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

: � �4: � 'CO: FROM: DATE: SUBJEC'C: �vY<�C I/ OA/ffs f�� 1�(17;7:``►I�ill�i Al Locke/Gordon Witcher Jane Zimmerntan March 18, 1996 Drainage Calculation Review, Office Max, Everett Mail Way I have the following comments regarding the drainage calculations for this project: � • �.�i�ll• •i �i A wetpond is required, per City standards, to ueat stormwater runoff from this site. The wetpond design volume is determined separately from detention volume. The wetpond design volume is equal to the volume of runoff from the wetpond design storm (6 month, 24-hour storm). The following two methods are acceptable for determining the wetpond design volume: 1) multiply the area of impervious surface in the contributing basin by the precipitation associated with the des�gn stonn, or 2) determine the total volume of stormwater runoff from contributing areas (impervious and pervious) using the Santa Barbara Urban Hydrograph method or comparable method. Note: the 6 month, 24-hour total precipitation is 64% of the 2 year, 24-hour precipitation. However, the 6 month, 24-hour storm volume is less than 64% of the 2 year, 24-hour storm volume as calculated using SCS based methods (the actual percentage varies depending upon the percent of impervious cover, with a maximum of approximately 59%, using the SBUH methodology, for a completely impervious site). City standards require a minimum of 50% of the wetpond design volume to be provided as permanent dead storage and for the entire wetpond design volume to be reached during the wetpond design storm. Routing of the wetpond design storm developed hydrograph through the pond is required to verify that the wetpond design