Coastal Problems and Solutions
Contemporary Sea-Level Changes
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| Future sea-level rise, due to melting of polar ice, would flood many coastal cities. |
People tend to view a shoreline as a permanent entity. But in fact, shorelines are ephemeral geologic features. On a time scale of hundreds to thousands of years, a shoreline moves inland or seaward depending on whether relative sea level rises or falls or whether sediment supply increases or decreases. In places where sea level is rising today, shoreline towns will eventually be submerged. For example, the Persian Gulf now covers about twice the area that it did 4,000 years ago. And if present rates of sea-level rise along the East Coast of the United States continue, major coastal cities such as Washington, New York, Miami, and Philadelphia may be inundated within the next millennium (figure above).
Beach Destruction-Beach Protection?
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| Examples of beach erosion. | |
In a matter of hours, a storm especially a hurricane can radically alter a landscape that took centuries or millennia to form. The backwash of storm waves sweeps vast quantities of sand seaward, leaving the beach a skeleton of its former self. The surf submerges barrier islands and shifts them toward the lagoon. Waves and wind together rip out mangrove swamps and salt marshes and break up coral reefs, thereby destroying the organic buffer that can protect a coast, leaving it vulnerable to erosion for years to come. Of course, major storms also destroy human constructions: erosion undermines shore-side buildings, causing them to collapse into the sea; wave impacts smash buildings to bits; and the storm surge very high water levels created when storm winds push water toward the shore floats buildings off their foundations (figure above a, b).
But even less-dramatic events, such as the loss of river sediment, a gradual rise in sea level, a change in the shape of a shoreline, or the destruction of coastal vegetation, can alter the balance between sediment accumulation and sediment removal on a beach, leading to beach erosion. In some places, beaches retreat landward at rates of 1 to 2 m per year.
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| Techniques used to preserve beaches. |
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In many parts of the world, beach front property has great value; but if a hotel loses its beach sand, it probably won’t stay in business. Similarly, a harbour can’t function if its mouth gets blocked by sediment. Thus property owners often construct artificial barriers to alter the natural movement of sand along the coast, sometimes with undesirable results. For example, beach-front property owners may build groins, concrete or stone walls protruding perpendicular to the shore, to prevent beach drift from removing sand (figure above a). Sand accumulates on the up-drift side of the groin, forming a long triangular wedge, but sand erodes away on the down-drift side. Needless to say, the property owner on the down-drift side doesn't appreciate this process. Harbour engineers may build a pair of walls called jetties to protect the entrance to a harbour (figure above b). But jetties erected at the mouth of a river channel effectively extend the river into deeper water and thus may lead to the deposition of an offshore sandbar. Engineers may also build an offshore wall called a breakwater, parallel or at an angle to the beach, to prevent the full force of waves from reaching a harbour. With time, however, sand builds up in the lee of the breakwater and the beach grows seaward, clogging the harbor (figure above c). To protect expensive shore side construction, people build seawalls out of riprap (large stone or concrete blocks) or reinforced concrete on the landward side of the beach (figure above d), but during a storm, these can be undermined.
In some places, people have given up trying to decrease the rate of beach erosion and instead have worked to increase the rate of sediment supply. To do this, they pump sand from farther offshore, or truck in sand from elsewhere to replenish a beach. This procedure, called beach nourishment, can be hugely expensive and at best provides only a temporary fix, for the backwash and beach drift that removed the sand in the first place continue unabated as long as the wind blows and the waves break.
Destruction of Wetlands and Reefs
Bad cases of beach pollution create headlines. Because of beach drift, garbage dumped in the sea in an urban area may drift along the shore and be deposited on a tourist beach far from its point of introduction. Oil spills, from ships that flush their bilges or from tankers that have run aground or foundered in stormy seas, or from offshore well leaks, have contaminated shorelines at several places around the world.
The influx of nutrients, from sewage and agricultural run-off, into coastal waters can create dead zones along coasts. A dead zone is a region in which water contains so little oxygen that fish and other organisms within it die. Dead zones form when the concentration of nutrients rises enough to stimulate an algae bloom, for overnight respiration by algae depletes dissolved oxygen in the water, and the eventual death and decay of plankton depletes oxygen even more. One of the world’s largest dead zones occurs in the Gulf of Mexico, offshore of the Mississippi River’s mouth.
