The values of the correction factor K2 should be applied only when the field conditions
are such that the entire length of the pier is subjected to the angle of attack of the
flow. Use of this factor will result in a significant over-prediction of scour if (1) a
portion of the pier is shielded from the direct impingement of the flow by an abutment
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
K 4 = 0.4 (VR )0.15
V1 - VicD50
VcD50 - VicD95
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)