Presenter/s: Aaron Kopp
Symposium Session: 2023 - 05 Restoration as Risk Reduction: Climate Resilience
Topics covered: climate change, fish-salmon, flood, floodplain, hydraulics, modeling, and risk and resilience
A flood hazard risk study was conducted on the Lower Skykomish River. The study provided not only the assessment of flood hazards but also the integration of fish habitat improvements with flood risk reduction. The intent is to offer a new approach to river management and floodplain land uses that is more sustainable, both economically and ecologically, than traditional river training. Our work will identify the locations where project actions would be the most effective, and least costly approach to flood risk reduction, climate change resilience, and restoration of ecosystems in a 50-year time frame.
The unique aspect of this study is that we’ve taken the projected climate change increases from the hydrologic analysis and promulgated those through the detailed hydraulic analysis and then used that to inform geomorphic and risk assessments, all on a very broad scale of the Lower Skykomish River (river miles 23 to 0). As a second step, we are currently conducting a vulnerability assessment and cost/benefit analysis of three basic alternatives (status quo O&M, levee breach, levee setback or similar).
The study provided a comprehensive summary of the recent projections on the hydrology and geomorphology of the Lower Skykomish River. Predictions emphasize future increases in peak flows, particularly in the 2- to 10-year frequency range (Mauger et al. 2018). The magnitude of these discharges is sufficiently large, and their recurrences sufficiently frequent, that they typically have the greatest influence on sediment transport and channel geomorphology. Their increase implies a future with more dynamic channel processes, with greater rates of bank erosion, wood recruitment, and sediment erosion and deposition. These impending changes, however, make quantitative prediction of long-term rates of channel change infeasible because our understanding of these rates is based solely on the historical record. A future that diverges from that record will be novel, and without precedent. It will present an even greater challenge to management of flood hazards in the valley, because larger flows will occur more frequently, but it also implies that those restoration actions specifically implemented to enhance channel processes are likely to become even more effective and beneficial over time.
The results showed that in the Pacific Northwest, climate change is expected to impact flooding via three primary mechanisms: sea level rise, more intense heavy rains, and reduced snowpack. Peak flows are expected to increase by 10% by the 2050s. The hydraulic analysis showed that this will correspond to an approximate 1-foot increase in Lower Skykomish in 10-year flood depth by 2050s and 1.4 feet in 2080s. Specific leveed areas, particularly adjacent to Haskel and Riley sloughs near Monroe, WA will experience more significant overtopping and greater inundation with projected climate change.