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Jean-Louis BRIAUD
Distinguished Professor
Texas A&M University
and President of FedIGS
OBSERVATION METHOD: A NEW TOOL FOR THE BRIDGE
SCOUR ENGINEER
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2Jean-Louis BRIAUD
Acknowledgements
• Texas DOT (John Delphia)
• Massachusetts DOT (Rich Murphy)
• Dr. Chen, Dr. Oliveira
• Anand Govindasamy, Dekay Kim, Inwoo Jung
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SCOUR DEPTHS
Normal Water Level
LC
z(abut)
z(abut) Applies z(cont) Applies
Probable Flood Level
z(pier) z(cont)
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4
0
100
200
300
400
500
600
700
800
900
1000
Co
ns
tru
cti
on
Co
nc
rete
De
teri
ora
tio
n
Ea
rth
qu
ak
e
Na
tura
l
Ste
el
Fir
e
Mis
c.
Ov
erl
oa
d
Co
llis
ion
Hy
dra
uli
c
Cause
Nu
mb
er
of
Failu
res f
rom
1966 t
o 2
005 (
1502 T
ota
l)
0%
10%
20%
30%
40%
50%
60%
Perc
en
t
SCOUR = NO.1 KILLER OF BRIDGES
1 BRIDGE FAILS EVERY 10 DAYS
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5Jean-Louis BRIAUD
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6Courtesy of the University of Kentucky at Louisville
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7
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8
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9
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10
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11
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12
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13
IT IS GETTING BETTER
0
20
40
60
80
100
120
140
160
1966 -
1970
1971 -
1975
1976 -
1980
1981 -
1985
1986 -
1990
1991 -
1995
1996 -
2000
2001 -
2005
Year
Nu
mb
er
of
Sco
ur
Failu
res
fro
m 1
966 t
o 2
005 (
878 T
ota
l)
0%
5%
10%
15%
Perc
en
t
RESEARCH PAYS OFF
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PROBLEM
1. Comparison between measured and calculated scour depths by current method exhibits a lot of scatter
2. Comparison between measured and calculated scour depths by current method shows excessive conservatism on the average
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HEC-18 RESULTS
0
1
2
3
4
5
6
0 1 2 3 4 5 6
Pre
dic
ted
max
. ab
utm
en
t sco
ur
de
pth
(m
)
Measurement (m)
Q100
Historic data
MEASURED ABUTMENT SCOUR DEPTH (m)
PR
ED
ICT
ED
MA
X. A
BU
TM
EN
T
SC
OU
R D
EP
TH
(m
)
BENEDICT DATABASEABUTMENT SCOUR
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HEC-18 RESULTS
0 2 4 6 8 100
2
4
6
8
10
Measured Landers-Mueller Pier Scour, zmax
(m)
Dete
rmin
istic
HE
C-1
8 S
and, z
ma
x (m
)
Zpredicted/Zmeasured = 3.26
LANDERS-MUELLER DATABASEPIER SCOUR
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17Jean-Louis BRIAUD
OMS-TEXAS & MASSACHUSETTS
y = 1.0669xR² = 0.8612
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0.00 1.00 2.00 3.00 4.00
Zp
re
dic
ted
(O
MS
)(m
)
Zmeasured (m)
0188-02-023
0072-04-020
170-0177-05-119
0382-05-021
B13001-1EA
B28032-0JC
D06002-0U4
B28009-0JD
D10005-367
D12026-1XX
E01001-41Q
Linear (Regression)
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J.-L. Briaud, Texas A&M University
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Jean-Louis Briaud – Texas A&M University
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EFA - EROSION FUNCTION APPARATUS
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Jean-Louis BRIAUD – Texas A&M University
POCKET ERODOMETERPET test result = Depth of hole in mm after 20 squirts at 8 m/s
22
$0.49 at
WalMart
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Jean-Louis BRIAUD – Texas A&M University
23
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EROSION CLASSIFICATION
0.1
1
10
100
1000
10000
100000
0.1 1.0 10.0 100.0
Velocity (m/s)
Very High
Erodibility
I
High
Erodibility
II
Medium
Erodibility
IIILow
Erodibility
IV
Very Low
Erodibility
V
Erosion
Rate
(mm/hr)
-Fine Sand
-Non-plastic Silt -Medium Sand
-Low Plasticity Silt -Fine Gravel
-Coarse Sand
-High Plasticity Silt
-Low Plasticity Clay
-All fissured
Clays-Cobbles
-Coarse Gravel
-High Plasticity Clay
-Riprap
- Increase in Compaction
(well graded soils)
- Increase in Density
- Increase in Water Salinity
(clay)
Non-Erosive
VI-Intact Rock
-Jointed Rock
(Spacing < 30 mm)
-Jointed Rock
(30-150 mm Spacing)
-Jointed Rock
(150-1500 mm Spacing)
-Jointed Rock
(Spacing > 1500 mm)
SPSM CL
CHML MHRock
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0.0001 0.001 0.01 0.1 1 10 100 1000 100000.01
0.1
1
10
100
1000
CRITICAL
VELOCITY,
Vc (m/s.)
