United States Department of Transportation
i
Evaluation of design methods to determine scour depths for bridge structures : [technical summary].
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2013-01-01
[PDF-300.83 KB]
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Abstract:Scour of bridge foundations is the most common cause of bridge failures. The Federal Highway Administration
(FHWA) has developed a design method, HEC-18, for the state Departments of Transportation (DOTs) to evaluate
the scour potential of existing bridges and predict the scour depths for new bridges. The scour models in the HEC-18
were based on a number of empirical equations that were developed primarily from laboratory fl ume studies with
limited fi eld data verifi cation. Because of the complex nature of scour process, these scour-prediction equations
recommended in HEC-18 may tend to provide conservative scour depth estimates to ensure that an adequate
factor of safety is considered for bridge scour design. Underestimation of scour depths may result in costly bridge
repairs or even catastrophic bridge failures, while overestimation may cause costly, unnecessarily deep foundations.
The scour potential evaluation for existing bridges is also important. Overestimation of scour depths causes more
bridges to be misclassifi ed as “scour critical,” thus resulting in unnecessary installation of scour countermeasures
or bridge replacements. Currently, the Louisiana Department of Transportation and Development (DOTD) uses
the HEC-18 method for bridge scour design. Costs associated with the current design methods that usually lead to
conservative estimation of scour depths can be very high. On the other hand, DOTD has developed and maintained
a large database for a large number of bridge structures that are prone to scour. Those bridges were monitored and
hydrologic and hydraulic data collected. In addition, diff erent bed materials scour at diff erent rates and, because of
this fact, HEC-18 does not always accurately predict the scour depth at a certain time. A more reliable scour prediction
method is needed, especially for the clay and silty clay soils common in Louisiana,
with distinct local climatic characteristics (e.g., heavy downpours, severe storms, and
hurricanes).
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