11102 EVERGREEN WAY 2018-01-01 MF Import

i The control structure must ensure that detention release rates do not exceed alloa�able release rates and that the detained volume in the por:d reaches or exceeds the requirad wet pond volume for the 2 ,year storm. Orifice calculations must be submitted showing that the control structure is meeting both these requirements. The following information regarding the detention/wetpond must be submitted to the City for review and approval: 1) completed City of Everett Drainage Calculation Form � , 2) wetpond sizing calculations 3) orifice sizing calculatione 4) stage-storage relationship of the pond 5) results of routing the 2 year design storm through the pond Netuond• The required we ��ond volume is equal to the volume of runoff from the 2 year, 12 (24) -hour storm (1.20(1.55) inches of precipitation) . Two methuds are acceptable for determining the required wet pond volume; either multiply the area of impervious surface in the contributinq basin by 1.20 (1.55) inches or determine the volume of the 2 year, 12 (24) -hour hydrograph for the entire contributing area. At least 50� of this volume is required to be permanent storage in the pond. The required detention volume must be provided as live storage in addition to the dead storage. The desiqn wetpond volu�;e must be reached or exceeded for 4 the 2 year, 24 (12) -hour storm. As a firs� guess, the primary orifice should be sized to release approximately the existing two year storm peak and the second orifice located at the elevation relating to the design wetpond volume. The 2 year, 24 (12j -hour storm should then be routed through the pond to ver.ify that the wetpond design volume is being reached or exceeded fur the design storm. The maximum storage in the wetpond (dead storage + live storage) during the design storm (2 year, 12 (24) -hour storm) must be equal to or greater than the calculated required wetpond volume. The design storm must be routed through the pond to demonstrate that this requirement is being met, unless the entire required wetpond volume is being provided � as dead storage. The minimum allowable orifice size for a restrictor device associated with a wetpond is 1 inch. If a 1 inch orifice doesn't result in the required storage in the pond, then the amount of the requi.red wetpond volume provided as dead etorage must be increased until the requirement is satisfied (i.e. , if use of a one inch orifice results in 500 cubic feet less stored volume for the 2 year,