THE DYNAMIC BARRIER ISLAND
A casual observer visiting the National Seashore for only a day might notice many natural processes and conditions that reflect the changing or dynamic nature of barrier-island environments. For example, a visitor standing in an active dune field in the face of a strong wind can observe the dynamics of these mobile systems. In addition, on a windy summer day, sunbathers on the beach quickly realize the advantage of lying on moist firm sand near the shoreline in order to avoid the dry loose sand that is being transported landward by persistent onshore wind. Swimmers in the surf zone might notice being tugged along rather strongly by currents that literally sweep the sand alongshore. Observers of the surf zone might see that waves generally break along three relatively distinct lines that coincide with offshore submerged sandbars. Campers who drive along the beach on the edge of the Gulf may discover that a smooth, flat beach can be changed in a few hours into an irregular surface consisting of a series of rhythmic ridges and troughs that make driving slow and rough. Fishermen along Mansfield Channel at the southern end of the National Seashore know that their fishing lines might be strung out by currents moving lagoonward on one occasion and gulfward on another. Visitors who are in the park during the passage of a cold front or norther might observe that a shift in wind causes a corresponding shift in the direction of sand migration. The north winds also cause a change in the direction of wave approach and produce a higher and rougher surf (Robert Whistler, personal communication, 1979).
These and other casual observations result in several questions: What happens to sand that is blown landward from the beach day after day and year after year? Why do the currents in the surf zone change direction from time to time, and what happens to the sand that these currents transport alongshore? Are offshore submerged sandbars always present or are they temporary features that disappear for much of the year? What causes rhythmic irregularities in the surface of the beach and why are some beaches composed mostly of shells and others of sand? What is the internal structure of a beach or of a dune? What effect does vegetation have on sand migration? How strong are tidal currents that flow in and out of Laguna Madre? Does the wind affect water levels in bays and lagoons? What happens to island environments when a hurricane strikes?
In the following sections many of these and other questions will be answered. Some of the answers might lead to more questions. Many natural processes and their effects are difficult to understand, partly because the processes are complex and cannot be duplicated under laboratory conditions, and partly because natural island features can be interpreted in different ways. The fact that many island features and processes are not entirely understood challenges both amateur and professional naturalists.
Perhaps the most dynamic environments on Padre Island are those along the Gulf shoreline, where processes of air, water, and land meet and interact. Gulf waves, produced by the wind, approach the shoreline, begin to "feel" bottom along the upper shoreface, steepen and break, reform and break again along a series of offshore submerged bars. The waves ultimately reach the beach, where their remaining energy is expended as they run up the beach and wash back to the Gulf, forming the swash zone (figs. 2 and 74).
Change is natural and common in these high-energy environments, where sediments are constantly being rounded and sorted, so that through time the particles become more and more uniform in shape, size, and composition. Clayey and muddy sediments that are transported to the Gulf by rivers are virtually absent from the beach and nearshore areas along Padre Island. These clay-sized particles are too fine (small) to settle in these high-energy environments; instead, they remain suspended in agitated waters and eventually settle in deep, calmer water offshore. Coarser sediments such as gravel do not reach the Gulf but are deposited in river channels in inland areas where reduced channel gradients cause a corresponding reduction in stream carrying capacity. Sand, however, composed predominantly of quartz and a small percentage of dark-colored heavy minerals, is small enough and durable enough to reach the Gulf, but is too coarse to be held in suspension. The sand rolls, bounces, and skips along the bottom of a stream until it is deposited near the mouth of the river. If the river discharges into the Gulf, then Gulf currents eventually sweep much of the sand along the coastline and concentrate it on beaches and the adjacent shoreface.
