FROM MODELING TO MONITORING: STREAM BED SCOUR AT HIGHWAY CROSSINGS IN ALASKA

Stream bed scour is the leading cause of bridge failure in the United States, resulting in nearly 60 percent of all failures. Bridges in Alaska are subject to adverse hydrologic conditions such as ice jams, glacial outburst floods, rain on snow events, and extreme tides that intensify scour. These conditions and the remoteness of Alaska’s limited transportation corridors make the stability of Alaska’s bridges a major concern. The U.S. Geological Survey and the Alaska Department of Transportation and Public Facilities are involved in a cooperative study to determine susceptibility of Alaska’s bridges to scour and to monitor those structures that are susceptible. Computer simulations of 100- and 500-year recurrence interval flood flows were used to determine hydraulic variables needed to estimate scour at 325 bridges. These initial scour estimates identified the bridges that were most vulnerable to scour and needed further study and monitoring. Locations were selected for intensive field data collection and more rigorous hydraulic modeling. Currently, streambed elevation is monitored in near real-time at 10 bridges in Alaska. This monitoring network provides information on the immediate safety of the structures and field data to enhance scour prediction equations.