FAU Geosciences

King tides and associated nuisance flooding impacts on South Florida's natural and human systems


Perigean, spring, and perigean spring tides

Simply stated, tides are waves moving through the ocean and along the shoreline. The causes of tides are primarily due to relationships between the sun, moon, and Earth. Sir Isaac Newton first documented in 1687 that the gravitational attraction (bodies with greater mass and closer distance) between the sun and moon influences the rising and falling of Earth’s oceans (Sumich JL, 1996; NOAA, 2004). Tides that are higher and lower than average can occur when the moon is at perigee (perigean tides), the moon is new or full (spring tides), or a combination of the two (perigean spring tides). The gravitational pull of the moon on Earth’s ocean surface is greatest when the moon’s monthly elliptical orbit brings it closer to the Earth (perigee), which causes a bulge in the oceans that increases high tides and decreases low tides once a month (NOAA, 2004; NOAA, 2015a). Here, these tides primarily influenced by the moon at perigee are referred to as perigean tides. When the sun, moon, and Earth are closely aligned during both new and full moons, the gravitational force of the sun is stronger as Earth’s yearly elliptical orbit brings it closer to the sun, which creates a bulging of the oceans and thus higher and lower tides twice a month referred to as spring tides (NOAA, 2004; NOAA, 2015a). The combined effect of a full or new moon at perigee occurs ~3-4 times a year in spring and fall and is known as perigean spring tides (NOAA, 2015a). The average mean sea-level (MSL) seasonal cycle for the Key West tide station demonstrates that tides are much higher in fall than spring when the peak of hurricane and tropical storm season causes changes in air and sea temperature, wind speed, barometric pressure, and coastal current speeds (see Figure 1). During the fall in late September through early November, perigean spring tides cause several days adding up to weeks of minor coastal flooding in South Florida.

seasonaltidesFigure 1 Average mean sea-level seasonal cycle at the Key West tide station. The seasonal cycle is influenced by fluctuations in coastal temperatures, salinities, winds, atmospheric pressures, and ocean currents. Source: http://tidesandcurrents.noaa.gov/sltrends/seasonal.htm?stnid=8723970

Minor or nuisance flooding and its impacts

Critical elevation thresholds where coastal elevation flooding may occur are defined by the National Weather Service (NWS). Although every situation is different, elevation thresholds for the Key West tide station are shown in Table 1. It is important to note that the impacts of coastal flooding are being documented as real-time reports from local partners and the public, which will be implemented into a local database where these thresholds will be updated accordingly (K. Kasper, personal communication, November 2015). A Coastal Flood Statement is issued by the NWS when minor or nuisance flooding occurs along the coast. Nuisance flooding is coastal flooding that occurs with high tides causing public inconveniences such as closing of roads and compromised infrastructure, which is now exacerbated during perigean spring tides due to rising sea levels (NOAA 2015b). In Fort Lauderdale, nuisance flooding from the Atlantic Ocean during fall perigean spring tides is causing longer inundation periods at higher levels. The impacts of nuisance flooding in South Florida include: seawater coming up through storm drains, groundwater coming up through the ground in some low-lying areas, streets are flooded and sometimes closed, some business are inoperable, and tidally influenced canals, rivers and waterways are overtopping and leaking through older seawalls (see Figure 2 below), and pollution. As in urban areas, natural areas are also experiencing longer inundation periods at higher levels causing groundwater to come up through the ground and saltwater inundation causing osmotic stress in freshwater plants. However, we have also observed more salt tolerant plants flourishing during these coastal flooding events.
           
Table 1 Guidance used for issuance of coastal flood statements, advisories, and warnings at the National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) in Key West, Florida. Where MLLW = Mean Lower Low Water (MLLW) tidal datum at the Key West National Oceanic Service (NOS) tide gauge.

National Weather Service critical elevation thresholds for Key West

Coastal Flood Statement

0.88 to 1 m above MLLW and/or 0 to 0.12 m above overwash level at other locations

Coastal Flood Advisory

1.03 to 1.15 m above MLLW and/or 0.15 to 0.24 m above overwash level at other locations

Coastal Flood Warning

1.18 m above MLLW and/or 0.30 m or higher above overwash level at other locations

 

Hannah Cooper

Figure 2 Nuisance flooding exacerbated by perigean spring tides on Mola Ave in Fort Lauderdale, Florida.

 

NOTE: at the time these pictures were taken (Figure 2), the $1.5 million newly engineered seawall being constructed on the east of Mola Ave was complete. Along with an extra 0.61 m (2") of earth added to the plot, it was successful at holding back the spring tides at that location. However, nuisance flooding still advanced from the west where seawalls are lacking or outdated.

References

Kasper K (2015). Telephone Interview.

NOAA (2015a) What is a perigean spring tide? Available at:
http://oceanservice.noaa.gov/facts/perigean-spring-tide.html

NOAA (2015b) What is nuisance flooding? Available at:
http://oceanservice.noaa.gov/facts/nuisance-flooding.html

NOAA (2004) Tides and Water Levels Tutorial. Available at:              http://oceanservice.noaa.gov/education/kits/tides/lessons/tides_tutorial.pdf

Sumich JL (1996) An Introduction to the Biology of Marine Life, sixth edition. Dubuque, IA: Wm. C. Brown. pp. 30-35.