In Beaches of the Gulf Coast, Richard A. Davis Jr., a veteran coastal geologist, explores the dynamics of beach formation, providing the reader with a basic understanding of the characteristics and behavior of the beach environment and what causes it to change. He compares natural beach environments with those that have experienced human intervention, and he profiles many of the common plants and animals that grow and live on and adjacent to the beach.
Following the coastline from the Florida Keys around the Gulf Coast to Varadero Beach in Cuba, Davis describes the major characteristics of beaches in each US state, with a final chapter on Mexico and Cuba. Focusing on public beaches, Davis emphasizes the special features of the beaches, indicating whether and how they are nourished—either naturally or artificially—and pointing out which beaches have problems and which ones are doing well.
Including photographs, satellite images, charts, and maps that reveal the natural processes of beach formation and erosion, Davis showcases the beauty of some of the Gulf’s “best” beaches, both popular and remote. Beaches of the Gulf Coast provides a broad range of basic knowledge for all who own beachfront property, who live near the beach, or who simply love the beach and want a better understanding of this special coastal environment.
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Beaches of the Gulf Coast
By Richard A. Davis Jr.
Texas A&M University PressCopyright © 2014 Richard A. Davis Jr.
All rights reserved.
WHEN we hear the word beach, the first thing that comes to mind is sand; the next is probably waves. Actually there are multiple processes that impact beaches and control their existence and appearance (figure 1.1). It is appropriate to begin with the most fundamental of these coastal processes: the weather. Then it is important to consider how the waves, which are a result of the weather, impact the beach. These waves also generate currents that are a major element of beach dynamics. Storms, especially hurricanes, are a significant factor in Gulf of Mexico beaches. A process that is always present but is not weather related is the ebb and flow of tides, but tides do not play a major role in Gulf Coast beaches.
The Gulf Coast is positioned in the latitudes that range from about 18° to 30° north of the equator. This range of latitudes experiences a fairly wide variation in weather patterns. As the seasons change, so do the weather patterns. During the summer the Gulf is within the Trade Winds belt, with the prevailing direction from the southeast. This is the time when tropical storms can impact this coast. In the winter the westerlies prevail as weather systems are moving from the northwest to the southeast. The changes from one pattern to another influence the way beaches respond to the wind and the waves produced by it.
In the midlatitudes, the weather typically moves from west to east. High-pressure systems tend to come from the higher latitudes and collide with low-pressure areas, causing precipitation. It should be noted that wind tends to flow from high pressure toward low pressure. As these systems move across the mainland of the United States, they dominate the weather. For much of the year, this type of weather does not influence the Gulf Coast, but in the winter these patterns move into more southern latitudes. For example, the Florida peninsular coast experiences prevailing wind from the southeast from about mid-March to mid-October. As the seasons change, the sun moves to the southern hemisphere and the frontal systems of westerly weather impact this peninsula.
The typical situation occurs when a cold front (high pressure) comes from the northwest and passes across the Texas coast (figure 1.2a). As the front approaches, the wind is blowing onto the coast from the southeast. Just behind the front the wind is from the northwest and is typically strong. Lower temperatures and dryer air are associated with such frontal passages. On the Texas coast intense frontal passages are commonly called "blue northers" because of the cold north wind associated with them.
These frontal systems move across the Gulf of Mexico toward the Florida peninsula. There is some loss of intensity due to the warming effect of the Gulf water. As the front approaches the Florida coast, the wind has a southerly component. There is a rapid change to the northwest with increased speed as the front passes (figure 1.2b). Under some circumstances these fronts pass across the Florida peninsula and may move northerly along the Atlantic Coast to become a nor'easter, the strong storms of the winter in the New England area.
This pattern continues for several months, with the intensity of each frontal system increasing as the winter weather gets colder. Such frontal passages tend to dominate the wave and current patterns along the Gulf, especially on the Texas and Florida coasts (figure 1.2c).
Tropical Storms and Hurricanes
During the warm months, intense storms that develop in the tropical eastern Atlantic can impact the Gulf Coast. These storms begin as so-called tropical waves off the west coast of Africa. As the system gains intensity, it becomes a circulating low-pressure system. Because of its position in the low latitudes, it moves westward as part of the Trade Winds system. Circulation in these weather systems is in an anticlockwise direction. As the barometric pressure lowers, the wind speed increases, eventually reaching tropical storm level (39 mph) and in some cases eventually reaches hurricane status (75 mph).
The tropical storm/hurricane moves west and north as it approaches North America. As it passes the Caribbean Sea and the Bahamas, such a storm will commonly move in one of two paths: up to the north along the Atlantic Coast or into the Gulf of Mexico. Those storms that enter the Gulf can have a range of tracks (figure 1.3): some move westward to Mexico, some swing up onto the northern Gulf Coast, and rarely they move onto the west coast of the Florida peninsula.
