CHAPTER 10 The Coast: Beaches and Shoreline Processes

1. CHAPTER 10 The Coast: Beaches and Shoreline Processes © 2011 Pearson Education, Inc.

2. Overview Coastal regions constantly change. The beach is a dominant coastal feature. Wave activity continually modifies the beach and coastal areas. Waves affect deposition and erosion of sand and subsequent coastal features. Sea level changes affect the coast. Humans have attempted various coastal stabilization measures. © 2011 Pearson Education, Inc.

3. Coastal Regions • General Features • Shore – the zone that lies between the low tide line and the highest area on land affected by storm waves • Coast – extends inland as far as ocean related features are found • Coastline – boundary between shore and coast © 2011 Pearson Education, Inc.

4. Beach Features • Beach – entire active area of a coast affected by waves. Consists of • Shore– divided into • Backshore – above high tide line; covered with water only during storms • Nearshore – from low tide water line to where waves break at low tide • Offshore – area beyond low tide breaking waves © 2011 Pearson Education, Inc.

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6. Other Beach Profile Features • Wave-cut bench – flat, wave-eroded surface • Recreational beach – area above the shoreline • Berm– dry, gently sloping region © 2011 Pearson Education, Inc.

7. Other Beach Profile Features • Beach face – wet, sloping surface between berm and shoreline • Longshore bar – underwater sand bar parallel to the coast • Longshore trough – separates longshore bar from the beach © 2011 Pearson Education, Inc.

8. Composition of Beaches Formed from locally available material May be coarse or fine grained sediment Boulders from local cliffs Sand from rivers Mud from rivers Significant biologic material on tropical beaches Example, Coral reef material Material is always in transit along the shoreline. © 2011 Pearson Education, Inc.

9. Sand Movement Along Beach Two Major Types Perpendicular to shoreline (toward and away) Swash– water rushes up the beach Backwash– water drains back to the ocean Parallel to shoreline (up-coast or down-coast) Longshore current – transports sand along the beach © 2011 Pearson Education, Inc.

10. Swash and Backwash © 2011 Pearson Education, Inc.

11. Longshore Transport © 2011 Pearson Education, Inc.

12. Wintertime Beach • Heavy wave activity • Backwash dominates • Sediment moved away from shore • Narrower beach • Flattened beach face • Longshore bars are present • Stormy weather © 2011 Pearson Education, Inc.

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14. Summertime Beach • Light wave activity • Wide, sandy berm • Steep beach face • Swash dominates • Longshore bars not present • Generally milder storms © 2011 Pearson Education, Inc.

15. Longshore Current • Parallel motion of water along shoreline • Caused by wave refraction • Causes zigzag motion of water in surf zone • Longshore currents travel at speeds up to4 km (2.5 miles) per hour © 2011 Pearson Education, Inc.

16. Longshore Transport Also called longshore drift, beach drift, or littoral drift Only occurs in the shallow water surf zone Transports beach sediment in a zigzag fashion in the direction of the longshore current Beaches sometimes called “rivers of sand” © 2011 Pearson Education, Inc.

17. Longshore Transport Millions of tons of sediment moved yearly Direction of transport changes due to wave approach In general, net sediment movement is southwardalong the Atlantic and Pacific coasts of the United States © 2011 Pearson Education, Inc.

18. Two Major Types of Shores • Erosional Shores • Well-developed cliffs • Exist where tectonic uplift of coast occurs • U.S. Pacific coast is one example • Depositional Shores • Gradually subsiding shore • Barrier islands and sand deposits are common © 2011 Pearson Education, Inc.

19. Erosional Shores Protruding bits of land called headlands absorb much wave energy. Wave cut cliffs and sea caves are other features carved out by wave activity. © 2011 Pearson Education, Inc.

20. Erosional Shores Sea arches form where sea caves in headlands erode all the way through. Sea stacks form when the tops of sea arches erode away completely. Bedrock uplift generates a marine terrace. © 2011 Pearson Education, Inc.

21. Erosional Shorelines • Wave erosion increases with • More shore exposed to open ocean • Smaller tidal range • Weaker bedrock © 2011 Pearson Education, Inc.

22. Depositional Shorelines • A bay barrier,orbay mouth bar, seals off a lagoon from the ocean. • A Tombolois an sand bar that connects an island to the mainland. • Barrier islands are long offshore sand deposits that parallel the coast. • A spit connects at one end to the mainland and hooks into a bay at the other. © 2011 Pearson Education, Inc.

23. Depositional Shorelines © 2011 Pearson Education, Inc.

24. Depositional Shorelines Tombolo Barrier island © 2011 Pearson Education, Inc.

25. Barrier Islands • Common along East and Gulf coasts of the United States • Do not exist along erosional shorelines • Protect mainland from high wave activity • Can migrate landward over time © 2011 Pearson Education, Inc.

26. Barrier Island Anatomy • Ocean beach • Dunes • Barrier flat • High salt marsh • Low salt marsh • Lagoon © 2011 Pearson Education, Inc.