Along some stretches of Padre Island, shells and shell fragments derived from sources alongshore and offshore are also important components of the high-energy beach and nearshore environments. Additional sediments have been supplied from offshore submerged sand bodies. These sand bodies were deposited by ancient rivers discharging seaward of the present shoreline during past "ice ages," when sea level was much lower as a result of continental glaciers. These various sources of sediments from rivers, shells, and offshore deposits interact with waves and wave-induced currents, with tides, with storm-driven water, with sea-level changes, and with the wind to create the dynamic conditions that are present along the Gulf shoreline and beach of Padre Island.
Waves and Longshore Currents
There are numerous types of currents in the Gulf of Mexico. The most important currents affecting the Gulf shoreline are those that result primarily from waves breaking alongshore. Swimmers in the surf are often pulled along by currents parallel to the beach. Such currents, which are capable of moving tons of sediment in the surf and swash zones, are significant in explaining many of the features and changes that occur along the Gulf shoreline. How do those currents form? What is their effect?
Generally, waves approaching land are not parallel to the shoreline. Instead, waves arrive and break at an angle to the shore. Currents that are produced move in a preferred direction, depending on the direction of wave approach (fig. 20). The movement of water alongshore in this manner is termed longshore or littoral drift. If waves approach a north-south oriented shoreline from the northeast, longshore drift will be toward the south. Conversely, waves approaching from the southeast will produce currents flowing north (fig. 21). Waves that are exactly parallel to the shoreline and, therefore, advancing in a direction that is perpendicular to it will not cause significant longshore drift. Depending on the size of the waves and the angle of wave approach, the current at times may be relatively strong (maximum movement occurs when waves approach at an angle of about 30°; King, 1972). Current velocities of up to 3.9 feet per second (fps), or 2.7 mph, have been measured along Mustang Island, immediately north of Padre Island. The average velocity, however, is less than 1 fps (Davis and Fox, 1972).
Winds exert stresses on open waters. These wind-generated stresses direct waves that break on the shoreline. Hence, knowledge of wind directions provides information on the directions of longshore drift. But wind direction is only part of the story. A glance at the Texas Gulf Coast (fig. 22) reveals a curved shoreline that is oriented approximately northeast-to-southwest along the upper coast and northwest-to-southeast along the lower coast. Because of this curvature, waves approaching the shore from any specific direction will be parallel to only one relatively short segment of the beach and shoreface. Along the shoreline at either end of such a segment, waves will approach at an angle, and currents (longshore drift) will be formed that flow parallel to shore and toward each other (fig. 22). The area in which the opposing currents eventually meet is known as a zone of convergence.
By studying winds that occur over Gulf waters, E. A. Lohse (1952, 1955) noted that geologically effective winds (those producing waves that expend the most energy doing geologic work) are more or less uniformly distributed from the northeast, east, and southeast (fig. 23). By comparing the shoreline trend with the wind directions, Lohse (1952) concluded that the zone of convergence is located near 27°N latitude, which places it along the Central Section of Padre Island National Seashore (pl. I). If this is true, net longshore drift and sediment transport should be in a counterclockwise direction north of 27°N latitude and clockwise south of it.
The phrase "net longshore drift and sediment transport" in the preceding sentence should be emphasized. Because wind directions vary seasonally, ranging generally between northeast and southeast, the zone of convergence does not remain stationary during the year but shifts in response to changes in wind direction, which controls wave approach. So, during winter northers when winds are blowing from the northeast, the zone of convergence shifts toward lower latitudes. When the winds blow predominantly from the southeast during summer months, it shifts back toward higher latitudes. The central area over which this convergence pendulum swings should reflect the area of net convergence and sediment accumulation. But is there physical evidence of net convergence along central Padre Island?
