Outline for class: Tuesday, 19 March

Topic: Coastal Mobility: the Evidence

I. Announcements

• Voyage 6 / Drill 6 due Thursday, 21 March, 11 AM

II. Review of sea waves and swell waves

• Sea waves are short wavelength, "choppy" waves that are being generated by the wind. Sea waves form anywhere the wind is blowing, whether that be far out in the middle of the ocean, or near the coast.
• As waves move away from the area where they were generated, they evolve into smooth crested, long wavelength waves. Thus, all swell waves start out as sea waves, and all swell waves have traveled some distance from the place where they were generated.

III. Wind waves and currents of the North Pacific gyre

• Winds blowing on water generate waves and the large-scale oceanic currents that we studies several weeks ago. How do they compare?
• Problem 1 from Problem Set for Waves (class handout or available on class preb page for 14 March): Typical wind waves that arrive at our coast have periods of 10 sec and wavelengths of 150 m (while still in deep water). At what speed would we expect these waves to be travelling? They are travelling at 15 m/s, which is equal to 54 km/hr. When we calculated the speed of the oceanic currents (from motion of dumped Nike sneakers!), we found that they moved about 0.5 km/hr. Thus, waves can move across the Pacific Ocean in a few days, whereas currents take months to move across the ocean.
• Waves are caused by storm winds that are locally based and temporary in time. Waves are the energy from the wind moving through the water, but the water itself does not move (except in orbital circles). Currents are caused by prevailing winds that persistently blow from the same direction. Currents are the actual water itself moving through the ocean, which is a much slower process.

IV. Wave interference

• On the ocean there can be wind blowing over the water at different locations and generating waves that travel in different directions. As these different sets of waves travel across the ocean, they interfere with each other either constructively or destructively. When we watch waves arriving at a coast, all waves are not the same height. This is because we are seeing the results of both constructive and destructive interference. We often observe what are called wave sets; that is, waves of smaller height followed by waves of greater height.
• Constructive interference: where wave crests intersect, wave height is additive and wave height is increased.
• Destructive interference: where wave crests and wave troughs intersect, wave height is subtractive and wave height is descreased.
• Constructive interference has caused many disasters at sea and is responsible for so-called rogue waves or episodic waves. Where waves interfere constructively, a large wave can appear to "come out of nowhere", because the height of one wave group adds to the height of another wave group.
• "The Perfect Storm", a real event in the North Atlantic that was the subject of a book and subsequent movie, was called the perfect storm because it is the perfect meteorological conditions to make a large storm. Storms converged on the same location in the North Atlantic, creating huge waves that impacted coastal New England and that resulted in the loss of boats at sea. A great read!
• Question 3 from Problem Set for Waves. Waves from separate seas travel across the ocean and intersect with each other. If Sea A has wave heights of 2 m and Sea B has wave heights of 4 m, estimate the height of waves that would result from constructive interference and destructive interference. Illustrate your answers (see Figure 9.14 in the text). Answer: If there was perfect constructive interference, the resulting wave height would be 6 m. If there was perfect destructive interference, the resulting wave height would be 2 m.

V. Waves and Ocean Beach

• At Ocean Beach a large sand bar outside of the entrance to the bay causes waves to break far from shore (at least when the waves are large). This bar is at a right angl to the beach at about Taravel. Although Ocean Beach looks like a uniform beach but the nearshore and offshore bathymetry are actually rather complex.
• I showed a bathymetric map of the bathymetry offshore from Ocean Beach. Copies of these maps are available for students who are doing their field projects at Ocean Beach. We also looked at an example of the Scripps wave height model where the bar was visible. Students doing projects at the beach should go to the web site and get the offshore data for the day they do their project.
• Ocean Beach has longshore currents created when waves approach the beach at an angle, and it has very strong rip currents. The beach is usual because it is so close to the bay and experiences tidal currents that also generate currents parallel to the beach.

