# erosion and deposition by waves

## landforms from wave erosion

Erosion by waves can create unique landforms (Figure 10.12). Wave-cut cliffs form when waves erode a rocky shoreline. They create a vertical wall of exposed rock layers. Sea arches form when waves erode both sides of a cliff. They create a hole in the cliff. Sea stacks form when waves erode the top of a sea arch. This leaves behind pillars of rock.

## landforms deposited by waves

Deposits from longshore drift may form a spit. A spit is a ridge of sand that extends away from the shore. The end of the spit may hook around toward the quieter waters close to shore. You can see a spit in Figure 10.16. Waves may also deposit sediments to form sandbars and barrier islands. You can see examples of these landforms in Figure 10.17.

## what are waves

All waves are the way energy travels through matter. Ocean waves are energy traveling through water. They form when wind blows over the surface of the ocean. Wind energy is transferred to the sea surface. Then, the energy is carried through the water by the waves. Figure 10.11 shows ocean waves crashing against rocks on a shore. They pound away at the rocks and anything else they strike. Three factors determine the size of ocean waves: 1. The speed of the wind. 2. The length of time the wind blows. 3. The distance the wind blows. The faster, longer, and farther the wind blows, the bigger the waves are. Bigger waves have more energy.

## wave erosion

Runoff, streams, and rivers carry sediment to the oceans. The sediment in ocean water acts like sandpaper. Over time, they erode the shore. The bigger the waves are and the more sediment they carry, the more erosion they cause.

## longshore drift

Most waves strike the shore at an angle. This causes longshore drift. Longshore drift moves sediment along the shore. Sediment is moved up the beach by an incoming wave. The wave approaches at an angle to the shore. Water then moves straight offshore. The sediment moves straight down the beach with it. The sediment is again picked up by a wave that is coming in at an angle. This motion is show in Figure 10.15 and at the link below.

## wave deposition

Eventually, the sediment in ocean water is deposited. Deposition occurs where waves and other ocean motions slow. The smallest particles, such as silt and clay, are deposited away from shore. This is where water is calmer. Larger particles are deposited on the beach. This is where waves and other motions are strongest.

## beaches

In relatively quiet areas along a shore, waves may deposit sand. Sand forms a beach, like the one in Figure 10.13. Many beaches include bits of rock and shell. You can see a close-up photo of beach deposits in Figure 10.14.

## breakwaters

Barrier islands provide natural protection to shorelines. Storm waves strike the barrier island before they reach the shore. People also build artificial barriers, called breakwaters. Breakwaters also protect the shoreline from incoming waves. You can see an example of a breakwater in Figure 10.18. It runs parallel to the coast like a barrier island.

## protecting shorelines

Shores are attractive places to live and vacation. But development at the shore is at risk of damage from waves. Wave erosion threatens many homes and beaches on the ocean. This is especially true during storms, when waves may be much larger than normal.

## groins

Longshore drift can erode the sediment from a beach. To keep this from happening, people may build a series of groins. A groin is wall of rocks or concrete that juts out into the ocean perpendicular to the shore. It stops waves from moving right along the beach. This stops the sand on the upcurrent side and reduces beach erosion. You can see how groins work in Figure 10.19.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

landform that results when a sandbar builds up enough to rise above the waters surface

a. spit

-->  b. barrier island

c. groin

d. sea stack

e. sandbar

f. sea arch

g. breakwater

artificial barrier parallel to a shore that reduces beach erosion

a. spit

b. barrier island

c. groin

d. sea stack

e. sandbar

f. sea arch

-->  g. breakwater

landform that results when waves create a hole in a wave-cut cliff

a. spit

b. barrier island

c. groin

d. sea stack

e. sandbar

-->  f. sea arch

g. breakwater

artificial barrier perpendicular to the shore that reduces erosion by longshore drift

a. spit

b. barrier island

-->  c. groin

d. sea stack

e. sandbar

f. sea arch

g. breakwater

landform that results when waves erode the top of a sea arch

a. spit

b. barrier island

c. groin

-->  d. sea stack

e. sandbar

f. sea arch

g. breakwater

underwater ridge of sand running parallel to shore that is deposited by waves

a. spit

b. barrier island

c. groin

d. sea stack

-->  e. sandbar

f. sea arch

g. breakwater

ridge of sand extending out from shore that is caused by longshore drift

-->  a. spit

b. barrier island

c. groin

d. sea stack

e. sandbar

f. sea arch

g. breakwater

Factors that determine the size of ocean waves include

a. speed of the wind.

b. length of time the wind blows.

c. distance the wind blows.

-->  d. all of the above

Sediments you are most likely to find on a beach include

a. clay.

b. silt.

-->  c. pieces of shell.

d. all of the above

Erosion by ocean waves can cause

a. sandbars.

b. spits.

-->  c. cliffs.

d. beaches.

Landforms created by longshore drift include

-->  a. spits.

b. sea arches.

c. sea stacks.

d. two of the above

A breakwater is most similar to a

a. spit.

-->  b. barrier island.

c. wave-cut cliff.

d. pillar of rock.

Landforms caused by ocean wave deposition include

a. groins.

b. sea stacks.

c. sea caves.

-->  d. sandbars.

Which series of landforms shows the correct order in which a stretch of rocky shoreline may be eroded?

a. sea arch, cliff, sea stack

-->  b. cliff, sea arch, sea stack

c. sea stack, cliff, sea arch

d. cliff, sea stack, sea arch

Bigger waves can carry more sediment.

-->  a. true

b. false

The smallest sediments in ocean water are deposited on the shore.

a. true

-->  b. false

Most waves strike the shore at an angle rather than straight on.

-->  a. true

b. false

Longshore drift carries sediments far inland.

a. true

-->  b. false

Groins are built to prevent the formation of sandbars.

a. true

-->  b. false

Sediment in ocean water scrapes rocks like sandpaper.

-->  a. true

b. false

Longshore drift moves sand opposite to the direction of prevailing winds.

a. true

-->  b. false

The end of a spit may hook around toward the open ocean.

a. true

-->  b. false

A barrier island is generally small and round in shape.

a. true

-->  b. false

Sand collects on both sides of a groin.

a. true

-->  b. false

## diagram questions

No diagram questions associated with this lesson