Shoreline Processes and Features 1:The Dynamic Ocean

Erosional Features 2

Many coastal landforms owe their origin to erosional processes. Such erosional features are common along the rugged and irregular New England coast and along the steep shorelines of the West Coast of the United States. Wave erosion is steadily wearing away the California coast. Where the coast is made up of sedimentary rock, average erosion is 15-30 centimeters per year. But where the coast consists of soil and sand, erosion can be as high as 2-3 meters per year. Coastal erosion is a hazard to structures built on cliffs and bluffs along the shore. The cliffs along California's coast form as tectonic processes slowly uplift coastal land. At the same time, the energy of ocean waves undercuts the cliffs. Over time, this process produces features such as wave-cut cliffs and platforms, sea arches, and sea stacks.

How are beaches formed?

Beaches are formed by sediment accumulating along the shore of a lake or ocean.

Depositional Features

A beach is the shore of a body of water that is covered in sand, gravel, or other larger sediments. Sediment eroded from the beach is transported along the shore and deposited in areas where wave energy is low. Such processes produce a variety of depositional features.

Which is one way that a barrier island may form?

Wave erosion separates a spit from the mainland.

How does coastal erosion affect the rocky California coast?

Wave erosion gradually breaks down the sedimentary rock.

Wave Refraction

Wave refraction is the bending of waves, and it plays an important part in shoreline processes. Wave refraction affects the distribution of energy along the shore. It strongly influences where and to what degree erosion, sediment transport, and deposition will take place. Waves seldom approach the shore straight on. Rather, most waves move toward the shore at a slight angle. However, when they reach the shallow water of a smoothly sloping bottom, the wave crests are refracted, or bent, and tend to line up nearly parallel to the shore. Such bending occurs because the part of the wave nearest the shore touches bottom and slows first, whereas the part of the wave that is still in deep water continues forward at its full speed. The change in speed causes wave crests to become nearly parallel to the shore regardless of their original orientation.

Wave-Cut Cliffs and Platforms

Wave-cut cliffs result from the cutting action of the surf against the base of coastal land. As erosion progresses, rocks that overhang the notch at the base of the cliff crumble into the surf, and the cliff retreats. A relatively flat, benchlike surface called a wave-cut platform, is left behind by the receding cliff. The platform broadens as the wave attack continues. Some debris produced by the breaking waves remains along the water's edge as sediment on the beach. The rest of the sediment is transported farther seaward.

How do waves affect a beach along the ocean shoreline?

Waves constantly move sediments to, from, and along the beach.

Spits, Bars, and Tombolos

Where longshore currents and other surf zone currents are active, several features related to the movement of sediment along the shore may develop. A spit is an elongated ridge of sand that projects from the land into the mouth of an adjacent bay. Often the end in the water hooks landward in response to the dominant direction of the longshore current. The term baymouth bar is applied to a sandbar that completely crosses a bay, sealing it off from the open ocean. A tombolo is a ridge of sand that connects an island to the mainland or to another island. A tombolo forms in much the same way as a spit.

Spits, bars, and tombolos all form from _____.

currents that deposit sand and other sediments

People sometimes build a series of groins along a beach. The purpose of the groins is to _____.

prevent sand from leaving the beach

What are groins, breakwaters, and seawalls?

structures built to protect beaches

On an irregular coastline, headlands tend to be eroded while bays tend to be lined with sandy beaches. Which process explains this difference?

wave refraction

Smooth, rounded stones and pebbles cover many ocean beaches. These rocks were shaped by the abrasive actions of _____.

waves

Longshore Transport

Although waves are refracted, most still reach the shore at a slight angle. As a result, the uprush of water, or swash, from each breaking wave is at an oblique angle to the shoreline. These angled waves produce currents within the surf zone. The currents flow parallel to the shore and move large amounts of sediment along the shore. This type of current is called a longshore current. The water in the surf zone is turbulent. Turbulence allows longshore currents to easily move the fine suspended sand and to roll larger sand and gravel particles along the bottom. For a 10-year period at Oxnard, California, more than 1.4 million metric tons of sediment moved along the shore each year. Longshore currents can change direction because the direction that waves approach the beach changes with the seasons. Nevertheless, longshore currents generally flow southward along both the Atlantic and Pacific shores of the United States.

