Geography of Marco Tides EarthCache

Ask almost anyone what causes the tide and the answer will be the moon and perhaps the sun. True, their gravitational pull provides the energy needed to create the tides; but how that energy plays out is dependent upon the geographic and topographic factors of each section of coastline. The location of this EarthCache under the north end of the Judge Jolly Bridge leading over to Marco Island provides a unique opportunity to see and understand how the Marco Island tides occur.

Location

This EarthCache is at a public area within the Rookery Bay National Estuarine Research Reserve under the north end of the Judge S.S. Jolly Bridge leading over to Marco Island. Easy access to and parking for the EarthCache is available on either side of the highway. The area, regularly used for fishing, is safe, well maintained for the public and offers a unique view of the Marco tidal river and island. Please be careful as you make the turn off of the main highway onto the access road. Traffic behind you is usually fast and heavy.

Education

The gravitational pull of the moon and the sun upon the ocean is sufficient to raise the level of the ocean only 18 inches. Yet, a tidal range from high tide to low tide can be as much as 50 feet, such as in the Bay of Fundy (which exhibits the most extreme tidal range of any place on Earth) or only a few feet, as experienced around Marco Island. The large difference in tidal range is a factor of the local geographic and topographic features.

As the 18 inch tidal wave generated by the gravitational pull of the moon and the sun approaches the coastline, the energy of that tidal wave begins to interact with, first, the continental shelf and coastline and, second, with the local topography. The effect of the continental shelf is to increase the frequency of the approaching tidal waves and to increase the height. The relatively shallow aspect of the Gulf of Mexico causes the height of the tidal wave to nearly double from an average of just 18 inches to upwards to 36 inches. Secondly, the shape of the semi-circular Gulf of Mexico coastline is just right to cause reflection of the tidal wave (it bounces back toward the direction from which it came); the reflected tidal wave interacts with the next approaching tidal wave. Along the upper coast of the Gulf of Mexico (New Orleans, Mobile) this interaction between the first tidal wave and the next tidal wave results in an annulation of the next high tide and causes a diurnal tide pattern (one high tide per day). However, Marco Island is situated on the bottom extremity of the Gulf of Mexico; here the impact of the reflected tidal wave is not sufficient to directly interfere with the next high tide. Therefore Marco Island tides are similar to those of the Atlantic coast, i.e. a semidiurnal (two high tides per day) tide.

Next, the near shore topography of Marco Island exerts its influence. High tide moves around the Gulf of Mexico in a clockwise direction. So, the high tide at the mouth of Big Marco River is about 45 minutes earlier than the high tide further south at Coon Key. Secondly, the relatively narrow opening to the mouth of of the Big Marco River (where the EarthCache is located) reduces the impact of the coming tidal wave and therefore reduces the height of the hide tide along the Big Marco River. Further south, at Coon Key the topography, a wide area open to the full impact of the tidal wave, allows for a stronger, higher, high tide there. Therefore, Coon Key typically has a slightly higher tide than the Big Marco River area.

Lastly, Marco Island is on the edge of the Everglades, a large estuary. This large region of shallow, tidal zone causes tidal trapping; i.e. a delay or reduction of the next high tide due to the large regions of shallow tidal area slowing the ebb tide. Tidal trapping along the Marco Island coastal area causes a more irregular pattern of high and low tides than would otherwise be the case. Along some coastal areas of the world, the tide pattern is so consistent that one can almost set your clock by it. Not so at Marco. The tide pattern is quite irregular.

Summary

This brief discussion presenting the key factors in the tidal pattern associated with Marco Island is only the tip of the proverbial iceberg, or should I say the sandbar; NOAA (National Oceanic and Atmospheric Association - the agency responsible for the published the tidal charts) identifies nearly 400 specific factors needed to calculate the tide patterns around the world. The typical tidal chart utilizes 37 key factors to predict the tides in order to have a reasonable accuracy. Hopefully, this discussion enhances your understanding of the tides and how the local geography and topography are the final determiners in the tides of any particular area. My source for this material is: McCully, James Greig. Beyond the Moon - A Conversational, Common Sense Guide Understanding the Tides. World Scientific Publishing Co., 2006

Logging this Cache

Please email <hiking2175@gmail.com> the cache owner the answers to the following questions in order to log a “find” for this EarthCache. Please do not write answers in the public log. Answers must be emailed to the owner separately.

Answers will vary depending upon the time of day of your visit.

1. What is the time and date of your visit?
2. In which direction is the current flowing at the time of your visit? (north or south)
3. Look at one of the bridge pillars near the bulkhead where you are standing. What marks do you see on the pillar that give indication to how high the tide can reach?
4. At the time of your visit how high (or low) was the tide relative to the marks on the pillar?
5. Measure approximately 100 feet along the bulkhead (using your average stride to do this is fine) and measure how fast the current is flowing (seconds per 100’) by timing a floating object (a stick is fine) as it is carried by the current over the 100 foot section you measured. What is the speed (seconds per 100 ft)?