MEAN GRAIN SIZE, D50 (mm) JOINT SPACING FOR
JOINTED ROCK
Vc=0.1(D50)-0.2
Vc=0.35(D50)0.45
Vc=0.03(D50)-1
US ARMY CORPS OF
ENGINEERS EM 1601
INTACT
ROCK CLAY SILT SAND GRAVELRIP-RAP &
JOINTED ROCK
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26
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27
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28Jean-Louis BRIAUD
OBSERVATION METHOD FOR SCOUR
• Developed at Texas A&M University for
TxDOT
• Because of experience with calculating
scour depths that seemed unreasonably
large compared to observations by
inspectors in the field
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29Jean-Louis BRIAUD
OBSERVATION METHOD FOR BRIDGE SCOUR
• Step 1: Observe maximum scour depth = Zmo
• Step 2: Find out the maximum flood the bridge
has been subjected to : Collect gage data, RI
from TAMU-OMS, Qmo/Q100, Vmo/V100
• Step 3: Extrapolate field measurements to
predict future scour depth
Zfut / Zmo = F (Vfut / Vmo)
• Step 4: Compare future scour depth to
foundation depth (pier) Zfut < Zfound / 2
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30Jean-Louis BRIAUD
Step 1: Observe maximum scour depth = Zmo
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31Jean-Louis BRIAUD
Step 2: Find out the maximum flood the bridge
has been subjected to = Vmo
930 Flow Gages in Texas
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32Jean-Louis BRIAUD
Step 2: Find out the maximum flood the bridge
has been subjected to = Vmo
Maximum flood analysis
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33Jean-Louis BRIAUD
Step 2: Find out the maximum flood the bridge
has been subjected to = Vmo
Maximum
RI map
between
1970 and
2005
Automated
with TAMU-
FLOOD
software
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34Jean-Louis BRIAUD
Step 2: Find out the maximum flood the bridge
has been subjected to = Vmo
Maximum
RI map
between
1920 and
2005
Automated
with TAMU-
FLOOD
software
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35Jean-Louis BRIAUD
GETTING QMO/QFUT FROM RIMO/RIFUT
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36Jean-Louis BRIAUD
GETTING VMO/VFUT FROM QMO/QFUT
Wide channel = 0.4
Narrow channel = 0.25
Most likely value = 0.35
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37Jean-Louis BRIAUD
GETTING THE VELOCITY RATIO
FROM THE RECURENCE INTERVAL RATIO
0.350.35 0.261 0.091
100 100 100 100
v Q RI RI
v Q RI RI
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38Jean-Louis BRIAUD
EVALUATING TAMU-FLOOD PRECISION
1
2
3
45
FLOW GAGE
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39Jean-Louis BRIAUD
PRECISION OF TAMU-FLOOD
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40Jean-Louis BRIAUD
Step 3: Extrapolates field measurements to
predict future depth
Zfut / Zmo = F (Vfut / Vmo)
•Known = Zmo and Vmo
•Choose Vfut
•Obtain Zfut from charts
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41Jean-Louis BRIAUD
Step 3: Extrapolates field measurements to
predict future depth Zfut
The Z-Future Charts were developed by performing a large number (~350,000) of HEC-18 Clay simulations using
−Varying pier, contraction & Abutment scour geometry−Varying soil conditions−Varying velocities−Varying age of the bridge
Zfut / Zmo = F (Vfut / Vmo)
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42Jean-Louis BRIAUD
Category III Materials
Upstream Water Depth (H1): 5 m to 20 m
Contraction Ratio (Rc) : 0.5 to 0.9
