Presenter/s: Curtis Loeb
Symposium Session: 2022 - 10 Complex, High-Risk, Climate-Resilient Floodplain Restoration
Topics covered: climate change, fish-salmon, floodplain, groundwater, risk and resilience, temperature, and wood
ABSTRACT
The Lower Columbia Estuary Partnership, Port of Camas-Washougal, U.S. Fish and Wildlife Service, Bonneville Power Administration, and others have partnered to reconnect 965 acres of historic Columbia River floodplain and reduce flood risks at the Steigerwald National Wildlife Refuge located southeast of Washougal in Clark County, Washington. Restoration and reconnection of the floodplain presented numerous technical challenges, including ensuring no adverse impacts to WA State Route 14 and designing a levee that meets strict regulatory, flood risk reduction, and habitat objectives. Reconnection was particularly challenging given anticipated changes in climate (higher winter flows and higher summer stream temperatures) and future increases in watershed development. Three distinct measures were developed to improve resiliency to climate risk and uncertainty. (1) A vegetated wind-wave overbuild berm was designed in lieu of riprap to protect the levee from extreme winds and associated wave erosion that is anticipated to increase in the future. The berm also accommodates transitioning wetland and riparian habitats up the topographic slope due to changing river stages. (2) When analyzing site hydrology and hydraulics, the design also incorporated peak flows scaled to account for anticipated future development and higher intensity winter storms, as predicted by climate models. Consequently, site infrastructure including the west setback levee/floodwall were designed for Gibbons Creek discharges that are 20% larger than current peaks. (3) Finally, the design targeted full floodplain connectivity (Stage Zero condition) in restoring Gibbons Creekâs 80-acre alluvial fan while also ensuring functionality of instream habitat at the base of the alluvial fan. Maximizing hyporheic exchange to cool water temperatures along with wood structures for initial floodplain roughness and cover habitat for juvenile salmonids in turn maximizes thermal refuge for salmonids throughout the warm summer months. These three climate resiliency design measures were based on several simple yet often underestimated concepts including space, scale, imprecision, and redundancy, and the focus of the presentation will be on how these ideas can be applied in constrained environments. This presentation will include both broad concepts and specific recommendations that are useful for landowners, planners, agencies, as well as restoration scientists and engineers.