Coasts

Key Concepts The location of a coast depends primarily on global tectonic activity and the ocean’s water volume The shape of a coast is a product of uplift and subsidence, the wearing down of land by erosion, and the redistribution of material by sediment transport and deposition

Key Concepts (cont’d.) Coasts may be classified as erosional coasts (on which erosion dominates) or depositional coasts (on which deposition dominates) Beaches change shape and volume as a function of wave energy and the balance of sediment input and removal Human interference with coastal processes has generally accelerated the erosion of coasts near inhabited areas

12.1 Coasts Are Shaped by Marine and Terrestrial Processes Coast – zone affected by coastal processes Shore – ocean meets land Location – depends on global tectonic activity and volume of ocean water Shape – depends on uplift, subsidence Erosion, and redistribution of sediment

Coasts Are Shaped by Marine and Terrestrial Processes Change of sea level Three factors cause eustatic change (global sea level) Amount of water in ocean Volume of Earth’s ocean basins Water temperature Local changes in sea level Tectonic and isostatic factors Wind currents, seiches, storm surges, El Niño or La Niña, or other water in motion

Coasts Are Shaped by Marine and Terrestrial Processes Figure 12.2 Sea levels past and future. a. Sea level rose rapidly at the end of the last ice age as glaciers and ice caps melted and water returned to the ocean. The rate of rise has slowed over the past 4,000 years and is now about 3 millimeters per year. b. Projections of sea level through the year 2100. Seven research groups (represented here by colored lines) have estimated future sea level based on historical observations and climate models. The most conservative of these predictions estimates a Figure 12.2 S ea levels past and future. 20-centimeter (8-inch) rise.

12.2 Erosional Processes Dominate Some Coasts High-energy coast – rapid erosion Low-energy coast – infrequent erosion Complex features Sea cliffs Sea caves Wave-cut platforms

Erosional Processes Dominate Some Coasts Figure 12.4 The results of wave action on a coast.

Results of Wave Action On a Coast Figure 12.5 Results of wave action on a coast.

Sea cliff Wave-cut Original land surface platform Notch eroded by waves Figure 12.5 Results of wave action on a coast. Wave erosion of a sea cliff produces a shelflike, wave-cut platform visible at low tide. Remnants of the original cliff can protrude as sea stacks. Stepped Art

Erosional Processes Dominate Some Coasts Wave energy Figure 12.6 Wave energy converges on headlands and diverges in the adjoining bays. a. The concentrated forces shape the headland into platforms and stacks. The accumulation of sediment from the headland in the tranquil bays eventually forms beaches and straightens the contours of the shore. b. Waves approaching a shallow Caribbean reef refract around it. Energy is clearly concentrated on the headland.

Land Erosion and Sea-Level Change Figure 12.7 Drowned river valleys: submerging coasts that filled with water as the last ice age ended. a. Delaware and Chesapeake bays on the East Coast of the United States. b. Sydney Harbour, Australia, called by its discoverer “The finest harbour in the world.”

Volcanism and Tectonism Figure 12.9 Volcanic alterations to coasts. a. Lava flowing seaward from an eruption on the island of Hawai’i forms a fresh coast exposed to erosion for the first time. b. Two volcanic cones on the southeastern coast of the Hawai’ian island of O’ahu. One of the volcanoes has collapsed, and its crater has filled with seawater.

12.3 Beaches Dominate Depositional Coasts Beach – loose particles Wave action, particle size, and permeability Swash – carries particles onshore Backwash – carries particles offshore Figure 12.13 b. The general relationship between grain size and beach slope.

A Typical Beach Profile Figure 12.14 A typical beach profile. The scale is exaggerated vertically to show detail.

Waves Transport Beach Sediment Longshore drift and longshore current Figure 12.16 a. A longshore current moves sediment along the shoreline between the surf zone and the upper limit of wave action. b. Groins built at right angles to the shore at Cape May, New Jersey, to slow the migration of sand. The groins interrupt the flow of longshore currents, so sand is trapped on their upcurrent sides. This view is toward the south; and south of the groins, on the downcurrent sides, sand is eroded.

