Barrier Islands

Barrier islands are narrow linear mobile strips of sand up to about 30-50 feet (10-15 m) above sea level, and typically form chains located a few to tens of miles offshore along many passive margins. They the 6-12 foot (2-4 m) deep depression with up to 25 feet (8 m) of water and leave a path of destruction where the torrents of water raged through the city. These levees also channel the sediments that would naturally get deposited on the flood plain and delta far out into the Gulf of Mexico, with the result being that the land surface of the delta south of New Orleans has been sinking below sea level at an alarming rate. A total land area the size of Manhattan is disappearing every year, meaning that New Orleans will be directly on the Gulf by the end of the century. Alarming poststorm assessments of damage from Hurricanes Katrina and Rita push that estimate forward by years.

The projected setting of the city in 2100 is in a bowl up to 30 feet (5 m) below sea level, directly on the hurricane-prone coast, and south of Lake Ponchartrain (by then part of the Gulf). The city will need to be surrounded by 50-100 foot (15-30 m) tall levees that will make the city look like a fish tank submerged off the coast. The levee system will not be able to protect the city from hurricanes any stronger than Katrina. Hurricane storm surges and tsunami could easily initiate catastrophic collapse of any levee system, initiating a major disaster. Advocates of rebuilding are suggesting elevating buildings on stilts or platforms, but forget that the city will be 3-6 meters below sea level by 2090, and that storm surges may reach 30-35 feet (10 m) above sea level. A levee failure in this situation would be catastrophic, with a debris-laden wall of water 45-50 feet (15 m) tall sweeping through the city at 30

miles (50 km) per hour, hitting these build-ings-on-stilts with the force of Niagara Falls, and causing a scene of devastation like the Indian Ocean tsunami.

Sea-level rise is rapidly becoming one of the major global hazards that humans must deal with, since most of the world's population lives near the coast in the reach of the rising waters. The current rate of rise of an inch (a couple cm) every 10 years will have enormous consequences. Many of the world's large cities, including New York, Houston, New Orleans, and Washington, D.C., have large areas located within 10-20 feet (a few meters) of sea level. If sea levels rise even a few feet (1 m), many of the city streets will be underwater, not to mention basements, subway lines, and other underground facilities. New Orleans will be the first under, lying a remarkable 10-15 feet (3-5 m) below the projected sea level on the coast at the turn of the next century. At this point governments should not be rebuilding major coastal cities in deep holes along the sinking, hurricane-prone coast. Governments, planners, and scientists must begin to make more sophisticated plans for action during times of rising sea levels. The first step would be to use the reconstruction money for rebuilding New Orleans as a bigger, better, stronger city in a location where it is above sea level, and will last for more than a couple of decades, saving the lives and livelihoods of hundreds of thousands of people.

New Orleans is sinking farther below sea level every year and getting closer to the approaching shoreline. Sea level is rising, and more catastrophic hurricanes and floods are certain to occur in the next 100 years. Americans must decide whether to spend hundreds of billions of tax dollars to rebuild a city with historic and emotional roots where it will be destroyed again, or to move the bulk of the city to a safer location before subsidence increases and another disaster strikes. The costs of either decision will be enormous. The latter makes more sense and will eventually be inevitable. The city could be moved in the slump following the destruction by Hurricane Katrina, saving lives, or residents could wait until an unexpected category five superhurricane makes a direct hit and kills hundreds of thousands of people. Katrina was a warning, New Orleans is sinking below sea level, and it is time to move to high and dry ground.


Beatley, Timothy, David J. Brower, and Anna K. A. Schwab. Introduction to Coastal Management. Washington, D.C.: Island Press, 1994. Davis, R., and D. Fitzgerald. Beaches and

Coasts. Malden, Mass.: Blackwell, 2004. Kusky, T. M. The Coast: Hazardous Interactions within the Coastal Environment. New York: Facts On File, 2008. Williams, S. J., K. Dodd, and K. K. Gohn. Coasts in Crisis. Reston, Va.: U.S. Geological Survey Circular 1075, 1990.

are separated from the mainland by the back-barrier region, which is typically occupied by lagoons, shallow bays, estuaries, or marshes. Barriers are built by vertical accumulation of sand from waves and wind action. Barrier islands are so named because they form a natural protection of the shoreline from the forces of waves, tsunami, tides, and currents from the main ocean. Many barrier islands have become heavily developed, however, as they offer beautiful beaches and resort-style living. The development of barrier islands is one of the most hazardous trends in coastal zones, since barriers are simply mobile strips of sand that move in response to changing sea levels, storms, coastal currents, and tides. storms are capable of moving the entire sandy substrate out from underneath tall buildings.