Critical Velocity (Vc) : 0.5 m/s
Pier Diameter (D) : 0.1m to 1.0 m
thyd = 25 years
Vfut/Vmo
2.00
Zfu
t/Zm
o
2.8
1.0
Step 3: Extrapolates field measurements to
predict future scour depth Zfut/Zmo = Vfut/Vmo
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43Jean-Louis BRIAUD
Step 3: Extrapolates field measurements to
predict future scour depth Zfut/Zmo = F(Vfut/Vmo)
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Zfu
t/Zm
o
Vfut/Vmo
Zfuture Chart-CategoryIII-Abutment
Vmo=1.5*VcVc=0.781Vratio:0 to 1.5Vfut= Vmo*VratioMnning Co: 0.035Slope=0.001thyd=25yrs
Vmo=2.0*Vc
Vmo=2.5*Vc
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44Jean-Louis BRIAUD
Step 4: Compare future scour depth to
foundation depth Zfut < Zfound / 2
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45Jean-Louis BRIAUD
OMS-TEXAS & MASSACHUSETTS
y = 1.0669xR² = 0.8612
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0.00 1.00 2.00 3.00 4.00
Zp
re
dic
ted
(O
MS
)(m
)
Zmeasured (m)
0188-02-023
0072-04-020
170-0177-05-119
0382-05-021
B13001-1EA
B28032-0JC
D06002-0U4
B28009-0JD
D10005-367
D12026-1XX
E01001-41Q
Linear (Regression)
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46Jean-Louis BRIAUD
Limitations
• Requires a good network of flow gages (and
rain gages). Interpolation could be refined
• Cannot be used directly for new bridges but
lessons learned (database) can be useful for
new bridges
• Estimate in filling (USGS research and a
TxDOT survey have found that it was rare
(10% of the time) and ranged from 2 to 4 ft)
• Not yet developed for layered soil (be careful
with thin hard layer over soft layer)
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47Jean-Louis BRIAUD
Advantages
• Valuable tool to prioritize bridge repairs,
countermeasure decisions, asset
management
• Can serve as an input to FHWA risk
approach
• Part of the practical design concept
• No need for erosion testing
• Actual soil
• Actual flow history
• Actual geometry
• Based on observed measurements
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48
COMPARISON
HEC 18 OMS
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49
DIFFERENCES BETWEEN HEC 18
AND OBSERVATION METHOD
HEC 18
• Flume tests (scale pb?)
• Wrong worst soil (Fine
sand)
• Simplified geometry
• Simplified single
velocity
OMS
• Full scale
• Right soil
• Exact geometry
• Exact velocity
hydrograph
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50Jean-Louis BRIAUD
CITY: SHEFFIELD HIGHWAY: MAPLE AVE RIVER: WATER HOUSATONIC RIVER
OMS RISK RATING = LOW
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51Jean-Louis BRIAUD
CITY: DEERFIELD HIGHWAY: US 5 RIVER: WATER DEERFIELD RIVER
OMS RISK RATING = MEDIUM
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52Jean-Louis BRIAUD
CITY: BUCKLAND HIGHWAY: STATE ROUTE 2 RIVER: DEERFIELD RIVER
OMS RISK RATING = MEDIUM
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53Jean-Louis BRIAUD
CITY: BUCKLAND HIGHWAY: STATE ROUTE 2 RIVER: DEERFIELD RIVER
OMS RISK RATING = HIGH
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54
1. TAMU-OMS significantly decreases the scatter between measured and calculated scour depths
2. TAMU-OMS eliminates the excessive conservatism
3. TAMU-OMS is a valuable new tool for the bridge scour engineer (e.g.: can be used to prioritize repairs, to evaluate risk, as a management tool, for practical design)
4. TAMU-OMS is available for all states in the country