Water Treatment Plant Seismic Risk Assessment for the Joint Water Commission, Hillsboro, Oregon

The 75 MGD Joint Water Commission (JWC) water treatment plant (WTP) serves potable water to 500,000 people southwest of Portland, Oregon. The project included a seven step process leading to a decision to seismically upgrade the WTP. The WTP is located in a seismically active area dominated by the Cascadia Earthquake Subduction Zone that can produce a magnitude 9 earthquake every 475 years. The probability of this large magnitude event coupled with liquefiable site soils that will amplify the ground motion pose a significant seismic risk to the WTP. The seven step evaluation process included: 1.) Defining the desired WTP level of service goals following the 72, 475, and 2,475-year return earthquakes in terms of WTP capacity and outage time. 2.) Quantifying the ground motion for the three levels of earthquakes, the probability of liquefaction and/or lateral spreading occurring, and the consequences is liquefaction and/or lateral spreading should occur. 3.) Evaluating the seismic performance of the WTP components and overall system when subjected to the three levels of earthquakes, in terms of structural capability. 4.) Evaluating the impact of the expected structural capability on unit processes and WTP flow capacity and outage/reduced capacity restoration time. 5.) Designing four seismic mitigation packages and developing their associated costs for later use in risk-based decision making. 6.) Evaluating the expected performance of the WTP for the three levels of earthquakes for each of the four upgrade packages in terms of flow capacity and outage/restoration time. 7.) Assessing the risk-based economic consequences of implementing the four improvement packages considering direct damage, the owner's cost resulting from outage and business interruption to the community. The analysis concluded that the lowest net cost and highest benefit-cost ratio was attained when the 72-year improvement package was implemented.