How is surfing like erosion?
Have you ever surfed or even body surfed? Have you felt a wave crash onto your body and then try to drag you offshore? Surfers use the power of waves for a wild ride. But that power can also be used to create landforms along a shoreline.
Wave energy does the work of erosion at the shore. Waves approach the shore at some angle so the inshore part of the wave reaches shallow water sooner than the part that is further out. The shallow part of the wave "feels" the bottom first. This slows down the inshore part of the wave and makes the wave "bend." This bending is called refraction.
- In this animation, notice how the wave refracts as it comes into the beach. http://www.grossmont.edu/garyjacobson/Oceanography%20112/Wave%20Model.htm
Wave refraction either concentrates wave energy or disperses it. In quiet water areas, such as bays, wave energy is dispersed, so sand is deposited. Areas that stick out into the water are eroded by the strong wave energy that concentrates its power on the wave-cut cliff (Figure below).
These colorful cliffs on Martha's Vineyard are eroded by wave action. Note that the topsoil and vegetation at the top of the cliff are undercut by the falling sand and clay beneath.
Other features of wave erosion are pictured and named in Figure below. A wave-cut platform is the level area formed by wave erosion as the waves undercut a cliff. An arch is produced when waves erode through a cliff. When a sea arch collapses, the isolated towers of rocks that remain are known as sea stacks.
(a) The high ground is a large wave-cut platform formed from years of wave erosion. (b) A cliff eroded from two sides produces an arch. (c) The top of an arch erodes away, leaving behind a tall sea stack.
Rivers carry sediments from the land to the sea. If wave action is high, a delta will not form. Waves will spread the sediments along the coastline to create a beach. Waves also erode sediments from cliffs and shorelines and transport them onto beaches. Beaches can be made of mineral grains like quartz, rock fragments, and also pieces of shell or coral (Figure below).
Quartz, rock fragments, and shell make up the sand along a beach.
Waves continually move sand along the shore. Waves also move sand from the beaches on shore to bars of sand offshore as the seasons change. In the summer, waves have lower energy so they bring sand up onto the beach. In the winter, higher energy waves bring the sand back offshore.
Some of the features formed by wave-deposited sand are in Figure below. These features include barrier islands and spits. A spit is sand connected to land and extending into the water. A spit may hook to form a tombolo.
Examples of features formed by wave-deposited sand.
Shores that are relatively flat and gently sloping may be lined with long, narrow barrier islands (Figure below). Most barrier islands are a few kilometers wide and tens of kilometers long.
Much of North Carolina's coast is protected by barrier islands that enclose Pamlico Sound. The thin white strips on the outer edges of the islands are beach sand.
In its natural state, a barrier island acts as the first line of defense against storms such as hurricanes. When barrier islands are urbanized, hurricanes damage houses and businesses rather than vegetated sandy areas in which sand can move. A large hurricane brings massive problems to the urbanized area.
Intact shore areas protect inland areas from storms that come off the ocean. Where the natural landscape is altered or the amount of development makes damage from a storm too costly to consider, people use several types of structures to attempt to slow down wave erosion. A few are pictured below (Figure below). A groin is a long, narrow pile of rocks built perpendicular to the shoreline to keep sand at that beach. A breakwater is a structure built in the water parallel to the shore in order to protect the shore from strong incoming waves. A seawall is also parallel to the shore, but it is built onshore.
(a) Groins trap sand on the up-current side so then people down current build groins to trap sand too. (b) Breakwaters are visible in this satellite image parallel to the shoreline. (c) Seawalls are similar to breakwaters except built onshore. Extremely large storm waves may destroy the sea wall, leaving the area unprotected.
People do not always want to choose safe building practices, and instead choose to build a beach house right on the beach. Protecting development from wave erosion is difficult and expensive.
Protection does not always work. The northeastern coast of Japan was protected by anti-tsunami seawalls. Yet waves from the 2011 tsunami that resulted from the Tohoku earthquake washed over the top of some seawalls and caused others to collapse. Japan is now planning to build even higher seawalls to prepare for any future (and inevitable) tsunami.
- Ocean waves have a tremendous amount of energy and so they may do a great deal of erosion. Some landforms created by erosion are platforms, arches, and sea stacks.
- Transported sand will eventually be deposited on beaches, spits, or barrier islands.
- People love the shore, so they develop these regions and then must build groins, breakwaters, and seawalls to protect them.
Use this resource to answer the questions that follow.
- What is the main agent of erosion at shorelines? How does more erosion take place?
- What is differential erosion? What is the result?
- When does the most erosion take place?
- What is hydraulic action?
- What causes abrasion?
- How does cliff retreat happen?
- How does a wave-cut platform form?
- How do caves form?
- How does a stack form?
- Describe how refraction concentrates wave energy so that some parts of a beach erode more.
- What processes cause spits and barrier islands to form?
- How do barrier islands protect beaches? What happens when these natural barriers are destroyed?