Correction Factor for Bed Material Size

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or another pier; or (2) an abutment or another pier redirects the flow in a direction

parallel to the pier. For such cases, judgment must be exercised to reduce the value

of the K2 factor by selecting the effective length of the pier actually subjected to the

angle of attack of the flow.

The correction factor K3 results from the fact that for plane-bed conditions, which is

typical of most bridge sites for the flood frequencies employed in scour design, the

maximum scour may be 10 percent greater than computed with Equation 7.6. In the

unusual situation where a dune bed configuration, with large dunes, exists at a site

during flood flow, the maximum pier scour may be 30 percent greater than the

predicted equation value. This may occur on very large rivers, such as the Mississip-

pi. For smaller streams that have a dune bed configuration at flood flow, the dunes

will be smaller and the maximum scour may be only 10 to 20 percent larger than.

Correction Factor for Bed Material Size

The correction factor K4 decreases scour depths for armoring of the scour hole for bed

materials that have a D50 equal to or larger than 2.0 mm and D95 equal to or larger than 20

mm. The correction factor results from research by Molinas et al. (1998) and Mueller (1996).

Molinas' research for FHWA showed that when the approach velocity (V1) is less than the

critical velocity (Vc90) of the D90 size of the bed material and there is a gradation in sizes in

the bed material, the D90 will limit the scour depth. Mueller and Jones (1999) developed a K4

correction coefficient from a study of 384 field measurements of scour at 56 bridges.

if D50 > = 2 mm and D95 > = 20 mm

then

K 4 = 0.4 (VR )0.15

(7.9)

where:

V1 - VicD50

VR =

(7.10)

VcD50 - VicD95

and

VicDx = Approach velocity corresponding to critical velocity (m/s or ft/sec) for incipient scour in

the accelerated flow region at the pier for the grain size Dx (m or ft)

7.18