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Story Tools

Date of Issue: November 04, 2009


Drifting the coast in search of places to lay sand

Anna Maria Island is all about water.

It surrounds us. It binds us to the Gulf of Mexico to the west, Tampa Bay to the north, Anna Maria Sound to the east, Sarasota Bay and Longboat Pass to the south.

Water inundates our shores and bayfronts during storms.

Everything is about balance. Too little, too bad. Too much, very too bad.

Water flows into our canals and bayous. It’s a happy, healthful thing for the wetlands. Water from the surrounding waters can sometimes also be a harmful thing.

But one of the least-acknowledged elements of water flow is what goes on offshore in the Gulf.

There’s a current called the littoral current, littoral drift, littoral stream or longshore drift that generally flows north-south offshore of all of Southwest Florida.

Although sometimes it flows south-north. Those pesky currents!

The general consensus is north to south. It’s a water flow that is nearshore and carries and leaves sediments in its wake.

Sediment, sand, can be good or bad. Good is when it deposits additional acreage to our beaches. Bad is when it flows into the bays and covers vital seagrass beds, home to most marine life at some point in their lives.

Our Longboat Key neighbors have proposed something that is basically a “sand sausage” off the northern tip of the key as a sand attractor. Coastal engineers have said the measure could keep the littoral current’s sand on the key’s shore. For them, that’s a good thing.

At least one Manatee County official, Commissioner Joe McClash, has suggested there are other environmental issues and he is challenging the proposal.

Putting sand-sausage revetments in the near-shore waters at Longboat’s north end will create public safety and boating hazards, he has said, and the use of man-made materials would "set a precedent" among Florida west coast erosion-control efforts.

How about some pass history about all this stuff, from the late Dr. Gustavo A. Antonini, David A. Fann and Paul Roat in a publication, “A Historical Geography of Southwest Florida Waterways, Volume One.”

Here’s a guide to the below: ebb = outside the pass; flood = inside the inlets.

Everything you ever wanted to know …

“Tidal inlets — Floridians sometimes call them passes — are highly dynamic and visible features of Southwest Florida's boating geography,” according to the book. “Inlets provide strategic points of entry and egress between the Gulf of Mexico and the inland waterways, but can be intimidating to navigate because of their shifting nature, strong ebb and flood currents, and wave action — including breakers, which may extend across the inlet mouth even in a buoyed channel.  

“Waves propagating into an opposing current experience an increase in height and decrease in length resulting in steeper waves that are more difficult to navigate. Offshore shoals continually shift because of the moving beach sand, and it is sometimes not feasible to keep buoys in the best water. Local watermen, under such conditions, often leave the buoyed channel guided by their knowledge of local conditions and of the dynamic history of inlet development, which enables them to pick the best depth and avoid uncharted obstructions.

“Longboat Pass, New Pass and Venice Inlet are federally maintained waterways between the Sarasota Bay system and the Gulf. They are periodically surveyed and, when shoaling occurs to a point where actual depths are less than the designed project depths, as dredged by the U.S. Army Corps of Engineers under the auspices of the West Coast Inland Navigation District.

“Inlets may close, open, migrate or stabilize in response to changes in sediment supply, wave climate, tidal regime, and back-bay filling or dredging. Changes in inlets occurring at different time scales ranging from hours during severe storm events to decades or even centuries.

“Sand is deposited as shoals just inside and outside the inlet due to the reduction in current speed in these areas. Ebb-tidal deltas occur at the seaward margin — outside — of the inlet and retreat or bend in response to the interaction between incoming waves and ebb tides. Large inlets, like Big Sarasota Pass, build extensive, visible ebb-tidal deltas. The sediment sources include material washed out from the bay, material eroded from the main ebb channel, and longshore drift.

“Longshore drift is sand that moves up and down the coast between the beach and the outer edge of the breaker zone due to waves approaching the shore at an angle.

