1450 80TH ST SW 2018-01-02 MF Import

Table 2. Historical Existing Peak Storm water Flows Storm Interval Historical Existing Peak Flow Rate, Existing Basin cfs Basins Hyd. # Y2 2-Year 0.03 2E 1 10-Year 0.22 10E 2 100-Year 0.53 100E 3 Developed Conditions: The proposed development will consist of the construction of a warehouse and office building during phase one. Phase two will likely consist of the construction of an additional warehouse on the southern portion of the site. The detention and treatment system will be designed to accommodate the future development of phase two. Phase one will create approximately 50,254 SF of impervious area. Phase two will create approximately 34,280 SF of impervious area. The existing ecology block wall pond will be utilized as a wet pond for water quality treatment and to provide detention storage volume during storm events. The existing fence surrounding the pond will be removed and metal grating installed to cover the pond. An underground pre -cast concrete vault will be required to provide additional detention volume and to serve as a wetvault for pre-treatment of runoff. Some additional detention volume will be provided in 24" ADS pipes which will also convey runoff to the underground vault. See Attachment 3 — Proposed Conditions. A volume correction factor for Streambank Erosion Control has been applied to the detention volume. Approximately 81% of the site will be covered with impervious area which results in a correction factor of 43%. The Time of Concentration for the Developed Basin, as shown on Attachment 3, was determined as follows: Pervious Time of Concentration: 170 If sheet flow through grass, S = 0.050 80 If sheet flow across pavement S = 0.020 800 If channel flow through pipes S = 0 010 Total 14.92 min. Impervious Time of Concentration Total Imporvious Time of Concentration = 7.00 min. "The minimum impervious tinge of concentration is 5.00 minutes. See attached Watemorks calculations. 1,1.tSDG0001V7-SDG-01Woes\Drainage Anslysis77.doc 5 6