Climate Impact Pathways through Sediment to Inform Coastal Resilience Planning
Eric Grossman, U.S. Geological Survey

Ongoing impacts of sediment aggradation in stream channels that affect flood risk and disturbance to essential habitats for salmon recovery (tidal marsh, channels, eelgrass) resulting from channelizing flow are projected to be exacerbated by climate change across the Pacific Northwest. Improved understanding of these impacts and development of metrics and models to detect change and monitor performance of land use actions are intended to help managers implement adaptive management strategies for increased resilience. An expected shift by 2080 to more precipitation as rain, especially on steep mountain slopes exposed by retreating glaciers and snowpack is projected to increase the intensity of floods and fluvial sediment loads in glacier-fed streams by 3-6 times present amounts. As sea level rise inundates coastal environments the extent and frequency that tides and storm surge retard stream flows will increase and promote stream channel sedimentation farther upstream. The current and historical loss of sedimentation on floodplains as fluvial sediment continues to be focused to the nearshore and deeper depths of Puget Sound through levees has led to ~1m of local subsidence across extensive and nationally important agricultural lands and greater flood risk and drainage challenges. New empirical data and models summarizing the extent and frequency of the joint occurrence in stream flows, tidal anomalies, suspended and bedload sediment flux and routing, vertical land movements, and coastal hydrodynamics (currents, waves, and regional sea level rise) that affect water levels and sedimentation across floodplain-estuary-nearshore environments, reveal that legacy land use effects have reduced coastal resilience and will challenge Puget Sound recovery and planning. Models linking climate change downscaling, sediment transport and coastal change help identify opportunities for coordinated investments to reduce coastal hazards while enhancing ecosystem functions that valued wildlife and people depend upon. Lessons learned from the largest estuary restoration in the Pacific Northwest at Nisqually Delta, the March 2014 SR530 (Oso) landslide, and recent avulsions on the Nooksack and Skagit rivers help to highlight information needs, metrics, and priorities for sequencing and implementing ecosystem restoration and adaptation planning across the dynamic floodplains, estuaries, and beaches of the Pacific Northwest. We will also describe research results summarizing the wave and sediment attenuation capacity of several Puget Sound tidal marsh plants, scales of nearshore roughness important to wave dissipation, and sensitivity of wave transformation models to rising sea level that are being integrated into the Puget Sound Coastal Resilience Tool (spatial.wwu.edu/coastal/resilience).