GSC 1530: Chapter 20

1. GSC 1530: Chapter 20 Shorelines

2. Shorelines • Shorelines, both ocean and lake, can be very beautiful settings • However, shorelines are some of the most geologically dynamic settings on Earth • Processes like shoreline erosion, sea level increases and deadly coastal storms (e.g., hurricanes) threaten the lives and property of billions of people worldwide • As of 2008 about 165 million people in the U.S. live within 50 miles of a marine coast

3. Shorelines Terminology • Oceanic shoreline – land line marking contact between land and sea • Oceanic shore – area between lowest high tide and highest land point affected by storm waves • Coast – area extending from the landward backshore boundary to the limit of ocean-related features like dunes • Coastline – coast’s seaward edge • All locations can vary

4. Coastal Processes Terminology • Current ( ) - unidirectional flow of water and energy caused by wind, water density differences and water temperature differences • Oceanic currents redistribute heat through the ocean and therefore play a critical role in climate (see figures)

5. Gulf Stream Warm, less salty surface current Gulf Stream: 50-90 miles wide; maximum discharge: 540 billion tons/hour Cold, salty deep current

7. Coastal Process Terminology • Tides ( ) - periodic, rhythmic rise and fall of water along coastlines due to the gravitational tug of the Sun and Moon on the Earth • Most oceanic coastlines experience two high tides and two low tides each day Spring tide Neap tide

9. Tidal Cycle

11. Coastal Process Terminology • Tsunami ( ) – omni directional flow of water away from a central point caused by any event (e.g., seafloor earthquake, submarine landslide) that vertically displaces water from its equilibrium position • Don’t call tsunamis “tidal waves” earthquake triggered tsunami

12. Japan Tsunami 2011 Japanese Tsunami Footage

13. Wave Terminology • Wave – undulation of the water surface • Currents, tides and tsunamis generate waves • Wavelength – the horizontal distance between two adjacent wave crests or two adjacent wave troughs (see figure) • Wave frequency – the number of waveforms that pass a fixed point per unit of time • The higher the wave frequency the greater the wave’s impact or penetration energy

14. Wave frequency – the number of crests, or troughs, of a waveform that pass a fixed point per unit of time Common units – cps (cycles per second); Hertz (Hz); 1 Hz = 1cps Wave base – the maximum depth to which energy from a surface wave is transmitted; equal to the wave’s wavelength/2

15. As a wave approaches a shoreline, waves of oscillation transform to waves of translation – the predominant water flow is horizontal

17. Note “breaking” waves; they can perform substantial shoreline erosion

18. Major Factors Controlling Shoreline Erosion • Topography (e.g., steep or shallow) of shoreline • Resistance of shoreline rocks or sediments to erosion • Erosive force of daily currents, tides • Frequency and intensity of high-energy storm events (e.g., hurricanes) or abnormal events like tsunamis • Frequency of tectonic events (e.g., earthquakes)

19. Sea arch

20. Wave undercutting

21. Sea arch Sea stack

22. Importance of recognizing the rates of shoreline erosion?

25. Evidence for long-term fluctuations in sea-level is found in wave-cut platforms in rocky coastlines (see figures)

26. What long-term cyclic process could cause significant fluctuations in sea level? wave-cut platforms

27. Source: Geology of Michigan and the Great Lakes, 2008

28. Emergent and Submergent Coasts • Emergent coast – coastline rises relative to sea level as a consequence of sea level decline or land uplift • Submergent coast – coastline is lowered (covered by water) relative to sea level as a consequence of sea level increase or land subsidence • Global warming and cooling and plate tectonic motion can obviously affect long-term sea levels; what rapid, high energy process could quickly create emergent or submergent coasts? (see slide)

30. Coastal Processes • Shoreline erosion and sediment movement are also influenced by wave refraction • Wave refraction – change in shape of a waveform as it approaches shore; occurs when portions of the wave pass into waters of unequal depth (see figure)

33. Coastal straightening due to long-term wave erosion

34. Wave Refraction • Wave refraction is also responsible for establishing the longshore current and initiating littoral drift (beach drift) (see slides)

35. = littoral drift or beach drift

36. Dynamic Coastal Settings • Mainland beaches, barrier islands, spits and estuaries are some of the most dynamic settings on Earth • Let’s examine the geological characteristics of these settings and the hazards associated with their development

37. Barrier Islands Mainland Beach Spit

38. Beach – an accumulation of unconsolidated sediment along the landward margin of an ocean or lake

39. Dynamic Coastal Environments

40. An elongate, narrow, low-elevation island, composed of unconsolidated sediment, that parallels the mainland

42. Aerial view of the NC “Outer Banks” barrier islands

43. Spit – an elongate ridge on unconsolidated fine-grained sediment that extends from the mainland into the mouth of an adjacent bay

45. Estuary – a coastal embayment formed by the flooding of a river’s mouth; often result from the rise in sea level that accompanies the melting of Ice Age ice sheets – an example of a submergent coast

46. Earth DIGIT CD

47. Estuary

48. Human Impacts • Let’s examine some of the impacts and consequences of human interaction with these dynamic coastal environments • Question to ponder: why do we develop land in these dynamic coastal environments? What ongoing controversy surrounds this development?

49. Long Island, New York

50. eye