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Skagit River Revised Flood Insurance StudySkagit River Revised Flood Insurance StudyLevee Scenario DiscussionLevee Scenario Discussion
Ted Perkins, Hydraulic EngineerTed Perkins, Hydraulic EngineerUS Army Corps of Engineers, Seattle DistrictUS Army Corps of Engineers, Seattle District
8/9/20078/9/2007
Skagit Levee Failure Scenarios Skagit Levee Failure Scenarios
•• Challenges of Defining Levee Failures Challenges of Defining Levee Failures •• FEMA RegulationsFEMA Regulations•• Implementation of RegulationsImplementation of Regulations•• Comparison to Actual ObservationsComparison to Actual Observations
Past Skagit River Levee FailuresPast Skagit River Levee Failures
Isolated Levee FailuresIsolated Levee Failures
SystemwideSystemwide Levee FailuresLevee Failures
General FEMA Levee Failure PolicyGeneral FEMA Levee Failure Policy
http://http://www.fema.gov/library/viewRecord.do?idwww.fema.gov/library/viewRecord.do?id=2206=2206
General FEMA Levee Failure PolicyGeneral FEMA Levee Failure Policy
• If the subject levee does not meet the requirements stated in Section 65.10 of the NFIP regulations, as verified by the RPO, the Mapping Partner shall recompute the 1-percent-annual-chance flood elevations as if the levee did not exist.
(page H(page H--11)11)
One Levee SystemOne Levee System
General FEMA Levee Failure PolicyGeneral FEMA Levee Failure Policy
• The above procedures for the determination of BFEs and regulatory floodways also apply to the conditions where levees exist on both sides of the stream. In these cases, the evaluation shall include the possibility of simultaneous levee failure, failure of only the left side, and failure of only the right side, and shall consider simultaneous levee failure for both the BFE and regulatory floodway computations.The Mapping Partner shall contact the RPO for guidance on the evaluation of levee systems under these circumstances.
(page H(page H--12)12)
Levees on Both Sides of the RiverLevees on Both Sides of the River
Levees InundatedLevees Inundated
General FEMA Levee Failure PolicyGeneral FEMA Levee Failure Policy
• For levee systems where an area of land may be totally or partially surrounded by levees or where two or more flooding sources join that have levees on both sides of the stream, the Mapping Partner that is performing the analysis shall contact the RPO before proceeding with any analyses for levee failures. For these complex situations, the flood hazard in the area that would have been protected by the non-failed levee(s) must be based on selection of failure scenarios that yield the highest BFE or flood hazard.
(page H(page H--12)12)
Surrounded by Levees
Levee Scenarios Levee Scenarios
All Levees Intact All Levees Intact
All Levees Intact All Levees Intact
Right Bank Levees RemovedRight Bank Levees RemovedBurlington to Samish and Padilla BaysBurlington to Samish and Padilla Bays
Dark Blue AreaDark Blue Area
Right Bank Levees on Right Bank Levees on MainstemMainstem and and North Fork Skagit River Removed While North Fork Skagit River Removed While
All Other Levees Remain Intact All Other Levees Remain Intact
Right Bank Levees on Mainstem and North Fork Skagit River Removed While All Other Levees Remain Intact
Right Bank Levees Removed Right Bank Levees Removed FlowpathsFlowpaths
FlowpathFlowpath 1 Burlington Area (Page 60)1 Burlington Area (Page 60)
FLO
OD
PR
OFI
LES
SK
AG
IT R
IVE
R D
ELT
A, O
VE
RB
AN
K F
LOW
PA
TH
1
FE
DE
RA
L E
ME
RG
EN
CY
MA
NA
GE
ME
NT
AG
EN
CY
SKAG
IT C
OU
NTY,
WA
LEGEND500-YEAR FLOOD
100 - YEAR FLOOD
50-YEAR FLOOD
GROUND ELEVATION
CROSS SECTION LOCATION
OVERBANK FLOWPATH DISTANCE IN FEET ABOVE POINT OF BIFURCATION TO FLOW PATHS 2 AND 3
ELEV
ATIO
N (F
EET
NAVD
88)
01P0 4000 8000 12000 16000 20000 24000
30
40
50
(AN
D U
NIN
CO
RPO
RATE
D AR
EAS)
30
40
50
FL
OW
SP
LIT
TO
FL
OW
PA
TH
S 2
AN
D 3
PU
LVE
R R
OA
D
BU
RLI
NG
TO
N N
OR
TH
ER
N R
AIL
RO
AD
I-5
S A
NA
CO
RT
ES
AV
EN
UE
N G
ARDN
ER R
OAD
& E
FAI
RHAV
EN A
VE
STAR
T O
F O
VERB
ANK
FLO
WPA
TH 1
AT
RM 2
0.9
Levee Failures 1917 Levee Failures 1917
Levee Failures 1917 Levee Failures 1917
Burlington 1917 Burlington 1917
Levee Failures 1909 and 1921 Levee Failures 1909 and 1921
Levee Failures 1909Levee Failures 1909
Levee Failures 1921 Levee Failures 1921
Newspaper Documentation Newspaper Documentation Burlington Burlington
•• Concrete Herald 12/17/1921Concrete Herald 12/17/1921““The entire city of Burlington was
flooded to a depth of from three to five feet”
Concrete Flow = 240,000 cfs•• Skagit County CourierSkagit County Courier
12/2/190912/2/1909“The water broke the dykes
guarding Burlington and almost the entire town was flooded. In the residence districts the water was over eight feet deep in places.”
