607 RIVERSIDE RD 2021-01-29 - Laserfiche WebLink

• • , Ms.Sabrina Fendler,PE May 21,2018 2170746.10 a o i. Report Section 3.5 BMP T5.13 Post Construction Soil Quality and Depth must be incorporated for all landscaping areas. The report should be revised accordingly and appropriate notes added to the plans. Response: Notes have been added to Sheet C3.0 and to the report regarding BMP T5.13.All pervious areas will be prepared per the Landscape Architect's plans and recommendations,which meet or exceed the requirements of BMP T5.13. ii. Report Section 3.6 must address the need for oil control BMPs. Will the truck traffic associated with the facility any of the criteria in Volume V, Section 2.1, Step 2 of the Surface Water Management Manual (SWMM)? Response: Oil control is required for the project. The plans and report have been updated to note the provided oil-water separator. iii. Revise Section 4.6 to identify the reason that the site only requires basic treatment is that the Snohomish River is listed as a Basic Treatment Receiving Water in Appendix I-C of Volume I of the SWMM. Response: The report has been revised to note this justification. iv. Discuss what happens to the stormwater treatment unit when the Snohomish River_is at flood stage. If backflow into the treatment unit will cause resuspen'sion of pollutants or damage to the cartridges then some type of backwater prevention will be required. Response: An offsite backflow valve is provided within the project's dedicated downstream drainage. SDMH#100 on the recently constructed improvements to 8th Street will limit any reverse flow from entering the project's drainage if the water surface elevation of the river rises above the water surface elevation on the site.Therefore, it is not anticipated that any reverse flow will occur to the system or the treatment unit. A note regarding this backwater valve has been added to the report. v. Section 4-3.1 of the City's Design and Development Standards and Specifications (DCSS)requires that stormwater conveyance systems be analyzed using the rational method. Since the rational method can't be used to model detention it is reasonable to use SBUH for the truck bay detention sizing. The system upstream of the detention area must be analyzed using the rational method with the associated water level in the storage used as a starting tailwater elevation. Refer to DCSS Section 4-2.2 (1)and (3)for allowable water surface elevations and requirements for the pipe system analysis. Response: A modeling scenario that uses the Rational Method has been added to the report.This scenario is for the 25-year onsite event with the river at Mean Higher High Water elevation. Refer to updated Appendix B-3 in the report. Some onsite pipes have been upsized to limit the amount of onsite ponding.There are two CBs (#11 and#12)that will have less than 4 inches of ponding during this event and the owner is aware of this