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Date of Issue: May 11, 2006

Sandscript

Probably more than you wanted to know about beaches

With sand seeming to be the subject du jour this week, as the beach renourishment effort on Anna Maria Island stumbles through Bradenton Beach with no real end in the long-term need for sand in sight, perhaps a primer on sand transport, currents, shores and all things beachy is in order.

So let's talk a bit about inlets, some of the most dynamic - changing - of the coastal systems in Southwest Florida.

The following comes from "A Historical Geography of Southwest Florida Waterways, Volume One: Anna Maria Sound to Lemon Bay." It was written by the late, great Dr. Gustavo Antonini, David Fann and myself, with graphics provided by Patti and Tom Cross, and edited by Cathy Ciccolella and moi, and published in 2000.

 

Inlet dynamics

Tidal inlets - Floridians sometimes call them passes - are highly dynamic and visible features of Southwest Florida's boating geography. 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 clear 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 Sarasota Bay and the Gulf. They are periodically surveyed and, when shoaling occurs, are dredged by the U.S. Army Corps of Engineers in cooperation with the

West Coast Inland Navigation District (WCIND).

Longboat Pass is a single-span lift bridge situated near the inlet mouth. The bridge at Longboat opens on demand for boaters from 6 a.m. to 6 p.m. and afterwards on three-hours notice.

As a historical note, six inlets have closed during the past century on this reach of the Florida coast: Bradenton Beach, Little Sarasota Pass, Midnight Pass, Casey's Pass, [Old] Stump Pass, and Bocilla Inlet. Another historic inlet probably existed at Buttonwood Harbor on Longboat Key prior to 1883.

Both current and historic inlets have formed, closed, and reopened over their life span, due in part to natural processes as well as to human intervention. Such events directly affect the amount of water flowing through an inlet during a tidal cycle, referred to as a tidal prism. Dredging inlet "A" can rob some of the tidal prism from inlet "B," situated several miles down the coast. Similarly, the tidal prism of an inlet may be affected by changing the area of the bay adjacent to it; an inlet may close due to an abundance of sediment and strong longshore drift coupled with a small tidal prism.

There is considerable debate regarding the role played by the dredging and filling of mangrove and marsh environments along bay margins on decreasing the tidal prism and the related closing of inlets. There is little disagreement, however, in the potential for storm overwash of the barrier islands and the creation of new inlets.

 

Inlet features

Inlets are natural or manmade channels connecting the coastal Gulf to estuaries with strong tide-induced currents which build up supplies of sand, called shoals, just inside or adjacent to their channels. 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 occur at different time scales, ranging from hours during severe storm events to decades or even centuries.

For the mariner running the inlet, the most recognizable feature is the steep groundswell which builds up across the inlet mouth, caused by resistance created by the sea bottom where offshore swells run into shoal water. The transport of sediment along the beach face, referred to as longshore drift, occurs on the Gulf side of barrier islands and is generally a north-to-south event.

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. It's also called littoral drift.

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.

Material brought out on the ebb tide is deposited on the swash platform - call them sandbars outside an inlet. 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. These channel features, at boat deck level, appear to have the smoothest water surface and absence of breakers and, under favorable weather, may offer the mariner an alternative shorter route through the inlet.

Spits occur where there is a high rate of sediment transport along shore and a small tidal prism. Spit growth eventually may restrict tidal flow in the main channel and cause downdrift migration or closure of the inlet.

Flood (incoming) tide transports sediment landward through the inlet via the main channel, producing a similar shallow water, delta-like feature on the bayside 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.

 

Types of inlets

Tidal and wave energies determine 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.

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. Another factor leading to inlet alteration is beach renourishment activities, which can contribute to pass shoaling through sand transport via longshore drift.

The signature features of tide-dominated inlets are a well-defined main ebb channel with deposits of beach sand on adjacent Gulf shores. Longboat Pass and Venice Inlet fall under this category. These 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, the Longboat Pass entrance channel is more than 1 mile long and the bridge must be negotiated within the throat of the main ebb channel, an area where currents are particularly strong. Venice Inlet is jettied and, while passage through this entrance channel is less than 0.5 mile, currents and eddies adjacent to the rock revetments make for potentially precarious conditions.

 

Longboat Pass

Longboat Pass pre-development conditions, based on a 1883 map, appears to have a mixed-energy offset form. Flood channels on the bayside are extensive to the north and south of the inlet. Storm overwash may have created the small inlet approximately 0.5 mile to the north. Photographs from 1977 indicate the inlet had a similar shape and was in approximately the same location. The channel has been dredged, and the bayside, it follows the natural flood course, but on the Gulf side it cuts directly across the swash platform. A recurved spit, Beer Can Island or Greer's Island, as it is also known, has developed at the north end of Longboat Key and is a popular destination for weekend boaters. The flood (bayside) tidal delta is extensive and the Intracoastal Waterway has been dredged through the shoal. The 1995 aerial shows present conditions at Longboat Pass.

 

And the point?

There have been many changes in the location, shape and dynamics in the Sarasota Bay system during the past 100 years. These changes have affected the mariners' ability to enter and leave inland waters and make passages in the Gulf of Mexico. Natural processes and human intervention have influenced the evolution of these inlets. 

Notwithstanding the history of change, mariners can use this knowledge of inlet history and fathom the inlet's behavior and navigable condition. While the focus of concern for safe navigation often is on the Gulf side, it is important to remember that the bayside of inlets, particularly their flood tidal deltas, play an important role in the creation of important recreational fisheries and bird rookery habitats. 

 

Sandscript factoid

Yep, there apparently were inlets at about 14th Street South in Bradenton Beach, and near Buttonwood Harbor on Longboat Key.

The book quoted above is no longer in print, but is available for you Web browsers. I'd give you all the letters and numbers to find it, but it's probably going to be easier for you to just go to Google, type in the title, "A Historical Geography of Southwest Florida Waterways, Volume One: Anna Maria Sound to Lemon Bay,"  and download from there.

There's also a second volume that runs farther to the south.

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