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In response to feedback, the King County Flood Control District has identified the I-90 Flood Risk Reduction Project as a long-term project in the South Fork Snoqualmie River Corridor Capital Investment Strategy, with possible implementation sometime after 2027.

About the project

The I-90 Flood Risk Reduction project was created to address potential flooding on Interstate 90 (I-90) that could occur during a 50-year or greater flood event. Depending on the size of the event, flooding of I-90 could last for 24 to 60 hours, creating significant impacts to public safety and costing the regional economy an estimated $14 to $17 million (South Fork Snoqualmie River Early Action Concept Development Technical Memorandum, King County, Inc. 2015). Flooding of I-90 could potentially damage the road requiring closure for repair, further exacerbating impacts to transportation and the regional economy.


Potential flood water on I-90 originates from the South Fork Snoqualmie River when it overtops the McConkey Levee and combines with water from Clough Creek. The King County Flood Control District (external link) authorized and funded this project in 2014.


The I-90 project includes detailed analysis of flooding patterns, geomorphology, critical areas, and feasibility of alternatives to reduce the risk from flooding. Based on the project analysis, and public and stakeholder input, the King county Flood Control District will design and implement a solution to reduce the risk of I-90 flooding.


The following alternatives have been assessed for effectiveness at reducing flooding in a cost effective manner:

  • Narrow, medium and wide Si View Levee setback on the right bank of the South Fork Snoqualmie River downstream of I-90.

  • Gravel dredging in the existing river channel between river mile 3.9 and 4.4.

  • Gravel dredging from the South Fork Snoqualmie River channel, between river mile 3.9 and 4.4.

Project objectives

  • Reduce flood risks to I-90 while reducing long term maintenance and repair costs of the levee system and improving habitat conditions.
  • Reduce flood hazards that currently exist in the Clough Creek neighborhood and area south and west of I-90.
  • Decrease existing impacts from the South Fork levee system on fish habitat, the riparian area, and water quality.
  • Make room for sediment being deposited within the project area.
  • Mitigate upstream and downstream impacts associated with modifications to the levees.
  • Address structural problems with the levees in the project areas that are identified in the South Fork Snoqualmie River Levee Characterization Report.
  • Identify a cost-effective solution, considering both implementation and long term costs.

Project Chronology

July 2013  I-90 flooding risk identified as part of South Fork Snoqualmie Corridor Plan 
Nov. 2013 Project added to King County Flood Control District 6-year Capital Investment Plan
July 2014 - May 2015   Concept development phase - Screened 16 ways to reduce I-90 flooding risk
July 2015   Met with community, cities and tribes to share findings and get feedback 
July 2015 - Aug. 2016  Alternatives analysis
Aug. 2015  Added Dredge/Gravel Management 
Nov. - Dec. 2016         Meeting with community, cities and tribes to share results and get feedback 


For information about the I-90 Flood Risk Reduction Project, please contact Mark Ruebel, Supervising Engineer, King County River and Floodplain Management Section.