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Effects of Climate Change on Natural and Regulated Flood Risks in the Skagit River Basin and Prospects for Adaptation. Alan F. Hamlet Se- Yeun Lee . Dept. of Civil and Env . Engineering, UW Skagit Climate Science Consortium (SC 2 ) Climate Impacts Group, UW.

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Effects of Climate Change on Natural and Regulated Flood Risks in the Skagit River Basin and Prospects for Adaptation

Alan F. Hamlet

Se-Yeun Lee

Dept. of Civil and Env. Engineering, UW

Skagit Climate Science Consortium (SC2)

Climate Impacts Group, UW

study area the skagit river basin
Study Area : The Skagit River Basin
  • Ross dam on the Upper Skagit River and Upper Baker dam on the Baker River provide storage for flood control.
  • Inflows to Ross and Upper Baker dams are relatively small portion (less than 40 %) of the total flows in the lower Skagit River.

21st Century Climate Impacts for the Pacific Northwest Region

Mote, P.W. and E. P. Salathe Jr., 2010: Future climate in the Pacific Northwest, Climatic Change, DOI: 10.1007/s10584-010-9848-z


Seasonal Precipitation Changes for the Pacific Northwest

Mote, P.W. and E. P. Salathe Jr., 2010: Future climate in the Pacific Northwest, Climatic Change, DOI: 10.1007/s10584-010-9848-z


Research Context

  • Warmer, wetter conditions are projected to substantially increase flood risks in the Skagit River Basin under natural (or unregulated) conditions (Tohver and Hamlet, 2010; Hamlet et al. 2010).
  • In order to increase flood protection in the flood plain, Skagit County and others are considering proposed modifications of current flood control operations (Steward and Associates, 2005; Skagit County, 2008).

Are current or proposed alternative flood control operations adequate for managing the projected increases in flood risk in the Skagit River Basin?


The Skagit River Simulation Model


Upper Baker



Lower Baker

The Baker River

The Upper Skagit River


Mount Vernon

The Sauk and Cascade Rivers

: Storage

  • Evaluate Current and Proposed Flood Control Operations
  • Include Flood control, Hydropower, Instream Flow

: Run of River

flood control curves
Flood Control Curves





changes in flood risks under natural conditions
Changes in Flood Risks under Natural Conditions


  • Timing shifts of peak flows from spring to winter
  • Increases in magnitude of peak flows



Hybrid Delta Scenarios

simulated 100 year flood statistics for the skagit river near mount vernon
Simulated 100-year Flood Statistics for the Skagit River near Mount Vernon

+ 23%

+ 40%

Baseline Condition

20 %

24 %

  • Current flood control operations significantly reduce flood risk for future conditions in comparison with natural conditions
  • 100-year flood risks are reduced only 3 % for the 2040s and 1 % for the 2080s under the alternative flood control curves.
  • A warmer climate and wetter winters are projected to shift the seasonal timing of peak flows in the Skagit River from spring to fall/winter and increase the risk of flooding.
  • Current and proposed alternative flood control storage in the headwaters helps mitigate the impacts of natural floods, but impounds a relatively small portion of the total flow in the lower Skagit River Basin.
  • Increasing flood storage and/or moving evacuation timing earlier in the year have relatively little effect on regulated peak flows in our simulations.
conclusion cont d
Conclusion (Cont’d)
  • These results support the argument that climate change adaptation efforts in the Skagit will need to focus primarily on improved management of the floodplain to reduce vulnerability to increasing flood risk.
  • Hamlet, A.F., Lee, S.Y., Mantua, N.J., Salathe, E.P., Snover, A.K., Steed, R., and Tohver, I., 2010. Seattle City Light climate change analysis for the city of Seattle, Seattle City Light Department, The Climate Impacts Group, Center for Science in the Earth System, Joint Institute or the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington.
  • Skagit County, 2007. The Skagit River flood risk: Will you be prepared when disaster strikes? Flood Awareness Week, October 7-13, 2007, Washington.
  • Steward and Associates, 2004. Environmental effects of additional flood control on the Baker river. Snohomish, Washington.
  • Tohver, I.M., and Hamlet, A.F., 2010. Impacts of 21st century climate change on hydrologic extremes in the Pacific Northwest region of North America. The Columbia Basin Climate Change Scenarios Project (CBCCSP), chapter 7.