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How Coastlines Erode
Waves, longshore drift, and sea level rise
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Coastlines are always changing because waves, currents, wind, storms, and sea level all move rock and sediment. Erosion matters because many people live, travel, and build near beaches, cliffs, roads, and ports. A sandy beach can shrink after a storm, while a rocky cliff can be undercut until pieces collapse. Places such as California and North Carolina show how powerful coastal erosion can be when waves meet steep cliffs, barrier islands, and rising seas.
Waves erode coastlines by pounding the shore, forcing air and water into cracks, scraping rock with sand and pebbles, and carrying sediment away. When waves reach the beach at an angle, longshore drift moves sand along the coast in a zigzag path. Storm surge raises water levels during storms, letting waves attack dunes, seawalls, and cliffs higher than usual. Sea level rise makes erosion more likely over time because waves can reach farther inland and damage areas that used to be protected.
Key Facts
- Wave energy increases with wave height, so taller storm waves usually cause more erosion.
- Wave speed in deep water can be estimated by v = wavelength / period.
- Longshore drift happens when waves approach the shore at an angle and move sand along the beach.
- Cliff undercutting occurs when waves erode the base of a cliff, making the upper cliff unstable.
- Storm surge is a temporary rise in sea level caused by storm winds and low air pressure.
- Relative sea level change = sea level rise + land subsidence, so sinking land increases coastal risk.
Vocabulary
- Erosion
- Erosion is the wearing away and movement of rock, soil, or sediment by water, wind, ice, or gravity.
- Longshore drift
- Longshore drift is the movement of sand and sediment along a coast caused by waves hitting the shore at an angle.
- Undercutting
- Undercutting is erosion at the base of a cliff that removes support and can lead to collapse.
- Storm surge
- Storm surge is an abnormal rise of ocean water during a storm that can flood and erode coastal land.
- Dune restoration
- Dune restoration is the rebuilding or protection of sand dunes using plants, fencing, or added sand to reduce coastal erosion.
Common Mistakes to Avoid
- Thinking erosion only happens during big storms is wrong because everyday wave action and currents can slowly move sediment for years.
- Confusing weathering with erosion is wrong because weathering breaks material apart in place, while erosion moves the material away.
- Assuming seawalls always solve erosion is wrong because they can reflect wave energy and make beaches narrower in front of the wall.
- Ignoring wave direction is wrong because angled waves create longshore drift, which controls where sand is removed and where it is deposited.
Practice Questions
- 1 A wave has a wavelength of 24 m and a period of 6 s. Use v = wavelength / period to find the wave speed.
- 2 A beach loses 1.5 m of width per year for 8 years. If no sand is added, how much narrower will the beach become?
- 3 A town must choose between building a seawall and restoring dunes along a sandy beach. Explain one benefit and one drawback of each choice.