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305 Beaches, Shoreline Processes, and the Coastal Ocean ESSENTIAL LEARNING CONCEPTS At the end of this chapter, you should be able to: 10.1 Use appropriate beach terminology to specify how coastal regions are defined. 10.2 Explain how sand moves on the beach. 10.3 Describe the characteristic features of erosional and depositional shores. 10.4 Discuss how changes in sea level produce emerging and submerging shorelines. 10.5 Describe the types of hard stabilization and evaluate various alternatives. 10.6 Compare the various types of coastal waters. 10.7 Specify the issues that face coastal wetlands. T he coastal ocean is a very busy place. Humans have always been attracted to the coastal regions of the world for their mild climate, abundance of seafood, ease of transportation, abundant recreational opportunities, and other commercial ben- efits. Population studies, for example, reveal that about 50% of world population— some 3.5 billion people—live along coasts, and more than 80% of all Americans live within an hour’s drive from an ocean or the Great Lakes. In the future, these figures are expected to increase. In the United States, for example, 8 of the 10 largest cities are in coastal environments, and about 3600 people move to the coast every day. By 2025, as much as 75% of the global population is expected to live at the coast. Although coastal regions are desirable places to live, this rapid increase in human population negatively impacts coastal environments. The coastal ocean is also filled with marine life. About 95% of all fish caught in the ocean are obtained within 320 kilometers (200 miles) of the shore. In addition, coastal waters support about 95% of the total mass of life in the oceans. Further, coastal estuary and wetland environments are among the most biologically produc- tive ecosystems on Earth and serve as nursery grounds for many species of marine organisms that inhabit the open ocean. Coastal wetlands also serve as a vital natural cleanser of pollutants for storm runoff before it reaches the ocean. The coastal region is constantly changing because waves crash along most shorelines about 10,000 times a day, releasing their energy from distant storms. Waves cause erosion in some areas and deposition in others, resulting in changes that occur hourly, daily, weekly, monthly, seasonally, and yearly. In this chapter, we’ll examine the major features of beaches and shorelines, in- cluding the processes that modify them. We’ll also discuss ways people interfere with these processes, creating hazards to themselves and to the environment. And finally, we’ll examine the characteristics and types of coastal waters, including how human activities have impacted those regions. 10.1 How Are Coastal Regions Defined? The shore is a zone that lies between the lowest tide level (low tide) and the highest elevation on land that is affected by storm waves. The coast extends inland from the shore as far as ocean-related features can be found ( Figure 10.1 ). The width of the shore varies between a few meters and hundreds of meters. The width of the coast may vary from less than 1 kilometer (0.6 mile) to many tens of kilometers. The coastline marks the boundary between the shore and the coast. It is the landward limit of the effect of the highest storm waves on the shore. 10 Before you begin reading this chapter, use the glossary at the end of this book to discover the meanings of any of the words in the word cloud above you don’t already know. “The waves which dash upon the shore are, one by one, broken, but the ocean conquers nevertheless. It overwhelms the Armada, it wears out the rock.” —Lord Byron (1821) M10_TRUJ3545_12_SE_C10.indd 305 18/12/15 8:41 PM

306 CHAPTER 10 Beaches, Shoreline Processes, and the Coastal Ocean Beach Terminology The beach profile in Figure 10.1 shows features char- acteristic of a cliffed shoreline. The shore is divided into the backshore and the foreshore . 1 The back- shore is above the high tide shoreline and is covered with water only during storms. The foreshore is the portion exposed at low tide and submerged at high tide. The shoreline migrates back and forth with the tide and is the water’s edge. The nearshore extends seaward from the low tide shoreline to the low tide breaker line. It is never exposed to the atmosphere, but it is affected by waves that touch bottom. Beyond the low-tide breakers is the offshore zone, which is deep enough that waves rarely affect the bottom. A beach is a deposit of the shore area ( Figure 10.2 ). It consists of wave-worked sediment that moves along the wave-cut bench (a flat, wave-eroded sur- face). A beach may continue from the coastline across the nearshore region to the line of breakers. Thus, the beach is the entire active area of a coast that experiences changes due to breaking waves. The area of the beach above the shoreline is often called the recreational beach . The berm is the dry, gently sloping, slightly elevated margin of the beach that can be found at the foot of coastal cliffs or sand dunes. Because the berm is nor- mally composed of dry sand (Figure 10.2), it is a favorite place of beachgoers for activities such as lying in the Sun, beach volleyball, barbecues, and bonfires. Mov- ing offshore, the beach face is the wet, sloping surface that extends from the berm to the shoreline. It is more fully exposed during low tide and is also known as the low-tide terrace . The beach face is a favorite