Chapter 10 Review Questions
Describe differences between summertime and wintertime beaches. Explain why these differences occur.
Summertime beaches experience light wave activity and have a wide sandy berm and generally a steep beach face. The longshore bar is small to non-existent. A wintertime beach experiences heavier wave activity and hence has a more flattened beach face with a narrow rocky berm. Longshore bars are prominent.
Overall, does a groin add any additional sand to the beach? Explain.
Groins trap sand on the upcoast side, creating the illusion of adding sand to the beach on that side; however, downcoast of the groin, more erosion immediately occurs. Because of the downcoast erosion, more groins are needed to prevent excess sediment loss and this is why groin fields are created.
Discuss the causes and effects of eustatic changes in sea level.
Eustatic sea level changes are a worldwide phenomenon resulting from changes in the holding capacity of the ocean basins or the amount of water in them. The most readily identifiable cause of eustatic sea level changes is the formation and melting of continental glaciers. Other types of eustatic sea level change include those due to thermal expansion or contraction of water due to global warming or cooling, and from changes in the shape of the seafloor due to increased or decreased rates of seafloor spreading. No matter what produced the sea level change, similar deposits are left as evidence. For relative rises in sea level, drowned river mouths and drowned beaches and dune topography are left as evidence. For relative drops in sea level, marine terraces and stranded beach deposits are left as evidence.
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?
Rip currents form as strong, narrow currents that flow away from shore and are concentrated backwash from breaking waves on the shoreline. The best way to escape a rip current is to swim parallel to the shoreline for a short distance in order to get out of the current's path rather than try to swim against the rip current.
Describe how an ice age affects sea level.
Ice ages produce eustatic sea level changes. Formation of land glaciers locks water up, effectively lowering sea level. Also, as seawater cools, it contracts, lowering sea level further.
How do beaches reflect the composition of locally available materials? Include examples in your answer.
Often, beach sediment is eroded material from the local continent. In this case, the beach mineral particles will have a coarser texture, as with sandy quartz beaches of the U.S. East Coast. If rivers feed into the area, sediment will be finer and mud flats will develop. Volcanic islands are made of basalt and these often are black sand beaches. Areas that have abundant biological communities offshore and low relief inland, such as Florida, may have beaches that are wave-worked shell fragments or coral pieces.
List the two basic processes by which coasts advance seaward, and list their counterparts that lead to coastal retreat.
Coasts may advance seaward as a result of uplift or deposition. Retreating coasts can result from subsidence or erosion.
List the types of hard stabilization. Choose one type and describe what it is intended to do.
• Groins are barriers to sediment flow built perpendicular to a coastline. They are constructed of many kinds of material, but they are usually made of large blocky material called riprap. Sometimes, groins are even constructed of sturdy wood pilings (similar to a fence built out into the ocean). A groin is constructed with the express purpose of trapping sand. A groin interrupts the flow of longshore drift and will cause sediment to be deposited in its upstream side as the current is slowed and experiences a decreased ability to carry sediment. Downstream from the groin, waves begin to erode sediment from the beach because the wave's energy that was once used to transport sediment along the length of the shore is now available to pick up and transport other sediment. Until the sediment moves around the groin, erosion downstream of the groin will continue. • A jetty is similar to a groin in that it is a structure that is built perpendicular to the shore, and it is also usually constructed of riprap. Jetties are built to protect harbor entrances from wave action and they secondarily trap sand. Jetties are often built in closely spaced pairs, and they are usually longer than groins. As a result, they usually cause more pronounced upstream sediment deposition and downstream sediment erosion. • Breakwaters are hard stabilization built parallel to the shoreline that often protects a harbor or creates boat anchorage. From the experience at Santa Barbara and Santa Monica, California, breakwaters cause a build-up of sand inshore of them and unwanted erosion downstream from them. • Seawalls are built parallel to the shore along a beach to protect the coastline from high water and wave energy. Once waves begin breaking against a seawall, the turbulence generated by the abrupt release of wave energy quickly erodes the sediment on its seaward side, causing it to collapse in the surf. Where they have been used to protect barrier island property, the seaward slope of the island beach has steepened, increasing the erosion rate. Seawalls destroy recreational beaches.
List and discuss four factors that influence the classification of a coast as either erosional or depositional.
Whether the dominant process along a coast is erosion or deposition depends on the combined effect of many variables. These variables include: degree of exposure to ocean waves, tidal range, composition of coastal bedrock, tectonic subsidence or emergence, isostatic subsidence or emergence, and eustatic sea level change.
When a breakwater was built in Santa Monica, what unexpected problem occurred? What was done to alleviate the problem (before the breakwater was destroyed by waves)?
A beach bulge was created because the natural transport of sand was disrupted; the breakwater blocked the waves responsible for sand movement. Sand was dredged and pumped downcoast to alleviate the problem.
Describe the origin of these depositional features: spit, bay barrier, tombolo, and barrier island.
A spit is a beach extension that protrudes into the open water of the bay. It will usually have a curved end due to the flow of tidal currents in and out of the bay, and it will often be elongated in the predominant direction of longshore drift. A bay barrier (bay-mouth bar) is a depositional feature that extends completely across the bay mouth and usually results from the depositional extension of a spit. A tombolo is a depositional feature that connects an island with another island or the mainland. It forms in the wave shadow of the island, and it is an extension of a beach deposit away from the landmass to which it is attached. Barrier islands are depositional features separated from the mainland by a lagoon. They may only be a few hundred meters wide but tens of kilometers in length (see Figure 10.9).
What specific features are included in a typical beach?
