The 1935 Labor Day Hurricane was a compact intense tropical cyclone (TC) that passed over the middle Florida Keys on the 2nd of September (Labor Day) before tropical cyclones were given names. It had very low atmospheric pressure in the eye of the storm, and very high wind velocities surrounding the eye.

This storm, and Hurricane Donna in 1960, are the last two tropical cyclones with eye of the storm passing over the Central Keys. In the timescale of hurricanes, that was not very long ago, and more storms like those are likely in the future.

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

Florida is a peninsula in the southeastern United States. It is the southern-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 almost reaches the mainland (at Blackwater Sound), separating Biscayne Bay to the north from Florida Bay to the SW.  Biscayne Bay is an inlet of the Atlantic Ocean. Florida Bay is an inlet of the Gulf of Mexico.

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 1935 hurricane eye passed near Long Key. Railroad viaducts and dikes (limestone earthen and rubble fill) on both sides of Long Key held back ebb current (storm surge return flow), causing railroad dikes to the NE of Long Key to burst and wash away (between Lower Matecumbe and Plantation Key).

To the west and south of the Central Keys are the Lower Keys (not shown), consisting of 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.

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 meter (m) at the gulf and ocean edges of the bay, and about half or one-third of a meter far inward.

The ocean side of the Central Keys is itself an underwater estuary, with coral reefs 2 to 5 km offshore forming an underwater bay called Hawk Channel between the reefs and the Central Keys. The reefs are formed with medium and coarse limestone rubble transported out from the Keys with ebb current of tropical cyclones (storm surge return flow). In that system, Hawk Channel is the back bay of the reefs.

The storm ebb current medium size rubble transport requires water current of 4 knots (nautical miles per hour). Tide fluctuations cause daily and nightly currents of 1 knot, in some places higher but not enough to transport medium size debris. Tropical cyclone ebb currents achieve 4 to 5 knots, occuring often enough to perform this cycle (every few decades, plus or minus), until now that the storm return flows are blocked (to never happen again).

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

The dikes (also called fill, embankments or causeways) consist of rubble piled up that blocks water flow. The viaducts are like a bridge, but with very large transverse water-blocking cross section, especially at heights higher up from 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 after initial construction 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.

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

The next two openings connecting the Atlantic and Gulf after Snake Creek – Whale Harbor and Indian / Lignumvitae – had longer dikes, also washed out by the storm ebb current (storm water return flow).

The next opening connecting the Atlantic and Gulf was previously wide open water more than 6 kilometers (km) wide between Lower Matecumbe Key and Long Key, but which had been (and still is) entirely blocked off with dikes and concrete viaducts (Coch, figure 8.6). And after Long Key, there are many more kilometers of dikes and viaducts, also blocking every possible opening, all still in place to block water release from future hurricanes.

The Long Key bridge, newer than the viaduct, has much less environmental impact than the viaduct (discussed below). 

Notice, in the photograph above, tidal sedimentation (lighter color) stripes on the gulf and ocean floor in line with the viaduct openings.

Tidal flow retardation is creating sedimentation stripes in line with the viaduct openings. These stripes are occuring on both sides of the viaducts, because the tide changes direction every day.

The following image shows sedimentation stripes on the Atlantic side of that viaduct:

The stripes are testament to the laminar flow tendancy of the region, newly restricted by the viaducts.

Besides not supporting occasional transport of medium size rubble to the reef shelf, holding back storm surge return flow will also cause more terrestrial damage (that will worsen with higher sea levels), and damage to mangroves in Florida Bay by prolonged smothering.

High winds of major hurricanes shear the tops off mangroves in Florida Bay, after which the plant only survives if it does not remain submerged for a prolonged period by flood waters that historically would have been allowed to flow out of the bay.

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

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

These affordable bridges can be higher above the water with longer spans than historical bridges – high enough to clear storm surges, with spans long enough to allow more laminar flow of water beneath the bridge.

In temperate and boreal climates, biomaterials (wood, hemp, etc.) could be used to replace some of the concrete in a box girder bridge, reducing life cycle emissions of the bridge.