William Sweet

Increased nuisance flooding along the coasts of the United States due to sea level rise: Past and future

Author(s): Moftakhari, HR; AghaKouchak, A; Sanders, BF; Feldman, DL; Sweet, W; Matthew, RA; Luke, A | Abstract:

Global and regional sea level rise scenarios for the United States

Environmental issues and disasters/Climatic and atmospheric; Environmental issues and disasters/Flood

In Tide’s Way: Southeast Florida’s September 2015 Sunny-day Flood

The Flood Event. High tides on 27 September 2015 f looded several Miami-region communities with 0.57 m of ocean water. The flooding was concerning because of the sunny-day conditions and awareness that trends of such events are accelerating within U.S. Atlantic Coast cities from rising seas (Sweet et al. 2014; Ezer and Atkinson 2014; Sweet and Marra 2016). It was the sixth largest flood measured by the National Oceanic and Atmospheric Administration (NOAA) tide gauge in Virginia Key, Florida (Miami region), since its 1994 installation (Fig. 6.1a). The five higher floods were in response to hurricanes. The f lood had substantial astronomical underpinnings (Fig. 6.1b); it occurred during spring tides and near the peak of the seasonal mean sea level (MSL), the lunar 8.8-year perigee, and the 18.6-year nodal cycles. These factors explain the 0.24-m NOAA tide prediction relative to mean higher high water (MHHW) tidal datum that delineates typical tidal inundation (Schureman 2001; Parker 2007). Yet, tide forcing alone was insufficient to produce the observed impacts as minor “nuisance” flooding begins in excess of 0.4 meters in this region (Sweet et al. 2014). Other dynamics were at play. A nontidal sea level anomaly (Fig. 6.1b, green line), which exceeded 0.15 m for a month starting September 22, reached 0.33 m during the flood and even higher for weeks afterwards. Strong high pressure over Eastern Canada (Fig. 6.1c) with >15 m s−1 northeasterlies offshore of the mid-Atlantic Bight (not shown) drove an Ekmanrelated setup along much of the U.S. East Coast. During the flood, setup was >20 cm along the southeast Florida coast as modeled by NOAA’s extratropical surge and tide operational forecast system (Funakoshi et al. 2013). Local winds, however, were calm (<3 m s−1; http://tidesandcurrents.noaa.gov/met), inverse barometer effects nonexistent (Fig. 6.1c), and dynamical wave effects minimal as inferred by the ~1 cm standard deviations during tide measurements (Sweet et al. 2015). Interestingly, Gulf Stream transport measured upstream in the Florida Current (FC) slowed to a monthly minimum of 23.4 Sverdrup (Sv; 1 Sv ≡ 106 m3 s−1) on 25 September (Fig. 6.1d), which persisted through the flood. Transport slowdowns raise MSL along the US southeast (Zhao and Johns 2014; Ezer 2016) and Florida coasts (Park and Sweet 2015) from adjustments to meridional Ekman transport (Lee and Williams 1988) and shelf-wave dynamics (Czeschel et al. 2012; Ezer 2016). Previous studies report a 0.5–1.5 cm rise in coastal MSL per 1-Sv decline in Gulf Stream system transport (Ezer et al. 2013; Woodworth et al. 2014; Goddard et al. 2015; Ezer 2016); when it slows, local tidal-flood risks increase (Sweet et al. 2009; Ezer and Atkinson 2014; Wdowinski et al. 2016). Here, we derive a contemporary return period of the flood using a time-dependent extreme value statistical model. Then, we assess the degree that (i) seasonal variability, (ii) tide cycles, (iii) FC monthly transport minimums, and (iv) a multidecadal trend have independently affected Virginia Key’s extreme water level distribution and estimate their attribution during the f lood. We conclude by analyzing how the flood’s return period changes under future SLR projections for the Miami region forced by three representative concentration pathways (RCP).

Recent and Future Outlooks for Nuisance Flooding Impacts on Roadways on the US 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.