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

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configuration may also change with a change in water teMperature or change
in concentration of silts and clays. The type of bed configuration and a
change in bed configuration will effect flow velocity, sediment transport,
and scour. Highways in the River Environment (6) discusses bed
configuration in detail.

Ice and debris by increasing the width of the piers, changing the shape of
piers and abutments, increasing the projected length of an abu~nt or
causing the flow to plunge downward against the bed can increase both the
local and contraction scour. The magnitude of the increase is still
largely undetermined. Debris can be taken into account in the scour
equations by estimating how much the debris will increase the width of the
pier or length of the abutment. Debris and ice affects on general
(contraction) scour can also be accounted for by estimating the amount of
flow blockage (decrease in width of the bridge opening) in the equations
for contraction scour. Field ..asurements of scour at ice jams indicate
the scour can be as much as 10 or 20 feet.

F. CLEAR-WATER AND LIVE-Bm SCOUR

There are two conditions of local scour. These are (1) clear-water scour and
(2) live-bed scour. Clear-water scour occurs when there is no movement of the
bed material of the stream upstream of the crossing but the acceleration of the
flow and vortices created by the piers or abutments causes the material at
their base to move. live-bed scour occurs when the bed material upstream of
the crossing is also moving.

Bridges over coarse bed material streams often have clear-water scour at the
lower part of a hydrograph, live-bed scour at the higher discharges and then
clear-water scour on the falling stages. Clear-water scour reaches Its maxi~
over a longer period of time than live-bed scour, figure 2.2. This is because
clear-water scour occurs mainly on coarse bed material streams. In fact cle&r-
water scour may not reach its MlXi~ until after several floods. Also,
maximum clear-water scour is about 10 percent greater than the maximum live-bed
scour. Live-bed scour in sand bed streams with a dune bed configuration
fluctuates about an equilibrium scour depth. The reason for this is the
fluctuating nature of the sedi..nt transport of the bed material in the
approaching flow when the bed configuration of the stream is dunes. In this
case (dune bed configuration in the channel upstream of the bridge) maximum
depth of scour is about 30 percent larger than equilibrium depth of scour.

The maximum depth of scour is the same as the equilibrium depth of scour for
live-bed scour with a plain bed configuration. With anti dunes occurring
upstream and in the bridge crossing the maximum depth of scour from the limited
research of Jain and Fisher (1979) is about 20 percent greater than the
equilibrium depth of scour.

For a discussion of bed form in alluvial channel flow the reader is referred to
chapter 3 of Highways in the River Environment (6). Equations for estimating
local scour at abutments or piers are given in Chapter 4. Note that these
equations were developed from laboratory experiments for clear water scour and
limited field data.

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