Chehalis Basin Strategy
Causes of Extreme Flooding
October 11, 2016
Policy Workshop
Agenda
• Hydrology
o Precipitation
o Flow
• Climate Change Effects
• Land Use
o Channel straightening and incision
o Riparian zone clearing
o Forest practices
o Urban runoff (increased impervious surface)
2
Precipitation and Flow
3
Chehalis Basin Hydrometeorologic
Zones
• 3.5% Snow-dominated
headwaters (dark blue)
• 17.5% rain-snow
transitional (light blue)
• 79% rain-dominated
(light green)
Source: Perry et al. 2016
Source: United States Average Annual Precipitation, 19812010, published in 2012 by PRISM Climate Group, Oregon
State University
Average Annual Precipitation
• 80 inches average over the entire basin
(approximately 1% in the form of snow)
• Range from 46 to 50 inches in low-lying valley areas
near Centralia and Chehalis
• To more than 140 inches in the Willapa Hills and
200 inches at some locations in the Olympics
6
Atmospheric Rivers
• Atmospheric Rivers are almost always the cause of
major flood events (Nieman et al. 2011)
• Between 1980 and 2009 Atmospheric Rivers
accounted for
o 46 of the 48 peak flow events studied in Western
Washington
o All 12 of the flood events studied in the Satsop River
o All peak daily flows that exceeded a 5-year return
• Heavy precipitation results when moisture source
(atmospheric river) interacts with topography
7
100-year Precipitation Map
(24 hour totals)
Source: WSDOT (data analysis
by Oregon Climate Service)
8
Estimated Peak Flows at Grand
Mound Gage
Percent Chance
Exceedence
Return Interval
Flow (cfs)
0.2
0.5
1
2
4
10
20
50
500
200
100
50
25
10
5
2
100,300
85,200
74,700
64,900
55,800
44,600
36,500
25,600
9
Recent Historical Flood Events
• February 1996 – broadly distributed rainfall and
widespread flooding. Peak Flow at Grand Mound =
74,700 cfs
• December 2007 – focused in Upper Chehalis and
Olympics, Skookumchuck flow reduced by dam.
Peak Flow at Grand Mound = 79,100 cfs
• January 2009 – focused in Cascade tributaries and
lower Chehalis Basin, still had large contribution
from upper Chehalis Basin. Peak flow at Grand
Mound = 50,700 cfs
10
Recent Historical Flood Events
Relative Contributions from Upper Chehalis and
Cascade Tributaries for Top 10 Historical Floods1
Upper Chehalis Contribution
Average: 66%
Range: 58% to 85%
Cascade Tributaries Contribution
Average: 34%
Range: 15% to 42%
1Based
on USGS historical peaks and mean daily flows
11
Relative Contribution of Flooding
Gage Name
Water
Date
Year
2008
04-Dec-07
1996
09-Feb-96
1990
10-Jan-90
1987
25-Nov-86
2009
08-Jan-09
1972
21-Jan-72
1938
29-Dec-37
1991
25-Nov-90
1934
21-Dec-33
1976
05-Dec-75
1971
26-Jan-71
CHEHALIS RIVER
NEAR GRAND
MOUND, WA
(100-year peak =
75,000 cfs)
CHEHALIS RIVER
NEAR DOTY, WA
(100-year peak =
35,000 cfs)
SOUTH FORK
CHEHALIS RIVER AT
BOISTFORT, WA
(100-year peak =
15,200 cfs)
Peak flow (cfs)
Peak flow (cfs)
Peak flow (cfs)
Peak flow (cfs)
Peak flow (cfs)
79,100
74,800
68,700
51,600
50,700
49,200
48,400
48,000
45,700