place for runners because the sand is wet and hard packed. Offshore beyond the beach face is one or more longshore bars —sand bars that parallel the coast. A longshore bar may not always be present throughout the year, but when one is, it may be exposed during extremely low tides. Longshore bars can “trip” waves as they approach shore and cause them to begin breaking. Separating the longshore bar from the beach face is a longshore trough . Beach Composition Beaches are composed of whatever material is locally available (see the chapter- opening photos). When this material—sediment—comes from the erosion of beach cliffs or nearby coastal mountains, beaches are composed of mineral particles from these rocks and may be relatively coarse in texture. When the sediment comes pri- marily from rivers that drain lowland areas, beaches are finer in texture. Often, mud flats develop along the shore because only tiny clay-sized and silt-sized parti- cles are emptied into the ocean. Such is the case for muddy coastlines such as along the coast of Suriname in South America and the Kerala coast of southwest India. Other beaches have a significant biological component. For example, in low- relief, low-latitude areas such as southern Florida, there are no mountains or other sources of rock-forming minerals nearby. As a result, beaches in these areas are generally composed of shell fragments, broken coral, and the remains of organisms that live in coastal waters. Many beaches on volcanic islands in the open ocean are composed of black or green fragments of the basaltic lava that comprises the islands or of coarse debris from coral reefs that develop around islands in low latitudes. Regardless of the composition, though, the material that comprises the beach does not stay in one place. Instead, the waves that crash along the shoreline are constantly moving it. Thus, beaches can be thought of as material in transit along the shoreline . Offshore Nearshore Shore Beach Foreshore Backshore Coast Breakers Low tide breaker line Low tide shoreline Wave-cut bench Longshore trough Longshore bar Wave-cut cliff Notch Coastline Coastal bluff High tide shoreline Berm Beach face Figure 10.1 Diagrammatic view of a cliffed coastal region showing beach terminology and landforms. The beach is de- fined as the entire active area of a coastline that is affected by waves; it extends from the low tide breaker line offshore ( left ) to the far end of the berm ( right ). Although most of these beach features can be found at any beach, not every beach has coastal cliffs. 1 The foreshore is often referred to as the intertidal zone , or littoral ( litoralis = the shore) zone . Figure 10.2 Photo of a typical beach. Many of the features of a typical beach are seen in this photo, such as the berm , which is the dry, flat area close to land, and the beach face , which is the wet, gently-sloping area ( left ). M10_TRUJ3545_12_SE_C10.indd 306 18/12/15 8:41 PM

308 CHAPTER 10 Beaches, Shoreline Processes, and the Coastal Ocean Movement Parallel to the Shoreline At the same time that movement occurs perpendicular to shore, movement parallel to the shoreline also occurs. MECHANISM Recall from Chapter 8 that within the surf zone, waves refract (bend) and line up nearly paral- lel to shore. With each breaking wave, the swash moves up onto the exposed beach at a slight angle; then gravity pulls the backwash down the beach face perpendicular to the shore. As a result, water moves in a zigzag fashion along the shore. LONGSHORE CURRENT AND LONGSHORE DRIFT (LONG- SHORE TRANSPORT) The zigzag movement of water along the shore is called a longshore current ( Figure 10.4 ). Longshore currents have speeds up to 4 kilo- meters (2.5 miles) per hour. Speeds increase as beach slope increases, as the angle at which breakers arrive at the beach increases, as wave height increases, and as wave frequency increases. Swimmers can inadvertently be carried by long- shore currents and find themselves carried far from where they initially entered the water. This demonstrates that longshore currents are strong enough to move people as well as a vast amount of sand in a zigzag fash- ion along the shore. Longshore drift (also called longshore transport , beach drift , or litto- ral drift ) is the movement of sediment in a zigzag fashion caused by the long- shore current (Figure 10.4b). Both longshore currents and longshore transport occur only within the surf zone and not farther offshore because the water is too deep there. Recall from Chapter 8 that the depth of a wave’s wave base is one-half its wavelength, measured from still water level. Below this depth, heavy wave activity produces a narrow rocky berm and an overall flattened beach face—a wintertime beach — and builds prominent longshore bars ( Figure 10.3b ). A wide berm that takes several months to build can be destroyed in just a few hours by high-energy wintertime storm waves. RECAP Smaller, low-energy waves move sand up the beach face toward the berm and create a summertime beach; larger, high-energy waves scour sand from the berm and create a wintertime beach. Waves approach the beach at an angle ... ... causing a zig-zag longshore current of water in the surf zone... ... and an associated longshore transport of sand. Movement of sand particles with each wave W a v e c r e s t Longshore transport Surf zone Longshore current Downcoast Upcoast (b) A longshore current, caused by refracting waves, moves water in a zigzag fashion along the shoreline. This