Five years of measuring the multiple benefits of a flood protection project: the Lower White River

Year: 2023
Presenter/s: Alex Lincoln
Symposium Session: 2023 - 07 - Restoration as Risk Reduction: Flooding
Topics covered: adaptive management and monitoring, fish-salmon, flood, floodplain, geology, lessons learned, risk and resilience, sediment transport, and urban


In 2009, extensive flooding on the White River impacted homes and caused flood damage in the City of Pacific, Washington. Following this flood event, the Countyline Levee Setback project was completed in 2017; this multi-benefit project reconnected 120 acres of historical floodplain to reduce flood risk for hundreds of residential and business properties in the City of Pacific and also restore habitat for three species of ESA-listed salmonids, including White River spring Chinook. It is the largest levee setback project completed to date by the King County Flood Control District. In the five years following floodplain reconnection, ongoing effectiveness monitoring is showing the physical, biological and flood risk reduction response to project actions. Using high-density LiDAR, bathymetric survey and cross-sections, aerial imagery, stage gages, and field surveys of large wood, vegetation, low-velocity aquatic habitat, and juvenile salmonid abundance, we have documented the ongoing transformation of this reach of the lower White River. The previously single-thread, confined mainstem river channel has become an evolving multi-thread (anastomosing) system, doubling the length of the pre-project channel. Mainstem water surface elevation at high flow decreased post-implementation, indicating reduced flood risk, and patterns of sediment deposition in the reach continue to influence flow patterns as the reach adjusts. Habitat improvements have also been observed; low-velocity edge habitat critical for juvenile salmonid rearing has increased substantially, juvenile salmonids including Chinook and coho have reoccupied the formerly isolated floodplain, and over 60,000 installed native plants have become established in addition to robust natural recruitment of native vegetation. Project monitoring will continue through 2028 to assess whether project goals are achieved, report on performance standards, inform design of future capital projects, and guide potential adaptive management actions.