A new risk-based measure of link criticality for flood risk planning

The importance of a reliable transportation system is often taken for granted but its value becomes undeniable when the system is severely disrupted with little warning, the area of impact is extensive, and the amount of time required to restore system connectivity is prolonged. For many states, the largest single cost of flood damage comes from damage to transportation infrastructure and the ensuing disruption of personal, commercial, and emergency-response travel. The goal of transportation risk planning at the state or provincial level is to consider the risks imposed by the vulnerability of network elements to strengthen the system’s ability to withstand a disruptive impact. Although separate measures of vulnerability and criticality are useful, a true risk-based measure would use the vulnerability of links in the network to calculate criticality, so links that are unlikely to be disrupted by a given hazard are left intact in the calculations. This paper details the development and application of the Network Criticality Index (NCI), a new link-specific, risk-based performance measure that uses flood-risk vulnerability scores and a Monte Carlo simulation process for simulating disruptive impacts and measuring link criticality. The paper includes an application of the NCI for flood risk planning to the Deerfield River basin in Vermont. Highly critical segments can be identified as those with the highest NCI values, and mitigation projects can be designed to mitigate the risk. The application finds that relatively high traffic flows can make highly vulnerable links critical due to a lack of redundant routes with adequate capacity to handle flows when they are disrupted by floods. Reducing risk to the highway system from flooding means either (1) improving the segment or crossing and thereby reducing its vulnerability score, or (2) influencing regional traffic flows to prevent disruptive effects on travelers when a disruption occurs. Addressing the vulnerability of the segment or crossing can be done with an engineering design that is sensitive to the project context and the need to improve its ability to withstand higher stream flows in the future.