44,800
40,800
52,600
28,900
27,500
17,900
20,100
22,800
20,713
9,542
12,900
13,300
10,400
10,700
13,000
9,770
3,600
11,300
8,540
3,379
10,500
8,190
10,300
8,400
8,020
8,390
6,110
6,630
11,664
6,540
20,600
17,400
9,612
6,593
3,526
SKOOKUMCHUCK
NEWAUKUM RIVER RIVER NEAR BUCODA,
NEAR CHEHALIS, WA
WA
(100-year peak =
(100-year peak =
14,400 cfs)
12,900 cfs)
12
Relative Contribution of Flooding
Water Year
2008
Date
4-Dec-07
Peak flow at Grand
Mound Gage
(cfs)
79,100
From Upper
Chehalis
2
DA = 437 mi
85%
From Cascade
Tribs
2
DA = 400 mi
15%
1996
9-Feb-96
74,800
67%
33%
1990
10-Jan-90
68,700
72%
28%
1987
25-Nov-86
51,600
69%
31%
2009
8-Jan-09
50,700
63%
37%
1972
21-Jan-72
49,200
68%
32%
1938
29-Dec-37
48,400
1991
25-Nov-90
48,000
66%
34%
1934
21-Dec-33
45,700
1976
5-Dec-75
44,800
69%
31%
1971
26-Jan-71
40,800
53%
47%
13
Climate Change and the
Chehalis River
Guillaume Mauger, Se-Yeun Lee, Christina Bandaragoda,
Yolande Serra, and Jason Won
Summary of CIG Study
• Storm dynamics (e.g., wind speeds) are not
projected to change, but ocean warming will bring
heavier precipitation
• Flooding is projected to increase
• Low flows are projected to decrease
• Models suggest drier conditions in summer
• New methods have led to improved streamflow
estimates, but there remains a large spread among
models
15
Flooding:
Uncertainty Is Larger than Differences Among Tributaries
KEY:
Projected Change in 100-year FLOOD for all Bias-Corrected Chehalis streamflow sites
2050s:
Average for all sites and all “MACA” models:
ChehalisR-atPorter
ChehalisR-nrDoty
ChehalisR-nrGrandMound
NewaukumR-nrChehalis
SatsopR-nrSatsop
SkookumchuckR-blwBldyRunCr
SkookumchuckR-nrBucoda
SkookumchuckR-nrVail
WynoocheeR-abvBlackCr
WynoocheeR-abvSaveCr
WynoocheeR-nrGrisdale
Average: All sites
Average: Key sites (*)
rawWRF, SRES A1B
VIC
DHSVM
min max min max
47
15
16
-28
45
-1
34
-11
96
28
14
7
53 128
55
49
89
48
59
25
7
-21
19
-39
9
-25
1
-27
27
-25
2
-13
45
18
69
54
67
47
57
3
87
47
36
20
59
17
33
4
66
19
35
10
bcWRF, SRES A1B
VIC
DHSVM
min max min max
47
7
23
-33
29
27
41
-15
55
5
34
-9
74
53
33
10
49 131
19
15
16
-24
20
-46
3
-30
17
-46
3
-40
11
-41
86
16
50
18
26
-4
43
-8
89
-2
25
23
51
5
29
-12
61
18
31
-9
bcMACA, RCP 4.5
VIC
DHSVM
avg min
avg min max
-1
52
65
-21
20
-29
38
-39 102
31
-5
54
-20 128
27
-14
64
-15 230
46
-32
71
-22 170
44
-4
46
-42 108
24
-4
48
-46 105
22
-10
37
-37 135
27
-24
63
-21 208
40
-7
42
94
-24
21
-9
31
-14 104
27
-13
50
-27 132
30
-16
56
-26 144
33
rawWRF, SRES A1B
VIC
DHSVM
min max min max
-------------
bcWRF, SRES A1B
VIC
DHSVM
min max min max
-------------
bcMACA, RCP 4.5
VIC
DHSVM
avg min
avg min max
6
67
68
-35
16
-4
82
-39 114
37
0
71
82
-32
24
100-year = +66%
10-year = +35%
2-year = +16%
≥+50%:
0:
≤−50%:
wetter
no change
drier
max
225
211
233
244
225
238
196
199
263
142
97
207
228
bcMACA, RCP 8.5
VIC
DHSVM
avg min
avg min max
5
76
-8 122
40
-13
-7 135 102
58
22
88
10 166
66
14
91
40 188
85
-35
26
47
-26
13
-5
65
-17 161
56
-6
58
-10 111
40
-1
69
