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

o assess the expected profil. and plan fOnD changes:

o adjust the fixed bed hydraulics to reflect any expected long tenD
profile or plan form changes;

o estimate contraction scour using the empirical contraction formula and
the adjusted fixed bed hydraulics;

o estimate local scour using the adjusted fixed bed channel and bridge
hydrau 11 cs; and

o add the local scour to the contraction scour to obtain the total scour.

STEP 4. COIITRACTION SCOUR

Contraction scour can be caused by different bridge site conditions. There are
4 main conditions (cases) which are as follows:

Case 1. Overbank flow on a flood plain being forced back to the main
channel by the approaches to the bridge.

Case 2. The normal river channel width becoming narrower either because of
the bridge itself or by the bridge site being on a narrower reach
of the ri ver .

Case 3. A relief bridge in the overbank area with little or no bed
material transport in the overbank area.

Case 4. A relief bridge over a secondary stream in the overbank area.

These 4 cases are illustrated in Figure 4.1: the equations for solving
each case are presented in the following sections:

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