East Coast MARE Ocean Lecture May 16, 2012 - Surf's Up! All About Waves at the Coast
- 1. Surfs Up! All About Waves at the Coast Prof. Tom Herrington Ocean Engineering Program Director Stevens Institute of Technology
- 2. Tonight’s Lecture • What happens when waves reach the coast • Surf zone currents • Wave & beach interactions • Shoreline evolution • Shoreline of the future
- 3. Anatomy of a Wave L H 2 h T T (Always Constant for an individual wave) Wave Number: k 2 Wave Phase Speed C L L T
- 4. Wave Growth
- 7. Interesting Mathematical Wave Properties • Wave are dispersive (longer waves move gk tanh( ) 2 kh 2 2 2h 2 faster) period and length are not independent! g tanh T L L • In deep water (h>L/2) Lo gT 2 C L gT 2 T 2 tanh(kh) ~ 1.0 • In shallow water (h<L/20) tanh(kh) ~ kh L ghT C gh • Between deep and 2 2 2 2h g tanh shallow water must use T L L full equation
- 8. -In Deep-Water - Waves have Ideal Shape and thus Propagate Energy but not Mass
- 9. Deep and Shallow-Water Wave Regions
- 10. Wave Motion and Particle Motion Progressive Waves Waves which interact with the sea floor are known as shallow-water waves. The orbits of the water molecules become elliptical.
- 11. Wave Refraction
- 12. Wave Refraction: Bending of Shallow- Water Wave Fronts Due to Change in Bottom Depth. The Leading Edge of a Wave Front Enters Shallower Water and Slows While the Remaining Front Continues at Higher Speed. The Net Result is a Rotation of Wave Fronts To Become Parallel with Bottom Depth Contours.
- 13. Wave Focusing and Spreading
- 14. Examples Headland Focusing Embayment wave spreading
- 15. Wave train breaking Really amazing rule of thumb Breaking wave height can be estimated by: Hb 0.78h
- 16. Waves Break by Plunging and Spilling • depends on the slope of the bottom
- 17. Surf Similarity Parameter • Ratio of slope steepness and local wave steepness ξ> 2.0 – surging/collapsing breaker 2.0 <ξ< 0.4 – plunging ξ< 0.4 – spilling
- 18. Plunging waves Plunging Breaker at Avon-by-the-Sea, NJ Spilling Breaker at Ocean City, NJ
- 19. Surf Zone Physical Processes • Currents in the surf zone are generated by the variation in Wave Height across surf zone and the angle at which the waves approach the coast. • Generates both cross-shore and alongshore currents
- 20. Cape May, NJ oblique wave approach
- 21. Flow in the Surf Zone is Very Complex! From Svendsen and Lorenz (1989)
- 22. Alongshore Current Forcing VL 41.4S gHb sin b cos b S : beach slope g : gravity Hb : breaking wave height b : wave angle at breaking Longuet-Higgens (1970)
- 24. Cross-shore wave generated motion Svendsen, et al. (1987)
- 25. Cross-shore Current Forcing Wave setup balances the gradient in the cross-shore directed radiation stress dS xx d R gh dx dx 3 S xx gdH 2 Masselink and Black (1995) 16
- 26. What is a Rip Current? • Narrow seaward moving current of up to 3 knots • Related to mass transfer of water trough the breaker zone
- 27. Flow through gaps in the sandbar system Hass, et al. (2000)
- 28. Rip Current Formation between Groins
- 29. When do Rip Currents Occur? …Extremely Large Wave Events… …Tropical Cyclone Swells… Hurricane Fabian April 18, 2003 Ocean City, NJ Sept. 4, 2003 44009: Hm0=13 ft, Tp =10 sec Surf City, LBI Surfer/Lifeguard drowned at this beach 44009: Hm0=5.75 ft, Tp=14 sec
- 30. …Low energy Wave Events… July 5, 2003 44009: Hm0=3 ft, Tp=6 sec Over 100 rescues along NJ coast
- 31. Analysis of Observed Rip Current Events In New Jersey • Revealed 2 conditions: 1. Extreme waves (> 8 ft) with periods > 8 sec 2. Long-period swell of any height • Rate of wave energy propagation to coast appears to be important P nEC
- 32. Rip Current Index • In order to weight large swell higher than wind waves and smaller swell a Rip Current H Pswell Index (RI) that is the ratio of RI 0.1* * swell energy flux to wind h Pwindwave wave energy flux multiplied by the ratio of the wave height to water depth appears reasonable. • Developed from wave buoy data located 20 n.m. off NJand Stevens Coastal Monitoring Network
- 33. Evaluation of Rip Current Index Against known Events in 2003 Isabel July 4th Fabian Juan
- 34. What about Notched Groins? What are these things?
- 35. Current and Sediment Measurements Near Notched Groin
- 36. -5 -10 10 0 -10 5 -15 -5 -15 10 -15
- 37. Measured Alongshore Current through Notched Groin Rankin, et al. (2003)
- 38. Waves as Agents of Coastal Change • Shoreline Changes occur on many temporal scales: – Seconds (Every wave) – Daily (Tides) – Seasonally (Changing wave climate) – Decadal (Extreme storms) – Centuries (Sea level changes) – Millennia (Global climate changes)
- 39. Seasonal Coastal Changes • The cross-shore extent of the beach undergoes erosion and accretion on a seasonal basis – In the summer and fall, small waves transport sand up onto the beach – In the winter and spring, large storm waves erode sand – Transition provides natural protection for the beach.
- 40. How Do We Know This?
