Evaluating Changes in Design Flow for Road-stream Crossings in West Tennessee using CMIP6-based Hydroclimate Projections

West Tennessee is an ~28,000 sq km region in the state of Tennessee that lies between the Tennessee River and the Mississippi River. It is characterized by flat terrain, channelized streams, and vast agrarian lands. Flooding and channel aggradation-degradation are common in this area. Road crossings over the vast network of streams in this region range from small pipe culverts to multi-span bridges. Many of these structures are aging and were not adequately designed. They are often incapable of draining water during high flow events creating flow obstruction and traffic disruption. This is likely to exacerbate in the future as prevailing design standards do not always account for the intensified extreme events under climate change. Therefore, the objective of this study is to evaluate the design flow changes at existing road-stream crossings under current and future climate conditions. We use Open Street Map and National Hydrography Plus version 2.1 to identify the locations of existing road-stream crossings. We estimate design flow (at 1% annual exceedance probability) at road-stream crossings using United States Geological Survey flood-frequency prediction methods for unregulated streams. The relative change in the future design flow is then estimated using ensemble hydrologic projections from the VIC-RAPID hydrologic model driven by downscaled Climate Model Intercomparison Project Phase 6 (CMIP6) global climate models. The ensemble streamflow projections are for 1980–2019 in the historic period and 2020–2059 in the near-term future period under the high-end (SSP585) emission scenario. The results project about a 30% to 50% increase in design flow in most streams, thus, suggesting the need for further evaluation. This study will help inform potential mitigation strategies and upgrading cost of existing structures to enhance climate resilience in West Tennessee.