Topic 23: SHORELINES Ocean Waves - receive their energy from winds that blow across the water surface. The size of a wave depends on how fast, how far, and how long the wind blows. Wave Motion - Wave length - Horizontal distance between two wave crests. Wave height - Vertical distance between crest and trough. Wave Base - a depth of half the wave length is the lower limit to effective wave motion. Breaking Waves - As a wave nears the shore, its base encounters frictional resistance by the sea floor. This causes the height of the wave to increase and the wavelength to decrease. A breaker is a wave that has become so steep that the crest of the wave topples forward, moving faster than the main body of the wave. Surf - Water in the surf zone looses its orbital motion and moves back and forth as a turbulent, often foamy, mass. Wave Refraction - Often one end of a wave reaches shallow water first. This end of the wave "feels bottom" and slows down while the rest of the wave continues at its deep-water speed. As a wave slows progressively along its length, the wave crest changes direction and becomes more nearly parallel to the shoreline.. Coastal Erosion and Sediment Transport - Longshore Currents - Waves that arrive at an angle to the shore pushes more water parallel to the shoreline than waves that arrive normal to the shore. Beach Drift - the zigzag movement of water on the shoreline carries sand and pebbles first up, then down the beach slope. Offshore Transport and Sorting - seaward of the surf zone, bottom sediments are moved by currents and by unusually large waves during storms. Beach - is a strip of sediment (usually sand or gravel) that extends from the low-water line inland to a cliff or zone of permanent vegetation. Foreshore - The distance extending from the lowest tide to the average high-tide level. Berm - A wave-deposited sediment platform that is flat or slopes slightly landward. Back shore - A zone extending inland from the berm to the farthest point reached by the surf. Wave-cut Cliff - Steep slopes that retreat landward by mass wasting as wave erosion undercuts them. Wave-cut Bench - a horizontal platform or rock formed beneath the surf zone as a coast retreats by wave erosion. Major Coastal Landforms: Marine Deltas - constructional processes may prograde (build out) the coastline more rapidly than it can be destroyed by surf. The extent to which a marine delta projects seaward from the land is a compromise between the rate at which a river delivers sediment at its mouth and the ability of currents and waves to erode the sediment. Spits - a finger like ridge of sediment that extends out into open water. Baymouth Bar - a ridge of sediment that cuts a bay off from the ocean. Barrier Islands - ridges of sand that parallel the shoreline and extend above sea level. Organic Reefs - a limestone reef built by vast colonies of tiny organisms, principally corals, that secrete calcium carbonate. Such organisms require shallow, clear water in which the temperature remains above 18¡C. Atoll - A roughly circular coral reef enclosing a shallow lagoon Tsunamis - A seismic sea wave generated by a strong earthquake or undersea landslide. Drowned Coasts - submergent coasts occur where sea level has been rising or tectonics has lowered the land surface. Drowned river mouths called estuaries, mark many such coasts today. Drowned coasts may also be marked by fjords such as found on the west coast of Norway. Uplifted Coasts - emergent coasts have been elevated by deep-seated tectonic forces. Often these coasts are characterized by marine terraces.