3301 SEAWAY BLVD WHOLE SITE 2025-10-30

Stormwater Reviewer, Jane Zimmerman, Comments: Bertch Capital resubmittals: Civil Plans dated 2/9/2016 (17?) and Stormwater Site Plan (Drainage Report) dated 2/15/2017 Calculations/Drainage Report (Stormwater Site Plan): 1. The east basin has been significantly reduced in size due to the resizing of the pond area. Therefore, the area of the east basin must be recalculated and the currently proposed pond area used for the purposes of WWHM modeling. 2. The submitted WWHM report has an error on page 13 — at the top of the report, it states "The Facility PASSED". However, the results table and the note at the end of the results table both indicate that the pond has failed to meet the duration standard. In addition, the pond analysis screen in WWHM2012 shows that the pond fails to meet the flow duration standard of Minimum Requirement #7. 3. The source of the "Detention Pond Volume Summary" in the report must be clearly labeled — how was this table created? From what information? 4. The trapezoidal pond approximation used in WWHM2012 to model the detention pond may not result in more area/volume at any storage elevation within the pond's WWHM2012- generated stage -storage relationship than the actual pond's area/volume at any active storage elevation. 5. Once the pond design is finalized, the flow splitting calculations (orifice design) for each control structure must be updated (and the correct information shown on the details of the control structures). 6. The downstream (tailwater) elevation for pipe 1 in the backwater analysis must be the maximum design water elevation in the MWS unit of 425.47'. Although a tailwater elevation of 425.37' is called -out at the beginning of the computer results, the headwater elevation in CB1 is calculated from an elevation of 423.00'. 7. The amount of precipitation becoming groundwater for the developed portion of the site, in both the pre- and post -development scenarios, must be determined in order to identify an allowable leakage rate (ALR) for the liner system. This calculation can be done in WWHM2012 by partitioning the groundwater component of the site's pre- and post - developed hydrological response to a separate point of compliance and running the same type of analysis as is performed for surface runoff and interflow. MWS-Linear Modular Wetland: 1. Ecology's Conditions of Use from the General Use Level Decision (GULD) document very clearly states that "Each site plan must undergo Modular Wetland Systems, Inc. review and approval before site installation. This ensures that site grading and slope appropriate for use of a MWS- Linear Modular Wetland Stormwater Treatment unit [emphasis added]." A letter must be provided to the City from the manufacturer indicating that they have reviewed and approved the site plan before the City can legally approve the use of this device to meet Minimum Requirement #6 for runoff treatment. ,Page-2�f-