PAST AND PRESENT INUNDATION EPISODES ON THE US EAST COAST: SHORT-TERM VARIABILITY AND ADAPTATION TO LONG-TERM SEA LEVEL RISE

Many global processes drive coastal sea level variability, e.g., melting land-based ice, crustal subsidence due to glacial isostatic adjustment, and steric effects on sea-surface height, but the societal impacts of coastal sea level rise are often local issues. Documentation of past and current inundation in specific areas may enhance outreach regarding the need for coastal adaptation. For example, many coastal cities now publicize “king tides” during the predicted yearly maxima of astronomical tides.

Here we use tide gauge data to show increased coastal inundation in recent years. We informally define inundation episodes as times when water levels remain >10 cm above local Mean Higher High Water for one or more hours. High water during storms is nothing new for coastal residents, but fair-weather coastal flooding has become more frequent and observable. A LIDAR-generated hypsometric curve of topography shows that the most frequently flooded (lowest elevation) areas occur along bays, sounds and estuaries, not the sandy barrier beachfronts. We also note that the stepped nature of coastal topography implies that the increase in flooded coastal areas will not be a smooth function of sea level rise magnitude.

To place sea level rise in a long-term context, we use published data from regional Holocene sea level reconstructions to explicitly illustrate the contribution of ongoing crustal subsidence due to glacial isostatic adjustment (GIA) at East Coast sites. In North Carolina crustal subsidence has been the dominant cause of sea level rise for most of the last 2000 years. Presently, however, GIA-driven crustal subsidence accounts for only about 1/3 of the sea level rise seen on the NC coast, whereas about 2/3 of the rise rate now being observed is due to anthropogenic warming.

Western boundary current (e.g., Gulf Stream) variability also affects coastal inundation. Rapid changes in East Coast water levels (± 25 cm in a few days) accompany changes in Gulf Stream speed and position, and thus can exacerbate coastal flooding due to sea level rise. We demonstrate the inverse relationship between Florida Current Transport rate and coastal water levels at many East Coast tide gauges, and show that brief (8-10 day) intervals of reduced Florida Current Transport increase coastal inundation during both fair weather and storms.