MONITORING SCOUR AT THE STATE HIGHWAY 15 BRIDGE ACROSS THE LEAF RIVER AT BEAUMONT, MISSISSIPPI
by K.V. Wilson, Jr.Prepared by the U.S. Geological Survey
1991
PROCEEDINGS OF THE 21st MISSISSIPPI WATER RESOURCES CONFERENCE
CONTENTS
Introduction
General scour
Constriction scour
Local scour
Date collection
Results of Surveys
Conclusions
References
FIGURES
1. Location of Leaf River at State Highway 15 at Beaumont, Mississippi
2. Approximate thalweg profiles of Leaf River at Beaumont, Mississippi
3. Variation of minimum bed at cross section 4, at upstream side of State Highway 15 main channel bridge at Beaumont, Mississippi
TABLES
1. General scour at cross sections 1, 5, and 7 (about 950 feet downstream, about 300 feet upstream, and 1,300 feet upstream from main channel bridge, respectively)
2. Constriction scour at cross sections 2, 3, and 4 (about 50 feet downstream from, downstream side of, and upstream side of main channel bridge, respectively)
ABSTRACT
Scour of channel-bed material around bridge piers and in the vicinity of bridges is a potential problem that must be considered when constructing bridges spanning waterways. Erosive action of flowing water may expose or undermine bridge-pier and bridge-abutment foundations and thereby induce structural failure. Accurate estimates of potential scour are essential in the design, con- struction, and maintenance of bridges. The collection of scour data at bridge sites during floods provides a better understanding of scour and provides the information necessary to improve methods that can be used in bridge design to estimate scour. Because of the difficulty of obtaining scour data around bridges during floods, few data have been collected.
The term "scour", as used in this report, is defined as the lowering of the channel bed by erosion below an assumed natural level or other appropriate datum; "scour depth" is the depth to which material is removed below the stated datum. Scour, a phenomenon that is a cause of concern for alluvial streams in Mississippi, can be attributed to three interrelated phenomena:
General scour: progressive degradation of the channel bed caused by natural processes or by changes in channel control that may occur over a long channel reach and, possibly, over many years. General scour could also be a temporary fluctuation about some mean level. This type of scour may occur in a channel, even if no bridge is present.
Constriction scour: channel-bed erosion caused by increased flow velocities through a bridge opening due to the decreased flow area formed by the bridge, its approach embankments, and its piers.
Local scour: erosion of the channel bed caused by local disturbances in the flow, such as vortices and eddies in the vicinity of piers and abutments.
Although these components of scour are not completely independent, general practice in bridge design is to compute each component of scour separately, using equations based on scale-model laboratory measurements, and then combine these predicted scour depths to estimate the total scour depth at a bridge site. Scour data at bridge sites are needed to verify and improve the scour-predicting equations. However, in evaluating field data for a natural stream at a bridge site, it is usually difficult to identify whether the scour measured is general, constriction, or local scour. Measurements of scour depth are complicated by channel-bed conditions, debris, effects of multiple piers, flow distribution, and definition of the reference elevation used to determine scour. Also, measured scour depths could be partially due to extreme floods that have occurred prior to data collection, and, therefore, total scour depths could be incorrectly associated with measured floods.
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For questions or comments, contact K. Van Wilson.