Simulating hydraulic interdependence between bridges along a river corridor under transient flood conditions.

Structural alterations to bridges may result in unintended consequences up- or downstream, such as changes in discharge, velocity, stream power, and water levels. This work presents a framework and methodology to analyze interconnected bridge-stream interactions under flood conditions. Such analysis may help prioritize limited resources available for bridge and river rehabilitations, facilitate holistic design of bridges and better-informed cost/benefit analyses, and address stakeholder concerns raised in response to planned bridge and infrastructure alterations. A two-dimensional unsteady HEC-RAS hydraulic model of the Otter Creek between Rutland and Middlebury, VT is used to simulate the impact of individual structures on the bridge-stream network, as well as potential sensitivity to those impacts, during extreme flood events. The presence of a bridge and approach roadway may induce measurable changes in peak discharge throughout the entire 46 miles of modeled river. These may be by as much as 10% at adjacent structures, down to 1% at structures as far as six miles upstream and nine miles downstream. Depending on their characteristics, bridges and roadways may either attenuate or amplify peak flood flows up- and downstream, or have little to no effect at all, suggesting that there is no easily predictable impact, and that hydraulic modeling is necessary for such analysis on rivers. Alterations to structures that develop substantial backwaters may result in the most dramatic impacts to the network, which can be both positive and negative. Affected bridges may or may not be sensitive to these changes in discharge; structures that do not experience relief (e.g., roadway overtopping) may be most sensitive to any distant perturbations.