FLOOD01169

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Title: Design Guidelines and Criteria Channels and Hydraulic Structures on Sandy Soil
• Date: 6/1/1981
• Prepared For: UDFCD
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Educational/Technical/Reference Information

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47

Y 2 = general scour of flow depth at the bridge,

.. DSO= the median diameter of the bed material at the bridge.

3.7 Local Scour

Local scour occurs in the bed at embankments due to the actions of

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vortex systems induced by obstruction of the flow. Local scour occurs in

conjunction with, or in the absence of, degradation, aggradation, and general

scour. The basic mechanism causing local scour is the vortex of fluid re-

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suiting from the pileup of water on the upstream edge and subsequent accel-

eration of flow around the nose of the embankment. The action of the vortex

is to erode bed materials away from the base region. If the transport rate

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of sediment away from the local regi on is greater than the transport rate

into the region, a scour hole develops. As the depth is increased, strength

of the vortex is reduced, transport rate is reduced, equilibrium is re-

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established, and scouring ceases.

The depth of scour varies with time because sediment transported into

the scour hole from upstream varies depending upon the presence or absence

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of dunes. A mean scour depth hetween the oscillation or scour depth is

referred to as equi1ihrium scour depth.

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3.7.1 Local Scour Around Embankments

Detailed studies of scour around embankments have been performed mostly

in laboratories. According to the studies of Liu et a1. (1961), the equic

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ljbrium scour depth for local scour in sand at a spill slope when the flow

is subcritical is determined by the expression

..

Y
s
Yl

1.] (

a
~

)0.40 Fr 0.33
1

(3-7)

where,

Y
s

= the equilihrium scour depth measured from the mean bed level to
the bottom of the scour hole,

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