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Transportation Infrastructure

Florida Keys Hurricane 1935

Hydro-Geomorphology

The word Florida means “Land of Flowers”, making reference to the state’s default natural landscape of semi-tropical jungle with flowers and supporting forests.

Figure 1:  Florida orchid, in southern Florida. [USDA]

The Florida peninsula geology, like Cuba, quickly flushes out rains in all offshore directions, unlike other U.S. states.

One of those directions of hydrological water flow is toward the Florida Keys, via Florida Bay.

Florida Bay also experiences tidal and storm current exchange with the Gulf of Mexico and the Atlantic Ocean.

As mentioned in the Geography page of this report:

The Ocean that is between the Keys and reefs is called Hawk Channel. It is much shallower than the rest of the Atlantic Ocean, but not as shallow as Florida Bay which is a shallow inlet of the Gulf of Mexico.

“Because reefs grow vertically to the low tide level, they provide very effective shelter to the coast… The more closely a reef is located to the coastline, the greater the shelter will be… Reef development may take place some distance offshore, in which case greater fetch and depth between reef and shore leads to higher energies impinging on the coast.”
— 
Charles R.C. Sheppard, “Coral Reef Coasts”, in Encyclopedia of Coastal Science, p. 339.

The reefs of the Florida Keys are not very close to shore, but are not too far either, providing some coastal protection.

During the 1935 Labor Day Hurricane, a storm surge 6 meters high (18–20 feet) blew water from the Atlantic Ocean over the Keys into Florida Bay. Outdated causeways (mainland-style dikes with viaducts), that are still in place today, constricted the subsequent ebb current, preventing water from flowing back out to the ocean. Constricting the ebb current caused storm damage from hydraulic head water pressure.

“To predict what may happen when a major hurricane hits an area where the tidal prism has been greatly reduced we can apply the lessons of the 1935 storm”
— 
Nicholas K. Coch, “Anthropogenic Amplification of Storm Surge Damage in the 1935 ‘Labor Day’ Hurricane”, in C.W. Finkl (ed.), Coastal Hazards, p. 212.

Florida Bay is not deep, but is much wider than Hawk Channel, so it can hold much more water than Hawk Channel. The storm dumped a huge amount of water into Florida Bay. Then the ocean level dropped, and Hawk Channel drained into the ocean (laminar flow) much faster than Florida Bay could drain into Hawk Channel (retarded flow), causing hydraulic head water pressure in the ebb current direction.

The reefs are formed with medium and coarse limestone rubble transported out from the Keys by ebb current of tropical cyclones (storm surge return flow). In that system, Hawk Channel is the back bay of the reefs.

Medium size rubble transport, by storm ebb current, 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, occurring often enough to perform this cycle (in century timescales).

“An inconsistency occurs between the position of rubble at the edge of the Florida platform and that of ‘reef’ debris at the foot of ancient escarpments. Our conclusion is that the ancient rubble is a result of some combination of materials transported sea-ward by storm-ebb tidal currents opposite topographic lows of the platform edge, material slumped from platform edge, or an accumulation of material that was eroded from an ancient platform surface during times of emergence.”
— 
Ball, Shinn, Stockman, “The Geologic Effects of Hurricane Donna in South Florida”, The Journal of Geology, Sept. 1967, p. 593-4.

The Central Keys were formerly crossed with many creeks and streams that were perpendicular (transverse) to the arc of the Keys and highway. Erosion would have split each Key into multiple islands, each new island oriented in the transverse direction and growing in that direction. This process was stopped by building dikes in the arc direction, blocking the natural transverse water flow.

“Shallow creeks were filled in to form long causeways. These raised causeways and viaduct approaches essentially formed a wall between the ocean and the Gulf of Mexico.”
— 
Coch, in Coastal Hazards, p. 214-215.

Removal of this dike system will allow island formation in the transverse direction through natural processes. Artificial islands should be oriented in those directions, transverse to the arc of the Keys, parallel to water flow, not in the direction now used which is perpendicular to water flow.

Erecting the artificial islands (called fill) perpendicular to tidal and storm water flow, as now done, essentially forms dikes that are as inefficient as possible, slowing water flow adding sedimentation.

The sedimentation is deposited by the slower speed of the water flow (because slower water takes longer to exit thereby giving it more time to deposit sediments), and through erosion of force-inefficient orientation of fills.

“In the tidal passes between the keys, currents cut out hundreds of cubic yards of artificial fill. …This erosion was not so prevalent, however, on the naturally occurring mud accumulations on the bay side of the tidal passes.”
— 
Ball, Shinn, Stockman, “The Geologic Effects of Hurricane Donna in South Florida”, The Journal of Geology, Sept. 1967, p. 590.

Aligning new artificial islands parallel instead of perpendicular to water flow would be more force-efficient.

Incentive to build artificial islands (of fill) will arise from insufficient land masses for bridge connections. For example, Cuban scientists recommended against building the pedraplenes:

“When decisions had to be made regarding the construction of causeways linking Cuba to Cayo Coco to facilitate the development of tourism centers, Cuban scientists proposed that the road link be constructed using bridges spanning gaps between naturally existing landmasses, thereby permitting water circulation vital to the survival of marine species.”
— 
Sergio Diaz-Briquets and Jorge Perez-Lopez, Conquering Nature, University of Pittsburgh Press, 2000 (online book), p. 274

That was over-ruled by the military, due to their lack of civil engineering capability (despite having islands closer together than in the Keys). Essentially, that is what happened with the railroad in the Florida Keys: lack of ability to design bridges prevented effective infrastructure design in the Keys.

Building affordable bridges is now possible in the United States. However, spans in the Florida Keys are longer than in Cuba. There will be incentive to create some artificial islands in the Keys to facilitate future bridge building. Those artificial islands could be constructed with fill removed from the dikes and viaducts, and made to be oriented parallel instead of perpendicular to water flow.

Improving the transportation infrastructure of the Florida Keys is a long term endeavor that can be done in stages with planning ahead of time.


Mangroves

Mangroves are shrub trees that grow in saline or brackish water. Mangrove forests grow on shorelines and shallow shoals of the Florida Keys and Florida Bay, providing habitat for aquatic life.

Figure 2:  Mangrove forests in the Everglades, at low tide.  During high tide, salt or brackish water covers the bottom of the plants.

Mangroves receive nutrients from the air (atmosphere). 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.


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