Anthropogenic Amplification of Storm Surge Damage in the 1935 “Labor Day” Hurricane
On Labor Day, September 2, 1935, the most powerful hurricane in U.S. history (Herbert et al. Greatest storms of this century (and other frequently requested facts) NOAA Technical Memorandum NWS NHC-31, U.S. Department of Commerce, Washington, D.C, 1992) made a landfall in the Middle section of the Florida Keys. At that time, this was a sparsely populated area. However, the 600 World War I veterans engaged in building U.S. 1 had recently increased the population significantly. The hurricane had winds exceeding Category 5 levels 155 miles/h (69.3 m/s) on the Saffir-Simpson Scale (Weatherwise 27:169–186, 1974) and surge levels up to 18 ft (6 m). Superposed wind waves caused damage at much higher levels. Some natives blamed the filling of tidal channels in the building of the Florida East Coast Railway extension in the early 1900s for impounding floodwaters. However, no one has been able to verify their hypothesis or to quantify the degree of tidal prism restriction.
The storm was moving exceptionally slowly at 5 miles/h (2.2 m/s). Normally a slow moving storm draws up all of the warm water above the thermocline and weakens. However, in the Bahamas the slow-moving storm intensified to a Category 5+ event, in 1 day, as it traveled the 100 miles to Florida. Data cited here allows us to explain the exceptional intensification.
A synthesis of archival material, original Florida Overseas Railroad drawings, survivor interviews, meteorological data, and field studies have been used to explain the surge dynamics and rapid intensification in the 1935 hurricane. Filled railroad embankments totally restricted the return flow, while large (12 f. wide) stone piers restricted tidal flow by 19%. As the hurricane moved onshore, the ocean submerged the Keys and surface structures were obliterated. When the hurricane moved off the Keys, offshore winds started to blow the floodwaters back into the ocean. The bay waters in front of the Keys withdrew relatively quickly across the reef. However, the earthen fills blocked the waters behind the Keys and the flow was restricted as it passed between the stone piers of the railroad viaducts. A significant hydraulic head developed on the ocean side of the Keys. Fluid pressure built up steadily on the Gulf side of the fills, and eventually they were breached.
The slow storm movement actually aided in hurricane intensification in this case, the storm moved over the hot waters of the Bahama Platform and then traversed the Florida Straits, underlain by the 900 ft deep, very warm waters of the Florida Current. The lessons from this hurricane are applied to the rapidly developing coastal areas of Tampa-St. Petersburg and similar U.S. coastal sections.
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