607 RIVERSIDE RD 2020-09-18 - Laserfiche WebLink

In order for Riverside Road (N-S), Riverside Road (E-W), and the proposed Eighth Street to drain runoff up to the 100-year return event (as required by current code)to the pumped storm drain system,the tributary area for the pumps (and therefore the pond as well) is reduced. As part of this reduction, all allotments from the interior Riverside Business Park have been removed. As these individual interior lots develop, separate water quality systems will need to be constructed and discharge separately from the Riverside Road pumped storm drain system. Additionally, a portion of Riverside Road (E-W) and the overpass is redirected from the pumped storm drain system to the second cell of Stormwater Quality Pond 2. Just prior to discharge, the redirected stormwater is routed through a pretreatment unit. See Appendix C for Amendment 2, March 8, 2017, by KPFF and the Conveyance section below describing the modifications to the existing pumped storm drain system. As the direct discharge to the second cell of Stormwater Quality Pond 2 bypasses the presettling cell of the pond,that function is replaced with a Contech CDS unit. This unit has a General Use Level Designation (GULD)from the Washington State Department of Ecology(WSDOE)for pretreatment. As the second cell of Stormwater Quality Pond 2 may create a tailwater condition in the CDS unit, a downstream valve is included to facilitate vactor cleaning. See Appendix C for Amendment 2, March 8, 2017, by KPFF describing the pretreatment unit. In the finished condition,the existing Stormwater Quality Pond 2 will have a tributary area much smaller than the original design. This will result in reduced maintenance needs and a greater water quality performance. See Appendix A for the site development plans showing the proposed improvements. F. CONVEYANCE As mentioned above, the Riverside Road pumped drainage system is being modified to accommodate the 100-year return event from a reduced tributary area. Additionally, the pumped systems have been interconnected to allow them to work together to provide the required pumping capacity. Storm drain pipes have been added to connect the existing catch basins in Riverside Road (N-S). The interconnecting storm drain pipes will allow the water level in this pipe system to function as a reservoir. At events approaching the 100-year return, the water level in all of the pump stations will remain constant with all of the pumps working together. As all of the pumps have differing operating points and tributary areas, an individual pump may be undersized for its particular drainage basin. The addition of the interconnecting pipes allows the water level to rise and spill into the neighboring pump's system to utilize its additional capacity. See Appendix C for Amendment 2, March 8, 2017, by KPFF describing the flows tributary to each pump station. See Appendix C for the Stormwater Site Plan (Comprehensive Hydraulics Report),April 2000, by Reid Middletown documenting the original storm drainage pump design. With the reduced area now tributary to the Riverside Road pumped storm drain system, the pumped system has capacity to convey Eighth Street runoff. The flat nature of the existing site and utility conflicts in Riverside Road (N-S) make it infeasible to drain Eighth Street via gravity to the pumped system in Riverside Road (N-S). Storm Drain Lift Station 5 is proposed to convey Eighth Street runoff to one of the proposed interconnecting storm drain pipes on Riverside Road (N-S). Storm Drain Lift Station 5 receives the flow from Catch Basin 1 in Eighth Street.The duplex system employs two 4.91 horsepower pumps to convey the 100-year storm event(902 GPM) at a total dynamic head of 29 feet. In order to ensure a longer-lasting system,the first pump will run for a minimum of 5 minutes and the second pump will be triggered to turn on when the flows exceed the first pump's capacity. The discharge will flow through a 375-foot-long, 6-inch pressurized PVC force main into Catch Basin 28A, located within Riverside Road. Catch KPFF Consulting Engineers 8