12.4 Larger-Scale Features Accumulate on Depositional Coasts Figure 12.18 A composite diagram of the large-scale features of an imaginary depositional coast. Not all these features would be found in such close proximity on a real coast.

Barrier Islands and Deltas Figure 12.21 b. The heavy black lines south of Ocean City represent jetties constructed in the 1930s to protect the inlet. The jetties disrupt the north-to-south longshore current. As a result, Assateague Island has been starved of sediment and has migrated about 500 meters (1,640 feet) westward. Figure 12.23 a. The bird’s-foot shape of the Mississippi Delta is seen clearly in this photograph. Lobed and bird’s-foot deltas form where deposition overwhelms the processes of coastal erosion and sediment transportation. The sediment-laden water looks brown or tan in this photograph taken from low orbit.

12.5 Biological Activity Forms and Modifies Coasts Coral reefs Fringing reefs Barrier reefs Atolls Figure 12.26 a. A fringing reef forms around an island in the tropics. The island sinks as the oceanic plate on which it rides moves away from a spreading center. In this case, the island does not sink at a rate faster than coral organisms can build upward. The island eventually disappears beneath the surface, but the coral remains at the surface as an atoll.

12.6 Freshwater Meets the Ocean in Estuaries Mixing fresh and salt water Estuaries classified by origin Characteristics influenced by water density and flow Figure 12.29 Types of estuaries in vertical cross sections. The salinity values show the amount of mixing between freshwater and seawater in the various types. Green color represents freshwater.

12.7 The Characteristics of U.S. Coasts Pacific Actively rising Recent tectonic activity Atlantic Passive margin Gulf coast Smaller wave size Smaller tidal range

12.8 Humans Interfere in Coastal Processes Engineering to prevent/slow erosion Figure 12.35 Some measures taken to slow beach erosion.

Some Measures Taken to Slow Beach Erosion Figure 12.35 Some measures taken to slow beach erosion.

(a) Groin Groins are structures that extend from the beach into the water. They help counter erosion by trapping sand from the current. Groins accumulate sand on their updrift side, but erosion is worse on the downdrift side, which is deprived of sand. Current (b) Seawall Seawalls protect property temporarily, but they also increase beach erosion by deflecting wave energy onto the sand in front of and beside them. High waves can wash over seawalls and destroy them and property. (c) Importing sand Importing sand to a beach is considered the best response to erosion. The new sand often is dredged from offshore, can cost tens of millions of dollars, and can disturb aquatic biodiversity. Because it is often finer than beach sand, dredged sand erodes more quickly. Figure 12.34 Some measures taken to slow beach erosion. Stepped Art Figure 12-34 p379

Chapter in Perspective Location of coast depends on global tectonic activity and ocean water volume Shape produced by many processes Classified as erosional or depositional Beaches are accumulations of sand Wave action shapes beaches Coral reefs and estuaries are biologically active Humans interfere with coasts

Some Questions from Students What are those little white pellets I find on the beach along the high-tide line? Surfers call them “nurdles.” I’ve noticed that beaches composed of pebbles and cobbles tend to be fairly steep, but beaches made of fine sand are relatively flat. Why is that? Those ubiquitous, insidious particles are the raw material for molded plastic goods. They are transported from producers to fabricators in containers loaded onto container ships. The pellets escape if a container is mishandled, breaks open during a storm, or is lost overboard. Virtually indestructible, “nurdles” float with the winds and currents until they encounter a shore. One researcher calculated that just 25 containers would carry enough plastic pellets to spread 100,000 “nurdles” per mile along all the seashores of the world! You’ll learn more about the problem of plastic in the ocean in this book’s last chapter. A beach’s slope is determined by the energy necessary to move the sand grains of which it is composed. Shallow water, smaller waves, and coarse grains combine to form steep beach slopes, but large waves can very easily move small particles and smooth out the slope.