Evidence of Converging Longshore Currents
Because different rivers intersect, erode, and transport rock materials of differing composition, each river entering the Gulf can be characterized by the suite or assemblage of minerals it transports and deposits. Rivers such as today's Rio Grande, Colorado, Brazos, and Mississippi discharge directly into the Gulf and contribute sand to the longshore dispersal system of the Gulf. The sands of each river consist not only of quartz but also of heavy, dark minerals such as magnetite, garnet, zircon, tourmaline, and many others. Assuming that each river carries a distinctive suite of heavy minerals, an analysis of beach sands should allow one to estimate the sediment contribution made by the various rivers and thus to infer the net direction of longshore sediment transport. In 1942, F. M. Bullard initiated a study to do this. He discovered that minerals characterizing the Rio Grande sediment were traceable northward along Padre island beaches from the mouth of the Rio Grande to central Padre Island (in the vicinity of 27°N latitude), north of which there was a significant decline in the Rio Grande heavy mineral suite. Bullard also found that heavy mineral assemblages distinctive of rivers to the north of Padre Island, such as the Colorado and the Brazos, could be traced southward to central Padre Island.
Thus, the distribution of minerals observed in Bullard's study and in later studies (van Andel and Poole, 1960; Hayes, 1965) supported the theory of a zone of sediment convergence on Padre island (Central Section, pl. I). There are complications, however, if one bases conclusions entirely on sediments presently supplied by modern rivers. Sea level was much lower only about 10,000 years ago (fig. 16), and rivers discharged sediments gulfward of their present mouths. Many geologists believe that the sediments that compose today's barriers are partly derived from the offshore shelf sediments that were deposited by these ancient rivers. Because of past variations in shoreline configuration, and perhaps changes in dominant wind directions, directions of longshore sediment transport and river sediment distribution may have varied from those in existence today (Curray, 1960). Consequently, the distribution of minerals along the beaches may reflect, in part, past currents and conditions. Because of this, other evidence is needed to help resolve the question of longshore drift direction and convergence.
EFFECTS OF JETTIES ON LONGSHORE DRIFT
Man-made structures, such as jetties, constructed perpendicularly to the Gulf shoreline at inlets and channels interfere with longshore sediment transport and trap sediments on the upcurrent side. This clearly occurs at Mansfield Channel at the south end of the National Seashore (pl. I). These jetties are south of 27°N latitude, and aerial photographs show that sediments have accumulated along the south side of the southernmost jetty (figs. 13 and 24). The accumulation of sand has created a sand deficit on the downcurrent (northern) side of the channel, resulting in a narrow, erosional beach immediately north of the jetties. At Mansfield Channel, net sediment transport is apparently northward, thus toward 27°N latitude.
Other jetties are present on Mustang Island, just north of Padre Island (fig. 1). Since 1972, sediments have accumulated on both the north and the south sides of the jetties at the Water Exchange Pass (fig. 1); this indicates that longshore drift periodically changes direction at this location. Behrens and others (1977) substantiated that winds control the two-way littoral drift in this area. During winter, when winds accompanying passage of a cold front blow from the north and northeast, longshore drift is usually southward. During summer months, when southeasterly winds prevail, longshore sediment transport is northward. Although sediments are transported back and forth during the year, there is a net sediment transport southward toward central Padre Island, where the area of net longshore drift convergence apparently occurs (Behrens and others, 1977).
Additional evidence that sediment drift is southward along Mustang Island was presented by W. A. Price in 1933. He observed that before it was stabilized by jetties, Aransas Pass, which is a natural tidal inlet located at the north end of Mustang Island, moved southwestward in the direction of longshore drift. Sediments accumulated on the north side (upcurrent side) of the inlet, while sediments on the downcurrent side (south side) were being eroded as the channel moved southward.
ACCUMULATION OF SHELLS IN ZONE OF CONVERGENCE
The occurrence of beaches composed predominantly of shells and shell material provides additional evidence of converging longshore currents along central Padre Island. Southward from Malaquite Beach development, the sandy beaches change gradually to shelly beaches, as indicated first at Little Shell Beach and finally at Big Shell Beach (pl. I).
