Ask A Scientist
Why do waves on the ocean approach the coastline at an angle?
Asked by: Maria Mirabito
School: Sidney High School
Teacher: David Pysnik
Answer from John Bridge
Professor of Geological Sciences, Binghamton Unive
St. Andrews University, Scotland
Earth surface processes and sedimentology
Different kinds of waves occur on the surface of the ocean. The waves that we see approaching the coast, and that break and rush up the beach, are caused by the wind blowing on the water surface. These are called wind waves. Waves caused by the gravitational attraction of the Sun and Moon on the ocean are called tidal waves, and are responsible for the flooding and ebbing of the tide. Also, isolated events such as submarine earthquakes, landslides, and volcanic eruptions produce the potentially damaging tsunami waves. Actually, tsunamis are often referred to wrongly as tidal waves, especially in the movies. I will only talk about wind waves here. The wind waves that we see approaching the coast during calm weather are normally formed somewhere else. Wind waves are formed mainly in a local area of the ocean where strong winds are acting. This so-called wave-generation area may be underneath a major weather system such as a hurricane or some other kind of storm at sea. The waves then move out of the generation area in various directions, but mainly in the prevailing wind direction. Wind waves can travel long distances over the ocean, and those formed by storms around Antarctica can reach Alaska. It is the location of the wave-generation area and the direction of the prevailing wind that determine the direction from which the waves approach the coast. For example, if waves were formed in the northern Atlantic Ocean and started moving to the southwest towards the Virginia coast, which is in a roughly north-south direction, the crest-lines of the waves would approach the coastline at an angle of about 45o. However, like most things in life, things are more complicated than this. As wind waves move into the shallow water near a coastline, their crest-lines can be refracted (or bent), similar to what happens to light waves when they pass through a lens. Wave refraction actually causes a wave crest that approaches a coastline at an angle to bend round so that it becomes almost parallel to the coastline. Just as with light waves approaching a mirror, wind waves can also be reflected off coasts, especially those with rocky cliffs and steep beaches. Just as with light waves, the angle of reflection of the waves is equal to the angle of approach. If a coastline is irregular, with rocky headlands and bays for example, reflected waves move in many different directions, and interfere with incoming waves. This results in very complicated patterns of wave crests that can approach the coast at many different angles.