- 42. We even know where motion stops… Depth of Closure Hallermeier (1981) and Birkemeier (1985) H e2 hc 1.75 H e 57.9 2 gT e
- 43. …and which way the sand is moving! 3 H H critical 0.00070 wT L s 3 Profile Transitions between eroded H H critical wT 0.00070 (“bar”) profile and accreted (”berm”) profile on a seasonal basis L s Kraus and Larson (1988)
- 44. Episodic Change: Driven by 3 Components 1. Extreme astronomical tides 2. Storm surge generated by intense storms or prolonged onshore winds can generate significant departures from predicted water elevations and wave attack high up on beach berm. 3. Large waves generate mass transport of water toward coast, increasing flood levels and extent of wave attack.
- 45. Prolonged storm surge at Atlantic City during March 1962 Nor’easter Water Level recorded at Atlantic City Steel Pier Storm Evolution March 5 – 8, 1962
- 46. Large-scale Damage Ocean City, NJ. Note the transport of water across the island by waves
- 47. How can we predict potential storm damage? Not Easily!
- 48. Results Not so conclusive • Longer storm duration more important than wave height and surge unless… • If duration is about equal wave heights dominate
- 49. Wave Climate Long Term Coastal Change
- 50. How do we know this? • Wave Hindcasts based on historic weather data • Offshore Buoy Data • Neashore Wave Data
- 51. Long Term Wave Climate Wave Height Distribution by Month (1980 - 1999) 5.00 4.50 4.00 3.50 Wave Height (ft) 3.00 2.50 2.00 127 130 1.50 135 1.00 138 0.50 140 0.00 145 1 2 3 4 5 6 7 8 9 10 11 12 Month of Year Pecent Occurrence of Wave Direction November (1980 - 1999) 12.0 127 130 10.0 135 138 140 8.0 Occurrence (%) 6.0 4.0 2.0 0.0 0 45 90 135 180 225 270 315 360 Direction (deg N)
- 52. We can use this to our advantage! WAVE DIRECTION & PERCENT OCCURRENCE Long Branch, NJ 40 35 30 Occurrence (%) 25 20 15 10 5 0 0 45 90 135 180 Direction (deg N)
- 53. Long Branch Feeder Beach
- 54. Shoreline Evolution • Elevation Change • Offshore contours are becoming parallel to shoreline • Deflation of feeder feature • 100,000 cu.yds. Per year transport to north
- 55. Long Term Change (Sea-level Rise)
- 56. Recent Sea Level Changes • 18,000 years ago, at the height of the last glaciation, sea level was 130 m lower than today. • Sea level continues to rise by about 1 foot per century in New York City. • A rise in sea level of up to a meter is predicted for the coming century.
- 57. Global Temp and Sea level changes for past 18,000 years based on radiometric age dating of corals
- 58. Modern Record Sea level rise about 30 cm/century speculation: greenhouse gas>>global warming >>melting ice NJ Tide Gauge Records 3.8 mm/yr in New Jersey
- 59. Falling Rising 1-3mm 1-3mm Rising Rising 1-3mm 3-5mm Falling 5+mm Rising 5-10mm
- 60. Tectonic Factors Affecting Sea Level Glacial Rebound
- 61. North American Glacial Rebound Rebound occurs much more slowly than ice melting. Even though the ice has been gone for 10,000 years, North America is still rebounding at 1 to 2 mm/yr.
- 62. What does the future hold? Projected sea-level rise for the 21st century. The projected range of global-averaged sea-level rise from the IPCC (2001) assessment report for the period 1990–2100 is shown by the lines and shading (the dark shading is the model average envelope for the range of greenhouse gas scenarios considered, the light shading is the envelope for all models and for the range of scenarios, and the outer lines include an allowance for an additional land-ice uncertainty). The AR4 IPCC projections (90% confidence limits) made in 2007 are shown by the bars plotted at 2095, the magenta bar is the range of model projections and the red bar is the extended range to allow for the potential but poorly quantified additional contribution from a dynamic response of the Greenland and Antarctic Ice Sheets to global warming.
- 64. What does this mean for the Coast? INCREASED FLOOD LEVELS Adjusted Flood Levels at Atlantic City StormMeas. Elev.1Surge2012221003 Sept. 1944 8.96 ft 4.17 ft 9.81 ft 11.43 ft March 1962 8.80 ft 3.43 ft 9.42 ft 11.05 ft Dec. 1992 9.14 ft 4.28 ft 9.39 ft 11.01 ft Oct. 1991 8.93 ft 4.48 ft 9.19 ft 10.81 ft 1. Relative to MLLW at Atlantic City 2. Adjusted for historic sea level rise of 3.8 mm/yr 3. Adjusted for IPPC Max Sea Level Rise Projection of 1.9 ft by 2100
- 65. What does this mean for the Coast? Change in MHHW Line 2100 1990 80 – 100 ft
- 66. Food for Thought ? May 4, 2012 Changes in the speed that ice travels in more than 200 outlet glaciers indicates that Greenland's contribution to rising sea level in the 21st century could be significantly less than the upper limits some scientists thought possible. The finding comes from a paper funded by the National Science Foundation (NSF) and NASA and published in today's journal Science.
- 67. Thank you Useful Data Sources: Waves: http://www.ndbc.noaa.gov/ Tides: http://tidesandcurrents.noaa.gov/ Stevens: http://hudson.dl.stevens-tech.edu/maritimeforecast/ Stevens Storm Surge: http://hudson.dl.stevens-tech.edu/SSWS/ NJ Beach Data: http://www.gannet.stockton.edu/crc/index.asp