causes a net movement of sand grains (longshore drift) from upcoast to downcoast ends of a beach. (a) Waves approaching the beach at a slight angle near Oceanside, California, producing a longshore current moving toward the right of the photo. SmartFigure 10.4 Longshore current and longshore drift. https://goo.gl/SqWFnp TABLE 10.1 CHARACTERISTICS OF BEACHES AFFECTED BY LIGHT AND HEAVY WAVE ACTIVITY Light wave activity Heavy wave activity Berm/longshore bars Berm is built at the expense of the longshore bars Longshore bars are built at the expense of the berm Wave energy Low wave energy (nonstorm conditions) High wave energy (storm conditions) Time span Long time span (weeks or months) Short time span (hours or days) Characteristics Creates summertime beach: sandy, wide berm, steep beach face Creates wintertime beach: rocky, narrow berm, flattened beach face Web Animation Longshore Current and Longshore (Beach) Drift http://goo.gl/UlwJjR M10_TRUJ3545_12_SE_C10.indd 308 18/12/15 8:41 PM

10.2 How Does Sand Move on the Beach? 309 60°N 50°N 40°N 30°N 20°N 70°W 80°W 20°W 40°W 50°W 60°W 90°W 110°W 120°W 130°W 140°W 150°W 160°W 170°W 180° 50°N 60°N 70°N 40°N 30°N 20°N T r o p i c o f C a n c e r Arctic Circle NORTYH ATLANTIC OCEAN NORTH PACIFIC OCEAN ARCTIC OCEAN Gulf of Alaska Bering Sea Hudson Bay Baffin Bay Labrador Sea Beaufort Sea B e r i n g S tr a i t Gulf of Mexico Caribbean Sea G u l f o f C a l i f o r n i a A l e u t i a n I s . M i s s i s s i p p i R . M i s s o u r i R . R i o G r a n d e C o l u m b i a R . S n a k e R . C o l o r a d o R . Y u k o n R . M a c k e n z i e R . S t . L a w r e n c e R . L . S u p e r i o r L . M i c h i g a n L . E r i e Lake Ontario Lake Huron Great Slave Lake Great Bear Lake A P P A L A C H I A N M T S . R O C K Y M O U N T A I N S B R O O K S R A N G E A L A S K A R A N G E New York Los Angeles Chicago Miami Montréal Seattle Denver San Francisco San Diego Jacksonville San Antonio Boston Houston Anchorage Halifax St. John’s New Orleans Juneau Bermuda (U.K.) GREENLAND (DENMARK) C A N A D A M E X I C O CUBA BAHAMAS JAMAICA HAITI DOM. REP. U N I T E D S TAT E S ICELAND 0 250 500 Miles 0 250 500 Kilometers North Pacific storm center North Atlantic storm center C a l i f o r n i a C u r r en t G u l f S t r e a m Longshore currents are created by predominant wave direction. Waves radiate out from storm centers, creating a southward- moving longshore current (purple arrow) in the surf zone close to shore. Note how the longshore current in the North Atlantic moves in the opposite direction of the Gulf Stream. North Storm center Longshore current/Longshore transport Ocean surface current Figure 10.5 Major storm centers and the development of longshore current and longshore transport along U.S. coasts. Map showing the regions of major storm centers in the North Pacific and North Atlantic Oceans. Waves generated from these storm centers radiate out and create southward-moving longshore currents and longshore transport along both the Pacific and Atlantic coasts. For comparison, the main offshore surface current for both regions is also shown. waves don’t touch bottom, and they don’t refract; as a result, longshore currents can’t form. THE BEACH: A RIVER OF SAND By various processes, both rivers and coastal zones move water and sediment from one area ( upcoast or upstream ) to another ( downcoast or downstream ). As a result, the beach has often been referred to as a “river of sand.” There are, however, differences between how beaches and rivers transport sediment. For example, a longshore current moves in a zigzag fashion, while rivers flow mostly in a turbulent, swirling fashion. In addition, the direction of flow of longshore currents along a shoreline can change, whereas rivers always flow in the same basic direction (downhill). The longshore current can change direction because the waves that approach the beach typically come from differ- ent directions in different seasons. Nevertheless, the longshore current generally flows southward along both the Atlantic and Pacific coasts of the United States ( Figure 10.5 ). RECAP Longshore currents are produced by waves approaching the beach at an angle and create longshore drift, which transports sand along the coast in a zigzag fashion. M10_TRUJ3545_12_SE_C10.indd 309 18/12/15 8:41 PM

Headland Sea arches Sea stack Figure 10.7 Sea arches and sea stack at Praia da Marinha Beach, near Armacao de Pera, Algarve, Portugal. When the roof of a sea arch ( behind ) collapses, a sea stack ( middle ) is formed. 10.3 What Features Exist along Erosional and Depositional Shores? 311 Waves pound relentlessly away at the base of headlands, undermining the up- per portions, which eventually collapse to form wave-cut cliffs . The waves may form sea caves at the base of the cliffs. As waves continue to pound the headlands, the caves may eventually erode through to the other side, forming openings called sea arches ( Figure 10.7 ). Some sea arches are large enough to allow a boat to maneuver safely through them. With continued erosion, the tops of sea arches eventually crumble to produce sea stacks (Figure 10.7). Waves also erode the bedrock of the bench. Uplift of the wave-cut bench creates a gently sloping marine terrace above sea level ( Figure 10.8 ). In some regions that experience episodic uplift, such as the islands offshore of Southern California, a whole series of progressively older marine terraces—the oldest at the top—exist above sea level ( Figure 10.9 ). Figure 10.8 Wave-cut bench and marine terrace. A wave-cut bench is exposed at low tide along the California coast at Bolinas Point near San Francisco. An elevated wave-cut bench, called a marine terrace, is shown at right. Marine terrace Wave-cut bench Multiple marine terraces have been formed during uplift of the land. Figure 10.9 Marine (wave-cut) terraces. Each marine terrace on San Clemente Island offshore of Southern California was cre- ated by wave activity at sea level. Subsequently, each terrace has been exposed by tectonic uplift. The highest (oldest) terraces near the top of the photo are now about 400 meters (1320 feet) above sea level. M10_TRUJ3545_12_SE_C10.indd 311 18/12/15 8:41 PM

