Chapter 20: Shorelines

The study of shorelines requires understanding several key distinctions: the shoreline represents the fluctuating boundary between land and ocean, the shore encompasses the area between lowest tide and storm wave reach, the coast extends inland to include ocean-influenced features, and beaches are accumulations of sediment in constant motion. Waves, primarily generated by wind energy, possess characteristics including height, length, and period that depend on wind speed, fetch distance, and duration of wind action. As waves travel from deep water toward the coast, they experience shoaling, a process where the wave slows, wavelength decreases, and wave height increases until breaking occurs in the surf zone. This breaking wave energy drives shoreline erosion through mechanisms including hydraulic impact, compression, and abrasion. Sediment transport along coasts occurs through swash and backwash motions, combined with longshore currents and beach drift, which move sand in characteristic zigzag patterns and create landforms such as spits, baymouth bars, and tombolos. Wave refraction concentrates erosional energy on headlands, producing distinctive features like wave-cut cliffs, sea arches, and sea stacks, while simultaneously promoting sediment deposition in adjacent bays. Barrier islands, prevalent along Atlantic and Gulf coasts, consist of low sandy ridges that migrate in response to storm activity and sea-level fluctuations. The chapter contrasts emergent coastlines, formed by tectonic uplift or falling sea levels, with submergent coastlines, developed where rising sea levels flood river valleys to create estuaries. Hurricanes represent the most destructive coastal hazards, forming over warm tropical waters and causing damage through storm surge, extreme winds, and inland precipitation. Coastal protection strategies include hard stabilization structures such as jetties, groins, breakwaters, and seawalls, along with alternatives including beach nourishment and managed retreat. The chapter concludes with tidal mechanics, explaining how gravitational forces from the Moon and Sun create predictable ocean-level oscillations that vary globally in timing and range, generating tidal currents that significantly influence sediment dynamics and coastal morphology.