These severe storms have a major influence on the coast, especially the beach environment. The speed of the wind and the size of the waves generated by this wind are dependent on the intensity of the storm. Hurricanes are rated on a scale that ranges from category 1 to category 5 as the wind speed increases. The increase of wind causes a related increase in the storm surge, the increase in water level produced by the friction of the wind over the water surface (figure 1.4). Such a phenomenon produces dangerous flooding in coastal areas during these storms. Storm surge at the coast is also related to the adjacent continental shelf. A shallow and wide shelf, such as on the Florida peninsula, will cause water to build up and increase the storm surge. A more narrow and steep shelf, such as the one adjacent to the Florida Panhandle, will have a much lower surge given the same wind conditions.
Because these intense storms have a cyclonic (anticlockwise) circulation, there is also offshore wind as the storm passes. Depending on the specific location of the coast relative to the storm system, there will be onshore wind producing a large storm surge as well as offshore wind that could diminish the storm surge.
The friction between the wind and water produces waves that are a regular disturbance of the water surface. Waves may occur at any location where wind blows over the water, but they are especially important along the coast. The beach is largely a result of wave action that may either erode the beach or build one. The size of waves in open water is the combined result of the speed and duration of the wind and the fetch, the distance over which the wind blows. In some basins there is a theoretical limit to the size of the waves because of the size of that basin. These fetch-limited basins include the Gulf of Mexico and the Mediterranean Sea. Large waves may develop in the Gulf, but there is a limit to how big they may be. Waves that are directly under the influence of the wind are called seas, and waves that have moved beyond the direct influence of the wind are called swell.
There is a wide range in the size of the waves as measured by the period, the time that it takes for a complete wave length to pass a point in space. In the Gulf of Mexico the wave period commonly ranges from about 3 seconds up to 6 or 7 seconds under normal conditions. Storms can generate waves that are significantly longer—up to 10 seconds in the Gulf. Wave period may exceed 20 seconds in the Pacific Ocean.
The motion of the water in a wave is essentially circular; that is, the wave form moves progressively, but the water does not. If it did, then we would have huge piles of water at the coast. This circular path of water within the wave at the surface has a diameter that is essentially equivalent to the height of the wave. The diameter decreases with depth until there is no motion at a depth of about one-half the wave length (figure 1.5). The wave length is the distance from crest to crest or trough to trough. A scuba diver likes to be at a depth equivalent to at least half the wave length. Less deep than that will cause the diver to move in a circular path as the water does and perhaps cause seasickness.
When waves move into shallow water and approach the shoreline, they are influenced by the bottom surface. As soon as the wave reaches a water depth equivalent to half the wave length, the bottom interferes with the wave motion and the circular motion becomes elliptical (figure 1.6). This causes the waves to compress in length and increase in height. As this process continues, the waves eventually become unstable and break. Breaking waves are very important in beach processes because they cause large amounts of sediment to become suspended and to move. There are three primary types of breaking waves: spilling, plunging, and surging (figure 1.7). Spilling waves break relatively gently and slowly, like spilling water from a glass. These breakers generally develop from waves that are directly under the influence of the wind. It is common for there to be multiple rows of spilling waves as they break over sandbars in the near-shore. Plunging waves break violently with a large "crash" and typically form from swell waves. Surging waves move up the foreshore as the last breaking wave on the beach.
Because wave breaking is the result of instability caused by shallow depths interfering with wave motion and propagation, there is an effect of water depth as the wave enters shallow water and approaches the beach. Along most coasts, especially in the Gulf of Mexico, sandbars are present parallel to the shoreline. There are commonly two or three sandbars, but the number varies. It is typical for waves to break over the bars and then re-form because all of their energy is not lost during breaking (figure 1.8). As a consequence we can see multiple rows of breaking waves parallel to the shoreline (figure 1.9). The band of breaking waves is the surf zone.
Wave Refraction and Longshore Currents
There is another very important aspect of the waves moving into shallow water that has a major impact on the beaches—wave refraction, which is the "bending" of the wave crest as it moves into shallow water at an angle. It is quite rare for waves to approach the shoreline exactly parallel to it. Most commonly, waves approach at an acute angle to the coast based on the direction of the wind that generates these waves. As the wave enters shallow water and slows, eventually to break, it does so at different times and positions along the wave crest. This causes the wave to refract or bend (figures 1.10 and 1.11). This phenomenon causes water to be transported along the surf zone in the direction into which the angle of approach opens. These are called longshore currents and may transport considerable sediment. Their speed is dependent on the size and speed of the waves, along with the angle of approach.
Long-period swell waves may refract in relatively deep water because of their wave length. If this refraction is complete, the waves will approach almost parallel to the shoreline. Under these circumstances, there would be an insignificant longshore current and sediment transport.
The speed of longshore currents is generally highest in the troughs just landward of the sandbars. Under strong wind the speed may exceed a meter per second. The combination of the energy expended by breaking waves and the longshore currents can cause sediment, virtually all sand but with some shell material, to be carried along the shoreline in the surf zone. The rates of sediment movement in this zone can be quite high and may have a major influence on the dynamics of the adjacent beach. Various obstructions, some anthropogenic and some natural, can interrupt these currents and the sediment they transport.