OCEANS AND PEOPLE DIVING DEEPER 10.1 WARNING: RIP CURRENTS . . . DO YOU KNOW WHAT TO DO? T he backwash from breaking waves usu- ally returns to the open ocean as a flow of water across the ocean bottom, so it is commonly referred to as “sheet flow.” Some of this water, however, can return to the ocean in strong, narrow surface currents called rip currents that flow away from shore and are generally oriented perpendicular to the beach. Rip currents are between 15 and 45 me- ters (15 and 150 feet) wide and can attain velocities of 7 to 8 kilometers (4 to 5 miles) per hour—faster than most people can swim for any length of time. In fact, it is useless to swim for long against a current stronger than about 2 kilometers (1.2 miles) per hour. Rip currents can travel hundreds of meters from shore before they break up. If a light-to-moderate swell is breaking, numerous rip currents that are mod- erate in size and velocity may develop. A heavy swell usually produces fewer, more concen- trated, and stronger rips. They can often be rec- ognized by the way they interfere with incoming waves, by their characteristic brown color caused by suspended sediment, or by their foamy and choppy appearance ( Figure 10A ). The rip currents that occur during heavy swell are a significant hazard to coastal swim- mers. In fact, rip currents cause an estimated 70 to 100 drownings annually in the United States, and 80% of rescues at beaches by life- guards involve people who are trapped in rip currents. What is the best thing to do if you are caught in a rip current? Swimmers can escape rip currents by swimming parallel to the shore for a short distance (simply swimming out of the narrow rip current) and then riding the waves in toward the beach. However, even excellent swimmers who panic or try to fight the current by swimming di- rectly into it are eventually overcome by exhaus- tion and may drown. Even though most beaches have warnings posted and are frequently pa- trolled by lifeguards, many people tragically lose their lives each year because of rip currents. Figure 10A Rip current and warning sign. A rip current ( red arrow ), which extends outward from shore and interferes with incom- ing waves, and a warning sign ( inset ). GIVE IT SOME THOUGHT 1. Describe the formation of rip currents. What is the best strategy to ensure that you won’t drown if you are caught in a rip current? Web Video Rip Current http://goo.gl/FqwlWb Rates of coastal erosion are influenced by the degree of exposure to waves, the amount of tidal range, and the composition of the coastal bedrock. Regardless of the erosion rate, all coastal regions follow the same developmental path. As long as there is no change in the elevation of the landmass relative to the ocean surface, the cliffs will continue to erode and retreat until the beaches widen sufficiently to prevent waves from reaching them. The eroded material is carried from high-energy areas and deposited in low-energy areas. M10_TRUJ3545_12_SE_C10.indd 312 18/12/15 9:22 PM

314 CHAPTER 10 Beaches, Shoreline Processes, and the Coastal Ocean sand usually less than 1 meter (3.3 feet) above average sea level, perma- nent buildings are often constructed on them. A tombolo ( tombolo = mound) is a sand ridge that connects an island or a sea stack to the mainland ( Figure 10.11b ). Tombolos can also connect two adjacent islands. Tombolos form in the wave-energy shadow of an island and as a result are usually oriented perpendicular to the average direction of incoming waves. BARRIER ISLANDS Extremely long offshore deposits of sand that are parallel to the coast are called barrier islands ( Figure 10.12 ). They form a first line of defense against rising sea level and high-energy storm waves, which would otherwise exert their full force directly against the shore. The origin of barrier islands is complex, but many appear to have developed during the worldwide rise in sea level that is associated with the melting of glaciers at the end of the most recent ice age, about 18,000 years ago. According to a recent study of global satellite images, 2149 bar- rier islands have been identified worldwide in every climate and every Figure 10.12 Examples of barrier islands. Maps and aerial photo of barrier islands in (a) North Carolina, (b) Texas, and (c) New Jersey. 