Recent and Future Outlooks for Nuisance Flooding Impacts on Roadways on the U.S. East Coast


Tidal floods (i.e., “nuisance” flooding) are occurring more often during seasonal high tides or minor wind events, and the frequency is expected to increase dramatically in the coming decades. During these flood events, coastal communities’ roads are often impassable or difficult to pass, thus impacting routine transport needs. This study identifies vulnerable roads and quantifies the risk from nuisance flooding in the Eastern United States by combining public road information from the Federal Highway Administration’s Highway Performance Monitoring System with flood frequency maps, tidal gauge historic observations, and future projections of annual minor tidal flood frequencies and durations. The results indicate that tidal nuisance flooding across the East Coast threatens 7508 miles (12,083 km) of roadways including over 400 miles (644 km) of interstate roadways. From 1996–2005 to 2006–2015, there was a 90% average increase in nuisance floods. With sea level rise, nuisance-flood frequency is projected to grow at all locations assessed. The total induced vehicle-hours of delay due to nuisance flooding currently exceed 100 million hours annually. Nearly 160 million vehicle-hours of delay across the East Coast by 2020 (85% increase from 2010); 1.2 billion vehicle-hours by 2060 (126% increase from 2010); and 3.4 billion vehicle-hours by 2100 (392% increase from 2010) are projected under an intermediate low sea-level-rise scenario. By 2056–2065, nuisance flooding could occur almost daily at sites in Connecticut, New Jersey, Maryland, the District of Columbia, North Carolina, and Florida under an intermediate sea-level-rise scenario.

Transportation infrastructure in coastal regions is vulnerable to coastal extreme events today; this vulnerability will increase with sea level rise (SLR), enhanced storm surge from tropical and nontropical storms, and land subsidence (1,2). Hurricanes have caused billions of dollars in direct damage to coastal roadways and bridges. Significant economic losses have also occurred due to transport disruption during and after these storms. Currently, 60,000 miles (96,561 km) of roadways are exposed to coastal storms, as noted in Douglass et al. (2). In the future, rising seas will cause more frequent disruptions, more damage, and storm surge impacts to extend further inland.

In some coastal areas, impacts are not limited to storm events. Critical transportation infrastructure is at risk from SLR alone. SLR-induced coastal flooding decreases service and increases maintenance costs for existing facilities; mitigation strategies will be an increasingly critical aspect of transportation planning, design, and operations and maintenance (3–5). Numerous transportation agencies have identified assets vulnerable to permanent inundation for sea-level-rise scenarios for the United States that range from an increase in 1–8.2 feet (0.305–2.5 m) by the year 2100 with sea-level-rise values along the U.S. Atlantic and Gulf Coasts likely to be greater than the global average (6).

SLR is also contributing to an increasing frequency of tidal floods (i.e., “sunny day,” “recurrent,” “shallow coastal,” and “nuisance” flooding), occurring more often during seasonal high tides or minor wind events. Some portions of the U.S. coast, including much of the Atlantic coast, are seeing an accelerating frequency of such events (7, 8). Flooding that surpasses local emergency thresholds for minor tidal flooding (i.e., “nuisance” levels of about 30–60 cm (1–2 ft)) result in flooded infrastructure including roadways. These nuisance floods have increased 5- to 10-fold or more since the 1960s along the U.S. coastlines (8–12). For example, cities such as Annapolis (Maryland), Norfolk (Virginia), and Miami Beach (Florida), are now flooded numerous times per year (42, 11, and 17 days in 2016, respectively) (13). With rising sea levels, such flooding is expected to increase dramatically in the coming decades (8, 10, 14–16).

The damage to coastal infrastructure from recurrent flooding is less well understood than that due to extreme coastal events or gradual SLR. In some regions, the cumulative cost of nuisance flooding could be comparable to, or exceed costs from, extreme coastal storm events (9). Dahl et al. determined that, currently, there are 91 communities where 10% or more of livable land area is flooded at least 26 times per year and that number will be likely to nearly double by 2035 (14). A primary impact of nuisance flooding in communities is rendering roads impassable or difficult to pass so frequently that routine transport needs are not met. Although the importance of transportation disruptions from nuisance flooding is broadly understood, few studies (16) have quantitatively addressed how nuisance flooding impacts roadway flooding.

The goal of this study is to broadly understand the type and extent of roadway infrastructures that are vulnerable to nuisance flooding and the transportation impacts now and in the future due to SLR. This study focuses on the East Coast of the United States and includes all coastal states from Maine to Florida, including the Gulf Coast of Florida and Key West. In this study, we determine the number and length of roadway segments (not including bridges, tunnels, or causeways) at risk from tidal nuisance flooding. The number of days per year that these vulnerable roadways are flooded from nuisance flood is estimated for current, mid-century, and late century. Current traffic counts are combined with the total hours per year that the roads are inundated to determine annual transportation impacts for these same periods. Results are summarized by each state and differentiated by the road type using the Federal Highway Administration (FHWA) functional classifications.


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Transportation Research Record: Journal of the Transportation Research Board



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