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Tropical Cyclone
Florida Keys 1935

Labor Day Hurricane 1935

Florida Keys

The 1935 Labor Day Hurricane was a compact intense tropical cyclone (TC), with very low atmospheric pressure in the eye of the storm, and very high wind velocities surrounding the eye. It occured before tropical cyclones were given names.

Figure 1:  Relative storm sizes and shapes at landfall of three intense tropical cyclones. For comparison, storm track directions are rotated to point upward. Dashed lines for Gilbert show extent of that storm at second landfall. [NOAA]

The Labor Day Hurricane of 1935 tracked toward the United States north of Cuba, turned northward, bisecting the Florida Keys, into the Gulf of Mexico (GOM), crossed into northern Florida and Atlantic US states, then back out to the Atlantic Ocean, becoming extra tropical near Greenland.

Figure 2:  Storm track of Labor Day Hurricane of 1935. [NOAA]


Ecosystem

Florida is a peninsula in the southeastern United States. It is the southeastern-most state of the 48 contiguous US states. The Florida Keys are an arc of islands off the southern coast of the Florida mainland.

The arc of the Keys is north-south for the Keys near Miami, which include Virginia Key, Elliot Key and northern Key Largo. In that portion of the Keys, Biscayne Bay separates the Keys from the mainland.

Central Key Largo forms a land bridge with the mainland, which separates Biscayne Bay to the north from Flordia Bay to the south. Those two large bays are inlets of the Atlantic Ocean and Gulf of Mexico respectively.

Southern Key Largo transitions the arc of the Keys to begin an east-west orientation, and also changes the geology of the Keys to only limestone that cannot hold surface water and therefore lacks native animal species that require drinking fresh water.

This arc of islands, referred to as the Central Keys, is concave northward (focal direction toward Cape Sable on the southwestern corner of the Florida mainland). The Central Keys are where the Labor Day Hurricane of 1935 crossed northward, and is the area of interest for this article.

To the west and south of the Central Keys are the Lower Keys, which are limestone with covering that allows pooling of freshwater for native animals like the Key deer that do not occur in the Central Keys where fresh water pools are not available.

Figure 3:  Satellite imagery of the Florida Keys. Red arrow points to land bridge of Key Largo and Florida mainland. Yellow arrow points to Cape Sable. Orange line separates Lower Keys (to the left) from Central Keys (to the right). Upper left area of image is the Gulf of Mexico (GOM). Lower and right portions of image is the Atlantic Ocean, which is very deep here and includes the Gulf Stream. Florida Bay is a shallow inlet of the Gulf of Mexico, with mangrove islands and average water depth of 1.5 m. [NOAA]

The Lower Keys are essentially out in the ocean, where the Gulf of Mexico meets the Atlantic Ocean. The Central Keys are an estuary, with Florida Bay as the back bay of the estuary, having tide fluctuation (difference between high tide and low tide) of 1 m on the ocean side of the Central Keys.

The ocean side of the Central Keys is itself an underwater estuary, with coral reefs offshore forming an underwater bay called Hawk Channel between the reefs and the Central Keys. The reefs are formed on medium and coarse limestone rubble transported out from the Keys with ebb current of tropical cyclones. In that system, Hawk Channel is the back bay of the reefs.

The storm ebb current medium size rubble requires water current of 4 knots (nautical miles per hour) to transport. Tide fluctuations cause daily and nightly currents of 1 knot. That is not enough to transport medium size debris. Tropical cyclone ebb currents achieve 4 to 5 knots.

Coral reefs require prevention of siltation, which is why rubble must not be too fine. Siltation prevention is possible in the Atlantic Ocean close to the Gulf Stream, because the Gulf Stream provides enough continuous current to prevent siltation.

Tidal currents could provide enough velocity to prevent siltation, but are not continuous enough, instead slowing and reversing direction several times per day. Some reef building is possible in these tidal currents, but not as much as by the Gulf Stream.