97°W 96°W 95°W 26°N 27°N 28°N 29°N Corpus Christi Bay Matagorda Bay Galveston Bay Vaso Palito Blanco L a g u n a M a d r e Gulf of Mexico Padre Island Mustang Island San Jose Island Matagorda Island Matagorda Peninsula Galveston Island BRAZORIA N.W.R. SAN BERNARD N.W.R. BIG BOGGY N.W.R. ARANSAS N.W.R. PADRE ISLAND NATIONAL SEASHORE LAGUNA ATASCOSA N.W.R. Port Lavaca Harlingen Galveston Corpus Christi Matamoros San Antonio Houston Brownsville T E X A S M E X I C O 0 20 40 Kilometers 0 20 40 Miles 77°W 78°W 76°W 34°N 35°N 36°N Cape Fear Cape Lookout Long Pt. Bluff Pt. Long Shoal Pt. Sandy Pt. Cape Hatteras Smith I. Portsmouth Island Ocracoke Island Hatteras Island Bodie Island Roanoke Island N e u s e P a m l i c o C u r r i t u c k S o u n d A l b e m a r l e S o u n d P a m l ic o S o u n d ATLANTIC OCEAN Phelps Lake New Lake Lake Mattamuskeet O U T E R B A N K S PEA ISLAND N.W.R. CAPE HATTERAS NATL. SEASHORE CAPE LOOKOUT NATL. SEASHORE Wilmington Jacksonville NORTH CAROLINA VIRGINIA 0 20 40 Kilometers 0 20 40 Miles Barrier islands along North Carolina’s Outer Banks. Barrier islands along the south Texas coast. (a) (b) PACIFIC OCEAN ATLANTIC OCEAN (a) (c) (b) North North North 0 .5 1 Kilometer 0 .5 1 Mile A portion of a heavily developed barrier island near Tom’s River, New Jersey. (c) Barrier Island Mainland L a g o o n M10_TRUJ3545_12_SE_C10.indd 314 18/12/15 8:41 PM

10.3 What Features Exist along Erosional and Depositional Shores? 315 tide–wave combination. Nearly 300 barrier is- lands ring the Atlantic and Gulf coasts of the United States ( Figure 10.13 ). They exist from Massachusetts to eastern Florida in a nearly con- tinuous line; they also occur discontinuously in the Gulf of Mexico from western Florida into Mexico. Barrier islands may exceed 100 kilome- ters (60 miles) in length, have widths of several kilometers, and are separated from the mainland by a lagoon. Notable barrier islands include Fire Island off the New York coast, North Carolina’s Outer Banks, and Padre Island off the coast of Texas. Human Impact on Barrier Islands One hu- man-related environmental issue of barrier islands is their attractiveness as building sites be- cause of their proximity to the ocean. For exam- ple, the population densities of barrier islands are three times higher than those of adjoining coastal states. In addition, the overall population on bar- rier islands increased 14% from 1990 to 2000 and continues to increase. Although it seems unwise to build a coastal structure on a narrow, low-ly- ing, shifting strip of sand, many large buildings have been constructed on barrier islands (see Figure 10.12c). Some of these structures have ei- ther fallen into the ocean or have needed to be moved (see MasteringOceanography Web Diving Deeper 10.1 ). Features of Barrier Islands A typical barrier island has the physiographic features shown in Figure 10.14a . From the ocean landward, they are (1) ocean beach, (2) dunes, (3) barrier flat, (4) high salt marsh, (5) low salt marsh, and (6) lagoon between the barrier island and the mainland. During the summer, gentle waves carry sand to the ocean beach , so it widens and becomes steeper. During the winter, higher-energy waves carry sand offshore and produce a narrow, gently sloping beach. Winds blow sand inland during dry periods to produce coastal dunes , which are stabilized by dune grasses. These plants can withstand salt spray and burial by sand. Dunes protect the lagoon against excessive flooding during storm-driven high tides. Numerous passes exist through the dunes, particularly along the southeastern Atlan- tic coast, where dunes are less well developed than to the north. The barrier flat forms behind the dunes from sand driven through the passes during storms. Grasses quickly colonize these flats, and seawater washes over them during storms. If storms wash over the barrier flat infrequently enough, the plants undergo natural biological succession, with the grasses successively replaced by thickets, woodlands, and eventually forests. Salt marshes typically lie inland of the barrier flat. They are divided into the low marsh , which extends from about mean sea level to the high neap-tide line, and the high marsh , which extends to the highest spring-tide line. The low marsh is by far the most biologically productive part of the salt marsh. New marshland is formed as overwash carries sediment into the lagoon, filling portions so they become intermittently exposed by the tides. Marshes may be poorly Figure 10.13 Locations of barrier islands along the U.S. Atlantic and Gulf coasts. Barrier islands occur from Maine to eastern Florida along the Atlantic coast and from western Florida to Mexico along the Gulf coast. Barrier islands do not occur along the Pacific coast. 80 °W 75 °W 70 °W 40 °N 35 °N 30 °N 25 °N 85 °W 90 °W 95 °W ATLANTIC OCEAN Gulf of Mexico U N I T E D S T A T E S 0 150 300 Kilometers 0 150 300 Miles North Web Animation Movement of a Barrier Island in Response to Rising Sea Level http://goo.gl/WqJoKR M10_TRUJ3545_12_SE_C10.indd 315 18/12/15 8:41 PM

10.3 What Features Exist along Erosional and Depositional Shores? 317 developed on parts of the island that are far from floodtide inlets. Their develop- ment is greatly restricted on barrier islands, where people perform artificial dune enhancement and fill inlets in an attempt to prevent overwashing and flooding. Barrier Island Migration The gradual sea level rise experienced along the eastern North American coast is causing barrier islands to migrate landward. The movement of the barrier island is similar to a slowly moving tractor tread, with the entire island rolling over itself, impact- ing structures built on these islands. Peat deposits , which are formed by the accumulation of organic matter in marsh environments, provide further evidence of barrier island migration ( Figure 10.14b ). As the island slowly rolls over itself and migrates toward land, it buries ancient peat deposits. These peat deposits can be