Presenter/s: Derek Booth
Symposium Session: 2023 - 07 - Restoration as Risk Reduction: Flooding
Topics covered: adaptive management and monitoring, lessons learned, riparian, and wood
ABSTRACT
King County (Washington State) has undertaken a variety of large-river restoration projects for over a decade, with an associated commitment to monitor their performance over time and draw lessons for future project designs, here and throughout the greater Pacific Northwest. A retrospective evaluation of six such projects, from 1 to 13 years post-construction, has yielded the following generalized findings:
The geomorphic responses to a restoration project do not depend only on the specific actions taken at the site. They also express:
Geomorphic context, at the spatial scales of the valley and the entire watershed;
Historical context, such as the prior disconnection of historical floodplains due to anthropogenically induced channel incision;
Hydrologic context, particularly the size and sequencing of geomorphically “significant” flows (that may or may not align with preestablished post-project monitoring schedules);
Other stochastic conditions and events, particularly the accumulation of large wood with the potential to create or initiate channel-shifting log jams;
Other reach-scale conditions not affected by the project, such as the continued presence of fixed bridge openings or adjacent levees.
Regardless of contextual differences, channel widening and associated channel shallowing is a near-ubiquitous response to levee removal.
Edge habitat (i.e., the shallow, low-velocity habitats formed adjacent to bar, bank, backwater, and side channel units) generally increases, reflecting the release of lateral constraints and resulting greater morphologic heterogeneity of the channel.
Occupation of side channels through a newly accessible floodplain is readily achieved as a simple function of grading and invert elevations. Substantial flow capture and/or full avulsion, however, does not typically occur without associated flow diversions (e.g., using in-channel engineered log jams).
Downstream effects of typical-scale projects (100s to 1000s of feet) are generally not discernable, but onsite ecological improvements are broadly significant, measurable, and accrue from the variety of project elements: increased inundation area, invasive plant removal and native riparian plantings, imported LWD, and reduced geomorphic constraints.