bridge sour

7/23/2019 Bridge Sour

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BRIDGE SCOUR


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BRIDGE SCOUR

COURSE OUTCOME (CO3)

-Analyse and evaluate scour at piers & abutment


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Components of Scour

1. General scour

2. Contraction scour

3. Abutment scour4. Pier scour


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1. General scour

Related to aggradation and degradationFactor that affect long term bed changes

a. Dams & reservoirs upstream & downstream of bridge

b. Changes in watersheds- urbanization & deforestration

c. Channel stabilization & rectificationd. Natural / artificial cutoff of a meander bend

e. Changes in downstream base level of the bridge reach

f. Gravel mining from the streambed

g. Diversion of water in/out of stream

h. Lateral migration of river bend


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2. Contraction scour

V1= approach velocity

h1= approach flow depthW1= width of approachV2= contracted velocityh2= contracted flow depthW2= contracted width

=

= =

=

=


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2. Contraction scour (cont.)

a. Clear Water Contraction Scour- Occurs without transport of bed material in theupstream reach into the bridge cross section


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2. Contraction scour (cont.)b. Live Bed Contraction Scour

- Occurs when there is transport of bed material in theupstream reach into the bridge cross section

=

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k1

k2

Wherey1= average depth in the upstream main channel, my2= average depth in the contracted section, my0=existing depth in the contracted section before scour, m

Q1= flow in the upstream transporting sediment, m3/sQ2= flow in the contracted channel, m

3/sW1= bottom width of the upstream main channel, mW2= bottom width of the upstream main channel in the contracted section, mn1= Mannings for the upstream main channel

n2= Mannings for the contracted sectionk1 & k2 = exponents depending on the mode of bed material transport


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3. Abutment scour

Abutment scour (after Richardson et al., 1990).

= approach velocity

abutment & embankment length measured at the top of water surface

equilibrium depth of abutment scour

ave upstream flow depth


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3. Abutment scour (cont.)

= 1.1

.

.33

Equilibrium scour depth for local live bed scour in sand near aspill through abutment under subcritical flow

If abutment terminates at a vertical wall and the wall on theupstream side is also vertical, the scour hole nearly double

= 2.15

.

.33

When > 25

= 4

.33


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4. Pier scour

Pier scour (after Richardson and Davis, 1995).

aused by horseshoe vortex induced by secondary flow at pier base

Square nose

Round nose

Circular cylinder

Sharp nose

Group of cylinders


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4. Pier scour (cont)

Pier-type correction factor K1(after Richardson et al., 1990)

Pier type K1

Square nose 1.1

Round nose 1.0

Circular cylinder 1.0

Sharp nose 0.9

Group of cylinders 1.0

= 2.0

.6

.3

Attack Angle(degree)

Correction factor, K2

L/a = 4

L/a = 8

L/a = 12

0 1.0 1.0 1.0

15 1.5 2.0 2.5

30 2.0 2.75 3.5

45 2.3 3.3 4.3

90 2.5 3.9 5.0

Flow-angle correction factor K2(after Richardson et al., 1990)

p= skew angle of flowLp= pier length

a = pier width


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Design Approach

1. Determine the fixed-bed channel hydraulics

2. Determine the long term impact of degradation or

aggradation on the bed profile

3. If degradation occurs, adjust the fixed bed hydraulics to

reflect the changes

4. Compute the bridge hydraulics


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Evaluating scour at bridge

FHWA Hydraulic Engineering Circular No.18

1. Determine the scour analysis variables

2. Analyze long term bed elevation change

3. Evaluate the scour analysis method4. Compute the magnitude of contraction scour

5. Compute the magnitude of local scour of pier

6. Compute the magnitude of local scour at abutments

7. Plot and evaluate the total scour depth


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Example 1A 198.12m long bridge is to be constructed over a channelwith a spill-through abutments (slope of 1V:2H). The leftabutment is to set to approximately 60.5m back from thechannel bank, The right abutment is set at the channelbank. The bridge deck is set at elevation 6.71m and has agirder depth of 1.22m. Six round nose piers are evenlyspaced in the bridge opening. The piers are 1.52m thick,12.19 m long and are aligned with the flow. The 100 yeardesign discharge is 849.51m3/s. The 500 year flow of1444.16 m3/s was estimated by multiplying the Q100by 1.7

since no hydrologic records were available to predict the500 year flow.

1. Determine the type flow condition

2. Compute the flow


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Example 2

A 200-m-long bridge is to be constructed over a sand-bed channelwith 300-m-long spillthrough abutments 1V:2H. Six rectangularbridge piers measuring 1.5 m thick and 12 m long are alignedwith the flow. At a design 100-yr flow discharge of 850 m3/s, the

upstream flow velocity is 3.75 m/s and the flow depth is 2.8 mupstream of the piers. Estimatea. the abutment scour depth andb. the pier scour depth.


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Solution to Example 2

Abutment scour

1. Calculate =

2. Check value

3. Findz

Pier scour

1. Calculate =

2. Check value

, K1and K2

3. Findz

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