We provided project management, predesign, design, and construction phase services for this complex project to eliminate Combined Sewer Overflows into Lake Washington from the Henderson relief structure, upstream of the pump station, and from the M.L. King overflow structure.

Work included identification of design parameters; preparation of cost estimates; sampling of CSO, CSO tank numerical and physical models; wet well modeling; negotiation of a National Pollutant Discharge Elimination System discharge permit (including King County, Seattle Public Utilities, Washington State Department of Ecology, City of Seattle, and permitting agencies); and design of a 15-foot-diameter, 3,100-foot-long storage tunnel; 60-inch and 72-inch-diameter microtunnels; upgrade of the Henderson pump station, 8,000 feet of conveyance pipelines ranging in size from 20-inch to 72-inch diameter and junction and regulating structures.

Our design of the 4-million-gallon CSO storage tunnel incorporated critical design issues to provide an effective facility to meet design objectives, including:

• Odor and corrosive gas control in the tunnel. We designed ventilation that pulls air from the tunnel through a carbon scrubber.
• Cleaning of the tunnel of settled solids after an event. A flushing gate was incorporated into the head of the tunnel to send a scouring wave through to remove solids after it was drained.
• Scouring settled solids at low flows and after the flushing gate is opened. A cunette, a small channel at the invert of the tunnel, was incorporated into the design of the tunnel. The effectiveness of this design was verified in the startup and testing of the tunnel.