L. Chapter 16 - Chapter 16 Shorelines and Shoreline...

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Chapter 16 Shorelines and Shoreline Processes Def’n: Shoreline – the area between low tide and the highest level on land affected by storm waves. This is a long, narrow zone where marine processes (waves, tides, nearshore currents) modify existing shoreline features. Shorelines are dynamic areas with the interaction of two of Earth’s systems (Hydrosphere and solid Earth). energy is expended (by waves and tides) erosion occurs sediment is transported and deposited While shoreline processes are restricted to a narrow zone at any particular time, it is important to remember from our study of marine transgressions and regressions that beach and nearshore sediments may be deposited over vast regions. Shoreline Processes 1. Tides Def’n: the regular fluctuation (twice daily) in the ocean’s surface in response to the gravitational attraction of the moon and sun. Moon – largest influence on tides Sun – more massive, but much farther away Tides are important shoreline processes because the area of wave attack constantly shifts onshore and offshore as the tides rise and fall. 2. Waves - Waves are directly or indirectly responsible for most erosion, sediment transport, and deposition in coastal areas. Def’n: oscillations of a water surface Waves are generated by the action of wind blowing over water (for example, under a storm center). As the fluid air moves over the fluid water, friction between the two results in energy transfer to the water causing the water to oscillate. Waves may develop in any body of water (lakes, ponds, oceans), but are best developed in oceans because of the fetch , the distance the wind blows over a continuous water surface. In an area where waves are generated; sharp-crested, irregular waves develop – seas . As seas move out from their area of generation, they are sorted into broad swells with rounded, long crests and all are about the same size. Swells lose little energy as they travel across the ocean. But when they enter shallow water, the wave shape changes and water is displaced in the direction the wave is travelling. Broad, undulating deep-water waves are transformed into sharp-crested waves. 1
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At a water depth equal to wave base (½ wavelength), the wave “feels” the seafloor, and the circular motion of water within the wave is disrupted. The speed of wave advance and the wavelength decrease, but the wave height increases. The wave becomes oversteepened as the wave crest advances faster than the wave form. The crest plunges forward as a breaker. As a wave breaks, it expends its kinetic energy on the shoreline. The water rushes onto the shore, then drains seaward to become part of another breaking wave. Types of Breakers Plunging – steep offshore slopes Spilling – gentle offshore slopes Shoreline Processes (cont.) Nearshore Currents Nearshore zone 1. Breaker zone – area where waves break 2 Surf zone – area where water from breaking waves rushes forward, then flows seaward as backwash.
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