“Material brought out on the ebb tide is deposited on the swash platform. The breaking waves that the mariner experiences at the inlet entrance are a dominant feature of swash platforms and help to create swash bars. Marginal channels may develop along the ends of barrier islands where incoming (flood) tidal flow is reinforced by wave-generated currents.

“Migration of barrier island spits along this reach of the Florida coast is southward, in the direction of net longshore transport.

“Flood (incoming) tide transports sediment landward through the inlet via the main channel, producing a similar shallow water, delta-like feature on the bay side of the pass. The interplay of ebb and flood tides on this bayside delta creates spits and spill-over lobes where ebb currents run strong. However, flood tidal deltas are less prone to change than ebb tidal deltas along this reach of the coast. Over time, they become stabilized by seagrasses and mangroves. They serve as nurseries for juvenile fish and are important fishing grounds.

“Tidal and wave energies shape the form of seaward flowing ebb-tidal deltas. The varying mix of these two forces determines the movement and deposition of sediments. The character of an inlet — its shape, dynamics, navigability — may change over time as the inlet adjusts to changes in the way tides and waves interact. Since Southwest Florida is a low wave energy coastline and the mean tidal range is relatively small (2 feet), a delicate balance exists between tide and wave dominated conditions. A slight decrease in tidal prism (e.g., due to bayside filling) may cause a change from tide-dominated to wave-dominated conditions in inlets. Likewise, a change in wave energy due to sediment accumulation and spit development along the beach face may cause development of an offset alignment to the ebb delta.”

If you’ve dozed off, the next part is kind of important in relationship to what’s going on at Longboat Pass.

“In addition to these natural forces, shoreline engineering through the construction of groins, jetties and bulkheads — features designed to stabilize the shoreline by holding beach sand in one place — can dramatically alter the supply of sediment and the course of development and shape of an inlet.”

Gus came up with four examples of inlets: tide-dominated, wave-dominated, mixed energy with straight shape, and mixed energy with offset shape. Gus determined that Longboat Pass was a tide-dominated inlet with a well-defined ebb channel.

“These types of inlets have relatively stable ebb-tidal deltas. Mariners should exercise caution in approaching tide-dominated inlets from the Gulf under ebb-tidal conditions because maximum ebb-current velocities are considerably higher than currents at flood stage at these locations. A combination of strong onshore winds and peak ebb tide can be especially hazardous due to the amplitude and steepness of the waves. Furthermore, Longboat Pass entrance channel is over one statute mile long and a lift bridge must be negotiated within the throat of the main ebb channel, an area where currents are particularly strong.”

Is the key’s position for some form of jetty or sand sausages at the southwest of Longboat Pass right or wrong? Dunno.

Is any placement of any form or hardening of anything in Gulf waters right or wrong?

Most coastal engineers say don’t do it.

Will the proposed sand sausages help or hurt beaches on Longboat Key or, more to our interest, hurt or help the shore of Anna Maria Island? Again, dunno.

But the continuing theme of Sandscript has been to not mess with Mother Nature, and I don’t remember sand sausages being a natural aspect of our offshore shores.

Sandscript factoid

Buttonwood Harbor is at the Manatee/Sarasota county line of Longboat Key.

Gus wrote:

“Buttonwood Harbor retains many bayside features of an historic antecedent inlet. The flood-tidal delta is one of the largest of all the inlets along the Gulf Coast Heritage Trail. Extensive beds of seagrass cover this feature and it is a prime recreational fishing area in Big Sarasota Bay. The access channel from the Bay to Buttonwood Harbor follows the relict flood channel. The barrier island at this location is prone to beach erosion, storm-wave attack, and potential breaching, and is one of the narrowest points on the key.”

Another historical inlet is just north of Coquina Beach, a skinny part of our own barrier island.

And if you want to read more about all this inlet/barrier island stuff, Google “A Historical Geography of Southwest Florida.” There are two volumes. A third was to be Tampa Bay north, but the project was halted when Gus was killed by a drunk driver while on a bike trip on his 67th birthday. The drunk truck driver also took the life of Gus’ stepson.

You can find the book links on The Islander Web site in the community links section.

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