Concrete Flow = 260,000 cfs
FlowpathFlowpath 2 Samish Area (Page 61)2 Samish Area (Page 61)
FLO
OD
PR
OFI
LES
SK
AG
IT R
IVE
R D
ELT
A, O
VE
RB
AN
K F
LOW
PA
TH
2
FE
DE
RA
L E
ME
RG
EN
CY
MA
NA
GE
ME
NT
AG
EN
CY
SKAG
IT C
OUN
TY, W
A
LEGEND500 - YEAR FLOOD
100 - YEAR FLOOD
50 - YEAR FLOOD
GROUND ELEVATION
CROSS SECTION LOCATION
STREAM DISTANCE IN FEET FROM SAMISH BAY
ELEV
ATIO
N (F
EET
NAVD
88)
02P0 4000 8000 12000 16000 20000 24000
10
20
30
(AND
UN
INCO
RPO
RAT
ED A
REAS
)
10
20
30
SA
MIS
H B
AY
ALL
EN
WE
ST
RO
AD
SU
NS
ET
RO
AD
FIE
LD R
OA
D
Newspaper Documentation Newspaper Documentation SamishSamish
•• Mount Vernon Argus 12/15/1921Mount Vernon Argus 12/15/1921““In some of the houses in the Samish flats
water was 6 or 7 feet and the occupants were forced to move to the second story.”
Concrete Flow = 240,000 cfs
Left Bank Levees RemovedLeft Bank Levees RemovedMount Vernon to StanwoodMount Vernon to Stanwood
Brown AreaBrown Area
Left Bank Levees on Left Bank Levees on MainstemMainstem and and South Fork Skagit River Removed While South Fork Skagit River Removed While
All Other Levees Remain Intact All Other Levees Remain Intact
Left Bank Levees on Left Bank Levees on MainstemMainstem and South Fork Skagit River and South Fork Skagit River Removed While All Other Levees Remain IntactRemoved While All Other Levees Remain Intact
Left Bank Levees on Left Bank Levees on MainstemMainstem and and South Fork Skagit River Removed While South Fork Skagit River Removed While
All Other Levees Remain Intact All Other Levees Remain Intact
FlowpathFlowpath 5 Conway Area (Page 67)5 Conway Area (Page 67)
FLO
OD
PR
OFI
LES
SK
AG
IT R
IVE
R, O
VE
RB
AN
K F
LOW
PA
TH
5
FE
DE
RA
L E
ME
RG
EN
CY
MA
NA
GE
ME
NT
AG
EN
CY
SKAG
IT C
OUN
TY, W
A
LEGEND500 - YEAR FLOOD
100 - YEAR FLOOD
50 - YEAR FLOOD
GROUND ELEVATION
CROSS SECTION LOCATION
STREAM DISTANCE IN FEET FROM LEVEE AT STILLAGUAMISH RIVER
ELEV
ATIO
N (F
EET
NAVD
88)
08P26000 30000 34000 38000 42000 46000 50000
0
10
20
30
(AN
D U
NIN
CO
RPO
RAT
ED A
REA
S)
0
10
20
30
MIL
LTO
WN
RO
AD
W J
OH
NS
ON
RO
AD
SK
AG
IT C
OU
NT
Y L
INE
FIR
ISLA
ND
RO
AD
CO
NW
AY
Levee Failures 1951 Levee Failures 1951
Levee Failures 1951 Levee Failures 1951
Conway 1951 Conway 1951
Newspaper Documentation Newspaper Documentation ConwayConway
•• Mount Vernon Argus 12/15/1951Mount Vernon Argus 12/15/1951““Conway residents declared the 1951 flood
was two feet, 10 inches below the 1921 inundation in their community...”