Barrier Islands Formation

Barrier islands probably formed in several ways. Some began as spits that were later cut off from the mainland by wave erosion or by the general rise in sea level following the last glacial period. Others were created when turbulent waters in the line of breakers heaped up sand that had been scoured from the bottom. Finally, some barrier islands may be former sand-dune ridges that began along the shore during the last glacial period, when the sea level was lower. As the ice sheets melted, sea level rose and flooded the area behind the beach-dune complex.

Beach Nourishment

Beach nourishment is the addition of large quantities of sand to the beach system. It is an attempt to stabilize shoreline sands without building protective structures. By building the beaches seaward, both beach quality and storm protection are improved. However, the same processes that removed the sand in the first place will eventually wash away the replacement sand as well. Beach nourishment can be very expensive because huge volumes of sand must be transported to the beach from offshore areas, nearby rivers, or other source areas for sand. Beach nourishment can also have detrimental effects on local marine life. For example, beach nourishment at Waikiki Beach, Hawaii, involved replacing the natural coarse beach sand with softer, muddier sand. Destruction of the softer sand by breaking waves increased the water's turbidity, or "cloudiness," and killed offshore coral reefs.

Why are beaches and shorelines constantly changing?

Beaches and shorelines are constantly changing because waves along the shoreline are constantly eroding, transporting, and depositing sediment.

Forces Acting on the Shoreline

Beaches and shorelines are constantly undergoing changes as the force of waves and currents act on them. A beach is the accumulation of sediment found along the shore of a lake or ocean. Beaches are composed of whatever sediment is locally available. They may be made of mineral particles from the erosion of beach cliffs or nearby coastal mountains. This sediment may be relatively coarse in texture. Some beaches have a significant biological component. For example, most beaches in southern Florida are composed of shell fragments and the remains of organisms that live in coastal waters.

Refaction

Because of refraction, wave energy is concentrated against the sides and ends of headlands that project into the water, whereas wave action is weakened in bays. This type of wave action along irregular coastlines is illustrated in the figure. Waves reach the shallow water in front of the headland sooner than they do in adjacent bays. Therefore, wave energy is concentrated in this area, leading to erosion. By contrast, refraction in the bays causes waves to spread out and expend less energy. This refraction leads to deposition of sediments and the formation of sandy beaches.

Wave Impact

During calm weather, wave action is minimal. During storms, however, waves are capable of causing much erosion. The impact of large, high-energy waves against the shore can be awesome in its violence. Each breaking wave may hurl thousands of tons of water against the land, sometimes causing the ground to tremble. It is no wonder that cracks and crevices are quickly opened in cliffs, coastal structures, and anything else that is subjected to these enormous impacts. Water is forced into every opening, causing air in the cracks to become highly compressed by the thrust of crashing waves. When the wave subsides, the air expands rapidly. This expanding air dislodges rock fragments and enlarges and extends preexisting fractures.

Protective Structures

Groins, breakwaters, and seawalls are some structures built to protect a coast from erosion or to prevent the movement of sand along a beach. Groins are sometimes constructed to maintain or widen beaches that are losing sand. A groin is a barrier built at a right angle to the beach to trap sand that is moving parallel to the shore.

Sea Arches and Sea Stacks

Headlands that extend into the sea are vigorously attacked by waves because of refraction. The surf erodes the rock selectively and wears away the softer or more highly fractured rock at the fastest rate. At first, sea caves may form. When two caves on opposite sides of a headland unite, a sea arch like the one in the figure results. Eventually, the arch falls in, leaving an isolated remnant, or sea stack, on the wave-cut platform.

Fill

In

Fill 2

In

Fill 3

In

Fill 4

In

Abrasion

In addition to the erosion caused by wave impact and pressure, erosion caused by abrasion is also important. In fact, abrasion is probably more intense in the surf zone than in any other environment. Abrasion is the sawing and grinding action of rock fragments in the water. Smooth, rounded stones and pebbles along the shore are evidence of the continual grinding action of rock against rock in the surf zone. Such fragments are also used as "tools" by the waves as they cut horizontally into the land, like the sandstone shown in the figure. Waves are also very effective at breaking down rock material and supplying sand to beaches.

