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As waves approach shore and enter shallow water, they slow, their wavelengths shorten, and their heights increase (shoaling). Waves approaching at an angle to shore refract toward the bathymetric contours, concentrating energy on headlands and spreading it in bays. Breaking waves release energy in the surf zone, driving longshore currents that transport sediment along the coast. Coastal morphology — beaches, sea cliffs, barrier islands, estuaries — reflects the balance between wave energy, sediment supply, and sea-level history.
Use maps showing wave orthogonals (perpendicular lines to wave crests) to visualize energy concentration. Trace longshore sediment transport pathways and understand how jetties or breakwaters interrupt this flow, starving downdrift beaches.
From your study of ocean surface waves, you know that waves are generated by wind and travel across the open ocean as organized oscillations of the water surface, carrying energy without transporting water itself. From sediment transport, you know that moving water can pick up, carry, and deposit particles depending on the flow's energy. Coastal processes describe what happens when these two systems collide: ocean wave energy meets the shoreline, and the resulting forces sculpt every beach, cliff, and barrier island on Earth.
As waves approach the coast and enter shallow water (where the depth is less than about half the wavelength), they undergo a transformation. The wave base begins to interact with the seafloor, friction slows the lower part of the wave, and the wave responds by shortening its wavelength and growing taller — a process called shoaling. Eventually the wave becomes too steep to sustain itself and it breaks, releasing its energy in the surf zone. The type of breaking (spilling, plunging, or surging) depends on the steepness of the beach and the wave characteristics, but in all cases the result is turbulent energy that can move enormous quantities of sand and sediment.
Wave refraction is the bending of wave crests as they approach a coastline at an angle. The part of the wave in shallower water slows down first, while the portion still in deeper water continues at its original speed, causing the wave crest to pivot and turn toward alignment with the depth contours. This has a critical consequence for coastal morphology: wave energy converges on headlands (points of land jutting into the sea, where the water shoals from multiple directions) and diverges in bays (where the concave coastline spreads the energy over a wider area). Headlands are therefore zones of intense erosion — wave energy is concentrated against them, undermining cliffs and carving sea stacks — while bays are zones of deposition where the weaker wave energy allows sediment to settle and beaches to form.
When waves arrive at an angle to the shore (which they almost always do, despite refraction reducing the angle), the breaking waves push water and sediment along the coast. This generates a longshore current flowing parallel to the beach within the surf zone, and the sediment it carries constitutes longshore drift — the primary mechanism by which sand moves along coastlines. A single stretch of coast may transport hundreds of thousands of cubic meters of sand per year in this manner. This has enormous practical implications: building a jetty or groin perpendicular to the shore interrupts the sediment conveyor, causing sand to accumulate on the updrift side while the downdrift beach is starved of its sediment supply and erodes. Understanding longshore drift is therefore essential to coastal engineering — every seawall, harbor breakwater, and beach replenishment project must account for how it will alter the natural sediment transport pathway.