Snoqualmie River Hydraulic and Hydrologic Study
"Snoqualmie H&H Study"
Many residents of the lower Snoqualmie Valley say they’ve seen greater flooding in the basin in recent years. In response, the King County Flood Control District (District) initiated the Snoqualmie River Hydraulic and Hydrologic (H&H) Study to investigate reported changes and concerns. By doing so, the District hopes to create a shared understanding of flooding changes that can inform future dialogue and action in the Snoqualmie Basin.
The Snoqualmie H&H Study was conducted in two phases. The first phase, the Snoqualmie River Hydraulic Study (“Phase 1 Study”), was completed in April 2016. It evaluated the downstream impacts of two large flood reduction projects that were constructed at Snoqualmie Falls between 2004 and 2012 (Army Corps 205 project and PSE Snoqualmie Falls project). According to the study, the projects increased the peak 100-year water level downstream of the falls by 0.1 feet and decreased the upstream 100-year water level in the City of Snoqualmie by 1.4 feet.
The second phase, the Snoqualmie River Hydrologic Study (“Phase 2 Study”), was a broader investigation into a variety of issues related to river gages, historical trends, basin hydrology, and recent flood events. The study looked for annual and seasonal trends in basin flood hydrology and evaluated possible causes of change, such as land development, forestry practices, sedimentation, and climate change. It also included a review of the USGS gaging program in the Snoqualmie basin and recommendations to improve the system for flood warning.
The Phase 2 Study found evidence that substantiates some of the reports that flooding has gotten worse in the Snoqualmie Basin. The following changes in flooding were detected:
- Small and moderate flood events appear to be occurring more often at several gages in the basin.
- There are increasing trends in fall and spring high flows. This corroborates residents' observations that spring and fall flooding has become more severe.
These trends are consistent with observed changes in precipitation patterns at several nearby weather stations, as well as climate change projections for the Pacific Northwest. Forestry, land development, and sedimentation were found to have a relatively minor effect on large flood events in the Snoqualmie Valley.
The analysis also revealed the complexity and variability of flood behavior in the Snoqualmie, and concluded that much of the change in flooding reported by residents may be attributed to the unique characteristics of each flood event.
Both phases were conducted by Watershed Science & Engineering on behalf of King County. Dr. Ed McCarthy conducted an independent technical review on behalf of Snoqualmie Valley residents and stakeholders to verify the integrity of the methods and to ensure the study addressed community concerns.
Lower Snoqualmie Valley near Duvall, December 9, 2015 flood.
For more information about the Snoqualmie River Hydraulic and Hydrologic Study, please contact Chase Barton, Supervising Engineer, King County River and Floodplain Management Section.