Coastal wetlands and coral reefs are particularly susceptible to changes in the environment, and many of them have been destroyed in recent decades. Their loss both increases a coast’s vulnerability to erosion and, because they provide spawning grounds for marine organisms, disrupts the global food chain. Destruction of wetlands and reefs happens for many reasons. Wetlands have been filled or drained to be converted to farmland, housing developments, resorts, or garbage dumps. Reefs have been destroyed by boat anchors, dredging, the activities of divers, dynamite explosions intended to kill fish, and quarrying operations intended to obtain construction materials. Chemicals and particulates entering coastal water from urban, industrial, and agricultural areas can cause havoc in wetlands and reefs, for these materials cloud water and/or trigger algal blooms, killing filter-feeding organisms. Toxic chemicals in such run-off can also poison plankton and burrowing organisms and, therefore, other organisms progressively up the food chain.
Global climate change also impacts the health of organic coasts. For example, transformation of once vegetated regions into deserts means that the amount of dust carried by winds from the land to the sea has increased. This dust can interfere with coral respiration and can bring dangerous viruses. A global increase in seawater temperature may be contributing to reef bleaching, the loss of coral colour due to the death of the algae that live in coral polyps. The statistics of wetland and reef destruction worldwide are frightening ecologists estimate that between 20% and 70% of wetlands have already been destroyed, and along some coasts, 90% of reefs have died.
Hurricanes-A Coastal Calamity
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| Characteristics and paths of hurricanes in the western North Atlantic. |
Global-scale convection of the atmosphere, influenced by the Coriolis effect, causes currents of warm air to flow steadily from east to west in tropical latitudes. As the air flows over the ocean, it absorbs moisture. Because air becomes less dense as it gets warmer, tropical air eventually begins to rise like a balloon. As the air rises, it cools, and the water vapour it contains condenses to form clouds (mists of very tiny water droplets). If the air contains sufficient moisture, the clouds grow into a cluster of large thunderstorms, which consolidate to form a single, very large storm. Because of the Coriolis effect, this large storm evolves into a rotating swirl called a tropical disturbance. If the disturbance remains over warm ocean water, as can happen in late summer and early fall, rising warm moist air continues to feed the storm, fostering more growth. Eventually a spiral of rapidly circulating clouds forms, and the tropical disturbance becomes a tropical depression. Additional nourishment causes the tropical depression to spin even faster and grow broader, until it becomes a tropical storm and receives a name. If a tropical storm becomes powerful enough, it becomes a tropical cyclone. Formally defined, a tropical cyclone is a huge rotating storm, which forms in tropical latitudes, and in which winds exceed 119 km per hour (74 mph). It resembles a giant counter-clockwise spiral of clouds 300 to 1,500 km (930 miles) wide when viewed from space (figure above a). Such a storm is called a hurricane in the Atlantic and eastern Pacific, a typhoon in the western Pacific, and simply a cyclone around Australia and in the Indian Ocean.
Atlantic hurricanes generally form in the ocean to the east of the Caribbean Sea, though some form in the Caribbean itself. They first drift westward at speeds of up to 60 km per hour (37 mph). They may eventually turn north and head into the North Atlantic or into the interior of North America, where they die when they run out of a supply of warm water (figure above b). Weather researchers classify the strength of hurricanes using the Saffir-Simpson scale, which runs from 1 to 5; somewhat different scales are used for typhoons and cyclones. On the Saffir-Simpson scale, a Category 5 hurricane has sustained winds of >250 km/hr (>156 mph). The highest wind speed ever recorded during a hurricane was in excess of 300 km/hr.
A typical hurricane (or typhoon or cyclone) consists of several spiral arms extending inward to a central zone of relative calm known as the hurricane’s eye (figure above c). A rotating vertical cylinder of clouds, the eye wall, surrounds the eye. Winds spiral toward the eye, so like an ice skater who spins faster when she brings her arms inward, the winds accelerate toward the interior of the storm and are fastest along the eye wall. Thus, hurricane-force winds affect a belt that is only 15% to 35% as wide as the whole storm (figure above d). On the side of the eye where winds blow in the same direction as the whole storm is moving, the ground speed of winds is greatest, because the storm’s overall speed adds to the rotational motion.
Hurricanes pose extreme danger in the open ocean, because their winds cause huge waves to build, and thus have led to the foundering of countless ships. They also cause havoc in coastal regions, and even inland, though they die out rapidly after moving onshore. The coastal damage happens for several reasons:
- Wind: Winds of weaker hurricanes tear off branches and smash windows. Stronger hurricanes uproot trees, rip off roofs, and collapse walls.