VI. Preview Voyage 6 diagrams---Voyage 6 / Drill 6 due Thursday

VII. Our dynamic coastline

• The configuration of coastlines changes daily, weekly, yearly, and over thousands of years, because of processes that operate there (for example: waves, tides, currents, tectonic activity, and sea-level changes)
• Coastal erosion: The natural evolution of a coastline involves erosion of cliffs and beaches. If we build structures too close to the water's edge, we do so at our own peril.

VIII. Local examples of coastal erosion

• During the 1997-98 El Niño winter, parts of the coastal cliff in Pacifica (~10 miles south of SF) eroded 10 meters and houses were removed from the cliff edge.
• Daly City landslide: a large landslide at a location several miles south of SFSU where the San Andreas fault cuts through a high coastal cliff. Houses were built there in the 50s and 60s prior to some of the current-day legislation. Many houses had already been removed because of erosion and 17 more were condemned and evacuated in 2000. These people will receive little compensation to their houses.
• South end of Ocean Beach (at the west end of Sloat Boulevard): Continual problems with cliff erosion here has destroyed parts of a recently-built parking lot and threatens the water treatment plant located on the east side of the Great Highway. Piles of sand have been placed against the cliff face to slow the rate of cliff erosion.
• We must recognize that humans create the problem when they build structures too close to the coast where the natural processes of wave and tidal action occur. Laws now prevent most building in these dangerous zones, but problems still arise from past development. Structures like sea walls and sand replenishment are temporary measures. The most sane approach is to buy out property owners and convert areas to park land. However, local governments often are strapped for funds and must depend on the federal government. For example, in Daly City, the local government is requesting federal funding to buy out property owners living on the Mussel Rock landslide. The maximum payment, though, is likely to be only $140,000, not nearly what the houses are worth on stable land.

IX. Slide sequence to illustrate (will be completed on thursday): (1) How has sea level changed during the past 20,000 years; (2) how old is San Francisco Bay (3) What are the primary differences between the U.S. east coast (a passive continental margin) and the U.S. west coast (an active continental margin); (4) more about human interactions with the coast.

• The coastal environment is a good place to look at earth's systems; that is, how geology, meteorology, and other elements interact. We will look at some examples.
• World-wide sea level changes: increases and decreases in the amount of glaciers on land is the primary reason that sea level has changed during the recent past. During the past 20,000 years, global sea level has risen as glaciers melted and the water that was held as ice on land returned to the sea.
• We know that there were glaciers in places like the Sierra Nevada because of U-shaped valleys such as Yosemite Valley and because of rocks that were polished and striated as glaciers moved over them.
• As sea level has risen during the past 20,000 years, San Francisco Bay was created. About 10,000 years ago sea water began to enter the lowland area we now know as the Bay. Thus, SF Bay is a young feature, geologically speaking. Sea level is now rising at a very slow rate (a few mm/yr), but if that rate increases because of global warming and accelerated melting of glaciers, the low areas around San Francisco Bay would be flooded by marine water.
• Active and passive continental margins: active margins are located at plate boundaries; passive margins are not located at plate boundaries. The U.S. West coast is an example of an active margin and the East coast is an example of a passive margin.
• On the East coast, the topography is flat and gently sloping and sea level rise can flood large areas. Much development has occured right at the coast and engineering techniques are required to protect them. Sometimes the engineering "fixes" last for many years; sometimes for less than one year.
• On the West coast, there is higher topography near the coast because of faulting associated with the transform plate boundary located here. Evidence for uplift are flat terraces that rise from the beach. These terraces were formed as flat "wave-cut platforms" by waves at beach level. With time there are uplifted and removed from wave action. A new platform is then formed at the beach. For example, many terrace levels bevel the western edge of the Santa Cruz Mountains south of San Francisco.

X. Question for 3x5 card: What is the difference between an active continental margin and a passive continental margin (if not in class, see pages 220-221 in the textbook).

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