PROJECT CHIEF: Tom Edwards
PROJECT EXTENT: Selected sites
Several bridges have failed in the United States within the last 10 years and many of those failures were caused by scour around bridge piers or abutments. The most recent failure that resulted in loss of lives was of a California Interstate 5 bridge, in early March 1995. As part of a program to eliminate bridge failures in the United States, the Federal Highway Administration (FHWA) established a nationwide bridge inspection program. Using guidelines established by the FHWA, individual State department of transportation offices make these bridge inspections. Every bridge over water is inspected and given a classification. Those bridges classified as "scour critical" are given immediate attention; they are either repaired, replaced, or monitored.
The USGS has been monitoring scour at selected bridge sites for the Oregon Department of Transportation (ODOT) since October 1988. The first study monitored scour at a new highway bridge over the Alsea Bay near Waldport, Oregon (see U.S. Geological Survey Open-File Report 91-531). A second study, which involved using geophysical methods to monitor scour was conducted at several sites in Oregon (see U.S. Geological Survey Water-Resources Investigations Report 90-4199). The third study, started in 1991 and planned to continue until September 1999, is monitoring scour-critical bridges in Oregon (see U.S. Geological Survey Open-File Report 95-366). In conjunction with this study, the Oregon District has been involved with testing and evaluating new scour-monitoring devices.
Bridge-scour monitoring techniques have improved. The original method employed a depth sounder that was modified to output depth data to a data logger. Now, off-the-shelf sonar devices capable of transferring data to computers or data loggers are being used, as well as three different types of driven-rod scour-monitoring devices. The driven-rod devices will be tested during the 1996 water year.
Click here for an example diagram of a typical scour-monitoring installation.
The Sites are visited monthly during the high-water season to make field measurements of velocity, scour-hole depth, water-surface elevations, and to retreive the data. Attempts are also made to make field measurements during high water. The miscellaneous scour-monitoring sites are visited twice, once after the high-water season to determine maximum scour for the year, and once in the fall to prepare the equipment for the following high-water season.
There are six sites being monitored this fiscal year: three continuous- recording sites, two miscellaneous sites, and one test site for new methods of monitoring scour. The three continuous sites are (1) Wallowa River at Minam, Oregon, at Highway 92, (2) Sandy River at Troutdale, Oregon, at Interstate 84, and (3) Crabtree Creek near Scio, Oregon, at Highway 226. The two miscellaneous sites are Mill Creek at Salem, Oregon, at Highway 22, and Little Muddy Creek near Harrisburg, Oregon, at Interstate 5. A test site for new methods of scour monitoring was established at Waldport, Oregon, at Highway 101.
OFR 91-531. Bridge-scour data for the Highway 101 bridge over Alsea River Estuary at Waldport, Oregon--1988-90, by Milo .D. Crumrine.
OFR 95-366. Bridge-scour instrumentation and data for nine sites in Oregon, 1991-94, by M.D. Crumrine.
The URL for this page is < http://oregon.usgs.gov/projs_dir/pn380/pn380.html >