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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STEP 6. CQIlIlUTE LOCAL SCOUR AT PIERS

Local scour at piers is a function of bed material size, flow characteristics,
fluid properties and the geometry. of the pier. The subject has been studied
extensively in the laboratory but there is very little field data. As a result
of the many studies there are many equations. In general, the equations are
for live-bed scour in cohesionless sand bed streams, and they give similar
results.

Sterling Jones (1983) compared many of the more common equations. His
comparison of these equations is given in Figure 4.12. Some of the equations
have velocity as a variable (normally in the form of a Froude number).
However some equations, such as Laursen do not include velocity. A Froude
number of 0.3 was used (Fr = 0.3) in Figure 4.12 for purposes of comparing
commonly used scour equations. In Figure 4.13 the equations are compared with
some field data measurements. As can be seen from Figure 4.12 the CSU equation
encloses all the points but gives lower values of scour than Jain, Laursen and
Nie1's equations. The CSU equation includes the velocity of the flow just
upstream of the pier by including the Froude Number in the equation. Chang
(1988) points out that Laursen's (1980) equation is essentially a special case
of the CSU equation with the Fr = 0.5.

The equations illustrated in Figures 4.12 and 4.13 do not take into account the
possibility that larger sizes in the bed material could armor the scour hole.
That is, the large sizes in the bed material will at some depth of scour limit
the scour depth. Raudkivi (Raudkivi and Sutherland, 1981, and Raudkivi and
Ettema, 1983) developed an equation that takes into consideration large
particles in the bed. The significance of this factor of armoring of the scour
hole over a long time frame and over many floods is not known. Therefore,
their equation is not recommended for use at this time.

For the determination of pier scour, the Colorado State University's equation
is recommended for both live-bed and clear water scour. With a dune bed
configuration the equation predicts equilibrium scour depths and maximum scour
will be 30\ greater. For flow with plane bed configuration or antidunes, CSU's
equation gives the maximum scour.

The extent to which a pier footing or pile cap affects local scour at a pier is
not clearly determined. Under some circumstances the footing may serve as a
scour arrester, impeding the horseshoe vortex and reducing the depth of scour
hole. In other cases where the footing extends above the stream bed into the
flow, it may serve to increase the effective width of the pier, thereby
increasing the local pier scour. As an interim guide, if the top of the pier
footing is slightly above or below the stream bed elevation (taking into
account the effect of contraction scour), use the width of the pier shaft for
the value of Oa" in the pier scour equation. If the pier footing projects well
above the stream bed to the extent that it significantly obstructs the flow,
use the width of the pier footing for the value of "a". Interpolate between
these two values depending upon the extent to which the footing may be expected
to affect the local scour patterns.

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