The size of barrier islands ranges from narrow and discontinuous strips of sand that may be only a few hundred feet wide, to large islands that extend many miles in width and length. The width and length is determined by the amount of sediment available, as well as a balance between wave and tidal energy. Most barriers are built of sand, either left over from glaciations, as in New England, eroded from coastal cliffs, or deposited by rivers along deltas such as at the end of the Mississippi River in the Gulf

Beach Subenvironments
Photo of waves crashing on beach (Stephanie Coffman, Shutterstock, Inc.)

of Mexico. Barrier island systems need to be discontinuous, to allow water from tidal changes to escape back to sea along systems of tidal inlets.

subenvironments of barriers are broadly similar to those of beaches; they include the beach, barrier interior, and landward interior. The beach face of a barrier is the most dynamic part of the island, absorbing energy from waves and tides, and responding much as beaches on the mainland do. The backside of the beach on many barrier islands is marked by a long frontal or foredune ridge, followed landward by secondary dunes. Barrier islands that have grown landward with time may be marked by a series of linear ridges that mark the former positions of the shoreline and foredune ridges, separated by low areas called swales. The landward margins of many barriers merge gradually into mud flats, or salt marshes, or may open into lagoons, bays, or tidal creeks.

About 15 percent of the world's coastlines have barrier islands offshore, with most located along passive-margin continental shelves, which have shallow slopes and a large supply of sediment available to build the barriers. In the united states the eastern seaboard and Gulf of Mexico exhibit the greatest development of barrier island systems. It seems that areas with low tidal ranges in low to middle climate zones have the most extensively developed barrier systems.

Barrier systems are of several types. Barrier spits are attached to the mainland at one end and terminate in a bay or the open ocean on the other end. They are most common along active tectonic coasts, although Cape Cod in Massachusetts is one of the better-known examples of a spit formed along a passive continental margin. some spits have ridges of sand that curve around the end of the spit that terminates in the sea, reflecting its growth. These are known as recurved spits. sandy Hook, at the northern end of the New Jersey coast, is a recurved spit. spits form as longshore currents carry sediment along a coastline, and the coastline makes a bend into a bay. In many cases the currents that carry the sand continue straight and carry the sediment offshore, depositing it in a spit that juts out from the mouth of the bay. Many other subcategories of spits are known and classified according to specific shape. some, known as tombolos, may connect offshore islands with the mainland, whereas others have cuspate forms or jut outward into the open water.

In some cases barriers grow completely across a bay and seal off the water inside it from the ocean. These are known as welded barriers and are most common along rocky coasts such as in New England and Alaska. Welded barriers seem also to form preferentially where tidal energy is low, as this prevents the tides from creating tidal channels that allow salty water to circulate into the bay. some also form during onshore migration of barriers during times of sea-level rise, when the barrier sands get moved into progressively narrowing bays as they are forced to move inland. since they are cut off from the ocean, bays that form behind welded barriers tend to be brackish or even filled with freshwater.

Barriers form by a variety of different mechanisms in different settings, but the most common mechanisms include the growth and accretion of spits that become breached during storms, growth as offshore sandbars, and as submergence of former islands during times of sea-level rise. Barriers are constantly moving and respond to storms, currents, waves, and sea-level rise by changing their position and shape. Barriers moving onshore are known as retrograding barriers; they move by a process of rolling over, where sand on the outer beach face is moved to the backshore, then overrun by the next sand from the beach face. A continuation of this process leads the barrier to roll over itself as it migrates onshore. Prograding barriers are building themselves seaward, generally through a large sediment supply, whereas aggrading barriers are simply growing upward in place as sea levels rise.

Continue reading here: Tidal Inlets

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