“Sunny Day Flooding”, also known as tidal flooding, is a phenomenon that occurs when low-lying areas are temporarily flooded during periods of unusually high tides, such as during a full moon. This may happen when the sky is clear and sunny, with not a single cloud on the horizon. During a sunny day flood event, streets can be covered by water in an unexpected way, because residents are not prepared for floods in the absence of rain or storms. Nevertheless, this flooding does occur during dry weather, and the frequency of these floods is increasing in an alarming way.
This summer, the federal government issued a warning to those who live in and around low-lying zones. The National Oceanic and Atmospheric Administration (NOAA) has identified dozens of communities that will suffer an increase in sunny day floods, from Miami to San Diego. The increase that is expected during the coming year is attributed to an abnormally active El Niño weather pattern, combined with sea level rise that is happening across the globe. Rapidly melting ice caps and glaciers from Antarctica to Greenland continue to feature prominently in news coverage, and videos of huge chunks of polar ice calving into the sea remind us all that we can expect major changes, and soon!
Sunny day high tide nuisance flooding in Brickell, downtown Miami Florida. The morning high tide on October 17, 2016. Roughly 4.0 ft MLLW, +3 ft above MSL, 2 ft NAVD 88, about 1.75 feet MHHW for Miami, Virginia Key tide gauge. This location rounds to 0 meters/0 feet AMSL.
One of those changes is sure to be a flood-filled future. This year, with the hyperactive El Niño weather pattern in play, all types of floods are predicted to be a problem. But the “sunny day flood” brings a special kind of threat, mostly due to the general lack of preparation of those affected. Sea level rise, coupled with sinking coastlines along the eastern seaboard, has greatly increased the number of affected communities. During these “sunny day floods”, roads are often covered by water; and in many cases vital thoroughfares are closed until the water recedes. Flooded roads are subsequently plagued by potholes, erosion of shoulders, and instability of the roadbed, which can lead to washouts.
In addition to road damage, affected communities also suffer many other negative consequences from frequent flooding. Basements and underground parking structures for homes and businesses fill with water again and again, followed by days or weeks of backbreaking clean-up. Storm water systems can become overwhelmed and damaged, and septic systems can overflow, polluting property and waterways. Many homes in rural areas rely on septic systems for disposing of toilet wastewater, so one can only imagine how inconvenient it must be when these systems become temporarily unusable due to flooding.
Low-lying areas are particularly vulnerable, of course, and nowhere in the US is more threatened than Florida. With much of the state at or below 10' above sea level, every inch lost to sea level rise or subsidence (sinking) of the coastline translates to real hardship for the people living in these areas. After the floods have receded, many homeowners find the rebuilding process to be all but impossible. Flood insurance, for those lucky enough to have it, becomes prohibitively expensive after a series of floods. Insurance companies or community code may suddenly require that a building be raised on stilts, or that repairs shall not be possible at all. Some zones may be declared no longer supported for rehabilitation, and repair of buildings in these areas would become illegal. When that happens, the buildings must by law be left to slowly fall apart by attrition and neglect, even as the property owner may still have to pay a mortgage for the uninhabitable home.
This year may be particularly hard for home and business owners in these low-lying communities. While coastal zones have typically suffered about five days a year of “sunny day flooding”, this record will most likely be smashed during the 2019-20 flood season. But even this is nothing compared to the predictions for 2050, by which time “sunny day floods” are projected to become a very common nuisance. Within the next thirty years, it is thought that many unlucky communities will suffer sunny day floods up to 100 days out of each year!