to the existing culvert draining to the southwest. To avoid conflicts with the existing 30" Alderwood water line and the underground GTE fiber optic lines, this catch basin will be behind the proposed sidewalk. A new 12" pipe will convey discharged storm water to the existing catch basin to the southwest. Additionally, the enisting :2" storm pipe from the :,;��J 'i�`�s�j'� J:UtEPORTS—�00305�00305-D.dce rrs:`-`%" 3 ,�,.,>;t!¢ sg+�f+Pt`` ;a:4�:�s,: ��}�� *��'". -�.,� -:,��,..��,��M,r,A��"',�,�"'w"'r�,�s"�••,�.-�� -+'�'i *�t=��" . - .. Po�f��:t� -� '� � �� -�:�� . . ". . • '.�,�. ..rt��- �F"� "' tu �—°��twk' �_' IrHu._.._ `. •• �. . �