27. Barrier Island • Ocean Beach – closest part of the island to the ocean • Dune – stabilized by grasses; protect lagoon from strong storms • Barrier flat – grassy area that forms behind dunes © 2011 Pearson Education, Inc.

28. Barrier Island • High and low salt marshes – biologically productive wetlands • Generate peat deposits of decaying organic matter • Lagoon – water between barrier island and mainland © 2011 Pearson Education, Inc.

29. Barrier Islands • Migrate landward over time due to rising sea levels • Older peat deposits found on ocean beach © 2011 Pearson Education, Inc.

30. Deltas • Triangular deposits of sediment where rivers empty into oceans or seas • Distributaries carry sediment to ocean © 2011 Pearson Education, Inc.

31. Beach Compartments • Three major components: • Rivers that supply beach sediment • Beach itself • Offshore submarine canyons • Beach starvation – human activities block supply of sand to beach compartments. © 2011 Pearson Education, Inc.

32. Beach Compartments (Continued) © 2011 Pearson Education, Inc.

33. Emerging Shorelines • Shorelines above current sea level • Marine terraces– flat platforms backed by cliffs © 2011 Pearson Education, Inc.

34. Submerging Shorelines • Shoreline below current sea level • Features include • Drowned beaches • Submerged dune topography • Drowned river valleys (estuaries) © 2011 Pearson Education, Inc.

35. Changing Sea Level Two major processes can change sea level: • Local tectonic processes • Global (eustatic) changes in sea level © 2011 Pearson Education, Inc.

36. Changing Sea Level • Local tectonic processes • Example: the Pacific coast of the United States is currently being uplifted. • Isostatic adjustments – rebound of Earth’s crust after removal of heavy loads or sinking with application of heavy loads • Ice-loading from glaciers during ice ages © 2011 Pearson Education, Inc.

37. Changing Sea Level • Global (eustatic) changes in sea level • Sea level changes worldwide due to • Change in amount of available sea water • Change in ocean basin capacity © 2011 Pearson Education, Inc.

38. Eustatic Changes in Sea Level Some Mechanisms • Ice ages lock seawater up in ice (glaciation) – sea level goes down • Ice melting after an ice age (deglaciation) – sea level rises © 2011 Pearson Education, Inc.

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40. Eustatic Changes in Sea Level • Also caused by thermal expansion and contraction of seawater • Physical property of water: warmer water expands and cooler water contracts. • Sea level rises and falls in response to seawater temperature. • This is roughly 2 meters (6.6 feet) per 1°C (1.8°F) change in temperature. © 2011 Pearson Education, Inc.

41. Pleistocene Epoch and Today • From about 2 million to 10,000 years ago, a series of four ice ages affected Earth. • Sea level was at least 120 meters (400 feet) below today’s sea level. • If all remaining ice on Earth melted today, sea level would rise another 70 meters (230 feet). © 2011 Pearson Education, Inc.

42. Global Warming and Changing Sea Level • Globally averaged temperatures – about 0.6°C (1.1°F) warmer over last 130 years • Sea level rose 10-15 cm (4-10 in) over past 100 years • As global warming continues, we will see a higher sea level. © 2011 Pearson Education, Inc.

43. United States Coasts • Atlantic coast • Pacific coast • Gulf coast © 2011 Pearson Education, Inc.

44. Atlantic Coast • Most coasts open to storm wave attack • Barrier islands common from Massachusetts south • Bedrock • Florida bedrock is resistant limestone. • Northward through New Jersey is comprised of easily erodable recent deposits. • New York through Maine has glacier-affected rocks. © 2011 Pearson Education, Inc.

45. Atlantic Coast • Strong storms called nor’easters can damage the coast north of Cape Hatteras, NC. • Nor’easters can generate storm waves up to 6 meters (20 feet). © 2011 Pearson Education, Inc.

46. Atlantic Coast • Drowned river valleys common • Average erosion is 0.8 meter (2.6 feet) per year; sea is migrating landward • Delaware, New York, and Georgia have the most serious erosion problem. © 2011 Pearson Education, Inc.

47. Atlantic Coast • Barrier islands • Drowned river valleys © 2011 Pearson Education, Inc.

48. Gulf Coast • Low tidal range • Generally low wave energy • Tectonically subsiding • Mississippi delta dominates • Locally sea level rises due to compaction of delta sediments • Average rate of erosion is 1.8 meters (6 feet) per year © 2011 Pearson Education, Inc.

49. Pacific Coast • Tectonically rising • Experiencing less erosion than Atlantic or Gulf coasts • Open exposure to high energy waves • Average rate of erosion 0.005 meter (0.016 feet) per year © 2011 Pearson Education, Inc.

50. Hard Stabilization • Structures built to decrease coastal erosion and interfere with sand movement • Also called armoring of the shore • Often results in unwanted outcomes • Some structures may increase wave erosion © 2011 Pearson Education, Inc.