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King Tides: A Glimpse At South Florida’s Future

South Florida’s Vulnerable Coastline

In October 2014, astronauts aboard the International Space Station took a photograph of Florida (below), which clearly shows why Florida’s coastal regions are especially vulnerable to sea-level rise.

South Florida is ground zero for sea-level rise, and it is not only a problem that must be considered for the future. It is impacting the residents of South Florida right now. The average elevation of Miami-Dade County is just six feet.

And it’s not just Miami that is at risk. In a March 2016 Inside Climate News article, Katherine Bagley stated, “Scientists, city officials, planners and policymakers say that in the coming decades, climate change will impact nearly every aspect of life in Fort Lauderdale and the rest of South Florida, from the price of flood insurance to home values, drinking water supplies, infrastructure, the economy and health.”

Florida at Night (2014). The brightest continuous patch of lights is the Miami-Fort Lauderdale metropolitan area, the largest urban area in the southeastern U.S. and home to 5.6 million people. Image Source: NASA Earth Observatory.

Florida at Night (2014). The brightest continuous patch of lights is the Miami-Fort Lauderdale metropolitan area, the largest urban area in the southeastern U.S. and home to 5.6 million people. Image Source: NASA Earth Observatory.

 

According to a recent op-ed by environmental specialist Richard Wildermann, in Fort Lauderdale, “...water regularly spills over sea walls, lapping against foundations every few weeks. During king tides, which are the very highest predictable tides that occur twice a year, whole streets and neighborhoods are inundated. The 165 miles of canals in the city once effectively drained storm water into the ocean. Now seawater backs up through drain pipes into the canals and floods city streets. One-way valves are being installed in the drainage pipes to prevent the back up. But during high tides or heavy rain, the valves can’t open and the streets flood.”

The Southeast Florida Regional Climate Compact recently released their new projections for South Florida sea-level rise, and these projections show even greater sea-level rise over the next few decades. Note the difference between the “most optimistic scenario” and the “high risk projects” projection. What will these changes in sea level mean for the residents of South Florida?

Unified Sea Level Rise Projection from the Southeast Florida Regional Climate Change Compact, 2015. Image Source: www.southeastfloridaclimatecompact.org.

Unified Sea Level Rise Projection from the Southeast Florida Regional Climate Change Compact, 2015. Image Source: www.southeastfloridaclimatecompact.org.

 

Changing Tides

These long-term changes in sea level occurred over thousands of years, and they determined Florida geology. However, the state also sees variations in the ocean’s level every day because of changing tides. The changes are noticeable close to the shoreline, but they affect the entire ocean. What causes these changes in the tide?

While the moon plays the largest role in the gravitational pull that dictates the tides, the sun also plays a role. This gravitational pull from the moon and sun create two bulges on opposite sides of Earth, changing the height of the sea surface. These bulges, for the most part, follow the position of the moon.

During the new and full moon, the sun, Earth, and moon form a line in space. In this arrangement, the gravitational pull of the moon is increased by that of the sun, “pulling” the tide’s range to its maximum. These tides are known as spring tides, not because of the time of the year but because they “spring” upward. These spring tides are the highest and lowest tides of the month.

Around the first and last quarter moons, the moon and sun are at a right angle in respect to Earth. Therefore, instead of working together, the sun’s gravity is working against the gravity of the moon and this leads to tides levels at their minimum. These are known as neap tides and they occur twice a month. The cycle between spring and neap tides occurs approximately every seven days.

 

The Causes of Rising Sea Levels

The topic of sea-level rise has gained national media attention in the last several years. Earth has cooled and warmed—and, as a result, the area and volume of the ice on Earth’s surface has increased and decreased. In a period of a colder climate, there is more ice covering the planet, thus sea level is lower. During interglacial periods, ice melts and sea level rises. However, the origin of the melting ice makes a difference.

When ice over land melts, it adds to the volume of water in the oceans, causing the level of the water to rise. The melting of sea ice does NOT contribute directly to a rise in water level because the ice is already floating and displacing the water. Archimedes’ Principle explains why:

Water in the solid form (ice) is less dense than water, which causes it to float. Only about 10% of the ice floats above the surface, and when ice melts, the volume of the water that was displaced by the ice equals the melted ice or water. In other words, the change in volume exactly counterbalances the extra volume of the ice that was up above the water’s surface.

Another cause of sea-level rise is thermal expansion of the oceans. The oceans have a greater heat storage capacity than land due to water’s higher specific heat. Specific heat is defined as the amount of heat energy required to raise or lower the temperature of a gram of a substance by 1°C.

The ocean’s high heat capacity allows it to absorb and store a large amount of heat energy with a small increase in temperature. This unique property of water has allowed the oceans to absorb as much as 90% of the additional energy added to the climate system since the mid-1900s. But as the ocean absorbs and stores more heat energy, the volume of ocean water expands. This is referred to as thermal expansion. Therefore, thermal expansion causes an increase in sea level.

