12815 19TH AVE SE 2018-01-02 MF Import

Silver Lake Elementary Additions Schematic Design Narrative Water Sewer & Drainage October 19, 2006 A loading dock, service drive and three new staff parking spaces will be provided on the north side of Building A. The lowest elevation of the loading dock is proposed to be about three feet lower than the existing finish floor elevations of the building. Drainage requirements and noted design issues are as follows: • Discharge - Except for a short system collecting the westerly end of the service drive, all new tightline drainage systems will discharge to the northeast into and through the existing biofiltration Swale. • Flow Control — The gymnasium will be demolished leaving a new pervious area. The amount of net new impervious area will be minimal. Therefore, detention is assumed to not be required. • Water Quality Treatment — fhe loading dock, service drive and new parking spaces will be subject to water quality thresholds. 'The City of Everett has indicated they will likely require some water quality treatment mitigation. The proposed approach is to continue to use the existing biofiltration Swale for water quality treatment for the majority of this service drive area except for lower portions of the service drive as it connects to the west parking lot. This small area of service drive will be treated via existing media filter vault in the northwest parking lot. New water quality treatment systems have been assumed to not be required. • Conveyance - In order to feasibly connect to the existing biofiltration Swale system, the new tightline system on the north side of Building A will need to be constructed at minimum slope (0.5%). If roof drains and the loading dock cannot feasibly discharge to this point, other solutions will need to be explored. Once survey is available and more is known about the existing system, feasibility can be confirmed. • Hydraulics — The loading dock trench drain elevation is likely lower than hydraulic grade line of the biofiltration Swale or offsite detention pond during longer and higher intensity storm conditions. This may cause the loading dock area to be surcharged or flooded for given periods of time during the wet season. See Schematic Drainage Plan for more information. END OF SCHEMATIC DESIGN NARRATIVE MAIM 011loe 250 SWStleel Snmh 181a Aland. WA 90090 p 425.922 2014 1 / 425.827 50e9 paceenps CJq