found beneath the island and may even be exposed on the ocean beach when the barrier island has moved far enough. DELTAS Some rivers carry more sediment to the ocean than longshore cur- rents can distribute. These rivers develop a delta ( delta = triangular) deposit at their mouths. The Mississippi River, which empties into the Gulf of Mexico ( Figure 10.15a ), forms one of the largest deltas on Earth. Deltas are fertile, flat, low-lying areas that are subject to periodic flooding. Delta formation begins when a river has filled its mouth with sediment. The delta then grows through the formation of distributaries , which are branching chan- nels that deposit sediment as they radiate out over the delta in finger-like extensions (Figure 10.15a). When the fingers get too long, they become choked with sediment. Climate Connection Figure 10.15 Examples of deltas. Aerial photos showing (a) the Mississippi River Delta and (b) the Nile River Delta. EGYPT Mediterranean Sea Gulf of Mexico 0 5 10 Kilometers 0 5 10 Miles 0 20 40 Kilometers 0 20 40 Miles ATLANTIC OCEAN PACIFIC OCEAN ATLANTIC OCEAN Mississippi River Suspended sediment Distributary Nile River Suez Canal Nile River Delta (a) (b) Satellite image of the branching “bird’s foot” structure of the Mississippi River Delta, which flows into the Gulf of Mexico and shows suspended sediment in the water. Photograph from the Space Shuttle of Egypt’s Nile River Delta, which has a smooth, curved shoreline as it extends into the Mediterranean Sea. North North 10.1 Squidtoons https://goo.gl/i6QIQd M10_TRUJ3545_12_SE_C10.indd 317 18/12/15 8:41 PM

318 CHAPTER 10 Beaches, Shoreline Processes, and the Coastal Ocean At this point, a flood may easily shift the distributary’s course and provide sediment to low-lying areas between the fingers. When depositional processes exceed coastal erosion and transportation processes, a branching “bird’s foot” delta similar to the Mississippi River Delta results. When erosion and transportation processes exceed deposition, on the other hand, a delta shoreline is smoothed to a gentle curve, like that of the Nile River Delta in Egypt ( Figure 10.15b ). The Nile Delta is presently eroding because sedi- ment is trapped behind the Aswan High Dam. Before the completion of the dam in 1964, the Nile carried huge volumes of sediment into the Mediterranean Sea. BEACH COMPARTMENTS A beach compartment consists of three main compo- nents: (1) a series of rivers that supply sand to a beach; (2) the beach itself, where sand is moving due to longshore transport; and (3) offshore submarine canyons, where sand is drained away from the beach. The map in Figure 10.16 shows that the coast of Southern California contains four separate beach compartments. 33°N 34°N 119°W 118°W PACIFIC OCEAN Santa Cruz Island Santa Barbara Island San Nicolas Island Santa Catalina Island San Clemente Island Anacapa Island Hueneme Canyon Mugu Canyon Redondo Canyon Newport Canyon La Jolla Canyon Santa Barbara Los Angeles Santa Monica Oceanside San Diego San Pedro Compartment S a n t a B a r b a r a C o m p a r t m e n t Santa Monica Compartment O c e a n s i d e C o m p a r t m e n t A v e r a g e . . . . . . d i re c t i o n o f . . . . . . l o n g s h o r e . . . . . . t r a n s p o r t 3-D enlargement of beach compartment 0 15 30 Kilometers 0 15 30 Miles Sand is swept down coast by longshore current Longshore current Rivers supply sediment Submarine canyon drains sediment off beach 2 1 3 Southern California Bight Southern California has several beach compartments , which are comprised of (1) rivers that bring sediment to the beach, (2) the beach that experiences longshore transport, and (3) submarine canyons that remove sand from the beach. Average direction of longshore transport (red arrows) is toward the south. North SmartFigure 10.16 Beach compartments. https://goo.gl/sm2k0K Web Animation Movement of Sand in a Beach Compartment http://goo.gl/Uyh856 M10_TRUJ3545_12_SE_C10.indd 318 18/12/15 8:41 PM

320 CHAPTER 10 Beaches, Shoreline Processes, and the Coastal Ocean Features of Emerging Shorelines Marine terraces ( Figure 10.17 ; see also Figures 10.8 and 10.9) are one feature characteristic of emerging shorelines. Marine terraces are flat platforms backed by cliffs, which form when a wave-cut bench is exposed above sea level. Stranded beach deposits and other evidence of marine processes such as ancient sea cliffs may exist many meters above the present shoreline, in- dicating that the former shoreline has risen above sea level (Figure 10.17). Features of Submerging Shorelines Features characteristic of submerging shorelines include wave-cut benches below sea level that contain drowned beaches (Figure 10.17). Other features of submerging shorelines include submerged dune topography and drowned river valleys along the present shoreline. Changes in Sea Level What causes the changes in sea level that produce submerging and emerging shore- lines? One mechanism is the raising or lowering of the land surface relative to sea level through the movement of Earth’s crust. Another mechanism is the alteration of the level of the sea itself through worldwide changes in sea level. MOVEMENT OF EARTH’S CRUST The elevation of Earth’s crust relative to sea level can be affected by tectonic movements and by isostatic adjustment. 