System Controls Sediment Sources Waves, Tides and Wind Coastal Platform Climate Topography and Bathymetry Sea-Level Change Landform Frequency Margin Classification Energy Classification Development Prograding Barrier Retrograding Barrier Aggrading Barrier


(Photo by from lecture by J.M. Dirnberger, Kennesaw State University)
System Controls
Sediment Sources

This is the life's-blood of barrier systems. Barrier landforms are constantly attacked by waves, winds and tides, and must be continually supplied with sediment if they are to build and maintain their shape. Sediment can come from many sources, including longshore drift, input from rivers, reworking of the coastal platform, erosion of bluffs and headlands, nourishment from backshore dunes, deltaic deposits, glacial deposits, artificial nourishment, and biogenic material.

Waves, Tides and Wind

These forces shape all barrier landforms by reworking, transporting, depositing, and removing material. They influence their shape, size, orientation and characteristics, and not every location will be equally affected.

Coastal Platform

One of the best environments for barrier system development are trailing-edge margins because of their tendency to have large continental shelves. Not only is the shelf a great source for sediment, but it provides a stable building base for islands and spits.

Climate

Temperature and precipitation are important influences on the coastal environment, affecting local erosion, river size, wave environment and vegetation type.

Topography and Bathymetry

The shape of the land near a coast can influence drainage, erosion, and sediment supply and type. The shape of the coastline can also encourage the formation of some barrier landforms, such as spits, welded barriers and tomobolos, by influencing the direction of longshore drift, refracting wave energy, and encouraging barrier landform development where the coastline abruptly changes direction.
The depth of the sea, particularly close to the coast, can influence a barrier landform's ability to form and grow, and will affect the wave environment and the strength of the waves allowed to reach the shore.

Sea-Level Change

Barrier islands strive to keep a relatively constant distance between themselves and the shoreline, migrating shoreward or seaward with a rise or fall of sea level. Worldwide, a retreat of barrier islands is occurring due to a rise in sea level and the influence of human interaction and erosion prevention.
The speed of sea-level change can also influence the development of barrier systems: for example, a rapid sea-level rise can cause a quick shoreline transgression that prevents substantial reworking of coastal sedimentary deposits, and creating a smaller amount of barrier landforms. A slower sea-level rise, however, allows the waves to rework the shelf sediment over a long period of time, providing lots of sediment for barrier-building (Davis, 1994).

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Landform Frequency

Worldwide, oceanic coasts contain examples of all barrier forms imaginable. However, there are some classification systems that help describe common characteristics of groups of coasts. While not all coasts can fit perfectly into these classification systems or match their descriptions, the systems are helpful for a general understanding of barrier systems.


Margin Classification

This system is based on the type of tectonic margin the coastline occupies. The three major categories are the leading-edge coast (located along the actively colliding side of a continental plate), the trailing-edge coast (located on the passive side of the continent opposite the collision margin), or the marginal-coast (protected from the open ocean by geographic barriers)(Davis, 2004).

The characteristics of leading-edge margins include short and small rivers, narrow shelves, and rocky coastlines with small beaches. The formation of barrier landforms is limited by the amount of sediment available, usually preventing the development of barrier islands. Along subduction margins (where an oceanic place is subsiding underneath a continental plate), the dip of the subsiding marine plate controls the width of the shelf, limiting the space for barrier landforms: a gentle angle allows a broader shelf, while a smaller angle creates a narrow shelf. The most common barrier landforms found on trailing-edge coasts are spits.

Trailing-edge margins have broad continental shelves, low coastal relief, and large rivers with large drainage basins (and therefore plenty of supplied sediment). Unlike leading-edge margins, barrier island complexes are common.

Marginal coasts are protected by islands (such as the Sea of Japan) or by other geographic features (like the Gulf of Mexico, which is shielded by the Florida and Yucatan peninsulas). While these shores may experience reduced wave action, the development of barrier landforms is still dependent on the amount of sediment available and the size of the coastal platform (Davis, 2004).

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Energy Classification

An important characteristic for describing a barrier environment, especially its barrier islands, is its coastline classification. Originally developed by Davis and Hayes in 1984, this classification system describes the relative impact of tides and waves have on the movement of sediment on a particular coast. Tidal currents transport sediment during the ebb and flow of the tidal currents, and can indirectly affect the size and strength of incoming waves, especially storm waves. Waves themselves shape the coast through their direct impact and the creation of longshore currents, which transport sediment along the coast. (Davis, 1994).

  1. Tide-Dominated Coasts. As the name implies, these coasts are dominated a large tidal range. They typically have long embayments, extensive inter-tidal areas, and lack barrier islands.
  2. Mixed-Energy Coasts. In these coasts, both tidal and wave processes shape the barrier system. Barrier islands are short and broad, and marshes and tidal flats are common in the backbarrier environment. A large ebb-tidal delta causes wave refraction, leading to the reversal of the longshore drift direction on the downshore side of the tidal inlet, trapping sediment near the inlet and starving the downshore side.
  3. Wave-Dominated Coasts. The dominant process for these coasts is longshore drift, which often creates smooth shorelines, long and linear barriers, and extensive lagoons. Wave-dominated coasts typically have well-developed barrier islands which migrate due to high-wave overwash, long barrier spits, and few tidal inlets due to the reduced tidal influence.


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Development

Barrier systems develop when the right combination of factors is in place. This typically includes ample and continuous sediment input, the right wave action to form longshore drift, and a coastal platform large enough to support barrier landforms. Additionally, many believe that topographic highs are important for barrier system development (Bona, 1990). Once the landform is in place, it can be in one of three states: prograding, retrograding or aggrading.

Prograding Barrier

An important characteristic for describing a barrier environment, especially its barrier islands, is its coastline classification. Originally developed by Davis and Hayes in 1984, this classification system describes the relative impact of tides and waves have on the movement of sediment on a particular coast. Tidal currents transport sediment during the ebb and flow of the tidal currents, and can indirectly affect the size and strength of incoming waves, especially storm waves. Waves themselves shape the coast through their direct impact and the creation of longshore currents, which transport sediment along the coast. (Davis, 1994).

Retrograding Barrier

If there is not enough sediment supplied to maintain the barrier or keep pace with sea-level rise, the barrier will retreat towards the shore, moving into the bay or lagoon that is behind it. Sediment is lost to storms, overwash and longshore drift, and if there is not enough to replace the losses, the barrier will become narrower and lower.

Aggrading Barrier

If the barrier has not moved, but continues to grow upward, then it is called an aggrading barrier. This indicates an equal rate of sediment supply and loss to keep pace with sea-level change, and is fairly rare (Davis, 2004).

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A COMPARISON OF OCEANIC BARRIER SYSTEMS AND GREAT LAKES BARRIER SYSTEMS
Copyright 2004, Jennifer Bruce