6525 HARDESON RD BASE FILE 2016-01-01 MF Import

DRAINAGE CONCEPT Due to site topography it is feasible to detain all of the runoff :5om the 21.84 acre site in a single detention pond. Runoff from the site naturally drains towrrd the west into Merrill Creek, therefore the detention pond was designed according to the Washington State Department of Ecology Stream Protection Standards. A portion of the site (approximately 3 acres) appears to have drained toward the north. After walking north, downstream of the site, and after review of the Ciry's aerial topogaphic map (] 986), it appears drainage from this portion rejoins the Merrill - Ring Creek drainage basin approximately one-third of a mile to the north. The allowable release rate for the 2-year, 24-hour developed storm event is one-half of the peak rate from the 2-year, 24-hour pre-developed storm event. Also, the allowable release rates for tl�e developed ]0-year and 100-year storm events are the pre-developed peak rates from the 10-year and 100-year stortn events. In accordance with Department of Ecology standards, a volume correction factor (factor of safety) was applied to the detention pond. Water quality enhancement will be provided through the usc of dead storage in the detention pond. The volume of dead storage required is the amount of runoft' from the 6 month 24-hour developed conditions storm event using 64% of the 2- year precipitation. In addition to meeting the above requirements, the stortn drainage detention and water quality facility design will be oversized in order to accommodate possible future project and property modifications. Slnrm Dminnge Repnrt For Aing Exlrusion - Page J