Protective Structures 2

Protective structures can also be built parallel to the shoreline. A breakwater is one such structure. Its purpose is to protect boats from the force of large breaking waves by creating a quiet water zone near the shore. A seawall is another protective structure built parallel to the shore. A seawall is designed to shield the coast and defend property from the force of breaking waves. Waves expend much of their energy as they move across an open beach. Seawalls reduce this process by reflecting the force of unspent waves seaward. Protective structures often only offer temporary solutions to shoreline problems. The structures themselves interfere with the natural processes of erosion and deposition. Then more structures often need to be built to counteract the new problems that arise. Many scientists feel that using protective structures to divert the ocean's energy causes more harm than good.

Forces Acting on the Shoreline 2

Regardless of the composition, the sediment that makes up the beach does not stay in one place. The waves that crash along the shoreline are constantly moving it. Beaches can be thought of as material in transit along the shoreline. Earth's surface is constantly being reshaped as material is eroded from one place and deposited in another. Water, wind, ice, gravity, and volcanic lava all take part in changing the Earth's surface. Material along a shoreline is mainly eroded, transported, and deposited through the action of water and wind. Many types of shoreline features can result from this activity.

Sea Level Changes 2

Sea level changes can produce many shoreline features. When sea levels fall, erosional shoreline features, such as wave-cut cliffs, arches, and stacks, may be found in areas that are no longer anywhere near an ocean. Rias, fjords, and Dalmatian coastlines are features that form when sea levels rise. A ria is basically a flooded river valley. Rias have similar features to ordinary river valleys, but they have higher water levels. A fjord is a feature formed when a glacier retreats and the sea fills the deep, narrow valley carved by the glacier. Dalmatian coastlines are made when parallel valleys fill with seawater but the tops of the valleys remain above water. The result appears to be a series of islands that run parallel to the coast.

Sea Level Changes

Sea levels change. On a day-to-day basis, the levels of the oceans rise and fall due to tides. However, they can also change on a much longer time scale. Global sea-level change takes place because of two reasons: the shape of the ocean basins change or the volume of water does. Over time, tectonic movements can change the shape of ocean basins. If tectonic movements bring continents closer together, the ocean basins get smaller and sea levels rise. If tectonic movements push continents away from each other and make the ocean basins larger, sea levels fall. This process takes millions of years because tectonic plates move very slowly. Sea levels can also change on a much shorter time scale if the climate changes. At the beginning of an ice age, for example, sea levels drop as water is taken out of the hydrological cycle and frozen in glaciers and polar ice caps. Conversely, at the end of an ice age, sea levels rise as polar ice caps and glaciers melt and water is returned to the cycle.

Erosional Features

Shoreline features vary depending on the type of rocks exposed along the shore, the intensity of waves, the nature of coastal currents, and whether the coast is stable, sinking, or rising. Shoreline features that originate primarily from the work of erosion are called erosional features. Sediment that is transported along the shore and deposited in areas where energy is low produce depositional features.

Stabilizing the Shore

Shorelines are among Earth's most dynamic places. They change rapidly in response to natural forces. Storms are capable of eroding beaches and cliffs at rates that far exceed the long-term average erosion. Such bursts of accelerated erosion not only affect the natural evolution of a coast but can also have a profound impact on people who reside in the coastal zone. Erosion along the coast causes significant property damage. Huge sums of money are spent annually not only to repair the damage but also to prevent or control erosion.

Barrier Islands

The Atlantic and Gulf Coastal Plains are relatively flat and slope gently seaward. The shore zone in these areas is characterized by barrier islands. Barrier islands are narrow sandbars parallel to, but separated from, the coast at distances from 3 to 30 kilometers offshore. From Cape Cod, Massachusetts, to Padre Island, Texas, nearly 300 barrier islands rim the coast.

What factors affect the type of shoreline formed?

The type of shoreline features formed depends on the type of rocks exposed along the shore, the intensity of waves, the nature of coastal currents, and whether the coast is stable, sinking, or rising.

An irregular coastline includes headlands and bays. When refracting waves approach this kind of coastline, how is their erosive power distributed?

Their power is stronger on the sides and ends of headlands.