FLOOD04125

• County: Statewide
• Basin: Statewide
• Title: Interim Procedures for Evaluating Scour at Bridges
• Date: 9/1/1988
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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fQUATIONS FOR ESTIMATING CONT1lACTION SCOUR

CASE 1. Use Laursen's (1960) equation given below to predict the depth of
scour in the contracted section 'z.

lz = (gc)6/7(~~)K~(n2)Kz
,~ Qc Wz n~

(1)

,. = 'z -" Average scour depth

Where:
y~
yz
W~
Wz
Qc
Qc
nz
n~
e

= average depth in the main channel
= average depth in the contracted section
= Width of the main channel
= Width of the contracted section
= flow in the contracted section
= flow in main channel
= Manning n for contracted section
= Manning n for main channel
= transport factor from the following

V.c/w
<0.50
1.0
>2.0

K,
0.59
0.64
0.69

Kz
0.066
0.21
0.37

Mode of Bed Material Transport

~st1y contact bed material discharge
some suspended bed material discharge
~st1y suspended bed material discharge

e
0.25
1.0
2.25

= (gy,S~)o,s. shear velocity
= fall velocity of Oso of bed material
= gravity constant, 32.2
= slope, energy grade 1inl main channel

= ~2+e~
7 3+8
Kz = 6e
7(3+1)

V.c
w
g
5,
K,

Mote 1. The Manning n ratio can be significant for a condition of dune bed in
the main channel and a corresponding plain bed, washed out dunes or anti dunes
in the contracted channel (6).

Mote 2. The average width of the bridge opening (Wz) is normally taken as the
top width with the width of the piers subtracted.

Mote 3. Laurson's equation for a long contraction will overestimate the depth
of scour at the bridge if the bridge is located at the upstream end of the
contraction or if the contraction is the result of the bridge abutments and
piers. At this time, however, it is the best equation available.

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