Tidal Topics

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Newcomers to the Florida Keys are often surprised to learn that a high tide at one location may be three or four hours behind another spot only a mile or two away. Indeed, a perusal of the NOAA Tides and Currents website reveals nearly 160 separate locations between Card Sound and Dry Tortugas for which tide predictions are calculated! Knowledge of a local tidal cycle is important for many activities, including safe navigation, fishing, diving, and securing a vessel on a mooring. In addition, the tidal cycle relates to the evolution of current velocities in the channels which, in turn, influences vessel navigation, fish feeding cycles, and the height and steepness of wind-generated waves within the channels. Finally, the tidal cycle and observed water levels have a significant impact upon the extent of coastal flooding and inundation during storm surge events. Coastal natives and seafaring people throughout history have lived with a keen awareness of the daily rise and fall of the ocean as a natural and reliable cadence. The tides result from the gravitational forces of the moon and sun acting on the rotating earth. However, tidal behavior at a particular location is uniquely dependent upon ocean basin, slope of nearby continental shelves, nearshore ocean depths, the shape and length of channels, and land elevation. To complicate matters, actual observed water levels always are the result of astronomical, oceanic, and atmospheric forces. Most of the time, the forces of the ocean and atmosphere are negligible compared to those of the sun and moon when it comes to water level rise. Occasionally, however, a hurricane or other large storm may transport or pile up enough ocean water locally to result in extreme high tides and coastal flooding. A major Florida Keys coastal flood event occurred in association with Hurricane Wilma on October 24, 2005. Most communities were affected, with inundation ranging from a few inches to several feet, depending on property elevation. A minor coastal flood event occurred in the Florida Keys this past October and November (2012) when three large weather systems evolved over the western North Atlantic (including Hurricane Sandy). Water levels were up to a foot or two above average around the Florida Keys, and this likely was due to a combination of wind forcing and a reduction in the transport of the Florida Current (Gulf Stream) associated with those three large storm systems. Only two water level stations are operated by NOAA in the Florida Keys. One is located in Marathon on the bayside of Vaca Key. The other is located adjacent to the Truman Waterfront in Key West Harbor. The Key West station is one of the oldest in the network, with data going well back into the 19th Century. NOAA operates an excellent web page where you may view the real-time water levels at the Florida Keys stations or view lists or graphs of tide predictions at any one of the nearly 160 Florida Keys locations. There is a handy search box on the top of the page where you can simply enter the location for which you desire a tide prediction. Or, there is a Google Maps link where you may view and click for a desired tide prediction. To demonstrate the striking variability of the tidal cycle in the Florida Keys, let us look at the predictions for a single high tide around Key West on April 15, 2013 (Tax Day). According to NOAA predictions, high tide will be reached at Key West White Street Pier and Sand Key lighthouse at about the same time (1:09 a.m. and 1:14 a.m., respectively). It will take another 30–45 minutes for high tide to be reached at Riviera Canal (1:50 a.m.) and Key West Harbor (2:02 a.m.). Twenty minutes later, high tide will occur at Boca Chica Marina (2:22 a.m.). However, it will take an additional hour (3:25 a.m.) for high tide to reach the Boca Chica bridge (in plain sight of the marina). High tide will not be reached at Smith Shoal (15 miles northwest of Key West) until 4:03 a.m., and it will be 4:27 a.m. before high tide is reached at Key Haven (near Stock Island). You will note that well over three hours will elapse between the high tide at Key West White Street Pier and the one at Key Haven, less than a few miles away. Numerous examples like this one may be presented up and down the Florida Keys. So, why does the time of high and low tides vary so much around the Florida Keys? The primary reason has to do with the complex topography around the Florida Keys, both above and below the water surface. The Florida Keys archipelago lies between two distinct oceanic basins (Gulf of Mexico and Atlantic Ocean), with a large, shallow, continental shelf to its north and a barrier reef and deep strait to its south and east. Add to this numerous small islands, channels, flats, and basins, and you have a recipe for very complex water movement and resulting tidal variability. Some of the tidal currents in the various channels around the Florida Keys occasionally may exceed five knots. When wind-generated waves move in opposition to tidal currents, the result is an increase in both wave height and wave steepness. As always, remember to be weather-ready and stay safe!