Following flooding of the West Point Treatment Plant (WPTP) in 2017, King County reviewed infrastructure upgrade options to prevent similar events in the future. One of the options was to construct an emergency bypass weir (EBW) that could divert up to 19.3 m3/s of wastewater from an influent control structure (ICS) to an emergency bypass channel (EBC) even when an emergency bypass gate (EBG) fails to open. Under normal operating conditions, the EBC is closed, and wastewater flows to a raw sewage pump (RSP) station. Due to limited space, the only way to implement the proposed concept was to connect a 76-m-long surge channel (SC) to the ICS and use 19 surge weirs (openings on SC wall) as the EBW. The feasibility of the proposed EBW was evaluated by three hydraulic model studies. In addition to the bypass, a conveyance system model included tunnels transporting wastewater to the WPTP and an outfall in the Puget Sound. The model indicated that an EBW with a crest elevation of 33.5 m could convey 14.25 to 13.6 m3/s during mean higher high water and king tides, respectively. The conveyance capacity of the tunnels is constrained by set points in regulator stations outside the WPTP. An increase in the EBW crest elevation by 50.0 cm would reduce the conveyance capacity by about 4.4 m3/s. A transient model simulated the propagation of pressure upsurge waves created by the loss of power to the RSP. The model predicted that using the SC to carry wastewater from the ICS to the EBW, instead of damping the pressure upsurge waves, would not have significant adverse impacts. Finally, a computational fluid dynamics model verified that the discharge capacity of the EBW would exceed the conveyance capacity of the system as predicted by the conveyance system model.
Khan et al. (Sat,) studied this question.