228 SW EVERETT MALL WAY OFFICE MAX 2018-01-01 MF Import

J •.' volume is reached for the wetpond design storm. The size of the prirnary orifice can be decreased as necessary, to a minimum allowable orifice size of 1 inch, to achieve enough live storage. If a 1 inch orifice dcesn't result in the reguired storage in the pond, then the percentage of the wetpond design volume provided as dead storage must te increased until the combination of dead storage and peak live storage (during the wetpond design storm) equals the wetpond design volume. The control structure must ensure that detention release rates do not exceed allowable release rates �IIcl that the detained volume in the pond reaches or exceeds the wetpond design volume for the wetpond design storm. City standards require a two-cell wetpond design, with 3 to 6 feet of dead storage in the first cell (called the open wate� cell in the City's standards) or in a wetvault. The volume of dead storage provided in the open water cell must be stated in the calculation package. A maximum of 70% of the wetpond design volume may be associated with the open water cell. The volume of dead storage provided in the wetland treatment cell must also be stated. A minimum of 30% of the required permanent storage volume, with an average depth not exceeding 1.33 feet, is required to be associated with this cell. The design depth of dead and live storage in the wetpond must be given in the calculation package. The applicant may wish to consider the use of a wetvault on this site to eliminate the need for access to the bottom of the open water cell. A maximum of 70% of the wetpond design volume may be associated with the open water cell or wetvault. A minimum volume equal to a 15 minute detention time for the peak flow from the design storni (approximately 160 cubic feet per acre of contributing impervious surface for a 6 month, 24-hour storm and 270 cubic feet per acre of contributing impervious surface for a 2 year, 24hour storm) must be associated with the open water cell or a wetvault. The required detention volume must be provided as live storage in addition to the dead storage. Live storage provided to meet wetpond design volume requirements counu towards the required detention volume. The wetpond volume provided may be calculated using the detention/wetpond surface area resulting from application of the DOE correction factor (i.e., a correction factor dces not need to be applied to the wetpond design volume calculation). Venetsted Swale: Since a wetpond is required for stormwater treatment, the vegetated swale may be eliminated.