12 131
65
-31
36
47
-15
22
-10
35
95
-18
22
-19
24
63
-6
24
-7
61
-4 115
45
-6
68
-2 117
46
max
169
368
194
184
73
224
169
233
102
118
57
172
180
max
189
227
171
bcMACA, RCP 8.5
VIC
DHSVM
avg min
avg min max
11
76
95
-21
23
16
-22
82
-39 147
36
12
79
-4 164
41
max
233
207
260
Projected change for a high greenhouse gas scenario
(RCP 8.5), for 2040 to 2099 relative to 1951 to 2005
2080s:
ChehalisR-atPorter
ChehalisR-nrDoty
ChehalisR-nrGrandMound
Low Flows:
Projected to Decrease; Smaller Range
KEY:
Projected Change in 10-year LOW FLOW for all Bias-Corrected Chehalis streamflow sites
2050s:
Average for all sites and all “MACA” models:
ChehalisR-atPorter*
ChehalisR-nrDoty*
ChehalisR-nrGrandMound*
NewaukumR-nrChehalis*
SatsopR-nrSatsop*
SkookumchuckR-blwBldyRunCr
SkookumchuckR-nrBucoda*
SkookumchuckR-nrVail
WynoocheeR-abvBlackCr*
WynoocheeR-abvSaveCr
WynoocheeR-nrGrisdale
Average: All sites
Average: Key sites (*)
rawWRF, SRES A1B
DHSVM
VIC
min max min max
-22
-16
-18
-16
-24
-24
-20
-19
-24
-13
-21
-14
-20
-11
-16
-13
-33
-16
-22
-16
-28
-12
-28
-15
-17
-5
-27
-15
-25
-20
-13
-12
-70
-66
-29
1
-64
-39
-29
-16
-73
-58
-38
-24
-36
-25
-24
-14
-30
-22
-22
-13
bcWRF, SRES A1B
DHSVM
VIC
min max min max
-20
-15
-3
2
-20
-15
-7
-2
-22
-13
-3
6
-21
-12
5
6
-25
-14
-13
-7
-28
-3
-9
4
-19
0
4
12
-22
-14
-2
-1
-56
-24
-18
4
-67
-28
-19
-13
-79
-47
-28
-4
-34
-17
-8
1
-26
-13
-5
3
bcMACA, RCP 4.5
DHSVM
VIC
avg min max
avg min
-15
-33
-9
-5
-11
-18
-37
-3
-3
-7
-14
-31
-3
-5
-16
-15
-30
-5
1
-13
-23
-39
-10
-14
-21
-17
-45
5
-4
-21
-12
-37
18
-3
-26
-26
-44
-7
-3
-6
-47
-72
-10
-19
-32
-61
-73
-29
-26
-40
-66
-81
-32
-27
-41
-29
-47
-8
-10
-21
-21
-40
-3
-7
-18
rawWRF, SRES A1B
DHSVM
VIC
min max min max
---------
bcWRF, SRES A1B
DHSVM
VIC
min max min max
---------
bcMACA, RCP 4.5
DHSVM
VIC
avg min max
avg min
-11
-19
0
-4
-13
-14
-28
9
-4
-9
10-year = -6%
2-year = -14%
≥+50%:
0:
≤−50%:
wetter
no change
drier
max
4
6
6
11
-8
20
15
-1
6
-15
-8
3
6
bcMACA, RCP 8.5
DHSVM
VIC
avg min max
avg min
-13
-27
-4
-4
-10
-17
-36
-1
-3
-7
-13
-29
-1
-4
-14
-17
-33
-3
2
-11
-26
-40
-6
-15
-24
-18
-43
12
-2
-19
-10
-34
27
2
-19
-27
-51
-2
-2
-5
-48
-73
-21
-17
-38
-65
-78
-49
-28
-41
-70
-85
-58
-31
-44
-29
-48
-10
-9
-21
-21
-39
-1
-6
-18
max
4
0
bcMACA, RCP 8.5
DHSVM
VIC
avg min max
avg min max
-11
-22
2
-5
-14
17 1
-14
-33
0
-4
-10
2
Projected change for a high greenhouse gas scenario
(RCP 8.5), for 2040 to 2069 relative to 1970 to 1999
max
2
1
4
10
-5
12
28
1
5
-9
-11
3
6
2080s:
ChehalisR-atPorter*
ChehalisR-nrDoty*
Climate Change Effects on 100-year
Maximum Daily Flow
• For worst-case conditions, 100-year flow on upper
Chehalis River (near Doty) is estimated to increase
by 84% compared to historical flows (from 22,400
cfs to 40,400 cfs)
• 100-year flow on Newaukum River is estimated to
increase by 94% (from 12,400 cfs to 23,900 cfs)
• While percent increase is slightly higher on the
Newaukum, the magnitude of flow increase is
greater on the Chehalis River
18