While vulnerable coastal cities will not become inundated overnight, scientific data from numerous organizations show that sea-levels are increasing globally. The causes of sea-level rise are well known. The unknowns are how quickly sea level will rise and the risks and costs to people around the world.

 

Rate of Rise Since the Industrial Revolution

According to the Intergovernmental Panel on Climate Change (IPCC), thermal expansion and glacier melt have been the main contributors to 20th century global mean sea-level rise. After 2,000 years of little change, sea level rose about 0.2 meters (8 inches) during the past century. About 75% of the observed rise (high confidence) since 1971 is from thermal expansion and glaciers in Greenland. Thermal expansion has occurred as the top 30 meters (1,000 feet) has warmed by 0.3°C (0.5°F) over the past 50 years. Since the early 1990s, the contribution of ice sheets in Greenland and Antarctica to sea-level rise has increased, partly due to the warming of the adjacent ocean. The Greenland Ice Sheet is experiencing record surface melting with a record rate of loss in the past decade. If Greenland melts, there could be as much as a 7.2 meters (24 feet) rise in sea level. If the West Antarctica Ice Sheet melted, ocean levels could rise by approximately 5 to 6 meters (16 to 20 feet). If all the ice on Antarctica melted, global sea level would rise 61 meters (200 feet).

An example of porous shell limestone from South Florida. A limestone aquifer supplies about 90% of South Florida’s drinking water.

An example of porous shell limestone from South Florida. A limestone aquifer supplies about 90% of South Florida’s drinking water.

 

South Florida’s Unique Geology

The geology of southern Florida is unique. Just underneath its surface is a porous substrate made of calcium carbonate known as limestone. Throughout Florida’s geologic history, there have been changes in sea level. During interglacial periods, when glaciers melted, sea levels in Florida would have been higher. During these periods of higher water, limestone (built up from the shells and skeletons of organisms that once lived in the oceans) formed on the shallow sea bottom. This sedimentary rock consists of layers of mineral and rock particles. Then, during glacial periods, the water would recede as glaciers formed, revealing much of the Floridian platform upon which the state sits.

Florida’s changing coastline across the ages. In a period of a colder climate, there is more ice covering the planet; hence, sea level is lower. During integlacial periods, ice melts and sea level rises.

Florida’s changing coastline across the ages. In a period of a colder climate, there is more ice covering the planet; hence, sea level is lower. During integlacial periods, ice melts and sea level rises.

 

Extreme High Tides & King Tides

Spring tides (Perigean-spring tides) occur twice a month throughout the year. However, these tides are more extreme during the fall months due to the shorter distance between the moon and Earth. These extremely high tides are sometimes referred to as King Tides, and they will become more prevalent in the future.

Some areas in South Florida are now seeing tides so high that many streets become flooded. During the King Tide of October 2015, many tide stations along the east coast were well over 3 feet at high tide for most the month.

As sea levels continue to rise, these flooding events will occur more frequently. Just 1.5 feet of sea-level rise can cause approximately 573 flooding events per year. This means that, for approximately three quarters of the year, these areas will experience extreme high tide events twice a day. For much of that time, the water will be so high that it will not retreat for several days, causing constant flooding over this period. Consider how this will affect the lives of residents. What will they have to do differently?

Along South Florida coasts are dozens of wells that tap into a freshwater aquifer. During periods when there is little rain and these aquifers have not had time to “recharge,” any withdrawal can lead to saltwater intrusion. This is because South Florida residents withdraw so much freshwater that they start pulling up brackish water; this type of saltwater intrusion is referred to as vertical intrusion and can occur in the absence of sea-level rise. However, with rising sea levels and the increased withdrawals as population grows, saltwater intrusion will become more problematic. Wells that are closest to the coastline will experience the greatest impacts resulting from “lateral intrusion.” Many cities, such as Hallandale and Deerfield Beach, are closing wells because they are no longer usable. Other cities are rebuilding wells away from the coast to lessen the chances of saltwater intrusion.

According to NOAA, storm surge is “an abnormal rise of water generated by a storm, over and above the predicted astronomical tide.” In the presence of a storm, the water levels will change due to the strong winds from the storm. Therefore, it is expected that storm surge would be higher during hurricanes and that the higher the storm category, the stronger the storm surge. Now, factor in higher sea levels and consider how that might affect storm surge. The combination of higher sea levels and storm surge has the potential to cause devastating damage to South Florida’s coastal communities. The buildings near South Florida’s coastline were not designed to withstand these higher surges.

Therefore, coastal communities are likely going to need to make some changes to these vulnerable buildings. Some examples include elevating, “hardening,” or even relocating some of these structures. In some cases, abandonment may be the only option available.

As sea levels continue to rise, the areas at lower elevation will be at risk of flooding. What about inland areas that are higher in elevation? Would these areas be safe from sea-level rise flooding?

Saltwater intrusion is affecting coastal wells as well. The diagram shows saltwater coming in from under the freshwater. As sea levels continue to rise, South Florida will see the saltwater continuing to move inland. As it does, saltwater will push the freshwater further away from the coast and upward toward the surface.

