alan f. hamlet se- yeun lee
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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. HamletSe-Yeun Lee
Dept. of Civil and Env. Engineering, UWSkagit Climate Science Consortium (SC2)Climate Impacts Group, UW
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.
Floodplain Development in the Skagit
Mote, P.W. and E. P. Salathe Jr., 2010: Future climate in the Pacific Northwest, Climatic Change, DOI: 10.1007/s10584-010-9848-z
21st Century Climate Impacts for the Pacific Northwest Region
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
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).
Research Context
6
Are current or proposed alternative flood control operations adequate for managing the projected increases in flood risk in the Skagit River Basin?
Reservoir Operations Modeling
The Skagit River Simulation Model
Mount Vernon Concrete
Upper Baker
Lower Baker
Ross
Diablo Gorge
The Sauk and Cascade Rivers
The Baker River The Upper Skagit River
: Storage
: Run of River
Evaluate Current and Proposed Flood Control Operations Include Flood control, Hydropower, Instream Flow
0
40,000
80,000
120,000
160,000
9 10 11 12 1 2 3 4 5 6 7 8
Stor
age
(cfs-
days
)
Month
Upper Baker
Current Flood Control CurveAlternative Flood Control Curve
Flood Control Curves
9
550,000
600,000
650,000
700,000
750,000
9 10 11 12 1 2 3 4 5 6 7 8
Stor
age (
cfs-d
ays)
Month
Ross
Current Flood Control CurveAlternative Flood Control Curve
60 kcfs-days
90 kcfs-days
37 kcfs-days
75 kcfs-days
Simulated and Observed Daily Peak Flowsfor the Skagit River at Concrete
0
20,000
40,000
60,000
80,000
100,000
120,000
140,000
160,000
0 0.2 0.4 0.6 0.8 1
Dai
ly P
eak
Flow
(cfs
)
Probabilityof exceedance
USGS CurFC
Effects of Climate Change
on Flood Risks
Changes in Flood Risks under Natural Conditions
0
100,000
200,000
300,000
0 90 180 270 360
Peak
Flo
w (c
fs)
Peak Flow date (starting from October 1st)
His_unregulated
2040_unregulated
0
100,000
200,000
300,000
0 90 180 270 360
Peak
Flo
w (c
fs)
Peak Flow date (starting from October 1st)
His_unregulated2080_unregulated
Timing shifts of peak flows from spring to winter
Increases in magnitude of peak flows
2040s
2080s
ECHAM5 A1B Hybrid Delta Scenarios
0
100,000
200,000
300,000
400,000
Unregulated CurFC AltFC
Q10
0 F
low
(cfs
)
2080s
100 year Flood
HistoricalHybrid delta _A1BAverage
Simulated 100-year Flood Statistics for the Skagit River near Mount Vernon
20 % 24 %Baseline Condition
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.
+ 40%
0
100,000
200,000
300,000
400,000
Unregulated CurFC AltFC
Q10
0 F
low
(cfs
)
2040s
100 year Flood
HistoricalHybrid delta _A1BAverage
+ 23%
Contribution of Ross and Upper Baker Inflows to
Mount Vernon Peak Flowsfor the Top 5 Flood Years
29,858 29,159 29,59332,162
56,98273,579
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
His 2040s 2080s
Peak
Flo
ws (
cfs)
1921/12/12
ROSS Upper Baker Other
17,85027,198
37,819
9,536
35,163
68,196
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
His 2040s 2080s
Peak
Flo
ws (
cfs)
1983/1/10
ROSS Upper Baker Other
8,860
13,47118,859
11,925
25,549
42,059
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
His 2040s 2080s
Peak
Flo
ws (
cfs)
2006/1/11
ROSS Upper Baker Other
18,96823,531
27,835
13,01222,006
38,327
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
His 2040s 2080s
Peak
Flo
ws (
cfs)
1986/11/24
ROSS Upper Baker Other
24,841 26,135 28,10434,290 32,478 40,156
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
His 2040s 2080s
Peak
Flo
ws (
cfs)
1995/11/29
ROSS Upper Baker Other
Conclusions
• 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)
• 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.
Reference• 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. http://cses.washington.edu/db/pdf/snoveretalscl709.pdf.
• 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. http://www.skagitriverhistory.com/Skagit%20County%20Docs/Flood_Control_Impacts_Report_08%2011%2004.pdf
• 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. http://www.hydro.washington.edu/2860/report/
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