Coastal Processes and Hazards

1. Coastal Processes and Hazards

2. Outline • Why is this important? • Definitions • How waves work • Interaction at shoreline • Importance of beaches • Human impacts on beaches

3. Why is this important? • 70% of Earth’s surface is water • Means a lot of coastlines • In U.S. - 30 coastal states • Projection by 2010 75% of population will live within 75 km of coastline • 1,358 people/coastal mile • High concentration of people and property!

4. Possible Hazards • Already covered • Hurricanes, tsunami, nor’easters • Discuss today • Waves, tides, erosion, sea level rise

5. Coastlines • Regions where land meets sea • Can be of different forms • Long sandy beaches • Rocky cliffs • Coral reefs

6. Sandy N. Carolina coastline Rocky coastline of Maine

7. Waves and Tides • Key forces that act to alter coastlines • Important for erosion, moving material along coast

8. How waves work • Caused by wind blowing over water surface • Transfer energy from air to water • 5-20 km/hr breeze: small (< 1cm high) ripples • 30+ km/hr: full size waves

9. Wave height • Depends on • Wind speed • Direction of wind blowing • Length of water over which wind is blowing • Consistency of wind direction

10. What is Water Doing? • Particle of water rotates in place with circular orbit • Orbit decreases in size with depth

11. Orbital Motion You probably have felt the same motion in the waves!

12. Wave Description • Wavelength (L) • Wave Height • Period (T) • Related to velocity of the wave • V=L/T • Typical T of few-20 sec, L of 6-600 m means V of 3-30 m/s

13. Swells • Interference of many sets of waves • Usually related to storms, multiple storms • Occasionally constructive interference occurs • Produce very large waves (rouge waves) • Can sink ships, may impact shorelines

14. Waves Near Coastlines • 1st: orbital motion changes to elliptical when depth is < 1/2 L • Why? Friction with bottom deep-water wave shallow-water wave

15. Waves Near Coastlines • 2nd: wave slows down, L gets smaller • Leads to more water, energy in shorter length; taller waves

16. Waves Near Coastlines • 3rd: at certain height (1:7 height to wavelength), wave is too steep and breaks • Topples forward, forms the bubbly, foamy stuff

17. Slope of Near-shore • Impacts wave breaks • If gently sloping bottom, waves break farther from shore • If steeply sloping bottom, waves break closer to shore • Rocky cliffs: break directly on rocks with large force

18. Wave Refraction • As waves get closer to shore, they bend to a direction roughly parallel to shore • Wave refraction, similar to light • Important for areas with bays and headlands • Headlands: water depth shallows quickly, waves slow and converge at this point • Bays: water in center is deeper, area is more protected

19. Bending of wave crests due to refraction as waves slow down in progressively more shallow water depths

20. Wave refraction concentrates energy at headlands, thereby causing increased erosion Wave refraction decreases energy at bays, thereby causing increased deposition

21. Longshore Drift • Waves arrive at small angle to shore, go up on beach at an angle • Moves sand grains (and people) at an angle • Very efficient at transporting sand to/from beaches

23. Beach • Shoreline made of sand/pebbles • Important for recreation, housing • Also as a natural barrier to absorb energy in breaking waves • Various beach processes affect how much beach is present during the year

24. General Beach Cycle • Summer • Generally fewer storms, lower wind speeds, waves with shorter L and height • Act to push offshore sand onshore, build wide, sandy beaches

25. Summer beach near San Diego, CA

26. Winter beach (same one) near San Diego, CA Note sandy beach is gone, due to large storm waves

27. General Beach Cycle • Winter • More energetic storms, waves erode beach sand, carry offshore • With less sand, energetic waves can attack coastal features such as roads, houses

28. Human Impacts • Humans like to live near the beach! • Nice climate • Great views • Additional food sources • Want to minimize risk from big waves, hurricanes, erosion of cliffs and beaches

29. Human Impacts • In order to mitigate hazards, we build • Seawalls • Dams • Groins • Jetties • Structures have multiple impacts

30. Dams • Dam rivers that add water, sand into ocean • Many built to provide freshwater reservoirs for coastal communities • Problem: sand in rivers adds to beach development. By cutting off this supply, adds to problem of shrinking beaches

31. Seawalls/Cliff Protection • Build structure for protection of beach or cliff • Changes beach dynamics • What happens? • Ocean waves break on wall because beach narrows • Steepens slope offshore, leads to larger waves • Can over time erode seawall or undercut base

32. Seawalls can cause beaches to disappear, construction of new seawalls over time Also, reduce attractiveness of coastline, property values

33. Seawall, coastal GA

34. Groins and Jetties • Elongate mass (usually rock or concrete) built perpendicular to shoreline • Purpose: keep sand on the beach • Problem: longshore drift still occurs • Leads to deposition on 1 side, erosion on other side

36. Jetty in NJ - note longshore drift is from left to right here Groin leads to deposition updrift, erosion downdrift

37. Beach Replenishment • Actively transporting sand onto a beach • Usually pump it from offshore • Can be very expensive ($millions/mile of beach) • May have to be repeated every year • Examples: Waikiki, HI; Miami, FL

39. 1981 Miami Beach

41. Next Time • Stream/River Processes