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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Figure 2.1 Schematic representation of scour at a cylindrical pier.

Factors affecting local scour are: (1) width of the pier; (2) projected length
of the abu~nt into the flow; (3) length of the pier; (4) depth of flow; (5)
velocity of the approach flow; (6) size of the bed material; (7) angle of
attack of the approach flow to the pier or abu~nt (8) shape of the pier or
abutment; (9) bed configuration; (10) ice formation or jaas; and (11) debris.

1. Width of pier has a direct affect on the depth of scour. With an increase
in pier width there is an increase in scour depth.

2. Projected length of an abutment into the stream affects the depth of scour.
With an increase in the projected length of an abutment into the flow there
is an increase in scour. However, there is a limit on the increase in
scour depth with an increase in length. This limit is reached when the
ratio of projected length into the stream to the depth of the approaching
flow is 25.

3. Length of a pier has no appreciable affect on scour depth as long as the
pier is lined up with the flow. If the pier is at an angle to the flaw the
length has a very large affect. At the same angle of attack doubling the
length of the pier increases scour depth 33 percent. Some equations take
the length factor into account by using the ratio of pier length to depth
of flow or pier width and the angle of attack of the flow to the pier.
Others uses the projected area of the pier to the flow in their equations.

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