Chapter 11

1. Chapter 11 Coastal Hazard

2. Learning Objectives Understand coastal processes such as waves, beach forms and processes, and rising sea level Understand coastal hazards such as rip currents and erosion Know what geographic regions are at risk for coastal hazards Understand the effects of coastal processes such as rip currents, coastal erosion, and rising sea level

3. Learning Objectives, cont. Recognize the linkages between coastal processes and other natural hazards Know the benefits derived from coastal processes Understand how human use of coastal zone affects coastal processes Know what we can do to minimize coastal hazards Understand the adjustments that can be taken to avoid damage from coastal erosion and rising sea level or personal injury from strong coastal currents

4. Introduction to Coastal Hazards Continental and oceanic processes converge to produce landscapes that are capable of rapid change Coasts are greatly influenced by Plate Tectonics East Coast United States and Canada, Canadian Arctic is passive Because they are not close to convergent boundary Have wide continental shelves with barrier islands and sandy beaches West Coast United States and Canada is active It is close to convergent and transform boundaries Have sea cliffs and rocky shorelines

5. Introduction to Coastal Hazards, cont. Coast also influenced by climate and organisms Alaska, Canada, and Great Lakes affected by seasonal ice or glaciers Temperate region coastlines affected by marsh vegetation Tropical and subtropical coasts affected by mangroves and coral reefs Coastal hazards include Strong coastal currents Coastal erosion Storm surge Tsunamis

6. Coastal Processes: Waves Waves are caused by offshore winds producing friction over water Size of waves depend on Speed of wind Duration of wind Fetch – Distance wind blows over water surface Waves become sorted into groups as they move away from their origin Rogue waves are exceptions to these groups

7. Wave Description Wave height Distance from crest to trough Wavelength Distance from crest to crest Wave period Time between crests Figure 11.6a

8. Wave Motion Motion is circular in open ocean Circles decrease in diameter with increasing depth Waves in shallow water become ellipses as waves “feel bottom” When depth is ½ wavelength Figure 11.6b

9. Figure 11.6c

10. Waves and Wave Energy Wave sets generated by storms are called swells Swells enter shallow water, become unstable and break Mathematical equations can be used to predict wave height, period and velocity Waves move in open ocean with little energy loss Energy is spent on reaching coastline Wave energy is proportional to square of wave height On coast, wavelength and velocity decrease, wave height increases, wave period is constant

11. Wave Variations along a Coastline Irregularities in topography cause variations in wave height as it approaches shore A single wave is called a wave front Irregular coastlines have headlands The shape of the coast is similar underwater to that of the coastline Water gets progressively shallower close to shore As the wave approaches the shore, it slows at the headland first This causes the wave front to bend around the headland Refraction

12. Effects of Wave Refraction Picture wave normals as lines perpendicular to wave fronts Wave refraction causes normal to converge and diverge Convergence Wave heights and energy increases Waves are bigger here Divergence Wave heights and energy decreases

13. Figure 11.7a

14. Breaking Waves Plunging breakers Waves that pick up quickly Typical on steep beaches More erosive Spilling breakers Waves that spill gently Typical on wide, flat beaches More likely to deposit sand

15. Figure 11.8

16. Beach Form and Processes Beach consists of loose material which has accumulated by wave action on shoreline Type of beach material depends on source of sand White beaches from shell and coral Black beaches from volcanic rock Brown beaches from quartz and feldspar

17. The Beach Onshore Landward extent of a beach on seashore or lakeshore Line of sand dunes Line of permanent vegetation or Sea cliff or bluff Form from erosion of rock or sediment Beaches are divided into Berm Beach portion that slopes landward Formed by deposition of sediment by waves Beach face Beach portion that slopes toward water In the swash zone Where waves swash and backwash

18. Figure 11.9

19. The Beach Offshore Swash zone Zone where waves swash and backwash on the beach Surf zone Where turbulent waves move after waves break Breaker zone Where the waves become unstable, peak, and break Longshore bar forms beneath breakers Longshore trough forms landward from bar

20. Sand Transport Littoral transport Sand movement parallel to shore Beach drift Sand moving in zigzag pattern in swash zone Longshore drift Transport of sand by longshore currents Longshore currents Current that flows parallel to shoreline as a result of up and back movement of water in swash zone

21. Figure 11.10

22. Sea Level Change—Eustatic Sea Level Global sea level Affected by changes in amount of water in oceans Climate/Average air temperature Temperature increases cause volume of water to expand Temperature decreases cause more ice to form on land Volume of water in ice sheets, glaciers increases, ocean water decreases Tectonic processes Changes ocean basin shape over long period of time

23. Sea Level Change—Relative Sea Level Location of the sea at shoreline Glacier melt or earthquakes can cause uplifting of land Decrease in sea level Rates of deposition, erosion, or subsidence makes the level rise or fall Tides caused by gravitational pull of the moon cause daily and seasonal changes Weather conditions Winds and storm surges change relative sea level

24. Geographic Regions at Risk for Coastal Hazards Coastal hazards are present on both sea shores and lake shores Coastal erosion is a problem on all areas that border the ocean Strong nearshore currents are also common problems Coastlines that are subsiding are at particular risk