Even though the basic water motion in waves is circular, friction from wind and wave breaking does cause some landward motion so that water builds up slightly at the shore. This is called setup and produces a modest instability in the water level at the shore. Because of this unstable and slightly elevated level, the water must return seaward. Under most circumstances it does so as undertow, where water is simply transported along the bottom. This is mostly associated with fairly steep beaches. A more common condition occurs where the seaward return flow is focused in a narrow concentration called a rip current.
The rip current may be directed through a low area, or saddle, in the crest of the longshore sandbar (figure 1.12), or it may be the result of converging currents that move along the shore. It is this phenomenon that causes problems for beachgoers and swimmers. Rip currents may be strong enough to carry swimmers seaward to depths beyond their ability to stand. The standard solution to the problem of being caught in a rip current is to swim parallel to the shore. These systems are narrow, and after a few strokes one should be free of their influence. Most beaches now have warning signs, and some have good information on recognition of rip currents. Generally, they can be recognized by actually seeing the water moving seaward (figure 1.13), or the waves are commonly lower over the rip because the seaward-directed water stifles them somewhat.
Rip currents only extend across the longshore bars in the surf zone, directed by the saddles in the bars. Beyond that there is really nothing that confines them, and the moving water spreads out. Sometimes it is possible to see this phenomenon because of the sediment that is in suspension. It is common for strong rip systems to carry sediment seaward, but the volume is not significant in the scheme of the beach's sediment budget.
Tides are the regular and predictable change in water level due to the gravitational attraction between the sun, moon, and earth. The relationship is proportional to the masses of these celestial bodies and inversely related to the distance between them. In other words, because the sun is so far away, about 93 million miles (150,000,000 km), its influence on the earth is relatively low even though it is tremendously large. The moon is quite small but is only 239,000 miles (385,000 km) away. As a result, its influence on the earth is about twice that of the sun's. The mass attraction between the earth and the moon is the main reason that we experience tides along the Gulf Coast. It is also the reason that tidal cycles follow the lunar cycle.
The attraction between the earth and moon causes a bulge in the water of the earth's oceans. As the earth rotates, the bulge moves toward the shoreline twice each day: once on the side of the earth toward the moon, and the other on the opposite side (figure 1.14). As the lunar cycle changes over a lunar month, the amount of distortion in this bulge also changes. The maximum distortion occurs during the new moon and the full moon, a period of essentially two weeks. Under these circumstances the lunar and solar tides are superimposed. The minimum distortion occurs during the first quarter and third quarter of the monthly cycle when lunar and solar tides are at right angles. The tides during the maximum are called spring tides, and the minimum tides are neap tides.
In general, the change in water level during each tidal cycle is small in the Gulf of Mexico—less than 1 m on nearly the entire coast. Even so, the difference between spring tides and neap tides can be significant. Semi-diurnal tidal cycles occur twice daily, and diurnal tidal cycles occur only once a day. The Gulf of Mexico experiences mixed tides during a tidal month (figure 1.15); that is, the tides are diurnal on some days and semi-diurnal on others.
Along some coasts the tides are very large. In the United States the panhandle of Alaska has by far the largest tides, up to 12 m. In the lower 48 states, tides along the Oregon, Washington, and Maine coasts reach nearly 4 m. In some of these places the beach is essentially a wide tidal flat (figure 1.16). On the Gulf Coast things are quite different. The tides are quite small, and the intertidal zone is very narrow. It is generally easy to recognize the high tidal level when one goes to the beach. It commonly is marked by seaweed, shells, or other materials. The tidal stage does not, however, have a major impact on how we use the beach. About the only thing we need to know is where to put our beach chair or blanket so that the tide does not come up and cover it.
Although a very slow process, sea-level change is an important factor in the beach environment. This topic has been prominent recently in the various media due to an apparent increase in the rate of sea-level rise related to the warming of the earth's climate. Actually, both the rise and fall of sea level have an impact on the beach. As sea level falls, the beach tends to become wider and may be accumulating significant sediment. This is not the situation at the present time on the Gulf Coast, where erosional conditions dominate, but it is happening in the high latitudes of the northern hemisphere due to glacial rebound. As the glaciers have melted, the crust rises as it adjusts, and the result is a relative lowering in sea level. This adjustment takes place as a part of eustasy. The northern part of Scandinavia is experiencing this phenomenon now. In North America there are numerous beach ridges representing former shorelines along the margin of Hudson Bay in Canada (figure 1.17).
Excerpted from Beaches of the Gulf Coast by Richard A. Davis Jr.. Copyright © 2014 Richard A. Davis Jr.. Excerpted by permission of Texas A&M University Press.
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Table of Contents
PART I. GENERAL CHARACTERISTICS AND DYNAMICS OF BEACHES,
1. Coastal Processes,
2. Beach Geomorphology and Barrier Island Morphodynamics,
3. Beach Materials, Structures, and Sources,
4. Human Impact on Gulf Beaches,
5. Common Animals and Plants of the Gulf Beaches and Surf Zone,
PART II. BEACHES ALONG THE GULF OF MEXICO COAST,
6. Beaches of Florida,
7. Beaches of Alabama,
8. Beaches of Mississippi,
9. Beaches of Louisiana,
10. Beaches of Texas,
11. Beaches of Mexico and Cuba,