DISCUSSION: Flooding is generally associated with the presence and aftermath of strong storms and organized weather systems such as hurricanes. However, tidal flooding is an instance of flooding that often times goes unsaid. Even though no rain is associated, nuisance ponding of otherwise busy pedestrian sidewalks and local roads presents challenging situations, especially in high-impact areas of coastal U.S. cities including Miami, FL, Boston, MA, New York City, NY, and Atlantic City, NJ.
Through a basic understanding of physics, the nature of tides is quite intuitive – as the moon orbits Earth, the ensuing gravitational pull acts as a force on the oceans that cause the oceans to bulge ever so slightly towards the moon on both sides of Earth. During either a new moon or full moon, the alignment of the sun and moon cause the highest tides of the month known as spring tides, and this is where the highest high tides and lowest low tides are observed. Perpendicular orientation between the sun and moon leads to neap tides, or more tempered changes in tidal heights. Granted, the overall gravitational force of the moon is only a miniscule fraction compared to that of the Earth (and the sun), but the close proximity of the moon compared to that of the sun and Earth’s centrifugal force created by its own spin is what makes the moon the primary mechanism for generating tides.
Tides are innocuous enough and a normal, periodic harmony between Earth, solar, and lunar forces. At certain times of the year, though, the spring tides are exacerbated enough that tidal water encroaches on infrastructure and the public, causing tidal flooding. An example of this is the annual “king tide” flooding event that occurs primarily over portions of eastern south Florida during September and October. During the “king tide”, coastal cities and communities surrounding Miami are exposed to flood waters that overrun roads and ground floors of houses and businesses. Easterly winds and late rainy season thunderstorms compound the flooding situation by spreading the excess water inland. A more recent episode of significant high tide occurred in Venice, Italy where roughly 75% of the city was inundated to some degree with flood waters (in combination with heavy rains and strong onshore winds). Water levels in the Italian city rose well over 150 cm before gradually receding, leading to significant impacts.
A growing concern with these tidal floods is the interaction of tides with sea level rise. Previous research has shown that more tidal flooding events are likely to occur as mean sea level increases, accelerated primarily by the melting and runoff of land-based glaciers into the oceans. Whether or not this increase will be large or small with time will depend on many other factors, but projections from coupled ocean-atmosphere models yield a common result of increased flooding events. Alluding back to the effects in south Florida, most residents there source their drinking water from the nearby Biscayne Aquifer. With an increased frequency of coastal flooding events, salt water intrusion into the aquifer can adversely affect the water quality for a heavily populated area of the state. More frequent and prolonged coastal flooding events may lead to enhanced salination which can pose challenging problems for both infrastructure and agriculture. It goes without saying that these impacts are on a global scale and coastal low-lying cities in other countries are prone to the same issues and concerns.
As before, while the exact changes and impacts remain a challenge to pinpoint, it highlights the importance of monitoring. Enhancements to the coupled ocean-atmosphere models will allow for scientists to better understand the changes and subsequent impacts of coastal flooding events.
Image Credit: Stefano Mazzola/Awakening/Getty Images
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© 2018 Meteorologist Brian Matilla