Earth Science 16.3B Shoreline Erosional Features

Erosional Features Shoreline features vary depending on the types of rocks exposed along the shore, the intensity of waves, the nature of the coastline 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.

Erosional Features Many coastal landforms owe their origin to erosional processes. Such erosional forces are common along the rugged and irregular coast of New England and along the steep shorelines of the west coast of the United States.

Erosional Features Wave erosion is steadily wearing away the California coast. Where the coast is made up of sedimentary rock, the average erosion is 15-30 centimeters per year. Where the coast consist of just 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 near the shoreline. California Coastline

Erosional Features The cliffs along California’s coastline form as tectonic processes slowly uplift coastal land. At the same time, the energy of ocean waves undercuts the cliffs. Over time, these processes produce features such as wave-cut cliffs and platforms, sea arches, and sea stacks.

Wave-cut cliffs and platforms Wave-cut cliffs result from the cutting action of the surf against the base of a 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, bench-like surface, called a wave-cut platform, is left behind the receding cliff. The platform widens as wave erosion continues and the sediment left by the breaking waves gradually forms a beach.

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 lightly fractured rock at the fastest rate. At first, sea-caves form. When two caves at opposite sides of a headland unite, a sea arch forms. Eventually the sea arch falls in, leaving an isolated remnant or sea stack on the wave-cut platform.

Depositional features: Recall that 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 create a variety of depositional features; among them spits, bars and tombolos.

Depositional features: 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.

Depositional features: The term baymouth bar is applied to a spit or sandbar that completely crosses a bay, sealing it off from the open ocean. Baymouth bars form across bays where currents are weak. The weak currents allow a spit to extend to the other side and close off the bay forming a baymouth bar. Baymouth bar

Depositional features: 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, from the migration of sediment down the beach from longshore currents.

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.

Barrier Islands: From Cape Cod to Padre Island Texas, hundreds of barrier islands ring our coastline forming protective harbors. Barrier islands form in several ways. Some begin as spits of land that later get cut off from the mainland by erosion or a rise in sea level after the last glacial period. Others are created when turbulent waters in the line of breakers heaped up sand that had been scoured from the ocean bottom. Cape Hatteras Barrier Islands

Barrier Islands: Finally, some barrier islands may be former sand dune ridges that began along the shore during the last glacial period, when sea levels were lower. As the ice sheets melted, sea levels rose and the areas behind the dunes flooded separating them from the mainland. Cape Hatteras Barrier Islands

Stabilizing the Shore: Shorelines are among the Earth’s most dynamic places, always in periods of change. They change rapidly in response to natural forces. Storms such as hurricanes can erode beaches and cliffs at rates far faster than any long term erosion. Erosion from storms along coastlines causes significant property damage. Huge sums of money are spent annually not only to repair damage but also to prevent and control coastal erosion.

Stabilizing the Shore: Protective Structures: Groins, breakwaters, and seawalls are some of the 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 a beaches that are losing sand. A groin is a barrier built to trap sand that is moving parallel to the shore. Notice in the photo at top right how a series of groins traps sand along the shore.

Stabilizing the Shore: Protective Structures: Protective structures can also be built parallel to the shoreline. Certain types of breakwaters are sometimes built like this. A breakwater’s purpose is to protect boats from the force of breaking waves by creating a quiet water zone behind the breakwater near the shore.

Stabilizing the Shore: Protective Structures: 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 that would cause erosion. Waves expend much of their energy as they move across an open beach. Seawalls reduce this erosion process by reflecting the force of unspent waves seaward. 3 types of seawalls

Stabilizing the Shore: Protective Structures: Protective structures often offer only temporary solutions to shoreline problems. The structures themselves interfere with the natural processes of erosion and deposition. Than even more structures need to be built to counteract the new problems that arise. Many environmental scientists feel that protective structures to divert the ocean’s energy causes more harm than good.

Stabilizing the Shore: Beach Nourishment: Beach nourishment is the addition of large quantities of sand to a beach system. It is an attempt to stabilize shoreline sands without building protective structures. By building beaches seaward, both beach quality and storm protection are improved. However, the same process that removed the sand in the first place will eventually take away the sand that has been added by beach nourishment. Rebuild of Miami beach

Stabilizing the Shore: Beach Nourishment: Beach nourishment can be very expensive because huge quantities 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. Rebuild of Miami beach

Stabilizing the Shore: Beach Nourishment: For example, beach nourishment in Waikiki Beach in Hawaii involved replacing the natural course grain beach sand with a finer, muddier beach sand. Destruction of this softer sand by breaking waves increased the water’s turbidity, or “cloudiness”, and killed offshore coral reefs as a result. Waikiki Beach Hawaii