223 SW EVERETT MALL WAY 2016-01-01 MF Import

TO: FROM: DATE: SUBJECT: PU�L/C W0/4/ffS Wetpond/Detention: MEMORANDUM AI Locke/Gordon Witcher Jane Zimmerman July 23, 1996 Drainage Calculation Review, Everett Honda, Everett Mall Way The stage-storage relationship of the wetpond is required to show that the pond has the capacity to store the required wetpond permanent storage volume. The design depth of dead and live storage in the wetpond must be given in the calculation package. The open wa[er cell is required to have a dead storage depth of 3-6 feet, the wetland treatment cell may have a maximum average depth of 1.33 feet. The City standards require a minimum of 50% of the wetpond design volume to be provided '�� % we po d design s orm (the 6 monthh24-hour sto�o)d�deQsllgn�vol�e �et ond desien storginhe �•, i' '�. developed hydrograph through the pond_is required to_v_erify_thatihe wetpond-design�alu�ne % is reached for the wetpond design s[orm. -The size of the primary orifice can be decreased as necessary, to a minimum allowable orifice size of 1 inch, [o achieve enough live storage. If a i inch orifice doesn't result in the required storage in the pond, then the percentage of the wetpond design volume provided as dead storage must be increased until the combination of dead storage and peak live storage (during the wetpond design storm) equals the wetpond design volume. The control structure(s) must ensure that detention release rates do not exceed allowable release rates � that the detained volume in the pond reaches or exceeds the wetpond design volume for the wetpond design storm. Orifice calculations must be submitted showing that the control structure meets both these requirements. Since high flow velocities are generally not a concern in the wetpond, the storm sys[em should not be designed to bypass high tlows around the wetpond. With a bypass in place, short- intensity storms with a low volume but high peak flow will mostly bypass the pond. These types of storms wash a lot of pollutants off of paved surface and must be conveyed to the wetpond. Orifice sizine: Non-publicly maintained restrictor devices may have a minimum orifice size of 1 inch.