In a study of the origin of the shell beaches, Watson (1971) concluded that concentrated shells result from longshore drift convergence in the area of 27°N latitude. The smaller shells that characterize Little Shell Beach (fig. 25) were derived from beaches to the north, while the larger shells characterizing Big Shell Beach (fig. 26) were derived from a southern source area. A zone of transition that separates Little Shell and Big Shell contains a mixture of large and small shells (Watson, 1971). This transition zone coincides with the transition zone between heavy mineral suites derived from the northeast and from the south, identified by Bullard (1942) and van Andel and Poole (1960). Hayes (1965) identified a similar distribution of light mineral suites.
In explaining the origin of the shell beaches, Watson suggested that the converging longshore currents funnel both shells and sand into the area along central Padre Island. The shells accumulate and subsequently become concentrated by onshore winds that blow away the sand-sized particles. The sand accumulates in dunes, which may become stabilized by vegetation or which may migrate across the island and into Laguna Madre.
If sediments are funneled into the area of central Padre Island by longshore currents, then historical studies of the shoreline in this area should reflect it. This was confirmed by Morton (1977), who noted that although most of the Texas beaches are apparently undergoing a long-term net erosional trend, central Padre Island near 27°N latitude has exhibited long-term net shoreline accretion. Morton stated that "net accretion nearly coincides with the transition zones, established by heavy minerals, grain-size distribution and shell species," identified by Bullard (1942), van Andel and Poole (1960), Hayes (1965), and Watson (1971).
In conclusion, longshore or littoral drift and drift convergence are extremely important natural processes operating along the Gulf shoreline of Padre Island. There is substantial evidence that net convergence occurs in the area of central Padre Island near 27°N latitude. An understanding of littoral drift and net convergence helps to explain many natural shoreline phenomena, including accretion and erosion along jetties and inlets and the accumulation of shells at Little Shell and Big Shell Beaches. The importance of littoral drift is also reflected by the fact that it is the basis of one theory proposed to explain the origin of barrier islands along the Texas coast (fig. 14).
Effects of Waves on the Upper Shoreface and Beach
In addition to creating longshore drift, Gulf waves are responsible for numerous features occurring along the beach and immediately offshore on the upper shoreface (fig. 2). Gulf waves breaking along the shoreline of Padre Island are usually less than 1 m (3.3 ft) high, but occasionally reach heights of more than 2 m (6.6 ft) during fall, winter, and spring storms (Hill and Hunter, 1976). As waves break in the shallow water forming the surf zone, they constantly agitate the sediment, lifting sand grains into suspension so that even weak currents become effective in moving sand. W. Bascom (1964) noticed this constant motion of the sand grains:
Waves can be classified as (1) destructive or (2) constructive, depending on whether they tend to erode or deposit sediment along the beach. Laboratory studies using wave tanks show that destructive waves and constructive waves can be differentiated on the basis of wave steepness, which is the ratio of wave height (H) to wavelength (L) (King, 1972) (fig. 27). The flatter waves those with a low H/L ratio are more constructive and have a tendency to transport sand onto the beach; steeper waves are more destructive and tend to remove sand. Smooth, round, symmetrical waves, termed swell, generated by winds blowing over Gulf waters far offshore, are among the most constructive of waves. Local strong onshore winds tend to generate steep waves that are closer together and more destructive. These constructional and destructional waves explain, in part, many features that occur along the Gulf shoreline.
OFFSHORE SUBMERGED SAND BARS (WAVE BREAKPOINT BARS)
Immediately offshore and approximately parallel to it, there may be as many as three or four subparallel submerged sandbars separated from each other by troughs. If waves are sufficiently high when they arrive alongshore, they break along the bars (fig. 28), re-forming in the troughs that lie between them. Bars and troughs closest to shore are easily detected by swimmers and surfers who may seek temporary refuge in the troughs, where waves seldom break. Although the bars might shift back and forth, become discontinuous and curved, or disappear at times depending on wave characteristics and nearshore conditions, they are relatively stable features that remain in a sort of dynamic equilibrium with the waves and currents that form them.
