Providing Cost-Effective Solutions to Dam Safety and Additional Water Supplies

Scoggins Dam is a 151-foot-high zoned earthfill embankment located near the Cascadia Subduction Zone, which is capable of producing a magnitude 9.2 earthquake at the site with strong ground motion lasting from 90 to 400 seconds. Both the dam’s existing embankment and its soft clay foundation are subject to large deformations during such an event. Since 2010, we’ve been working with Clean Water Services and the U.S. Bureau of Reclamation to identify cost-effective solutions for addressing dam safety deficiencies and increasing storage capacity to secure additional water supplies for the future needs of the Tualatin Basin. 

Our geologists and geotechnical engineers, in partnership with the USBR, have designed and executed three phases of geologic and geotechnical site characterization and laboratory testing studies. We’ve performed geotechnical investigations and site characterization to estimate soil properties, completed seismic stability analyses of the existing embankment and spillway structure, participated in risk workshops, completed risk analysis and evaluated risk reduction benefits. 

We’ve provided dam safety evaluations, appraisal and feasibility level design, and risk-informed decision-making services on an alternative dam configuration. In partnership with USBR and Clean Water Services, different alternatives were considered. 

The USBR worked on an alternative to raise the dam and resolve the dam safety deficiencies at the existing dam. HDR worked on a different alternative to construct a new roller-compacted concrete dam downstream of the existing dam and, with that, increase water supply storage by as much as 50,000 acre-feet. Because the USBR owns Scoggins Dam, we worked closely with them to plan, execute and fully document structural and quantitative risk analyses, and we advised Clean Water Services on all aspects of risk-informed decision making related to the appraisal and feasibility design of the reservoir expansion project. All of the issue evaluation studies, design and risk analysis work we performed fully satisfied the USBR’s strict standards for risk processes and risk-informed decision making.