2 These are called changes in relative sea level , because it’s the land that has changed, not the sea. Tectonic Movements The most dramatic changes in sea level during the past 3000 years have been caused by tectonic movements , which affect the elevation of the land. These changes include uplift or subsidence of major portions of conti- nents or ocean basins, as well as localized folding, faulting, or tilting of the conti- nental crust. Most of the U.S. Pacific coast, for example, is an emerging shoreline because continental margins where plate boundaries occur are tectonically active, produc- ing earthquakes, volcanoes, and mountain chains paralleling the coast. Most of the U.S. Atlantic coast, on the other hand, is a submerging shoreline. When a continent moves away from a spreading center (such as the Mid-Atlantic Ridge), its trailing edge subsides because of cooling and the additional weight of accumulating sedi- ment. Passive margins experience only a low level of tectonic deformation, earth- quakes, and volcanism, making the Atlantic coast far more quiet and stable than the Pacific coast. Isostatic Adjustment Earth’s crust also undergoes isostatic adjustment : It sinks under the accumulation of heavy loads of ice, vast piles of sediment, or outpour- ings of lava, and it rises when heavy loads are removed ( Figure 10.18 ). For example, at least four major accumulations of glacial ice—and dozens of smaller ones—have occurred in high-latitude regions over the past 3 million years. Although Antarctica is still covered by a very large, thick ice cap, much of the ice that once covered northern Asia, Europe, and North America has melted. 2 Recall that isostatic adjustment of Earth’s crust is also discussed in Chapter 1. Emerging shorelines include ancient sea cliffs and marine terraces with stranded beach deposits. Submerging shorelines include wave-cut benches with drowned beach deposits. Uplift Present sea level Figure 10.17 Features of ancient emerging and submerging shorelines. STUDENTS SOMETIMES ASK . . . Because of plate motions, I know that the continents have not always remained in the same geographic positions. Has the movement of the continents ever affected sea level? R emarkably, yes. When plate motion moves large con- tinental masses into polar regions, thick continental glaciation can occur (such as in Antarctica today). Glacial ice forms from water vapor in the atmosphere (in the form of snow), which is ultimately derived from the evaporation of seawater. Thus, water is removed from the oceans when continents assume positions close to the poles that provide a platform for large land-based ice accumulation, thereby lowering sea level worldwide. M10_TRUJ3545_12_SE_C10.indd 320 18/12/15 8:41 PM

10.4 How Do Changes in Sea Level Produce Emerging and Submerging Shorelines? 321 The weight of ice sheets as much as 3 kilometers (2 miles) thick caused the crust beneath to sink (Figure 10.18). Today, these areas are still slowly rebounding, 18,000 years after the ice began to melt. The floor of Hudson Bay, for example, which is now about 150 meters (500 feet) deep, will be close to or above sea level by the time it stops isostatically rebound- ing. Another example is the Gulf of Bothnia (between Sweden and Finland), which has isostatically rebounded 275 meters (900 feet) during the past 18,000 years. Generally, tectonic and isostatic changes in sea level are confined to a segment of a continent’s shoreline. For a world- wide change in sea level, there must be a change in seawater volume or ocean basin capacity. WORLDWIDE (EUSTATIC) CHANGES IN SEA LEVEL Changes in sea level that are experienced worldwide due to changes in seawater volume or ocean basin capacity are called eustatic sea level changes ( eu = good, stasis = standing). 3 The forma- tion or destruction of large inland lakes, for example, causes small eustatic changes in sea level. When lakes form, they trap water that would otherwise run off the land into the ocean, so sea level is lowered worldwide. When lakes are drained and release their water back to the ocean, sea level rises. Another example of a eustatic change in sea level is through changes in sea floor spreading rates, which can change the capacity of the ocean basin and affect sea level worldwide. Fast spreading, for instance, produces larger rises, such as the East Pacific Rise, which displace more water than slow-spreading ridges such as the Mid-Atlantic Ridge. Thus, fast spreading raises sea level, whereas slower spreading low- ers sea level worldwide. Significant changes in sea level due to changes in spreading rate typically take hundreds of thou- sands to millions of years and may have changed sea level by 1000 meters (3300 feet) or more in the geologic past. Changes to Sea Level during Ice Ages Ice ages cause eu- static sea level changes, too. As glaciers form, they tie up vast volumes of water on land, eustatically lowering sea level. An analogy to this effect is a sink of water representing an ocean basin. To simulate an ice age, some of the water from the sink is removed and frozen, causing the water level of the sink to be lower. In a similar fashion, worldwide sea level is lower during an ice age. During interglacial stages (such as the one we are in at present), the glaciers melt and release great volumes of water that drain to the sea, eustatically raising sea level. This would be analogous to putting a frozen chunk of ice on the counter near the sink and letting the ice melt, causing the water to drain into the sink and raise “sink level.” During the Pleistocene Epoch , 4 glaciers advanced and retreated many times on land in middle- to high-latitude regions, causing sea level