11102 EVERGREEN WAY 2018-01-01 MF Import

The following standard review comments are included for the applicant�s interest. They may or may not be applicabla in � the next review depending upon the chanqes made in response ��to the above comments. The City's Design and Construction Standards should also be consulted for more qeneral design criteria. DETENTION CALCDLATIONS lPotoond/D�tention: A completed City of Everett Drainage Calculation Form musL be submitted, showing the required detention volume and the allowable release rate. � The stage-storage relationship of the pond must also be submitted to show that the pond has the capacity to store the required wetpond volume and the live detention volume. The design wetpond volume must be reached or exceeded for the 2 year, 24 (12)-hour storm. As a first guess, the primary orifice should be sized to release approximately the existinq 2 year storm peak and the second orifice located at the elevation relating to the design wetpond volume. The 2 year, 24 (12) -hour storm should then be routed through the pond to verify that the wetpond design volume is being reached or exceeded for 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 in the City of Everett for this application is 1 inch. If this size orffice doesn't result in the required storage in the pond, then the percentage of the required wetpond volume provided as dead storage must be increased until the requirement is satisfied. The required volume of the wetponds must be determined separately from detention volume. The required wetpond volume is equal to the volume of runoff from the 2 year, 12 (24)-hour storm (1.20 (1.55) inches of precipitation) . Two methods are acceptable for determining the required wet pond volume; either multiply the area of impervious surface in the contributing 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.