Exactly how these offshore submerged bars are formed and maintained is only partly understood. C. A. M. King (1972), from whom much of the following information is obtained, proposes that the submerged bars are identical to wave breakpoint bars that can be formed in laboratory wave tanks. The bars are formed along the usual breaking point of steep destructional waves. Bars in deeper water are formed by larger waves, and bars in shallower water by intermediate and smaller waves. By varying the size of waves generated in a wave tank, a bar that has formed will migrate into deeper or shallower water, depending on whether the waves increase or decrease in size, respectively. Inner bars (closest to shore) are most susceptible to change because they are affected by both large and small waves, in contrast to the outermost bars that are affected only by large waves. King (1972) observed that because submerged bars are formed at the breaking point of the wave, the bars cannot be built up above the still water level and, therefore, will never become emergent.
Swash bars are formed in the swash zone along the beach where waves rush up on the beach and wash back to the Gulf (King, 1972). These bars are parallel to the shore, but unlike the submerged offshore bars, they are formed by flat, constructional waves that break and transport sediment landward, depositing it in the form of a low ridge. As the swash bars are constructed, water may be trapped or ponded in shallow troughs that lie landward of the bars (fig. 29). Occasionally, the ponded water flows rapidly along the troughs and back to the Gulf through narrow channels eroded through the bars. Large, constructional waves form higher swash bars because they rush higher onto the beach to deposit sediments. A swash bar deposited during Hurricane Cindy had an average width of 50 feet and a height of 2 to 4 feet (Hayes, 1965).
Berms are relatively horizontal beach deposits generally formed by constructional waves that transport sediment and deposit it on the backbeach (figs. 30, 31a and 31b). The location of these deposits (backbeach) indicates that berms are constructed by larger than normal waves that can reach high up on the beach. More than one berm may occur on the beach, reflecting different wave characteristics and tidal levels.
The highest berms on Padre Island occur along shell beaches. Their profiles may be emphasized by a steep, erosional scarp that separates the berm from the more steeply gulfward-dipping fore-beach (fig. 30). Berms are normally constructed by large, flat, constructional waves. The height of a berm may be increased by steep storm waves that transport sediment over the berm crest to build it higher. Such storm waves generally erode the berm and remove sediment from its seaward slope; eroded sand is distributed seaward (Bascom, 1964). Storm tides and steep, destructional waves that accompany hurricanes and other storms may occasionally level and smooth the entire beach, temporarily eradicating the berms.
Along the Gulf beaches south of Malaquite, the forebeach becomes steeper where beaches are composed predominantly of shells. The steepness of a beach is directly related to the size of the sediments that compose it. The steepest beaches on Padre Island occur along Big Shell Beach where there is an abundance of large shells and shell fragments.
In addition to sediment size, wave steepness and wavelength also affect beach gradient, so that beaches of uniform composition may have varying steepness (King, 1972). In general, steep, destructive waves tend to erode material from the beach and move it seaward to form a more gently sloping beach. Flatter, constructional waves tend to increase the steepness of a beach. Because beaches partly reflect this variation in wave characteristics, the beaches and their profiles are constantly changing, while remaining in dynamic equilibrium (King, 1972).
Beach cusps are temporary features that occur along the Gulf shoreline, usually in a series of more or less uniformly spaced mounds or ridges called horns, separated by crescent-shaped troughs called bays (fig. 32). Cusps form in the swash zone with their horns oriented roughly perpendicular to the shoreline (figs. 32, 33, and 34). These rhythmic forms may develop in only a few hours and can change a smooth, relatively flat beach into an uneven surface, causing rough driving conditions along the gulfward edge of the island. Cusps may be formed at different levels along the beach as wave and tidal conditions change.
