FLOOD03064

• County: Statewide
• Basin: Statewide
• Title: Evaluating Scour at Bridges
• Date: 11/1/1990
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Floodplain Report/Masterplan

KZ = (100/90)0.13 = 1.01
a' = (1,S70 X 2,8)/ 4,8 = 1091 ft
V. = 17,700/(1,870 X 2.8) = 3.38 ft/s
Fr = 3.38 / (32.2 X 4,8)0.5 = 0.27
.

Ys/4.8 = 2,27 X 0.55 X 1.01 X (1091/4.8)0.43 X 0.270.6' + 1

Ys/4,8 = 6.85

Ys = 32.9 ft

Calculations of scour cepth using the depth of flow in the
channel (9,0) gave a scour depth of 25.5 ft.

In appendix B an equation is given for a/y, greater than 25. In
this problem a/y, = 1,870/ 6,0 = 312.

Therefore, try the equetion for Case 6 given in Appencix B.

0,33

Ys-
y,

=

4 Fr,

v, = (43,600/(391 X 9,O)} X 1.1 = 13.6
Fr, = 13.E/(32.2 X 9,0)0.50 = 0,80
Y s/ 9 ,0 = 4 ( 0 , 80) 0.33 = 3, 72
Ys = 9.0 X 3.72 = 33.4 ft

.

Comments
This is very deep! It could occur if the abutment was out in the
main flow. But with the bank of the main channel having trees etc
this depth probably will not occur. However, that much flow corning
back to the channel ( 17,700 cfs) the scour potential is large.
What to do?
Use ASSHTO GUIDELINES to design the foundation and set the toe
depth for the abutment. with this amount of overbank a ripraped
guide bank should be used to protect the abutment.
Increasing the width of the bridge opening should be considered.

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