USGS Home
Contact USGS
Search USGS

Click on a green dot () to return to the table of contents at any time.

 Problem Statement

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.

 Personnel

Richard J. Huizinga, P.E. (Project Chief) Hydrologist
Paul H. Rydlund Jr., E.I.T., Hydrologist

 Cooperating Agency

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.

Study Area.

 Objectives

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:

1. Develop a suitable screening process that MoDOT will employ to identify scour susceptible bridges. This effort includes planning, literature search, a field inspection technique, training MoDOT personnel, and development of a computer data base for MoDOT use. The USGS will assist by providing methodology to MoDOT in their application and quality assurance of the screening process. This objective was reached in 1994.
2. The USGS will establish an appropriate data-base management system that is transferable to MoDOT for their use in data entry and storage of Level I analyses and to evaluate scour susceptibility based on hydraulic and structural characteristics of the bridge crossing. The established data base will also be accessible by USGS. This objective was reached in 1996.
3. From the results of the screening process, the USGS will perform a more detailed Level I field review of those bridges identified by MoDOT as scour susceptible. This objective was modified in 1996 to incorporate the method for rapid estimation of scour at highway bridges based on limited site data (Level I+) developed in Montana. Approximately 1,300 bridge sites were identified as scour susceptible and were assessed using the Level I+ methodology. This objective was reached in April 1997, with approximately 225 bridges identified as requiring a detailed scour evaluation.
4. From bridges identified as most scour susceptible, the USGS will perform a Level II analysis and estimate values of scour using accepted hydraulic techniques for select sites and provide hydraulic information for possible implementation of countermeasures to MoDOT. This objective is in progress.

 Approach

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.

 Reports

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.

1. Clearwater contraction scour (152K) at left abutment, as viewed from the downstream face of the left abutment. Note the flow pattern in the grass.
2. Same as above, different angle. (152K)
3. Pier scour (165K) at a left overbank pier.
4. Field Procedures

 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.

Get Acrobat Reader

USGS Home Water Resources Biology Geography Geology Geospatial