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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Equations 6 and 7:

More recently, Laursen (1980) suggested two r.lationships
for scour at vertical wall abutments for Cas. 1. On. for
live-bed scour and another for clear-water scour d.pending
on the relative magnitude of the bed shear stresses to the
critical shear stress for the bed ..t.rial of the str....
For live-b.d scour (~1 > ~.), us. equations 6 or 7. For
other abutment types, see note 2 below.

! = 2.75 (l.) [( 1 Y. + 1)1.7
Y. Y1 11.5 Y.

- 1J

(6)

Simplified form: ;: = 1.5 (;1)0.48

(7)

Equation 8: For clear water scour (~. < t.):

! = 2.75 (l.)
Y. .Y.

( 1 Y. + 1)7/6
11.5 Y1

(8)

- 1

(1.)1/2
~.

t1 = shear stress on the bed upstream
t. = critical shear stress of the Dso of the
upstream bed material. The value of t.
can be obtained from Figure 4.5.

COMMENTS ON CASE 1 EQUATIONS

'Note 1. These equations are limited to cases where a/Y1 < 25. For a/Y1 > 25
go to Case 6.

Note 2. Scour values given by Laursen's equations are for vertical wall
abutments. He suggests the following multiplying factors for other
abutment types for small encroachment lengths:

Abutment Type
45 degree Wing Wall
Spill-Through

Multiplying Factor
0.90
0.80

Note 3. Laursen's equations are based on sediment transport relations. They
give maximum !S.!!!!.!: !M include contraction m!!J:. FOR THESE
EQUATIONS, DO NOT ADO STEP 4 CONTRACTION SCOUR TO OBTAIN TOTAL SCOUR
AT THE A8UTMENT. FOR MEllIOD 1 ANALYSES LOCAL ABUlMENT SCOUR BELOW THE
CONTRACTION SCOUR LINE IS EQUAL TO LOCAL ABUTMENT SCOUR - CONTRACTION
SCOUR.

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