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

CASE 3. Case 3 applies to a relief bridge on a floodplain where there is no
bed .aterial transport (clear water scour). Use Laurson's (1980) equation
given below:

l2 = (~.)6/7
y. W2

[ v! Jm
120 y.1/3 0502/3

(2)

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CASE 2. This case applies. where there is no overbank flow but the stre..
channel narrows either naturally or by the bridge abutments encroaching on the
channel. Use the Case 1 equation with Qt = Qc. If the decrease in width W2 is
less than 10 percent, then neglect.

The subscript 1 refers to the upstre.. conditions and 2 to the width and depth
in the relief bridge.

W. = Width upstream of the relief bridge. It is estimated by
assuming a point of stagnation between the main bridge and
the relief bridge.

V. = Average velocity on the floodplain one bridge length
upstream.

050 = Median diameter of bed material at relief bridge.
ALL OTHER SYMBOLS ARE AS PREVIOUSLY DEFINED .

CASE 4. Case 4 applies to a relief bridge with bed material transport. Use
the equation given for Case 1 with appropriate adjustments of the variables.
This case can occur when a relief bridge is over a secondary channel on the
flood plain.

SPECIAL CONDITIONS: General scour resulting from variable water surface
downstream of the bridge is analyzed by determining the lowest potential water
surface elevation downstream of the bridge in so far as scour processes are
concerned. Use computer programs such as the U.S. Corps of Engineers HEC 2 or
the FHWA/USES WSPRO program to determine the flow variables such as velocity
and depths through the bridge. With these variables determine general and
local scour depths.

General scour resulting from the flow through the bridge being concentrated in
one area is analyzed by determining the superelevation of the water surface on
the outside of the bend and estimating the resulting velocities and depths
through the bridge. The .aximum velocity in the outer part of the bend can be
1.5 to 2 times the mean velocity. A physical model study can also be used to
determine the velocity and scour depthS distribution through the bridge for
this case.

Estimating contraction scour for unusual situations involves particular skills
in the application of principles of river mechanics to the specific site
conditions and such studies should be undertaken by engineers experienced in
the fields of hydraulics and river mechanics.

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