25. Figure 11.11

26. Effects of Coastal Processes: Rip Currents Currents that move away from shore Develop when waves pile up water between longshore bar and swash zone Becomes concentrated in narrow zones In United States, 200 people are killed and 20,000 people are rescued from rip currents Currents are narrow and widen and dissipate once they reach line of breaking waves Escape requires swimming parallel to shore Don’t panic

27. Figure 11.14b Figure 11.15

28. Effects of Coastal Processes: Beach Erosion Beach Budget Input Longshore and beach drift bringing sediment from upshore Local erosion of dunes and cliffs Output Longshore and beach drift bringing sediment away from shore Storm waves On-shore winds Storage Sediment on the beach Beach grows when input exceeds output Beach erodes when output exceeds input

29. Effects of Coastal Processes: Cliff Erosion Sea cliffs and lakeshore bluffs erode due to wave action, running water, and landslides Causes the cliffs and bluffs to retreat Human activities increase erosion rate Increase surface runoff Increase groundwater discharge Addition of weight to cliff Can be monitored using LIDAR

30. Figure 11.16

31. Linkages with Other Natural Hazards Earthquakes, volcanic eruptions, tsunamis Change the shape of shoreline Storm waves, storm surge, and coastal flooding Increase coastal erosion Landslides Caused by eroding cliffs and bluffs Climate change Storm frequency and intensity change with climate conditions

32. Linkages with Other Natural Hazards: 2010 Oil Spill Predicting the movement of the oil along the beaches and in salt marshes requires detailed coastal information on Wave height and frequency Direction and rate of longshore transport Strength of tidal flow into and out of barriers and island inlets How far inland tidal flow inundates salt marshes How salt marsh vegetation and sediment interact with the oil The effects of the oil on life on beaches and salt marshes

33. Natural Service Functions Pleasing landscapes Cliffs and bluffs are result of erosion Beaches Maintained or created by erosion and deposition Recreation Swimming, sailing, fishing, and sunbathing

34. Human Interaction: The Atlantic Coast Characterized by barrier islands long narrow islands of sand separated from the mainland by a lagoon or bay Removal of coastal dunes Increased vulnerability to storms Jetty construction Interrupts longshore drift Increases erosion at some locations

35. Figure 11.23

36. Human Interaction: The Gulf Coast and Canadian Sea Coasts Gulf Coast erosion due to Coastal engineering structures Subsidence due to groundwater and petroleum withdrawal Damming of rivers Canadian sea coasts Sandy coastlines erode more quickly than rocky shores Entire islands off of Nova Scotia have disappeared

37. Human Interaction: The Great Lakes Severity of erosion depends on: Existence of coastal dunes Dune-protected bluffs erode at a slower rate Orientation of the coastline Sites exposed to high-energy storm winds erode faster Groundwater seepage Seepage at the base of a bluff causes slope instability and increased erosion Existence of protective structures Often accelerate coastal erosion in adjacent areas

38. Minimizing the Effects of Coastal Hazards Hard stabilization Creating structures meant to protect shoreline Soft stabilization Adding sand to depleted beaches

39. Hard Stabilization: Seawalls Seawalls Structures built parallel to shoreline Vertical design reflects waves and redirects energy to shore Promotes beach erosion Figure 11.25

40. Hard Stabilization: Groins Groins Structures built perpendicular to shoreline usually in groups Traps sand from longshore drift Causes increased erosion in downdrift area Figure 11.26a

41. Hard Stabilization: Breakwaters and Jetties Breakwaters Built parallel to shore Intercepts waves to protect boats or ships in harbor Blocks littoral transport, increasing erosion and deposition in different locations Jetties Built in pairs perpendicular to shoreline near river or inlet Designed to keep channel open Causes increased erosion and deposition in other locations

42. Figure 11.27

43. Soft Stabilization: Beach Nourishment Adding sand to replace sand that has eroded Aesthetically preferable to hard stabilizations Temporary solution Sand must be chosen carefully to match conditions at beach

44. Figure 11.29

45. Perception of Coastal Hazards Perception of erosion hazard depends on people’s experience, proximity to the coastline, and probability to suffering damage Many people are complacent about wave and current hazards

46. Adjustment to Coastal Hazards Education and awareness of hazard Restricting swimming, posting warnings, beach forecasts Beach nourishment and shoreline stabilization Land use changes “Managed Retreat Solution” Avoid building in hazardous areas and relocate threatened buildings

47. Managed Retreat Solution Determine rate of erosion Determine setback Distance from shoreline where development will be allowed Use information to manage coastal zone

48. Five Principles of Living with Coastal Erosion Coastal erosion is a natural process rather than a natural hazard Any shoreline construction causes change Stabilization of the coastal zone through engineering structures protects property, not the beach itself Engineering structures designed to protect a beach may eventually destroy it Once constructed, shoreline engineering structures produce a costly trend in coastal development that is difficult, if not impossible, to reverse

49. End Coastal Hazards Chapter 11