Concrete Flow = 139,000 cfsMount Vernon Flow = 144,000 cfs
Fir IslandFir IslandRed, Pink and Light Blue Areas Red, Pink and Light Blue Areas
Fir Island Levees Removed While All Fir Island Levees Removed While All Other Levees Remain Intact Other Levees Remain Intact
Fir Island Levees Removed While All Other Levees Remain Intact
South Fork Skagit River Right Bank South Fork Skagit River Right Bank Levee Removed While All Other Levee Removed While All Other
Levees Remain IntactLevees Remain Intact
South Fork Skagit River Right Bank Levee South Fork Skagit River Right Bank Levee Removed While All Other Levees Remain IntactRemoved While All Other Levees Remain Intact
North Fork Skagit River Left Bank North Fork Skagit River Left Bank Levees Removed While All Other Levees Removed While All Other
Levees Remain Intact Levees Remain Intact
North Fork Skagit River Left Bank Levees Removed While North Fork Skagit River Left Bank Levees Removed While
All Other Levees Remain IntactAll Other Levees Remain Intact
Fir Island Fir Island FlowpathFlowpath
FlowpathFlowpath 4 Fir Island Area (Page 65)4 Fir Island Area (Page 65)
FLO
OD
PR
OFI
LES
SK
AG
IT R
IVE
R, O
VE
RB
AN
K F
LOW
PA
TH
4
FE
DE
RA
L E
ME
RG
EN
CY
MA
NA
GE
ME
NT
AG
EN
CY
SKAG
IT C
OUN
TY, W
A
LEGEND500 - YEAR FLOOD
100 - YEAR FLOOD
50 - YEAR FLOOD
GROUND ELEVATION
CROSS SECTION LOCATION
STREAM DISTANCE IN FEET ABOVE COASTAL LEVEE AT SKAGIT BAY
ELEV
ATIO
N (F
EET
NAVD
88)
06P0 4000 8000 12000 16000 20000 24000
10
20
30
(AND
UNI
NCO
RPO
RATE
D AR
EAS)
10
20
30
MA
NN
RO
AD
FIR
ISLA
ND
RO
AD
MO
OR
E R
OA
D
CO
AS
TA
L LE
VE
E A
T S
KA
GIT
BA
Y
PO
LSO
N R
OA
D
ST
AR
T O
F F
LOW
PA
TH
4F
IR IS
LAN
D A
T S
PLI
T IN
TO
TW
O F
OR
KS
Fir Island 1949 Flood Fir Island 1949 Flood
Fir Island 1990 Flood Fir Island 1990 Flood
Fir Island 1990 Flood Fir Island 1990 Flood
Newspaper Documentation Newspaper Documentation Fir IslandFir Island
•• Seattle Times 11/13/1990Seattle Times 11/13/1990““Workers have punched a hole in a dike to drain flooded Fir
Island but water from the raging North Fork of the Skagit River is filling the island faster than the water can leave…Water is reportedly as deep as 10 feet…That break sent a wall of water 10 feet high over fertile farm land…Waves were 8 to 10 feet high…He estimates he has 9 to 10 feet of water at his house.”
Concrete Flow = 149,000 cfsMount Vernon Flow = 142,000 cfs
Big Bend AreaBig Bend AreaYellow Area Yellow Area
All Left Bank Levees Removed All Left Bank Levees Removed with Right Bank Levees Intactwith Right Bank Levees Intact
All Left Bank Levees Removed with Right Bank All Left Bank Levees Removed with Right Bank Levees IntactLevees Intact
Big Bend Flow PathBig Bend Flow Path
Flow Path 6 Flow Path 6 –– Big BendBig Bend
FLO
OD
PR
OFI
LES
SK
AG
IT R
IVE
R, O
VE
RB
AN
K F
LOW
PA
TH
6
FED
ER
AL
EM
ER
GE
NC
Y M
AN
AG
EM
EN
T A
GE
NC
Y
SKAG
IT C
OU
NTY,
WA
LEGEND500 - YEAR FLOOD
100 - YEAR FLOOD
50 - YEAR FLOOD
1984 FIS 100-YEAR
GROUND ELEVATION
CROSS SECTION LOCATION
OVERLAND DISTANCE IN FEET FROM LOWER BEND TO RIVERSIDE BRIDGE
ELEV
ATIO
N (F
EET
NAVD
88)
17P0 4000 8000
20
30
40
50
(AN
D U
NIN
CO
RPO
RATE
D AR
EAS)
20
30
40
50
RIV
ER
SID
E D
RIV
E
RE
-EN
TR
Y A
T R
M 1
3.8
I-5
ST
AR
T O
F F
LO
W P
AT
H 6
1917 Levee Failures1917 Levee Failures
1917 Levee Failures1917 Levee Failures
Newspaper Documentation Newspaper Documentation Big Bend Big Bend –– North Mount VernonNorth Mount Vernon
•• Burlington Journal 1/4/1918Burlington Journal 1/4/1918““The Riverside district south of the river on the Pacific
highway to Mt. Vernon’s limits suffered…The highway was under from two to ten feet of water…”
•• Mount Vernon Herald 1/3/1918Mount Vernon Herald 1/3/1918““The Riverside section was badly hit, in some places as
much as fifteen feet of water covered the ranch property.”
Concrete Flow = 220,000 cfs
Base Flood Elevation MapBase Flood Elevation Map
Questions/Comments/Concerns?Questions/Comments/Concerns?
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