7.7.4
FHWA HEC-18 Equation
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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