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Date of Issue: December 31, 2008


On tides and tidal inlets in Southwest Florida

Dr. Ernest Estevez is the keeper of all lore nautical in our part of the world. For more than three decades he has practiced his craft at Mote Marine Laboratory, publishing scores of technical papers on riverine, estuarine and all other ecosystems. He is truly a font of marine knowledge.

So Ernie was the perfect source for gaining a bit of insight about tides in Southwest Florida. Why are we supposed to have four a day only to sometimes have one tide in a 24-hour period? Why are some high tides higher than others? What about lower low tides?

The following is an essay he wrote to answer the tidal questions posed by visitors and residents:

Our relationship with the sea is affected mostly by our contact with its edges and surfaces. Even the most ephemeral sea-level event, the breaking wave, holds our fascination.

“I moved here from New England where you could set your clock by the tides, but down here the tides are different every day. And the bay is a whole lot shallower than it was this summer! What’s going on?” … is a question I’ve come to expect this time of year. The answer is not complicated, but it has several parts.

For starters, the Florida west coast experiences “mixed” tides, or a combination of diurnal (one high and low tide per lunar day) and semi-diurnal (two high and two low tides) patterns. Each by itself is regular, but when both patterns occur together we can have days with only one or, rarely, even five tides. Two to four are the most common, but even these will have unequal heights, causing us to speak of higher and lower high tides or higher and lower low tides.

Mean tide is the average of all high and low tides. To make boating as safe as possible, all navigation charts show depths relative to mean lower low tide. Now it just so happens that, in summer, more of the higher high tides tend to occur during daylight hours, whereas in winter the lower low tide tends to be more common during the day. The outcome of this shift is that daytime water levels in our bays appear higher in summer than in winter. All else being equal, there are just fewer people on the water, or looking at it, at night than during the day.

But there is more at work than that. The average level of the near-shore Gulf changes from month to month, being lowest around February and highest in late September to mid-October. The main reasons for this pattern are thermal expansion of water on the west-Florida Shelf (warm water occupies a larger volume) and runoff of river flow in summer months. The barnacle or oyster that settles on a seawall near high tide in summer will die from exposure to air when winter comes, just because of these differences.

Seasonal winds accentuate matters, too. Winds from the north and especially east tend to drive water out of rivers and bays along our coast, whereas west or south winds push water inland. Our prevailing winter winds are of the first kind, especially after cold fronts, causing the already-lowered tides to reach even lower levels. Folks are hard-put to float their boats in the middle of a winter day following a cold front, but it’s a great time to walk out on mudflats and sand bars looking for clams!

The savvy Floridian or visitor knows some other main points about coastal water levels. For example, the tide range is not the same everywhere along the Gulf coast. The difference between highest and lowest tides varies from 1 or 2 feet to 4 or almost 5 feet, and there is no regular north-south trend from the Panhandle to the Florida Keys. A main reason is the distance across the continental shelf each oceanic tidal wave must travel every day — where the shelf is very wide, for examples off the Ten Thousand Islands or Big Bend coastlines, the tide range is greater. Where it is relatively narrow, off the Panhandle for example, the tide range is small.

Also, the overall level of the sea is gradually rising as a remnant of global warming since the last ice age. For west-central Florida, the rate of sea-level rise has been about 9 inches per century, a seemingly slow rate but one with enormous ecological importance in so flat a state as ours. The rate of sea-level rise is expected to increase as a result of climate change.

The rare and most extreme sea level is probably the one we most anticipate — the hurricane storm surge. In a stairwell at Mote Marine Laboratory, I once marked the wall to show where computer models predict the water will reach with hurricanes of increasing intensity. A Category 5 surge would put 3 feet of water into the second floor of our barrier-island facility, and inundate all low-lying mainland areas from Tampa Bay to Charlotte Harbor.

 

Midnight Pass impasse yet again

Midnight Pass is an inlet that once separated Siesta and Casey keys in Sarasota County. About 25 years ago, the migrating pass threatened two homes and, with water lapping against the pool decks and crashing against the exterior of the expensive homes, the pass was allowed to close under the provision that it would be relocated at homeowner expense.

The pass relocation proved too expensive for the homeowners, and after eight or so attempts Midnight Pass was no more.

Sarasota County commissioners last year agreed to revisit the issue and authorized about $800,000 in engineering studies to come up with a way to re-open the inlet. The new pass would be wide, deep and provide tidal flushing between the Gulf of Mexico and Little Sarasota Bay. Oh, and there would be a speedy point of ingress and egress for boaters.

Engineering studies in hand, federal, state and regional environmental permits were sought. Earlier this month, the lead state agency told the county it could not grant permits for the pass opening. County leaders are expected to appeal the notice to deny the work at a cost of upwards of $500,000.

If the pass is ever re-opened, millions more will be needed for regular maintenance dredging to keep the waters flowing.

 

Sandscript factoid

Here’s a couple literary quotes that come to mind when discussing time and tides and our human whims to contain either.

"To progress from nature's despoiler to its custodian, we must first redefine our place in — not over — nature, accept the role of resident rather than architect and resist the temptation technology affords us to mold a world responsive to our whims alone." From “The World is Not a Theme Park” by Ted Gup.

"We all like to congregate at boundary conditions. Where land meets water. Where earth meets air. Where body meets mind. Where space meets time. We like to be on one side and look at the other." From “Mostly Harmless” by Douglas Adams.

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