Vulnerability-Based Risk Assessment for Stream Instability at Bridges

The Federal Highway Administration recommends that stream stability analyses at bridges over water begin with a Level 1 assessment. Following this assessment of the physical condition of the stream in the vicinity of the bridge, the user must determine whether or not the relative risk is low. If it is low, then no action is needed. If the risk is greater than low, then a Level 2 analysis is recommended; however, no method is given for determining the risk. In this paper, the relative risk of unsatisfactory conditions at bridge foundations is assessed as a simple function of vulnerability and criticality. Vulnerability is based on a stream stability assessment and data from the National Bridge Inventory (NBI). Criticality is determined indirectly as a function of the bridge serviceability and cost by using data extracted from the NBI. Relative risk is then qualitatively determined by combining vulnerability and criticality. Three examples in central Pennsylvania are provided in which the relative level of risk is used to determine the need for a Level 2 analysis.