Waves, Beaches & Coastal Erosion

1. Waves, Beaches & Coastal Erosion

2. Would You Live Here? Florida Keys

3. Waves • Transport energy over a body of water • Can cause coastal erosion

4. WavesGeneration • Generated by Force • wind or air • Generated at storm centers • Separation of waves by differing rates of travel - dispersion • Sea Wrinkles  ripples  wind waves  swells

5. WavesSize • Factors determining size: • Fetch • Distance wind blows over open ocean • Speed of wind • Wind velocity • Duration • Length of time wind blows

6. Wave BehaviorRefraction • Wave crests approach shore at an angle • Crest over shallow water slows; crest over deeper water does not • Therefore, Wave bends

7. Wave BehaviorBeach Erosion • Wave rays travel perpendicular to wave crest • Energy focused on headlands  erosion

8. Wave BehaviorLongshore Current and Transport • Current that moves sediment parallel to shore • Swash moves sediment onto beach at an angle • Backwash moves sediment down perpendicular to shore • Zig-zag motion has an overall net transport; longshore transport

9. Beach compartments in southern California Beach compartments include: Rivers Beaches Submarine canyons Figure 10-12

10. What is a beach? • Area along coast where sediment accumulates • Coastal zones begin when waves interact with seafloor

11. BeachesShoreline Divisions • Littoral zone • Part of coast that stretches from land plants to where waves start to break • Shoreface • Permanently covered by water • Foreshore • Exposed at low tide and covered at high tide • Backshore • above mean high tide – only affected by storms

12. Quartz wide Cobble Pink sand narrow sand Black sand Shell BeachesTypes • Shape and structure • Composition • Size • Color

13. summer winter BeachesSummer and Winter • Summer • Winter • Gentle waves • Erosive storm waves • Carry sand to shore but too weak to carry back to sea • Sand carried seaward to offshore sandbars • Wider beach results • Narrow beach

14. Sandy shore Rocky shore Beach DynamicsDeposition and Erosion • Deposition • Coasts build out by waves and currents • Erosion • Reshaping of coastline by wave action • Little incoming sand, usually no major river

15. BeachesDepositional Features • Longshore current deposits sand in a line downcurrent of a headland • Sand spits • Bay mouth bars • When a sand spit closes off a bay • Barrier island • Long sand island enclosing a bay or lagoon

16. North Carolina Barrier Islands • 15,000 years ago sea level was ~85 m below present • As sea level rose, a breach in ridge resulted in flooding • As sea level continues to rise, ridge migrates landward Galveston, Texas

17. Alabama Baymouth Bars • Bars are temporary structures • Beach through bar

18. New Zealand Sandspit • Depositional features are constantly migrating

19. Cliff Wave cut bench BeachesErosional Features • Wave cut bench • Cliff • Cliff bottom attacked by waves • Waves undercut cliff to form wave cut notch • Overhanging rock collapse • As cliff retreats, area of flat rock exposed

20. Wave Cut Terrace Santa Barbara, California

21. Beach ErosionCalifornia • Beach erosion is a serious problem for coastal communities • 86% of California coasts are classified as actively eroding • 1 in 4 homes within 500 feet of coast will be lost to erosion in 60 years • Some causes: • Sea level rise • Building near coast • Efforts to reduce erosion

22. Palos Verdes Peninsula Malibu, California

23. 2004 2002 2005 Beach ErosionCalifornia Santa Barbara, California

24. Sand Sources and Beach Supply Rivers and Streams In Southern California beaches are wider near river mouths Eroding beach cliffs – 5 – 10% of beach sand Generally sandstone and granitic cliffs are good sand sources Off-shore reefs may supply sand Sand from the seafloor immediately offshore Barrier islands of the Atlantic coast are examples

25. Threats to the Sand Supply Flood control dams They have virtually eliminated the sand supply to S. California beaches Ventura River sand supply is reduced to 66% of normal Southern California has 77 sand and gravel quarries in stream channels An annual sand and gravel production of 20 million tons Paved river channels – reduces channel widening Seawalls and riprap reduce cliff erosion and focus wave energy onto the beach

26. Coastal Erosion Erosion may be Seasonal Worst erosion may occur during severe storm Low pressure raises water level Waves stronger during storm A wave 10 feet high and 90 feet in wave length can exert 10 tons of pressure/m2 on a seacliff

27. Mechanism of Erosion Hydraulic impact – water driven into cracks and rock breaks Abrasion – sand and boulders abrade cliff Solution-- chemically dissolving rock Biological activity – animal borings, people

28. Factors Affecting How Much Erosion Occurs Natural factors Width of the beach Amount of wave energy Nature of the rock present

29. Factors Affecting How Much Erosion Occurs Humans Impacts Cliff tops are loaded with buildings Surface drainage is altered Roads, seawalls, etc. are constructed

30. Pacific Coast Erosion Sea cliff retreat rates vary from nearly zero to 9 feet per year Marine terraces in many places are eroding 1 foot/yr Erosion has caused some developments to establish a setback calculated to last 50 years. The setback serves as a common park area

31. Planning for Erosion Erosion must be incorporated into coastal land use plans Setbacks must be established Real estate agents must by law reveal problems to coastal buyers

32. Control of Coastal Erosion Riprap – large boulders at base of sea cliff Riprap should not be discontinuous Seawalls Accelerate erosion of adjacent property Groins – are built to block the longshore drift Breakwaters Reduce wave action on the coast Santa Monica breakwater requires continuous dredging Santa Barbara breakwater is attached to the coast and requires 350,000 M3/yr of dredging

33. BeachesHuman Impacts • Parallel concrete structures • Absorbs wave energy • Sea walls • Actually promotes erosion • Groins • Wall perpendicular to shore • Restore erosion • Erodes downcurrent side • Jetties • Designed to maintain navigation routes • Erosion and accretion • Breakwaters • Wall parallel to shore • Reduce wave energy • Interrupts beach drift

34. Tsunamis Cause erosion, destruction of property, loss of lives

35. Coastal Zone Management Federal Efforts Coastal Zone management Act of 1973 passed to support state efforts to manage coastal development Flood Insurance Act of 1973 Requires mapping and avoidance of coastal hazards Program has not worked well, owners rebuilding in sites of homes destroyed by storms State Efforts The Coastal Initiative passed in 1972 California Coastal Act established the Coastal Commission in 1976

37. Miami Beach after before BeachesHuman intervention • Beach nourishment • Import sand into coastal areas • Dredging • Underwater excavation to clear waterways

38. Let Nature Takes Its Course • Remove Artificial Barriers • Move houses away or back • Learn to live with nature not change it