Online

Seigel, Rachel M.

Civil and Environmental Engineering

2021

M.S.

The 2011 Tropical Storm Irene resulted in considerable property and infrastructure damage in Vermont and neighboring states, including damages to and failure of over 300 bridges and 800 km (500 miles) of roads in Vermont alone, which brought to light the vulnerability of regional transportation infrastructure to extreme flood events. The northeastern United States is experiencing more frequent precipitation events of longer duration (i.e., extreme events). Infrastructure therefore must be able to withstand more frequent flood events of greater magnitude. It is not feasible to analyze and retrofit each structure for the rigorous hydraulic demands of extreme flood events; so prioritizing limited resources to locations at greatest risk in order to minimize flood damage is critical. Current state of practice is often limited in scope to steady-state analysis in the immediate vicinity of a specific structure or feature, and the far-reaching impacts up- and downstream the river are often not understood and considered in decision making. To better understand the interactions among rivers, hydraulic structures and surrounding hydrogeological features, a two-dimensional (2D) transient HEC-RAS (Hydraulic Engineering Center's River Analysis System) model of a Mad River Reach was constructed and calibrated. Available 2D HEC-RAS models of two additional Vermont river reaches supplemented the study allowing comparisons across a range of river gradients. The analyses considered the 2011 Tropical Storm Irene, as well as flood events that have annual exceedance probabilities of 50%, 4%, 2% and 1%, to analyze hydraulic impacts and interactions surrounding transportation infrastructure. A screening framework, that uses the 2D hydraulic modeling results, was developed to identify bridges and sites best suited for hydraulic intervention such as floodplain lowering and reconnection and addition of culverts for mitigating the impacts of extreme flood events along the bridge-river network. These interventions were then simulated in the developed 2D HEC-RAS models of the three study reaches. The results of the baseline and intervention models were examined to quantify bridge-river interactions on a reach scale, evaluate the overall effectiveness of the screening framework, and identify reach-level impacts of flood mitigation interventions. The results indicate that the developed screening framework that combines geomorphic and hydraulic characteristics can identify suitable bridges and other locations along a river for flood mitigation intervention. The screening framework is comparatively more applicable to moderate to high gradient rivers, but may still be applied to lower gradient rivers with supplementary data from prior flood damage reports and inspection records. The results demonstrate that the interventions have cascading effects up and downstream of the intervention locations. Interventions simulated on a moderate or high gradient river have farther-reaching effects that are often less intuitive up and downstream compared to a low gradient river highlighting the importance of a transient, two-dimensional hydraulic analysis. Overall, the results suggest that bridge flood mitigation projects in similar geographic and climate settings should consider the up and downstream geomorphic and hydraulic characteristics to better understand the potential impact the intervention will have on the bridge-river network.