FLOOD04125

• County: Statewide
• Basin: Statewide
• Title: Interim Procedures for Evaluating Scour at Bridges
• Date: 9/1/1988
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

CASE 6 SCOUR AT A8lJ1lIIENTS WHEN a/y.. > 25

Field data for scour at abutments for various size streams are scarce, but data
collected at rock dikes on th. Mississippi indicate the equilibrium scour depth
for large a/y.. values can be esti~ted by equation 10:

0.33

y. = 4 F....

(11)

Y..

The data a..e scatte..ed, p..ima..ily because equilibrium depths were not measu..ed.
Dunes as la..ge as 20 to 30 feet high move down the Mississippi and associated
time fa.. dune movement is ve..y large in cOlParison to time ..equired to form
live-bed local scour hol.s. Neve..theless, it is believed that th.se data
..ep...sent th. limit in scal. fo.. scou.. depths as compa..ed to labo..atory data
and enables useful ext..apolation of laboratory studies to field installations.

Acco..dingly, It is ..ecommended that equations 3 th..ough 10 be applied fa..
abutments with 0 < a/y.. < 25 and equation 11 be used fa.. a/y.. > 25.

CASE 7 ABUTMENTS SKElIED TO llIE STREAM

With skewed c..ossings, the app..oach embankment that is angled downst..eaa has
the depth of scou.. ..educed because of the st..eamlining effect. Conve..sely, the
app..oach embankment which is angled upstre.. will have a deepe.. scour hole.
The calculated scou.. depth should be adjusted In acco..dance with the curve of
Flgu..e 3.14. which is patte..ned afte.. Ahmad (1953).

.. 1.4
.
.c:
.. 1.2
..
01:
... 1.0
Ii
.1
Xi Il
..... .6
0_
-e! .4 U
DoO I ~.
-II: .2 I
.... I
0'" /,
0.2 0
... 0 45 110 135 110
..
~ of inclinAtion, c#l, de,.
An,le

FIGURE 4.11. Scou.. estimate adjustment fa.. skew.

38

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