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Warm Up 2/27/08 • Which of the following is a tidal current? a. spring tide c. neap tide b. flood tide d. both a and c • The smallest daily tidal range occurs during which type of tide? a. spring tide c. neap tide b. flood tide d. ebb tide • Which tidal pattern has two high tides and two low tides each day? a. semidiurnal c. mixed b. bidiurnal d. diurnal Answers: 1) d. 2) c. 3) a.
Shoreline Features and Processes Chapter 16, Section 3
Forces Acting on the Shoreline • Waves along the shoreline are constantly eroding, transporting, and depositing sediment. Many types of shoreline features can result from this activity • The impact of large, high-energy waves against the shore can be very violent • Cracks and crevices quickly open in the cliffs, and water is forced through • Abrasion is the sawing and grinding action of rock fragments in the water • Waves are also very effective at breaking down rock material and supplying sand to beaches
Wave Refraction • Wave Refraction – the bending of waves • Most waves move towards the shore at an angle • When the waves get closer, they are refracted, bent, so that they come into the shore parallel • Because of refraction, wave energy is concentrated against the sides and ends of headlands that project into the water, whereas wave action is weakened in bays
Concept Check • What is wave refraction? • The bending of waves
Longshore Transport • Longshore Current – flow parallel to shore and move large quantities of sediment along the shore • Turbulence allows long shore currents to easily move fine suspended sand and to roll larger sand and gravel particles along the bottom • Longshore currents can change directions because the direction that waves approach the beach changes with the seasons • Longshore currents generally flow southwards along the Western coast of North America
Concept Check • What causes longshore currents? • The angling of waves in the surf zone
Erosional Features • Shoreline features that originate primarily from the work of erosion are called erosional features • Wave erosion is steadily wearing away the California coast • The cliffs along our coast are created when tectonic processes push the land up at the same time the ocean waves crash against them, eroding them away • Wave-cut cliffs result from the cutting action of the surf against the base of coastal land • As erosion continues, it will reduce the cliffs into a bench-like feature, called a platform • The surf can erode headlands, creating sea caves, when two sea caves meet, a sea arch will form • Arches all eventually collapse into sea stacks
Concept Check • How does a sea arch form? • Sea arches form when two caves (eroded by surf) on opposite sides of a headland unite.
Depositional Features • Sediment that is transported along the shore and deposited in areas where energy is low produce depositional features • Where longshore currents and other surf zone currents are active, several features related to the movement of sediment along the shore may develop • A spit is an elongated ridge of sand that projects from the land into the mouth of an adjacent bay • The term baymouth bar is used when a sandbar completely crosses a bay, cutting it off from the ocean • A tombolo is a ridge of sand that connects an island to the mainland or another island • Barrier Islands – narrow sandbars parallel to, but separated from the coast
Concept Check • What is a barrier island? • A barrier island is a narrow sandbar parallel to, but separate from, the coast at distances from 3 to 30 km offshore.
Stabilizing the Shore • Shorelines are among Earth’s most dynamic places and are changing constantly • Erosion along the coast causes significant property damage every year • Groins, breakwaters, and seawalls are some structures built to protect a coast from erosion or to prevent the movement of sand along a beach • A groin is a barrier on the beach to trap sand that is moving parallel to the shore • A breakwater protects ships from the force of large breaking waves as they pull out of harbor • A seawall is built parallel to shore and protects the shore from waves • Beach nourishment is the addition of large quantities of sand to the beach system
Assignment (Due 2/28/08) • Read Chapter 16 (pg. 448-467) • Do Chapter 16 Assessment #1-30 (pg. 471-472) • Study for the UNIT TEST!!!
Cool Down • What structures can be built to protect a shoreline? • Groins, breakwaters, and seawalls • How can beach nourishment be helpful? How can it be harmful? • Beach quality and storm protection are improved. The process is expensive and can harm local marine life. • What is one thing new you learned today? Explain.