Coasts on the Move
Waves erode coastal rock, move sediment along the shore and deposit it where conditions allow. These processes shape headlands, bays, beaches, spits and bars.
What a learner can do afterwards
- Explain how waves arriving at an angle move sediment along a beach
- Say how a headland and a bay form on a coast made of alternating hard and soft rock
- Name one coastal landform made by erosion and one made by deposition
1 · Read
When waves approach a beach at an angle, swash carries sand up the beach at that angle. Backwash returns down the slope under gravity, usually towards the sea. Repeated trips move sediment in a zigzag along the shore. This is longshore drift.
Where bands of more and less resistant rock meet the sea along a coast, less resistant rock usually erodes faster into bays. More resistant rock remains as headlands. Wave erosion can form cliffs, caves, arches and stacks. Deposition of sand or shingle can build beaches, spits and bars.
If longshore drift carries sand east, it can supply beaches further east. A groyne extends out across a beach. It traps some sediment on the side from which drift arrives. Less may reach the next beach, where erosion can then increase.
A seawall runs along the shore and protects land behind it from waves. It does not trap drifting sand in the same way as a groyne. On suitable sheltered shores, marsh stems can slow water and trap sediment. Roots help hold the soil. Protection depends on the site and storm.
Angled swash and returning backwash move sediment alongshore. Erosion shapes rock, while deposition builds sediment landforms.
Lesson, question and flashcard revisions written for this dataset.
Readings written for this dataset. Flashcards revised for this dataset.
More to read
Different levels of coastal risk
Two neighbourhoods can face different risks from the same coastal storm. Low ground may flood while higher ground stays dry. Access to transport, safe housing and money for repairs also differs. Residents can identify places that flood and services they depend on. A protection plan should consider these differences alongside waves, tides and erosion.
Investigating beach changes
Photographs taken from the same position can show where a beach has gained or lost sediment. They do not explain the change by themselves. Waves, currents and wind can all move material. To investigate longshore drift, also examine the direction of arriving waves and barriers across the beach. Compare observations from several dates before deciding which explanation fits.
What a soil model can test
A tray model can compare sediment washed from bare soil and plant-covered soil. Use equal slopes and equal amounts of water. Such a test concerns runoff under those model conditions. A coastal marsh also experiences tides and waves. Before applying the results to a coast, ask which processes the tray includes and which it leaves out.
Planning a seawall
A seawall needs a design suited to its shore. Engineers assess waves, ground conditions, drainage and nearby beaches. Reflected waves can increase erosion at the wall's base. Inspection and repair are also needed. Construction decisions belong with qualified specialists and the relevant authorities. This reading explains assessment questions, not instructions for building a seawall.
2 · Watch
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Where it sits
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Optional connections
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.