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Viaducts

Labor Day Hurricane 1935

Page 3:  Viaducts

The dikes (earth and rock fill embankments) in the Florida Keys have spanned openings between them to allow some water exchange.

The openings of the dikes in the Florida Keys are larger than the openings in the pedraplenes of the Cuban military tourism sites, partly because Florida Bay is much larger than the northern bays of Cuba (causing and requiring more ebb current), and also because the dikes were unpopular with local people in the Keys.

Florida Keys citizens had trouble navigating boats because of dikes blocking passage between the gulf and ocean sides of the Keys. And they recognized and complained that the dikes caused hurricane damage by constricting ebb current.

“Some local inhabitants suspected that the bridge piers and embankments reduced the tidal flow, leading to catastrophic consequences when the 1935 Hurricane hit.”
— 
Coch, in Coastal Hazards, p. 214

In the Keys, therefore, dikes were separated with viaducts, as a sort of compromise. That provides more water flow than culverts, but still blocks water flow with dikes. And the viaduct piers have such wide hydraulic cross sections that they retard (partially block) water flow.


Figure 3.1:  Dikes and viaducts to Long Key, 1939.


Fig. 3.1 shows dikes and viaducts from Lower Matecumbe Key to Long Key. The end of a long dike from Lower Matecumbe Key is labelled ‘A’ in this picture (lower-right). Long Key is in the distant background toward the left, where a couple of dark bumps on the horizon are probably forests on Long Key.

A dike called Craig Key (or Craig fill) is labelled ‘C’ and has a bend (turns toward the right in this picture). A viaduct labelled ‘B’ connects the Craig fill dike (C) to the Lower Matecumbe dike (A).

An even longer dike with a bend, labelled ‘E’ and ‘F’, reaches Long Key. That long dike extends to the right in this picture (E), and then turns toward the left (F). The portion of that dike that extends to the left of the label ‘F’ is the jetty in Fig. 2.3 on the previous page of this report.

Another viaduct, labelled ‘D’, connects the Craig fill dike (C) to the Long Key jetty (E–F).

A, B, C, D, E and F in Fig. 3.1 are all artificial, blocking and retarding water flow for more than 4 miles. This is depicted in the maps of Fig. 8.6 in Coch.

The viaducts B and D have been replaced by new bridges. But those viaducts have not been removed. The new bridges run parallel to the viaducts, and block much less water than the viaducts. Fig. 8.4 in Coch is a photograph of another viaduct retarding flood tide. Coch recommends removing the viaducts to improve water flow.

Parts of the viaducts are used as fishing piers. Those could be replaced with pedestrian bridges that have less impact.

The waterways under viaducts B and D are referred to as Channel 2 and Channel 5 repectively in nautical charts. There are no mentions of any channels numbered 1, 3 or 4 in nautical charts. Presumably those have been blocked by dikes A and C.


Figure 3.2:  Scan of a paper nautical chart, 1861. [LOC]


Fig. 3.2 shows a scan of a paper nautical chart published in 1861. This map uses the old spelling of Matecumbe. The dark spot under the letter ‘M’ of “Matacumbe” is probably a smudge or paper imperfection (not an island).

The 1861 map shows no land between Lower Matecumbe and Long Key, except one island NW of Long Key. That island is now called Fiesta Key. The 1861 map shows no land link between Long Key and Fiesta Key.


Figure 3.3:  2014 Nautical chart. Craig fill is not called a Key.


Fig. 3.3 is a modern nautical chart, displaying depths in feet at low tide, and red and green bouy/marker numbers in quotes. This chart shows Dike F from Long Key (left side of Fig. 3.3) to Fiesta Key and past Fiesta Key to the Channel 5 bridge (and viaduct it replaces but has not been removed). As shown in the nautical chart, part of the viaduct has been removed (in Channel 5).

Nautical charts tend to draw the viaduct as if it was land, since it is usually impassable for boats due to small openings and strong currents from tidal prism reduction. Following is a photograph of that new bridge, and the old viaduct behind it with portion of the viaduct removed in Channel 5.


Figure 3.4:  Channel 5 bridge.


Fig. 3.4 shows a recreational boat passing under the new Channel 5 Bridge, from Florida Bay in the background, to the Atlantic Ocean in the foreground. The viaduct openings are small compared to the boat wake.

On the other side of Long Key, the same type of viaduct was built to connect Long Key to Conch Key. That viaduct is called the Long Key Viaduct.


Figure 3.5:  Long Key Viaduct, 1926.


Figure 3.6:  Closeup of Fig. 3.5 showing water turbulence under the viaduct.


Fig. 3.6 shows turbulence under the Long Key Viaduct, as opposed to laminar flow that would occur without the viaduct.

Turbulence requires more energy to transport the same amount of water compared to laminar flow. In other words, for the same amount of energy, less water is transported, reducing tidal prism.

This corresponds to Coch, Fig. 8.4, which is a photograph of turbulence under this viaduct, and explains why Coch says that upon observing that turbulence he understood the problem of tidal prism reduction causing storm damage in the 1935 Hurricane. The Coch photograph shows laminar flow under the new bridge next to the viaduct, while water under the viaduct is turbulent.

In addition to showing turbulence, Fig. 3.6 above shows water level higher on the upstream side of a viaduct pier than on the downstream side, already creating hydraulic head without even a storm.

The following photograph shows construction of the viaduct.

Figure 3.7: Construction of the Long Key Viaduct.


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