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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CHAPTER 4
ESTIIllATING SCOUR AT BRIDGES

Before the various scour forecasting methods for contraction and local scour
can be applied, it is first necessary to (1) obtain the fixed bed channel
hydraulics; (2) estimate the long term profile degradation or aggradation; (3)
adjust the fixed bed hydraulics to reflect these changes, and; (4) CGlpute the
bridge hydraulics.

DESIGIl APPROACH

The steps recommended for estiaating scour at bridges are sunnarized below.
The procedures for each of the steps, including recommended scour equations,
are discussed in detail following the summary and illustrated in Appendix D.

STEP 1. Determine scour analysis variables.

STEP 2. Analyze long term bed elevation change.

STEP 3. Select the scour analysis method to be used.

STEP 4. Compute the magnitude of contraction scour.

STEP 5. Compute the magnitude of local scour at abutments.

STEP 6. Compute the magnitude of local scour at piers.

STEP 7. Plot the total scour depths

STEP 1. DETERMINING SCOUR ANALYSIS VARIABLES

A. . From a flood frequency study determine the magnitude of the discharges for
the floods in step 1 of the Design Procedure (Chapter Ill) including the
overtopping flood when applicable. If the magnitude of the 500 year flood
is not available use a discharge equal to 1.7 X Q100' Experience has shown
that the incipient overtopping discharge often puts the most stress on a
bridge. However, special conditions (ice jams, etc.) may cause a more
severe condition for scour with a flow smaller than the overtopping or 100
year floods.

B. Determine the water surface profiles for these discharges using the
FHWA/USGS program WSPRO or other existing methods that employ the FHWA
bridge analysis practices. Various conditions and discharges should be
used to determine the worst combinations of scour conditions that could
occur at the bridge. The engineer should anticipate future conditions at
the bridge, in the stream's watershed, and at downstream water surface
elevation controls.

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