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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STEP 5. LOCAL SCOUR AT ABUlMEIfTS

Equations for predicting scour depths are based almost entirely on either
laboratory data or inductive reasoning from sediment continuity equations.
There are little field data to compare abutment scour equations. Abutments can
be set back from the natural stream bank or can project out into the flow; they
can have various shapes (vertical walls, spill through slopes) and they can be
set at an angle to the flow. Scour at abutments can be live-bed or clear-watar
scour. Finally, there can be varying amounts of overbank flow that is
intercepted by the approaches to the bridge and returned to the stre.. at the
abutment.

In this section, scour at abutments is divided into its various cases and
equations are given for each case. No single equation is recommended for a
given situation when more than one equation is applicable, because with the
lack of field data for verification, it is not know which equation is best. It
is recommended that the designer determine what case fits the design situation
and then use all equations that apply to the case. (See Table 4.1 and Figure
4.2.) IT IS MOST IMPORTANT THAT THE CCMlENTARY ON EACH OF THE EQUATIONS BE
READ AND UNDERSTOOD PRIOR TO ATTEMPTING TO USE THE EQUATIONS FOR DESiGN .
PURPOSES. Engineering judgment must be used to select the depth of
foundations. The designer should take into consideration the potential cost of
repairs to an abutment and danger to the travelling public in selecting scour
depths. Finally design measures such as spur dikes and riprap should be used
to increase the safety of the bridge.

Comments on Table 4.1 and Figure 4.2
Note 1. Equations for these cases (except for Case 6) are based on laboratory
studies with little or no field data.

Note 2. The factor a/y~ = 25 as a limit for Cases 1-5 is rather arbitrary but
it is not practical to assume that scour depth, y., would continue to increase
with an increase in abutment length -a-.

Note 3. There are two general shapes for abutments. These are vertical wall
abutments with wing walls and spill-through abutments, Figure 4.3. Depth of
scour is about double for vertical wall abutments as compared with spill
through abutments.

Maximum Depth of Scour
Note 1. For live-bed scour with a dune bed configuration, the Maximum depth of
scour is about 30 percent greater than equilibrium scour depth given by Liu,
et al's equations (Equations 3 and 5). Therefore the values of scour that are
calculated for these equations should be increased by 30 percent when the bed
form is dunes upstream of the bridge. The reason for this is that the research
that was used for determining scour depth for the live-bed scour case was run
with a dune bed and equilibrium scour was measured.

Note 2. For clear-water scour (equation 8), the maximum depth of scour is
about 10 percent greater than live-bed scour; however, there is no need to
increase the scour depths because the equations predict the maxi.um scour.

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