Assessing bridge vulnerability and risk due to stream instability

The FHWA recommends that stream stability analyses begin with a Level 1 assessment. Following data collection and observations at the bridge, the user must determine whether the relative risk is low or not. If it is low, then no action is needed. If the risk is greater than low, then a Level 2 analysis is recommended. In this paper, the relative risk of failure due to stream channel instability at a bridge is assessed as a simple function of vulnerability and criticality. Vulnerability is based on a stream stability assessment and the National Bridge Inventory (NBI) ratings for channel condition for a particular bridge. Criticality is determined indirectly as a function of the bridge importance, using data extracted from the NBI. Relative risk is then qualitatively determined by combining vulnerability and criticality. An example is provided in which the relative level of risk is used to determine the need for a Level 2 analysis.