A typical beach has a foreshore and a backshore. The foreshore is covered with water during high tide. As a geologic deposit of a shore area, a beach has sediment that has been pounded and impacted by waves. Beaches typically have a wave-cut bench, along which sediment moves, a berm, which is dry and gently sloping, a beach face, which is wet and slopes to the shoreline, and a longshore bar, which is an underwater sand bar paralleling the coast.
Describe the response of a barrier island to a rise in sea level. Why do some barrier islands develop peat deposits running through them from the ocean beach to the salt marsh?
Barrier islands are often associated with subsiding shores, but they are thought to move similar to a tractor's tread (rolling itself and moving toward the mainland) in response to a relative rise in sea level. Evidence for such migration can be seen in peat deposits, which are the remnants of old marshes that lie beneath the barrier islands. As barrier islands migrate toward the continents, islands override the marshes so that peat deposits are exposed on the ocean beach (see Figure 10.10 b).
What variables affect the speed of longshore currents?
Beach slope increases, higher angle of breaking waves relative to the shore, wave height, and wave frequency will increase the speed of a longshore current.
Discuss the formation of such erosional features as wave-cut cliffs, sea caves, sea arches, and sea stacks.
If waves release their energy along the base of a coastal cliff, they erode the cliff by abrasion with the force of water aided by sediment carried within the water, creating a wave-cut cliff. Sea caves typically begin along a weakness in the cliff (such as a fracture) and are further enlarged by wave erosion. If a headland juts into the ocean, waves refract around it and concentrate their energy release there. Waves may erode completely through the headland to create an opening through the rock called a sea arch. Continued erosion of the sea arch will cause the sea arch roof to collapse, separating the seaward side of the arch from the headland and forming a column of rock called a sea stack.
Why does the direction of longshore current sometimes reverse in direction? What is the primary direction of longshore current along the Pacific coast? Along the Atlantic coast?
Longshore current direction can reverse seasonally since direction of approaching waves may also change. The longshore current generally moves southward along both the Pacific and Atlantic coasts
What is longshore drift, and how is it related to a longshore current?
Longshore drift is also called longshore transport. It is the movement down the beach of the sediment in a zigzag pattern. The longshore current is responsible for this transport. By breaking at an angle, waves move sand and water the beach in a zigzag manner.
Discuss the causes and effects of tectonic changes in sea level.
Tectonic changes in sea level result from uplift or subsidence of a continent along a shoreline segment as a result of processes related to global plate tectonics. Tectonic uplift is occurring along the west coast of the United States as a result of the collision of the North American Plate with the Pacific, Juan de Fuca, and Gorda Plates. Along the east coast, tectonic events of a more passive nature are causing the shoreline to subside due to thermal contraction as the continent moves farther from the Mid-Atlantic Ridge. Contributing to this subsidence is the increasing sediment load deposited at continental margins. No matter what produced the sea level change, similar deposits are left as evidence. For relative rises in sea level, drowned river mouths and drowned beaches and dune topography are left as evidence. For relative drops in sea level, marine terraces and stranded beach deposits are left as evidence.
Describe the tectonic and/or depositional processes that affect the Gulf Coast.
Tectonic subsidence is common and sea level rise is similar to that of the southeast Atlantic coast. Mississippi river sediment deposition causes subsidence due to its weight and a subsequent seawater rise along the coast. Marshland is being lost because of high rates of erosion and states along the Gulf Coast are experiencing a net land loss.
Describe the tectonic and/or depositional processes that affect the Atlantic coast.
The Atlantic coast is subsiding due to tectonic processes associated with thermal contraction of the lithosphere as the edge of the continent moves farther away from the Mid-Atlantic Ridge. This same process causes the oceans to deepen with increased distance from spreading centers. In addition, the sediment load that the thick wedge of sediment underlying the continental margin represents is pushing the lithosphere down into the asthenosphere. Although both of these processes are in effect, it may be that uplift occurring north of Cape Hatteras is a result of isostatic rebound following the removal of the continental ice sheet that once extended as far south as New York and New Jersey.
How does the berm differ from the beach face?
The berm is dry sand found at the base of coastal cliffs or sand dunes, whichever is present at that beach. The beach face is more oceanward of the berm. It is wet and sloping and extends down to the water's edge. The berm is not covered during high tide but the beach face may be partially covered.
Describe the tectonic and/or depositional processes that affect the Pacific coast.
The coast is tectonically rising and experiencing less erosion than either the Atlantic or Gulf coasts. Sea level is still showing some rise. The Pacific coast is exposed to high-energy waves that erode, but this coast in general has a low erosion rate. Some areas experience beach starvation due to coastal river damming. Washington State has a net sediment deposition due to a long, protected stretch of shoreline.
Explain the difference between the shore and the coast.
The shore is closer to the ocean. It starts at the lowest low tide level and extends inland to the highest elevation storm waves affect. The coast is more inland and ranges from the shore until ocean-related features can no longer be found. Refer to Figure 10.1.
List the alternatives to hard stabilization. Choose one alternative and describe potential drawbacks.
There are several alternatives to hard stabilization: restriction of building in flood-prone, erosional coastal areas; beach replacement (beach nourishment); and relocation. In some coastal areas, residents are prohibited from rebuilding following a natural disaster in the hope of reducing future losses (financial and environmental). Beach replenishment involves the replacement of lost sand to the beach, and this process can be expensive as it must be done on a continual basis. Offshore sand is either dredged and deposited on the beach or pumped into the longshore drift for deposition. The average cost of beach replenishment is around $5.00 to $10.00 per cubic yard of sand (0.76 cubic meters). In some cases the relocation of a valuable structure to an area that is not subject to erosion is necessary. The most notable example of relocation was moving the Cape Hatteras Lighthouse 884 meters (2900 feet) in 1999 at a cost in excess of 12 million dollars.