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

STEP 3. SELECT THE SCOUR ANALYSIS ME1lIOO TO BE USED - METHOD 1 OR METHOD 2

Two methods are suggested for combining the contraction and local scour
components to obtain total scour. The first method identified as Method 1 is
recommended where precise contraction scour estimates are deemed necessary.
The second method, Method 2, is reco~nded where a preliminary scour estimate
is needed or where contraction scour is relatively small and will not have a
significant influence on the foundation design.

METHOD 1

This analysis method is based on the premise that the contraction and local
scour components are inter-dependent. As such, the local scour estimated with
this method is determined based on the expected changes in the hydraulic
variables and parameters due to contraction scour. Through an interactive
process, the contraction scour and channel hydraulics are brought into balance
before local scour is computed. The general approach for this method is:

o estimate the natural channel's hydraulics for a fixed bed condition
based on existing site conditions;

o estimate the expected profile and plan form changes based on the
procedures in this manual and any historic data;

o adjust the natural channel's hydraulics based on the expected profile
and plan form changes;

o select a trial bridge opening and compute the bridge hydraulics;

o estimate contraction scour;

o revise the natural channel's geometry to reflect the contraction scour
and then again revise the channel's hydraulics. Repeat this iteration
until there is no significant change in either the revised channel
hydraulics or bed elevation changes (a significant change would be 5
percent or greater variation in velocity, flow depth, or bed elevation);

o using the foregoing revised bridge and channel hydraulic variables and
parameters obtained considering the contraction scour, calculate the
local scour; and

o extend the local scour depths below the predicted contraction scour
depths in order to obtain the total scour.

METHOD 2

This method is based on the assumption that the scour components develop
independently. Thus, the potential local scour to be calculated using this
method would be added to the contraction scour without considering the effects
of contraction scour on the channel and bridge hydraulics. The general
approach with this method is:

2f)

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