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

Factors affecting local scour are: 1) width of the pier, 2)
projected length of an abutment into the flow, 3) length of the
pier if skewed to flow, 4) depth of flow, 5) velocity of the
approach flow, 6) size and gradation of bed material, 7) angle of
attack of the approach flow to a pier or abutment, 8) shape of a
pier or abutment, 9) bed configuration, 10) ice formation or
jams, and 11) debris,

1. pier width has a direct influence on depth of local scour.
As pier width increases, there is an increase in scour depth,

2. Projected length of an abutment into the stream affects the
depth of local SCOLr, An increase in the projected length of
an abutment into tr.e flow increases scour. However, there is
a limit on the increase in scour depth with an increase in
length. This limit is reached when the ratio of projected
length into the flow to the depth of the approach flow is 25.

3. pier length has no appreciable affect on local scour depth as
long as the pier is aligned with the flow. When the pier is
skewed to the flow, the length has a significant affect;
i.e., with the same angle of attack, doubling the length of
the pier increases scour depth 33 percent,

4, Flow depth has an affect on the depth of local seGur. An
increase in flow depth can increase scour depth by a factor
of 2 or greater for piers. with abutments the i~c=aase is
from 1.1 to 2.15 depending on the shape of the abutmen~,

5. The approach flow velocity affects scour depth. The greater
the velocity, the deeper the sccur, There is a high
probability that scour is affected by whether the flow is
subcritical or supercritical. However, most research and
data are for subcr~tical flow; i,e" flow with a Froude
Number much less than one (Fr < 1 ),

6. Bed material characteristics such as size, gradation, and
cohesion can affect local scour, Bed material in the sand
size range has no affect on local scour depth. Larger size
bed material that can be moved by the flow or by the vortices
and turbulence created by the pier or abutment will not
affect the maximum scour but only the time it takes to attain
it. Very large pa:~ticles in the bed material, such as
cobbles or boulder:; I may armor the scour hole. Research at
the University of ;,ukland, New Zealand, and by the Washington
state Department of Transportation (10) (11) (12) (13)
developed an equatLon that takes into account the decrease in
scour due to the a:~moring of the scour hole. Richardson and
Richardson (14) co:nbined the work of Raudkivi, Ettema,
Melville, Sutherla~d, Cope, Johnson and MacIntosh into a
simplified equati01. However, there are inadequate field
data to support th~se equations at this time. The extent

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