625 RIVERSIDE RD SEPA 016 - 028 - FREIGHT HANDLING FACILITY Land Use Decision Documents 2025-04-21

INTRODUCTION The Stormwater Site Plan(Comprehensive Hydraulics Report) (Hydraulics Report)for Riverside Business Park(RBP),dated April 2000,provides a general overview of stormwater management for RBP that provides guidance for developing detailed stormwater plans on the lots and roadways. The Hydraulics Report also provides design detailing for two regional water quality treatment ponds and for Riverside Road. The active marketing of lots at RBP,recent pending sale of Lots 2, 3,4, and 5, lot revision to accommodate the pending sales, and recent stormwater improvements on Lot 8 necessitate revisions to the Hydraulics Report. Concepts for drainage infrastructure must be developed to a greater level of detail to apportion available capacity for conveyance, existing pumping capacity, and water quality treatment. Additionally, a concept plan for conveying"overflow bypass"(flows in excess of current pump design flows)must be provided and will discharge directly to the river(Hydraulics Report page 21). The overflow bypass will accommodate Riverside Road and possibly 8'h Street. Lots 2 and 4 will drain directly to the River with a conditional option to direct the Lot 4 water quality storm to Pond#2 for treatment(See note 1 figure 2A). This revision assumes Lot 5 drains to Pump Stations 2 and 3,using on- site detention storage to limit the peak rate of stormwater runoff to that indicated in the pump table on Figure 15 (per concept design submitted by Lot 5 purchaser). SCOPE OF AMENDMENT 1 Amendment 1 addresses stormwater drainage from lots 2, 3,4,and 5. A separate amendment will address the"overflow bypass"for Riverside Road and the storm drainage system for 8`h Street. Riverside Road and 8' Street are not included in Amendment 1. Roadway overflow systems will be addressed in a separate amendment. The separation of the comprehensive drainage plan amendment for the lots and for the roads allows development on lots that are served exclusively by private storm drainage systems (Lots 2, 3, and 4)or that has a specific metered flow rate to an existing pump station(Lot 5 to Pump Stations#2 and#3). Additionally, a separate amendment for addressing Riverside Road overflow can be coordinated and integrated with the design of 8' Street, a separate capital improvement project. Amendment 1 is structured as a supplement to the Hydraulics Report and is added to the report as Appendix F. This amendment includes: • Revised Figure 2—Developed Conditions Basin Map (Ponds), documenting general drainage sub-basins and where they will discharge—the major revisions are that Lot 2 and Lot 3 now drain directly to the Snohomish River(River)with the water quality treatment provided on each lot. Additionally, a table is provided documenting the available stormwater treatment capacity of Pond 2,how volume has been allocated to lots and roads, and how much capacity remains. • Revised Figure 15—Developed Conditions Basin Map(Pumps), adding a table for each pump station documenting design flow capacity, the pump flow rates allocated to lots and roads, and how much pump capacity remains. • New Figure 2A—Developed Conditions Lot Drainage Planning Information,providing a table for Lots 2 through 9, 8th Street, and Riverside Road. Each table includes general information as to area, where the parcel drains and outfalls,peak flow rate (if applicable),design storm, detention requirements (if necessary), and source of water quality treatment. Riverside Business Park Stormwater Site Plan and Stormwater Quality Pond October 19, 2016 Comprehensive Hydraulic Report (3749.-- Reid iddleton Amendment 1