Land Use
19
Changes in Land Use that Affect
Hydrology and Hydraulics
•
•
•
•
Channel straightening and incision
Riparian zone clearing
Forest practices
Land use activities such as increased impervious
surfaces, loss of vegetation, and development in
the floodplain
Floodplain storage
The Chehalis watershed has lost much of its natural
flood storage
Otter Creek (during Hurricane
Irene)
Time Delay
22
Peak Flow Reductions – Newaukum
Time
Delay
Peak
Reduction
Example from North Fork Newaukum River, just downriver from the confluence of South Fork and
North Fork Newaukum Rivers, or where North Fork Road meets Jackson Highway
23
Land Use: Forest Practices
PRISM annual average
precipitation in the Chehalis Basin
• Heavily managed
forest
• Increasing
precipitation with
increasing elevation
Source: Perry et al. 2016
25
Percent tree canopy cover in the
Chehalis Basin
• Heavily managed
forests
• Clearcuts on order of
1 square kilometers
(240 acres) common
Source: Perry et al. 2016
26
Hydrologic Processes with Snow
• Greater accumulation
in the open snow
lasts longer in the open
• Longer lasting snow
increased chance for
rain-on-snow events
• Rain-on-snow events:
forest cover reduces
snowmelt contributions
Source: Perry et al. 2016
27
Hydrologic Processes without Snow
• Less evapotranspiration
and interception in the
open
greater soil
moisture in the open
• Road effects: subsurface
flow interception routes
runoff more efficiently to
stream
• Road Maintenance and
Abandonment Plans
created to mitigate this
Source: Perry et al. 2016
28
Literature Review Key Findings
• Peak Flows
o Channel forming flows increase (statistically significant)
o High flows (5-10 year return period) and extreme event
flows likely increase, but with unknown statistical
significance
o Increase lasts 20 or more years
• Low Flows
o Low flows increase, and this lasts about 5 years
Source: Perry et al. 2016
29
Land Use: Increased Impervious
Surfaces and Development
Development/Impervious Surface
• Impervious surfaces are generally confined to
developed lands and transportation corridors
• Total developed area in the Chehalis Basin is 7%
(residential homes, shopping centers, industrial
facilities)
• Total impervious area in the Chehalis Basin in 2011
was approximately 1.49% (USGS 2011)
o 2001: 1.44% (USGS 2001)
o 2006: 1.47% (USGS 2006)
31
Twin Cities and Newaukum River
Area
32
Conclusions
• In the upper Chehalis, Newaukum, and Skookumchuck
River basins:
o Land cover is dominated by forestlands and contains lowdensity rural and agricultural development in river valleys
o These areas have much less impervious surface than the
Chehalis-Centralia area
• Extreme floods on the Chehalis River, such as those
experienced in 2007 and 2009, are the result of
atmospheric rivers that deliver high rates of rainfall in
the upper Chehalis Basin above the Chehalis-Centralia
area
• However, land uses and floodplain conditions also
influence downstream flood timing and extents
33
Questions