Flood Hazard Mapping

Flood Hazards Map
Map of flood hazards along the Skykomish River near Sultan
Purpose 
An important goal of the Community Floodplain Solutions (CFS) program is understanding and predicting future risks associated with changing river conditions. Over time, the Skykomish River moves across its floodplain. This channel migration can occur gradually as the river erodes one bank and deposits sediment along the other. It can also occur abruptly, when a channel shifts into a new or previously occupied location.

The purpose of flood hazard mapping studies funded through CFS is to model, assess and map river conditions to inform future projects. Flood hazard mapping typically looks at water depth -- how much water is in the river channel or on the floodplain. These studies also look at the river's changing shape and location, not just water depth, across a broader reach scale. 


CFS teams aim to understand and predict risk into the future by asking these questions:

  • What major factors shape how the river is moving today?
  • What will the river look like in the next 50 years?
  • How much rainfall will we receive? How much water will be coming down the river?
  • How will the river move within the floodplain? How deep and how fast will the water flow?
  • How will the land change in the process?

Benefits

 
With a better understanding of potential risks associated with rainfall and river movements, program partners can use this information to evaluate and assess potential projects and how they will affect the landscape. The goal is to provide integrated and sustainable actions that provide resilient outcomes for residents, farmers and fish interests. The main objectives are listed below.
  • Identify priority projects that offer multiple benefits, including:
    • Lower flood hazard risk to private property and public infrastructure
    • Improve habitat for endangered salmon species, and
    • Enhance agricultural viability
  • Implement a pilot program of voluntary property acquisitions and channel migration easements
  • Demonstrate an integrated approach for mapping flood hazards that can be replicated in other river basins

Location of CFS Study Area

Lower Skykomish River valley from Gold Bar to the mouth (Monroe), between river miles (RM) 0 to 23.

Studies Will Guide Actions

 

The results of these studies will inform public safety, climate change impacts, potential multi-benefit projects and options for landowners.
  • Hydraulic and Hydrologic Modelling (to be completed in early 2021) – University of Washington’s Climate Impacts Group (UW CIG) will use the best available data to model future climate change impacts on river flows. With accurate precipitation estimates and a two-dimensional model of river flow, we can better predict high water and floodplain inundation.
Hydraulic Report (PDF)
Hydrologic Report (PDF)

  • Geomorphic Assessment and Channel Migration Zone (CMZ) Studies (completed in early 2021) – By understanding where the river has been in the past, what the river is doing now and potential processes on the landscape in the future, CFS teams will have a technical foundation for understanding and predicting erosion risk into the future.
Channel Migration Zone (CMZ) Study (PDF)
Geomorphic Assessment Report (PDF)
Geomorphic Assessment - Appendix A (PDF)
Geomorphic Assessment - Appendix B (PDF)

  • Infrastructure Assessment (to be completed in early 2021) – Buildings, levees, revetments, railways, bridges, roads and other structures in the study area may face greater risk from high water, erosion, and channel dynamics. This study specifically looked at 11 different private and county-owned levee structures to assess their condition.
Infrastructure Assessment Report (PDF)
Skykomish River
Substantial gravel deposition has caused rapid bank erosion, which also added to the large wood debris (LWD) load of the Skykomish River near mile 0.5. 

​Funding

Flood hazard mapping studies in the Skykomish River basin have been funded wholly or in part by the United States Environmental Protection Agency (EPA) under assistance agreement PC-01J22301 through the Washington Department of Fish and Wildlife (WDFW). The contents of these documents do not necessarily reflect the views and policies of the EPA or the WDFW, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.