to fluctuate consid- erably. The thermal contraction and expansion of the ocean as its temperature decreased and increased, respectively, affected sea level, too. The thermal con- traction and expansion of seawater work much like a mercury thermometer: As Figure 10.18 Isostatic adjustment caused by glacial ice. Block diagram view of an area of Earth’s crust before an ice age. The weight of a thick mass of glacial ice causes the crust to subside by the process of isostatic adjustment; note the flow in the mantle below. When the ice melts, the weight is removed and the subsided crust isostatically rebounds (uplifts) in a process that takes thousands of years. (a) Before glaciation (b) During glaciation (a) After glaciation Continental crust Mantle Uplift Uplift Uplift Weight of ice Glacier Subsidence Subsidence Subsidence Mantle flow Mantle flow 4 The Pleistocene Epoch of geologic time, which is also called the “Ice Age,” occurred 2.6 million to 10,000 years ago (see the Geologic Time Scale, Figure 1.31). 3 The term eustatic refers to a highly idealized situation in which all of the continents remain static (in good standing ), while only the sea rises or falls. Web Animation Glacial Isostasy http://goo.gl/vz3ZDT M10_TRUJ3545_12_SE_C10.indd 321 18/12/15 8:41 PM

10.5 How Does Hard Stabilization Affect Coastlines? 323 More groins are needed to alleviate the beach erosion, and soon a groin field is created ( Figure 10.21 ). Does a groin (or a groin field) actually retain more sand on the beach? Sand eventually migrates around the end of the groin, so there is no addi- tional sand on the beach; it is only distributed differently . With proper en- gineering and by taking into account the regional sand transport budget and seasonal wave activity, an equilibrium may be reached that allows sufficient sand to move along the coast before excessive erosion occurs downcoast from the last groin. However, some serious erosional problems have developed in many areas because of attempts to stabilize sand on the beach through the excessive use of groins. Jetties Another type of hard stabilization is a jetty ( jettee = to project outward). A jetty is similar to a groin in that it is built perpendicular to the shore and is usually con- structed of rip-rap. The purpose of a jetty, however, is to protect harbor entrances from waves, and only secondarily does it trap sand ( Figure 10.22 ). Because jetties SmartFigure 10.20 Interference of sand movement. Aerial view of a coastal region with a type of hard stabilization called a groin, which interferes with the movement of sand along the beach. Note how the groin modifies the shape of the beach by causing deposition of sand upcoast of the groin and erosion immediately downcoast. https://goo.gl/9xaSdT W a v e c r e s t Groin Ocean Upcoast end Downcoast end Direction of longshore current and longshore transport Land The addition of the groin causes deposition of sand upstream ... ... and erosion of the beach downstream. Path of sediment Original shoreline Figure 10.21 Groin field. A series of groins has been built along the shoreline north of Ship Bottom, New Jersey, in an attempt to trap sand, altering the distribution of sand on the beach. The view is toward the north, and the primary direction of longshore current is toward the bottom of the photo (toward the south). M10_TRUJ3545_12_SE_C10.indd 323 18/12/15 8:42 PM

324 CHAPTER 10 Beaches, Shoreline Processes, and the Coastal Ocean Figure 10.23 Jetties at Santa Cruz Harbor, California. These jetties protect the inlet to Santa Cruz Harbor and inter- rupt the flow of sand, which is toward the right (southward). Notice the buildup of sand to the left (upcoast) of the jetties and the corresponding erosion to the right (downcoast). are usually built in closely spaced pairs and can be quite long, they can cause more pronounced upcoast de- position and downcoast erosion than groins ( Figure 10.23 ). Breakwaters A breakwater is a form of hard stabi- lization that is built parallel to a shore- line ( Figure 10.24 ). Figure 10.25 shows a breakwater that was constructed to create the harbor at Santa Barbara, California. California’s longshore drift is predominantly southward, so the breakwater on the western side of the harbor accumulated sand that had mi- grated eastward along the coast. The beach to the west of the harbor con- tinued to grow until finally the sand moved around the breakwater and be- gan to fill in the harbor (Figure 10.25). While abnormal deposition oc- curred to the west, erosion proceeded at an alarming rate east of the harbor. The waves east of the harbor were no greater than before, but the sand that had formerly moved down the coast was now trapped behind the breakwater. A similar situation occurred in Santa Monica, California, where a breakwater was built to provide a boat anchorage. A bulge in the beach soon formed behind Figure 10.22 Effect of jetties and groins. Groins are built specifically to trap sand moving in the longshore transport system and occur individually or as a groin field. Both jetties and groins cause deposits of sand on their upcoast sides and an equal amount of erosion downcoast. Jetties protect a harbor or bay entrance and usually occur in pairs. Average direction of longshore drift Erosion Erosion Deposition Erosion Deposition Deposition Jetties Downcoast Upcoast Groins Wave crests Web Animation Coastal Stabilization Structures https://goo.gl/Fb2StT Jetties North M10_TRUJ3545_12_SE_C10.indd 324 18/12/15 8:42 PM