Missouri Water Science Center
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The Federal Highway Administration (FHWA) has established a requirement that all state highway agencies evaluate the bridges on the Federal Aid System for susceptibility to scour-related failure. A schedule has been proposed that would have all on-system bridges screened and evaluated for scour susceptibility within the next few years.
In the state of Missouri there are about 4,700 state-owned bridges requiring screening for scour susceptibility. A preliminary assessment by the Missouri Department of Transportation (MoDOT) in 1991-1993 indicated that as many as 3,300 of these bridges require field inspections to determine which may be susceptible to scour because of unstable river-channel conditions.
With such a large number of state-owned bridges that need to be evaluated as to their vulnerability to damage or failure from scour and with limited resources to repair or replace them, there is a need for a procedure to quickly identify and evaluate bridges that are scour susceptible. Such a procedure should identify bridges that would require a detailed bridge scour evaluation.
Period of Study
1993 to Present.
Richard J. Huizinga, P.E. (Project Chief) Hydrologist
Paul H. Rydlund Jr., E.I.T., Hydrologist
Missouri Department of Transportation (MoDOT)
Description of Study Area
The study area (11.1K GIF) covers the State of Missouri, which contains 114 counties plus the city of St. Louis. The MoDOT has divided the State into 10 highway districts. Missouri has a total area of 69,674 square miles, and contains parts of the Central Lowlands, the Ozark Plateaus, and the Mississippi Alluvial Plain physiographic regions.
The general objectives of the study are to assist MoDOT in identifying those structures that are scour critical and to evaluate the potential for scour at bridges in Missouri, thus allowing MoDOT to implement countermeasures. A scour-critical bridge is defined as a bridge whose structural integrity is potentially jeopardized due to scour at piers and/or abutments due to streamflow conditions and/or lateral migration of the stream.
The specific objectives of the USGS study are:
From the list of scour susceptible bridges selected jointly by MoDOT and USGS based on Level I and Level I+ analyses and other considerations, detailed scour evaluations (Level II) have been performed at 253 sites (13.5K). Current Level II analysis is being conducted in the Mississippi Alluvial Plain. The Level II analysis procedure includes determination of T-year flood discharges and other flood data, as well as a site visit to obtain channel and bridge geometry data, sediment samples, photographs, and to identify special conditions (e.g. debris accumulation (81.2K), footing exposure (85.5K), channel meanders and cutbanks (98.0K), and changes in land use, etc.) that could influence bridge scour. Step-backwater computations are made using a computer program, WSPRO (Water Surface PROfiler), which provides surface-water profiles and parameters necessary to calculate potential pier, contraction, and abutment scour using HEC-18. The computed potential scour depths are then plotted (16.9K) using the bridge geometry data obtained during the site visit and bridge structure data from MoDOT plans.
Huizinga, R.J., and Waite, L.A., 1994, Methodology for the assessment of scour at bridge sites in Missouri: U.S. Geological Survey Open-File Report 94-544.
Items of Interest
Contraction scour measurements made after flooding in Missouri during May and June of 1995.
Explanation of scour processes.
Contraction scour refers to the general lowering of the channel bed under the bridge. Contraction scour is initiated because of increased flow velocities through the bridge opening, change in local base-level elevation, or flow around a bend. The most common cause of contraction scour is the contraction of flow by bridge approach embankments that encroach on the floodplain or the main channel, or both.
Local scour (87.2K) is the removal of material around piers, abutments, spur dikes, and embankments caused by flow acceleration and turbulence near bridge sub-structural elements and embankments. Local scour can be exacerbated by accumulation of debris in a bridge opening.
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