T. CALAPPI ET AL.
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ing design equations. Specifically, the Froehlich Design
equation is both based on field data and uses an additive
adjustment. It is also considered among the top perfor-
mers in the Mueller and Wagner [2] study. Additionally,
laboratory-based equations are not without problems
(idealized conditions and scale effects). In fact, deficien-
cies in the leading equations stem from their reliance on
laboratory data [11].
This analysis shows that developing a family of equa-
tions in a similar format to the current HEC-18 equation
(Equation (1)) reduces the mean square error of predic-
tion and reduces the overall amount of over-prediction.
This study and others show the current HEC-18 equation
significantly over predicts scour in most cases, resulting
in increased construction costs. As shown in this study,
using field-scale data, partitioning the data set and defin-
ing regression parameters for specific conditions leads to
significant reductions in estimated scour depths while
maintaining scour over prediction.
HEC-18 was chosen for the starting point for this mo-
del development because it is the current scour model
approved by the FHWA and therefore widely used. Many
additions were made to the field of pier scour prediction
since the FHWA implemented HEC-18. The FHWA is in
the process of evaluating these new models. While the
CSU-based equation may not always be the recommended
pier scour equation in HEC-18, the authors feel the
framework developed in this study can be applied to
wide array of base equations and datasets.
5. Acknowledgements
The authors would like to acknowledge the Michigan
Department of Transportation for support of Project
#108493-Contract 2007-0436 and Project #85106-Con-
tract 2006-0413. The authors also recognize the com-
ments of all reviewers including the MDOT Bridge Scour
Technical Advisory Group and Dr. Peggy Johnson.
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