What this means is that the South Florida water table, which is just below the surface, will be pushed upward by the intruding saltwater wedge. Even if a home is 15 feet above sea level, if the depth to the water table is just a few feet, then the addition of a few feet of sea-level rise will increase the chances of flooding in that area. With this higher water table, one bad storm could cause flooding for days. Notice on the map of Miami Beach that many areas inland are bright orange, indicating that the water table is less than 1 foot below the surface.

South Florida beaches are constantly changing due to both natural and anthropogenic forces causing erosion. Erosion occurs naturally as wave action pounds shorelines, causing the sand to be washed away. After a tropical storm or hurricane, these effects would be even more pronounced. However, the creation of inlets and other man-made structures along South Florida’s coast have added to this erosion process. On one side of the structure, sand will build up but on the opposite side there is more erosion.

As a major tourist destination, beaches have a high economic value to Florida. Therefore, maintaining beaches for tourism has been a major priority for coastal communities. Florida’s Department of Environmental Protection recently released a report that identified over 500 miles South Florida beaches as critically eroded. They define a critically eroded area as “a segment of the shoreline where natural processes or human activity have caused or contributed to erosion and recession of the beach or dune system to such a degree that upland development, recreational interests, wildlife habitat, or important cultural resources are threatened or lost.” The majority of the southeast Florida coastline is considered to be critically eroded.

Florida beaches are restored through beach nourishment. This is accomplished by collecting sand with a dredge from an offshore location and then piping it onto the beach. As it is piped onto the beach, it is a mixture of water and sand. Once the water drains away, the sand is left behind, which is then moved by bulldozers. By adding sand to sand to beaches through nourishment, it reduces the need for building coastal structures such as sea walls.

Map of the Miami area where colors indicate the shallow depth of the water table. The water table rises as sea level rises, increasing the salinity of freshwater aquifers. These aquifers provide much of the local drinking water supply. Credit: Dr. Keren Bolter, Florida Atlantic University Center for Environmental Studies (CES).

Map of the Miami area where colors indicate the shallow depth of the water table. The water table rises as sea level rises, increasing the salinity of freshwater aquifers. These aquifers provide much of the local drinking water supply. Credit: Dr. Keren Bolter, Florida Atlantic University Center for Environmental Studies (CES).

 

What effect will sea-level rise have on beach erosion? Sea-level rise may exacerbate beach erosion by allowing stronger currents to get closer to the shoreline. Also, consider that with higher sea levels, the beaches will now be underwater so the issue becomes inundation rather than erosion. Therefore, beach nourishment efforts that keep pace with sea-level rise, as well as replace sand washed away by wave action, may be more costly and not a viable solution in specific locations due to the physical conditions.

Almost all of the shoreline within Dade County, Florida is critcially eroded. Image from the publication Critically Eroded Beaches in Florida, Division of Water Resource Management, Florida Department of Environmental Protection, August 2016. www.dep.state.fl.us/beaches/publications/pdf/CriticalErosionReport.pdf.

Almost all of the shoreline within Dade County, Florida is critcially eroded. Image from the publication Critically Eroded Beaches in Florida, Division of Water Resource Management, Florida Department of Environmental Protection, August 2016. www.dep.state.fl.us/beaches/publications/pdf/CriticalErosionReport.pdf.

 

Persons interested in learning more about sea-level rise and saltwater intrusion in South Florida should consider registering for the Greater Everglades Ecosystem Restoration Conference to be held this year in Coral Springs, Florida on April 17-20, 2017. For more information, visit www.conferences.ifas.url.edu/GEER2017. Other topics include storm water treatment, nutrient loading, restoration tools, invasive species management, ecosystem resilience, wildlife population dynamics, modeling for restoration, and environmental stressors, among others. The conference is hosted by the University of Florida’s Institute of Food and Agricultural Sciences (UF/IFAS).

This article is adapted from materials contained in Florida Center for Environmental Studies at Florida Atlantic University (October 17, 2016). King Tide 2016: A Glimpse at What South Florida Could Look Like, Briefing Book for U. S. Congressman Ted Deutch. Retrieved from: www.ces.fau.edu/news/pdfs/king-tide-briefing-book.pdf.

Also cited: Richard Wildemann (February 21, 2017) , The Island Eye News, Adapting to Rising Seas. Retrieved from http://islandeyenews.com/op-ed-adapting-to-rising-seas/.

 

Further Reading:

  1. www.sciencedirect.com/science/article/pii/S0964569116300278

  2. www.jcronline.org/doi/abs/10.2112/JCOASTRES-D-11-00169.1

  3. www.businessinsider.com/cities-exposedto-rising-sea-levels-2014-4

  4. www.climatechange2013.org/images/report/WG1AR5_Chapter13_FINAL.pdf

  5. www.palmbeachpost.com/news/news/wall-of-saltwater-snaking-up-south-floridascoast/nLxg8/

  6. http://rossi.urs-tally.com

  7. www.dep.state.fl.us/beaches/publications/tech-rpt.htm

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