10521 19TH AVE SE 2022-05-23 - Laserfiche WebLink

• 0 PUBLIC WORKS 2. There is no minimum infiltration rate required for use of perforated stub -out connections. The documentation provided does not demonstrate the infeasibility of this BMP for managing roof runoff. Please update the infeasibility assessment of this BMP based on the design criteria provided in Section 3.1.3, Volume III of the SWMM. • MR 96 — Runoff Treatment: 1. Document the amount of pollution generating hard surfaces (PGHS) and all other surfaces tributary to the proposed runoff treatment BMP. See the related comment above regarding off -site tributary areas. 2. The drainage report text indicates that Silver Lake is a phosphorus sensitive water body. However, while there are ongoing concerns about nutrient loads to Silver Lake, there is no formal designation for phosphorus. The project is not required to provide treatment for phosphorus. 3. Sand filter design: The plans reference a sand filter detail on Sheet 6, however no detail is provided. A complete review of the sand filter design is not possible without a detail showing the proposed configuration of the vault. There are a number of concerns with the proposed treatment device which may be answered with the detail: The design plans call -out a linear sand filter, but the designs appear to be based on BMP T8.20 Sand Filter Vault. Use the specific BMP name and number for the proposed runoff treatment BMP in the Drainage Report and in the design plans. Ensure that all of the SWMM design requirements for that BMP are met. Note that the Linear Sand Filter requires an open top. ■ BMP T8.20 requires an elevation difference of 4 feet between the inlet and outlet. The designs need to be adjusted to meet this requirement or a different BMP that can meet the SWMM design criteria needs to be selected. BMP T8.20 notes that the BMP is not suitable where high water table is expected. High groundwater was documented as a concern for the site and a reason for deeming LID BMPs infeasible (see MR #5). Clarify why high groundwater does not make this BMP infeasible, but does make LID BMPs infeasible. Adjust designs as needed to account for high groundwater. Document whether the BMP is intended to be on-line or off-line. As shown in the design plans, the BMP is located upstream of the detention system (indicating intent for off-line). Off-line systems require a diversion structure to divert the design flow rate into the sediment chamber and bypass the remaining flow to the detention system. Update the designs to comply with the design requirements. (Page 4 of 6