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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Hodel Cooperative Agreement
with the U.S. Geological Survey

~ppendix A

Location

The proposed ~tudy will cover {'.,study are~":I. {::>number<.::j sites for
bridge-scour data collection will be selected in (::>study area,::) from
different geographical areas representing a wide range of drainage-basin
sizes and streambed compositions.

Problem

Scour of the streambed in the vicinity of b,'idge piers and abutments during
floods has resulted in more bridge failures than all other cause~ in recent
history (Murillo, 1987). A recent, notable example occurred in April, 1987;
local scour caused pier failures and subsequent COllapse of a five-span,
multilane, New rork State Thruway bridge spanning Schoharie Creek. Althougn
a huge economic loss was sUffereo, an even greater tragedy was the loss of
10 people's lives as a result of that bridge failure.

The term "scour", as used here, is defined as the Lowering of the streambed
below a natural level or below an assumed datum. Thus "scour depth" is the
depth to which material is removed below that natural level or assumed datum.
Some scour occurs naturally in nearly all 3treams, but is a major concern in
alluvial channels. Total scour around bridges can result from a combination
of three interrelated phenomena:

Local scour - erosion of the streambed caused by local di~turbances
in the flow, such as vortices and eddies in the vicinity of piers
and abutments.

Constriction scour - streambed erosion caused by increased flow
velocities in and near a bridge opening due to the decr~a~ed flvw
area formed by the bridge, its approach embankments, and piers.

Ceneral scour - progressive degradation of the streambed caused by
natural processes or by changes in channel controls that occur ~ver
a long channel reach and over a long periOd of time. ThLS type of
scour may occur in a channel even if no bridge is pre~ent.

Adequate definition of the potential s~our at bridge sites is es~entlal to tile
proper design, construction, and maintenance of bridges. Accurate estimate~
of scour depths are a prerequizite for safe, co~t-effective bridge designs.
Underestimated scour depths - hence, underestimatiun of depth~ of excavation
for foundations or depths to drive piles - puts the structure and human life
at risk. Grossly overestimated scour depths result in overdesign, which
translates into an economic loss in the form of higher construction costs.

Several published equations for predicting local scour depths are available,
but the wide range in the predicted depths have prevented designers from using
these equations with much confidence. Host of these equations are Dased upon
scale-model laboratory measurements and have not been validated due to the
lack of onsite high-flow scour data. Bridge designers and bridge inspectors
need more reliable methodology to estimate scour depths fO" the conditions
found within (::>study area<::), Collection of onslte scour data has been
recommended as a way to improve scour pre~iction for both design ~ld
evaluation purposes (Jones, 1984; Jarrett and Boyle, 1986). This study Is
designed to achieve that goal for r::~study area(::).

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