- Waves: Winds shearing across the sea surface during a hurricane generate huge waves. In the open ocean, these waves can
- capsize ships. Near shore, waves batter and erode beaches, rip boats from moorings, and destroy coastal property. 5 Storm surge: Rising air in a hurricane causes a region of extremely low air pressure beneath. This decrease in pressure causes the surface of the sea to bulge upward over an area with a diameter of 60 to 80 km. Sustained winds blowing in an onshore direction build this bulge even higher. When the hurricane reaches the coast, the bulge of water, or storm surge, swamps the land. If the bulge hits the land at high tide, the sea surface will be especially high and will affect a broader area.
- Rain, stream flooding, and landslides: Rain drenches the Earth’s surface beneath a hurricane. In places, half a meter or more of rain falls in a single day. Rain causes streams to flood, even far inland, and can trigger landslides.
- Disruption of social structure: When the storm passes, the hazard is not over. By disrupting transportation and communication networks, breaking water mains, and washing away sewage-treatment plants, hurricane damage creates severe obstacles to search and rescue, and can lead to the spread of disease, fire, and looting.
Nearly all hurricanes that reach the coast cause death and destruction, but some are truly catastrophic. Storm surge from a 1970 cyclone making landfall on the low-lying delta lands of Bangladesh led to an estimated 500,000 deaths. In 1992, Hurricane Andrew leveled extensive areas of southern Florida, causing over $30 billion in damage and leaving 250,000 people homeless. Hurricane Katrina, in 2005, stands as the most destructive hurricane to strike the United States. Let’s look at this storm’s history.
Hurricane Katrina
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| The devastation of coastal areas by Hurricane Katrina. | |
Tropical Storm Katrina came into existence over the Bahamas and headed west. Just before landfall in southeastern Florida, winds strengthened and the storm became Hurricane Katrina. This hurricane sliced across the southern tip of Florida, causing several deaths and millions of dollars in damage. It then entered the Gulf of Mexico and passed directly over the Loop Current, an eddy of summer-heated water from the Caribbean that had entered the Gulf of Mexico. Water in the Loop Current reaches temperatures of 32C (90F), and thus stoked the storm, injecting it with a burst of energy sufficient for the storm to morph into a Category 5 monster whose swath of hurricane-force winds reached a width of 325 km (200 miles). When it entered the central Gulf of Mexico, Katrina turned north and began to bear down on the Louisiana-Mississippi coast. The eye of the storm passed just east of New Orleans, and then across the coast of Mississippi. Storm surges broke records, in places rising 7.5 m (25 feet) above sea level, and they washed coastal communities off the map along a broad swath of the Gulf Coast (figure above a, b). In addition to the devastating wind and surge damage, Katrina led to the drowning of New Orleans.
To understand what happened to New Orleans, we must consider the city’s geologic history. New Orleans grew on the Mississippi Delta, between the banks of the Mississippi River on the south and Lake Pontchartrain (actually a bay of the Gulf of Mexico) on the north. The older parts of the town grew up on the relatively high land of the Mississippi’s natural levee. Younger parts of the city, however, spread out over the topographically lower delta plain. As decades passed, people modified the surrounding delta landscape by draining wetlands, by constructing artificial levees that confined the Mississippi River, and by extracting groundwater. Sediment beneath the delta compacted, and the delta’s surface has been starved of new sediment, so large areas of the delta sank below sea level. Today, most of New Orleans lies in a bowl-shaped depression as much as 2 m (7 feet) below sea level the hazard implicit in this situation had been recognized for years (figure above c).
The winds of Hurricane Katrina ripped off roofs, toppled trees, smashed windows, and triggered the collapse of weaker buildings, but their direct consequences were not catastrophic. However, when the winds blew storm surge into Lake Pontchartrain, its water level rose beyond most expectations and pressed against the system of artificial levees and flood walls that had been built to protect New Orleans. Hours after the hurricane
eye had passed, the high water of Lake Pontchartrain found a weakness along the floodwall bordering a drainage canal and pushed out a section. Breaks eventually formed in a few other locations as well. So, a day after the hurricane was over, New Orleans began to flood. As the water line climbed the walls of houses, brick by brick, residents fled first upstairs, then to their attics, and finally to their roofs. Water spread across the city until the bowl of New Orleans filled to the same level as Lake Pontchartrain, submerging 80% of the city (figure above d).
Floodwaters washed some houses away and filled others with debris (figure above e). The disaster took on national significance, as the trapped population sweltered without food, drinking water, or adequate shelter. With no communications, no hospitals, and few police, the city almost descended into anarchy. It took days for outside relief to reach the city, and by then, many had died and parts of New Orleans, a cultural landmark and major port, had become uninhabitable.
Figures credited to Stephen Marshak.