Railroad Infrastructure

An early type of railroad had been built in the Keys, using dikes and viaducts instead of bridges, since bridge engineering had not yet been developed.

The dikes, also called fill or embankments, consist of rubble piled up to block water flow. The viaducts are like a bridge, but with very large transverse cross section, especially at heights higher up from the surface of mean sea level where storm water would be blocked, almost blocking as much water as the dikes, essentially a dike with culverts.

The dikes do in fact have culverts, since it was readily observed that hydraulic head between the two sides of the dike needed to be alleviated even without a storm.

Hydraulic head is the water surface height difference of two sides of a dike or viaduct. In the Central Keys, with tide fluctuation of 1 m, much of each day and night will have substantial tidal hydraulic head.

Figure 4:  Railroad dike with concrete culvert in the Central Keys before 1935. Water level (hydraulic head) is higher on the left than on the right at this tide, causing constricted water to flow from left to right through the concrete culvert.

Figure 5:  Railroad dike at Snake Creek washed out by the Labor Day Hurricane of 1935, with make shift suspended foot bridge spanning Snake Creek after the storm. Person is standing on the near side of concrete culvert. Water in foreground (this side of the culvert) is flowing where dike previously stood.

“Fluid pressure built up on the gulf side of the fills and eventually they failed quickly and violently.”
— 
Coch, in Coastal Hazards, p. 223

These dikes, also called causeways, are similar to the military-designed pedraplenes of Cuba:

“Cuban tourism authorities have constructed causeways (or stone embankments) bridging barrier islands to the mainland and to one another called pedraplenes (see Map 10.1 in book). These pedraplenes block the movement of water in the intracoastal waters, exacerbating contamination and destroying coastal and marine habitats… several colonies of flamingos that used to nest in the Sabana-Camaguey sub-archipelago have left this area because of the destruction of their habitat resulting from tourism facilities and pedraplenes and settled in the Bahamas”
— 
Sergio Diaz-Briquets and Jorge Perez-Lopez, Conquering Nature, University of Pittsburgh Press, 2000 (online book), p. 264, 274

Figure 6:  Viaduct in Central Keys, 1926, blocking laminar water flow (reducing current velocity) even without a storm. Water level during 1935 storm reached the top of this viaduct. Most of the water was blocked by the viaduct from returning to the ocean, being forced instead to concentrate toward Cape Sable. This and all other viaducts continue to retard water flow.

Figure 7:  Closeup of preceding photograph showing hydraulic head and water turbulence under the viaduct (reducing water current velocity).

Not only are the retardation barriers (dikes and viaducts) reducing storm ebb current velocity and volume, but the reduced current velocity is nevertheless high enough to transport silting materials to the reefs – the opposite of what is needed.

This damage to underwater topography formation is in addition to more terrestrial damage caused by not allowing flood waters to be released.


Planning

Dikes and water blocking bridges must not be built for the purpose of transportation in estuaries. Modern affordable bridge construction should be used for that, e.g., low-cost long-span box girder prestressed concrete bridges.

Single box bridges are the most affordable, with room for two lanes of traffic and space on both sides for pulling over. These can be built on shore with portable prestressing rigs, and floated into position with a barge crane. Some newer methods allow transport of the bridge segments over previously installed bridge segments (instead of being floated out).

Figure 8:  Section of a prestressed concrete box girder bridge deck, single box. These affordable bridges can be high above the water with long spans. Each section is precast on shore in a rig against previous section for better installation fit afterward.

Figure 9:  Installation of a section of a two box concrete girder bridge high above a river. Two and three box girder bridges allow for more lanes of traffic. However, single box concrete girder bridges cost much less and are adequate for most estuaries including the Florida Keys.

References:

 1.  “The Florida Keys”, in Brian R. Chapman, Eric G. Bolden, Ecology of North America Second Edition, Wiley 2015, p. 278–280.

 2.  Nicholas K. Coch, “Anthropogenic Amplification of Storm Surge Damage in the 1935 ‘Labor Day’ Hurricane”, in Coastal Hazards (Springer 2013)


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