Beach cusps are related to wave characteristics, but their exact origin and their sometimes remarkably uniform spacing (distance between horns) are not entirely understood, even though they have been studied since the mid-1800's. Beach cusps can be formed in laboratory wave tanks. Several explanations have been proposed regarding their origin and spacing, but none are entirely satisfactory. There is evidence that the uniformity of cusps is due to nearshore water circulation cells and rip currents or to edge waves, which are nearshore waves trapped by refraction (Komar, 1971, 1973). Some studies show that the horns of cusps are primarily the result of depositional processes, but other studies (for example Sallenger, 1977) suggest that they can be formed by erosion of existing ridges or berms. Cusps appear to develop optimally at times when waves are parallel to shore, approaching it at a 90° angle, but they may also form when waves approach at an oblique angle.
The spacing of cusps becomes more uniform if favorable wave conditions persist. Pronounced longshore drift in the swash zone as a result of oblique waves will commonly cut into the horns, forming asymmetrical cusps with erosional scarps; eventually, such longshore currents will destroy the cusps. Large, steep, destructional waves that tend to smooth and flatten a beach will also erode the cusps.
R. J. Russell and W. G. McIntire (1965) made many observations of beach cusps and their formation along ocean beaches. Much of the following is based on their observations and conclusions.
Cusps may form along beaches composed of a variety of deposits, ranging from boulder-sized material to fine-grained sand. The most favorable conditions for cusp development seem to be during a period of decreasing wave energy when steep, destructional waves are replaced by waves of less intensity. Horns that are depositional in origin form on the seaward face of the berm and may grow across the forebeach as sediments are added by wave uprush. Horns are composed of coarser material (in many cases shell, where available) and have a steeper slope than the interlying bays. Newly deposited horns are generally soft and contain considerable interstitial water. Along Big Shell Beach on Padre Island, newly deposited, soft, and water-saturated shell sediments composing the horns of cusps make vehicular passage difficult, even for four-wheel-drive vehicles.
Distances between horns (cusp spacing) may range from 20 to 185 feet (cusps with spacing of less than 1 foot have been observed along the shores of lakes; Komar, 1973). Wider cusps seem to be produced by higher waves. Cusps at higher levels of a beach may remain undisturbed for a relatively long period (up to 2 years or more) until conditions similar to those that formed them recur.
The Beach A Source of Sand for Dunes
The Gulf beach is a complex environment where water and air interact and compete for sediments. Sediments deposited high on the beach by waves and currents are dried, picked up, and transported landward by persistent onshore winds. Much of the migrating sand is trapped along the back edge of the beach by a wide variety of salt-tolerant grasses and flowering plants (fig. 35a-f). Some of the plants thrive on the sand, keeping pace with its accumulation, and stabilizing it with roots and spreading vines. Along much of the Gulf shoreline, just landward of and parallel to the beach, a relatively continuous dune ridge has been established as a result of the onshore wind and the sand-stabilizing vegetation. The fore-island dune ridge traps additional sand and prevents it from migrating into back-island areas. Dunes along the fore-island area, however, might hold the sand only temporarily, because during storms, high tides cut into the dunes, washing the sand back to beach and Gulf. Yet it is possible for the sand to escape from the fore-island and be deposited on the back sides of barrier islands and in bays and lagoons. This has happened on Padre Island.
Vegetation is relatively dense on northern and central parts of the Seashore (fig. 36), but along southern stretches of Padre Island, vegetation is sparse (fig. 37). Along sparsely vegetated coastlines, fore-island dune ridges are not well developed and they contain gaps or breaks through which sand is blown. Gaps in dunes commonly result from hurricanes and rarely from human activities. Where fore-island dunes are poorly developed or breached, active blowout dunes and back-island dune complexes commonly form and move lagoonward across the island, incorporating additional sand along the way. As long as the gap exists in the fore-island dune ridge, a link with the beach allows nourishment of the active dune complex. If sand accumulates in the gap and is eventually stabilized by vegetation, the supply of sand from the beach is eliminated. But the active dunes that have already formed continue to move across the island. Unless these migrating dunes are stabilized by vegetation, they eventually migrate into Laguna Madre.
Last Updated: 28-Mar-2007