Coastal processes explain how waves, currents, tides, and wind shape the edges of continents and islands. This cheat sheet helps students connect moving water and sediment to landforms such as beaches, spits, cliffs, sea stacks, and barrier islands. It is useful for reviewing erosion, deposition, and the changing balance between ocean energy and coastal materials.
The most important ideas are that waves transfer energy, currents move sediment, and coastlines change when erosion and deposition are out of balance. Wave speed can be estimated with speed = wavelength / period, and wave frequency can be found with frequency = 1 / period. Longshore drift moves sand in a zigzag path along the beach, while tides raise and lower sea level because of gravity from the Moon and Sun.
Key Facts
- Wave speed is calculated as speed = wavelength / period, where wavelength is the distance between wave crests and period is the time between crests.
- Wave frequency is calculated as frequency = 1 / period, so shorter wave periods mean more waves pass a point each second.
- Longshore drift moves sediment along a coast when waves approach the shore at an angle and backwash moves straight downslope.
- Erosion is strongest where wave energy is high, rock is weak, or cliffs are exposed to repeated hydraulic action and abrasion.
- Deposition occurs when waves or currents lose energy and drop sediment, forming beaches, spits, bars, and barrier islands.
- A beach profile is shaped by the balance between swash, which moves sediment up the beach, and backwash, which moves sediment back toward the sea.
- Tidal range is calculated as tidal range = high tide level - low tide level.
- Coastal landforms change fastest during storms because storm waves have greater energy and can rapidly erode dunes, beaches, and cliffs.
Vocabulary
- Erosion
- Erosion is the wearing away and removal of rock, sand, or soil by waves, currents, wind, or ice.
- Deposition
- Deposition is the dropping of sediment when water, wind, or waves lose energy.
- Longshore Drift
- Longshore drift is the movement of sediment along a shoreline caused by waves hitting the coast at an angle.
- Tidal Range
- Tidal range is the difference in height between high tide and low tide.
- Barrier Island
- A barrier island is a long, narrow island of sand that forms parallel to the coast and helps protect the mainland from waves.
- Sea Stack
- A sea stack is an isolated column of rock left behind after waves erode a headland, arch, or cliff.
Common Mistakes to Avoid
- Confusing erosion with deposition is wrong because erosion removes sediment while deposition drops sediment in a new location.
- Assuming all coastlines change at the same rate is wrong because rock type, wave energy, storms, sea level, and human structures all affect coastal change.
- Forgetting units in wave calculations is wrong because speed, wavelength, and period must use matching units such as meters and seconds.
- Thinking longshore drift moves sand only straight offshore is wrong because angled waves move sediment sideways along the beach in a zigzag pattern.
- Calling every sandy coastal feature a beach is wrong because spits, bars, dunes, and barrier islands have different shapes and formation processes.
Practice Questions
- 1 A wave has a wavelength of 12 meters and a period of 4 seconds. What is its wave speed?
- 2 A shoreline has a high tide level of 3.8 meters and a low tide level of 0.6 meters. What is the tidal range?
- 3 Waves approach a beach from the southwest at an angle. In which general direction will longshore drift move sediment along the beach?
- 4 Explain why a rocky headland often erodes into features such as caves, arches, and sea stacks, while a nearby sheltered bay may collect sand.
Understanding Coastal Processes & Landforms
The shape of a coast depends greatly on the kind of rock beneath it. Hard rocks such as granite resist wear for a long time, so they often form headlands and steep cliffs. Softer rocks such as clay erode more quickly and tend to form bays.
This difference can produce a curved coastline where tough rock projects into the sea. Waves bend as they enter shallow water near a headland. Their energy becomes concentrated there, which increases erosion.
In a bay, the same waves spread out and lose energy. Sand and shingle can then build up. This pattern is called wave refraction, and it helps explain why erosion and deposition often occur close together.
Cliffs do not simply shrink from the top. Most erosion begins near sea level, where breaking waves attack the base. Water and trapped air are forced into tiny cracks in the rock.
Repeated pressure can widen the cracks. Stones carried by waves scrape and strike the cliff face. Salt water may enter cracks, then leave salt crystals behind as it dries.
The crystals can push rock grains apart. A hollow called a wave-cut notch develops at the base. When the overhanging rock becomes too heavy, part of the cliff collapses.
The fallen material is broken up and carried away. Over time, this leaves a broad rocky surface called a wave-cut platform.
Coastal landforms often develop in stages. A crack in a headland can grow into a cave as waves exploit a weak line in the rock. If the cave cuts through the headland, it becomes an arch.
The roof of an arch may collapse, leaving an isolated pillar known as a stack. Continued erosion reduces the stack to a low stump. These features show that coastlines are not fixed.
Their appearance records the direction of wave attack, the strength of the rock, and the presence of joints or faults. On beaches, sediment size gives useful clues. Large pebbles need strong water movement to be transported, while fine sand can settle in calmer conditions.
People can change sediment movement even when they are trying to protect the shore. Groynes are barriers built at right angles to a beach. They trap sand on one side and can make that section wider.
However, less sediment may reach beaches farther along the coast. Sea walls reflect wave energy and may cause scouring at their base. Managed retreat takes a different approach by allowing selected low lying land to flood, creating salt marshes that absorb wave energy.
When studying a coastal diagram or photograph, identify the wave direction, the rock type, the sediment source, and any human structures. These details help explain why one part of a coastline is growing while another is retreating.