The FHWA HEC-18 equation (Richardson and Davis 2001) to predict local scour depths at a

pier, based on the CSU equation, is recommended for both live-bed and clear-water scour.

The equation predicts maximum pier scour depths. The equation is:

0.65

a

ys

Fr10.43

= 2.0 K 1 K 2 K 3 K 4 K 5

(7.7)

y1

y1

where:

ys

=

Scour depth, m (ft)

y1

=

Flow depth just upstream of the pier, m (ft)

K1

=

Correction for pier shape from Table 7.1 and Figure 7.6

K2

=

Correction for flow angle of attack of flow from Table 7.2 and Equation 7.8

K3

=

Correction factor for bed condition from Table 7.3

K4

=

Correction factor for armoring by bed material size from Equation 7.9

K5

=

Correction factor for pier width from Equation 7.13 or 7.14

L

=

Length of pier, m (ft)

Froude Number directly upstream of the pier = V1/(gy1)1/2

Fr1

V1

=

Mean velocity of flow directly upstream of the pier, m/s (ft/s)

g

=

For round nose piers aligned with the flow the depth of scour has the following limits.

ys ≤ 2.4 times the pier width (a) for Fr ≤ 0.8

ys ≤ 3.0 times the pier width (a) for Fr > 0.8

The correction factor for angle of attack of the flow K2 given in Table 7.2 can be calculated

using the following equation:

.65

L

K 2 = Cosθ + Sinθ

(7.8)

a

If L/a is larger than 12, use L/a = 12 as a maximum in Equation 7.8 and Table 7.2.

7.16

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