Rainbow Bend: Lots of Change with Little Intervention

Year: 2022
Presenter/s: Michael Thai
Symposium Session: 2022 - 02 Big Challenges Need Adaptive Management
Topics covered: floodplain, hydraulics, lessons learned, modeling, monitoring, riparian, risk, sediment transport, stream, and wood


ABSTRACT

The unpredictable dynamism of the riverine environment makes it challenging for restoration professionals to decide which actions should be incorporated into a restoration project. In King County, design teams encounter this challenge at an enhanced level due to increased floodplain development and constraints on local river basins – staff strive to figure out how much action is needed to release sites from severely degraded conditions, while also managing risk to adjacent property and infrastructure. Over a decade of effectiveness monitoring at levee removal sites suggests restoration actions have varied from just the right amount to too little or occasionally too much action taken. Located in a reach of the Cedar River that had been locked in place since the 1930’s, the Rainbow Bend project is an example of evaluating and responding “just right” to the common problem of how to restore in a cost-effective manner. The project consisted of a levee removal, pilot channel excavation, and floodplain wood placement in 2013. These simple actions allowed natural processes to transform the site into one of the highest quality areas for salmon rearing and spawning within the basin.

Since the project’s completion, King County staff have tracked channel migration, sediment accumulation, wood recruitment, water surface elevation, edge habitat, and fish use. Through a series of Digital Elevation Models, aerial images, habitat surveys and water surface gaging, staff will demonstrate how natural processes have shaped the geomorphic and ecological site attributes since construction, within the context of the initial project actions. Staff will also discuss how what we know now about the project has shaped how we view the original design and influenced current and future restoration projects in King County. At this time, the project has far exceeded the expectations of the original design team and provides a dramatic example of how small actions, when combined with mother nature and a bit of good luck, can create big lifts in salmon habitat in developed floodplains. This presentation will also serve as an expanded, technical companion to the Rainbow Bend video that has been submitted to River Restoration Northwest.