SR 530 Slide Completed

YG00FSA - Sediment transport and channel change following the SR530 Landslide - Completed FY2017

Problem - USGS has responded to the SR530 landslide disaster with scientific support provided to first responders and local and regional decision-makers. In addition to short-term scientific information needs arising from this event, questions related to ongoing impacts of SR530 landslide are emerging. Predicting, assessing, measuring, and monitoring the movement of sediment through the flood-prone, ecologically sensitive reaches from the landslide to Puget Sound is paramount. In addition, assessing the long-term stability of the river channel through the landslide deposit and understanding the feedback of river migration to landslide initiation is critical to predicting sediment transport and assessing future landslide risk.

Objectives - The study has three primary objectives. First, USGS will collect the sediment and streamflow data that will allow the generation of a complete sediment budget for the reach of river network between the SR530 landslide and Puget Sound. Second, USGS will work with many partners (Snohomish County, FEMA, WSDOT, USACE, DNR) to analyze the downstream movement of sediment in the NF Stillaguamish River and assess where flood-conveyance capacity may be compromised, and where ecologists, flood-risk professionals, and resource managers may be concerned. And third, USGS will work closely with the landslide geotechnical team (Snohomish County, DNR, WSDOT, USGS) to determine the long-term stability of the river location through the landslide deposit and assess the potential for the river to migrate north or south.

Relevance and Benefits - The study will contribute to the USGS mission by increasing understanding of the fluvial geomorphology of landslide-dammed rivers, including stability of new channels through landslide deposits, sediment transport from the landslide debris, and downstream effects of that sediment on flood conveyance. The study will help Federal, State and local agencies assure the long-term (greater than 60 days) safety of life, property, and health of the ecosystem during recovery of a landslide-dammed river. The USGS will be working with many partners to monitor, assess, and conduct targeted research to better understand river response and interactions with the landslide so that policymakers and the public have the understanding they need to enhance preparedness, response and resilience.

Approach - The USGS will gage discharge and sediment concentration in the reach upstream and immediately downstream of the SR530 landslide to determine total sediment load from the landslide deposit thus providing both information on the rate of incision of the river into the landslide deposit as well as an early estimate of the total load of sediment entering the downstream river reach. Using principles of geomorphology and expertise gained in projects elsewhere, the USGS will analyze the downstream movement of sediment in the NF Stillaguamish River using existing hydraulic models combined with new bed-material and bedload data to assess sediment-transport potential. The USGS will map material facies of the landslide deposit and sample particle-size data at locations close to the current river channel to assess the caliber of sediment prone to erosion and downstream transport, and will use these data to assess the potential sediment transport of the current channel alignment or alternative channel alignments.