Field Observations and Evaluations of Streambed Scour At Bridges
CHAPTER 7: SUMMARY AND CONCLUSIONS
The analysis and prediction of scour at bridges is complex. Scale models
and design methodology must account for the variability of site conditions
and the potential interaction of the various components of scour. Scour
at bridges has traditionally been classified into the categories of degradation,
contraction scour, and local scour (abutment and pier). These categories
do not explicitly account for the natural scour and deposition that occurs
in a river during a flood or series of floods. Data collected at bridges
during floods must be carefully analyzed to determine the magnitude of
local and contraction scour. If appropriate reference surfaces are not
selected, the scour reported may reflect scour caused by other processes.
Researchers and design engineers have agreed that field data on bridge
scour are needed to validate laboratory experiments and to ensure the
reliability of design methodology. The USGS, in cooperation with FHWA
and many State highway agencies, has collected and compiled data on scour
at bridges. The national database has been expanded and now contains 493
local pier scour measurements, 18 contraction scour measurements, and
12 abutment scour measurements from 79 sites located in 17 States.
Various researchers have proposed many pier scour equations,
but none have accurately and conservatively predicted the scour observed
in the field. Most equations are based on scaled laboratory experiments
that did not account for the complexity of the field conditions: Relations
among dimensionless variables developed from these laboratory experiments
did not compare well with the field data. The Froehlich Design, HEC-18,
HEC-18-K4, HEC-18-K4Mu, HEC-18-K4Mo (>2 mm), and Mississippi equations
proved to be better than other equations for predicting pier scour for
design purposes. The comparison of the scour depths predicted from these
equations with scour depths measured in the field clearly showed that
processes are reflected in the field data that are not correctly accounted
for in these equations. Analysis of the pier scour data indicated the
importance of bed material characteristics as an explanatory variable
in the predictive equations. A new K4 term for the HEC-18
equation was developed based on the relative bed material size (b/D50).
Although caused by the same geometric contraction of the floodplain,
contraction and abutment scour have traditionally been studied and treated
separately. Contraction scour equations are based on theoretically developed
equations that do not account for the complexity of the flow conditions
in the field. Likewise, most abutment scour equations are developed from
laboratory experiments that oversimplify or ignore many complexities common
at highway bridges. Field data are needed to evaluate published approaches
to computing contraction and abutment scour. A review of the literature
found 29 references with mention of contraction and (or) abutment scour
data, but only one presented detailed data collected during floods. Only
two references included data on abutment scour. Published comparisons
of field data with computed scour showed mixed results. Four papers showed
the contraction scour equation typically overpredicted the observed scour
and in a few instances severely overpredicted the scour; however, one
paper showed the Laursen contraction scour equation underpredicted a number
of measurements, and no severe overprediction was observed. The accuracy
of the contraction and abutment scour equations recommended in HEC-18
may depend greatly upon the degree of contraction, the flow distribution,
the configuration of the approach, and how well the hydraulic model represents
the true flow distribution.
Comparison of computed abutment and contraction scour depths with depths
measured in the field for U.S. Route 12 and Swift County Route 22 over
the Pomme de Terre River in Minnesota provides insight to the capabilities
and limitations of using one-dimensional models and the available abutment
and contraction scour equations to predict scour at contracted bridge
openings. The application of the methods outlined in HEC-18 to these sites
showed a variability of results similar to the comparisons published in
the literature. HEC-RAS and the equations recommended in HEC-18 provided
reasonable predictions for maximum total scour at the two bridges; however,
the magnitudes of the individual components (abutment and contraction)
of scour did not compare well with the field data. Although field data
in the approach sections were inadequate to provide a comprehensive evaluation
of the ability of a one-dimensional model to represent a complex two-dimensional
flow field, the comparisons that could be made showed the one-dimensional
model computed flow distributions comparable to the field data for the
fully developed scour hole conditions, but were less accurate for initial
conditions and in areas of highly curvilinear flow.
Overall, the methodology for computing scour at bridges published in
HEC-18 provides estimates that are generally conservative, in that the
depth of scour is usually overpredicted. The complexity and variability
of conditions at bridges make the development of predictive methodology
difficult. The equations oversimplify most conditions, but modification
of the methodology to account for site complexity and variability is not
a simple task. New methodologies must balance the desire to fully explain
complex processes with the need to provide procedures that are time and
cost effective to apply. Additional field data and model studies are needed
to continue to improve scour prediction methodology.
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