Chapter 12: Coasts (after a brief review of Tides) 1
Questions from previous classes: What happens when a wave meets a current? wave = people walking current = bus If wave goes with the current, the wave will go faster and it will stretch out (wavelength is greater with higher speed). If going in opposite direction, wave can break (like a person tripping). Example: Columbia River + ocean It gets complicated when waves come from angle, but basically waves elongate or shorten. Energy and momentum are conserved. Question about springs/geysers? Waves are a transfer of energy. 2
Is this image showing spring tides or neap tides? Neap tides: High tides are reduced when Sun and Moon at right angles. This also means that the low tides are the highest low tides. 3
Earth s motion creates opposing dome on the other side, away from the Moon. Center of mass Moon Moon attracts ocean The Earth and Moon act as one system that has a single center of mass. When the Earth rotates around that center of mass, it creates a dome on the opposite side of the Moon. 4
Combined result: Moon 5
Why? Diurnal tides 1 high tide in 24 hrs. Semidiurnal tides 2 high tides in 24 hrs. Mixed tides Combination 6
Why? Diurnal tides 1 high tide in 24 hrs. Semidiurnal tides 2 high tides in 24 hrs. Mixed tides Combination A detailed analysis of all the variables that affect the tides at any particular coast reveals that nearly 400 factors are involved a completely mathematical model of the tides is beyond the limits of marine science Successful models must take into account at least 37 independent factors related to tides (the two most important are the Moon and the Sun) - Essentials of Oceanography textbook 7
Why? Diurnal tides 1 high tide in 24 hrs. positions of continents depth of the ocean (remember, a tide is a type of wave) coastline shape Semidiurnal tides 2 high tides in 24 hrs. Mixed tides Combination Example: Diurnal tides are common in shallow inland seas. 8
Amphidromic points: where no tides occur Notice how at each of these places the surrounding color the tidal force for that region is blue, indicating little or no apparent tide. These convergent areas are called amphidromic points. Tide waves move around these points, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. 9
Amphidromic points: where no tides occur What do you think is happening near the North Pole? Red = High tides are the highest, Low tides are the lowest. Why? The two domes of water on either side of the globe cannot really exist this way, due to the rotation of the Earth. Instead, the domes are broken up into several tidal cells that rotate around an amphidromic point. 10
Coasts Classified by processes shaping them such as? Affected by: 1) Waves (primary) 2) Volcanoes 3) Sea levels 4) River transport 5) Tectonic activity 11
Coasts can also be classified by the amount of energy they experience. (Energy from what?) WAVES 12
Coasts can also be classified by the amount of energy they experience. (Energy from what?) WAVES Classify the coasts or coastal features you see on the next slides: high energy? or low energy? 13
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Coasts - balance between creation & destruction deposition erosion 20
Learning Goals: Coasts A coast s location depends on: Movement of the Earth s plates and how this impacts the crust The ocean s water volume (in other words, sea level) A coast s shape depends on: Wave energy Whether the plates are moving up or down (remember: isostasy) How erosion is impacting the land How sediments are moved along the shoreline Biological activity (examples: coral reefs and mangroves) Two major types of coasts: 1) Erosional 2) Depositional Humans try to fix coasts, but tend to cause more erosion, rather than less erosion. 21
High energy coasts larger waves caused by continuous storms. 22
Features of High Energy Coasts 23
Where is most of the erosion happening in this picture? 24
Ocean wave refraction erodes protruding regions first. 25
Low energy coast - sediments accumulate on shore. 26
Low Energy Beaches 27
Low Energy Coasts Berms sediment accumulation by wave action Beach scarp wall from furthest berm caused by highest tide Bars sediment that accumulates offshore 28
Rip currents When waves enter faster then they can leave. A narrow channel of outflowing water forms. 30
Summer vs Winter Conditions 31
La Jolla, California Summer Conditions 32
La Jolla, California Winter Conditions 33
Tectonic Activity 34
This beach was created by tectonic activity, and it s in Alaska. What do you think created it? 35
This former seafloor at Prince William Sound, Alaska, was raised 3.5 meters (12 feet) above sea level by tectonic uplift during the great earthquake of 27 March 1964. The exposed surface, which slopes gently from the base of the sea cliffs to the water, is about 400 meters (14 mile) wide. The light-colored coating on the rocks consists mainly of the dried remains of small marine organisms. 36
Longshore drift - movement of sediment along a shore by wave action. 37
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Longshore currents can create: Sand spit sand deposition downstream of a headland Bay mouth bar sand spits close off a bay Barrier island sand deposition apart from mainland Tombolo sand creating bridge to off-shore feature 39
Coastal sediment transport cells 40
A bay mouth bar. The inlet is now closed, but increased river flow (from inland rainfall) or large waves combined with very high tides could break the bar. For an indication of scale, note the freeway bridges at the top of the photograph. 41
Barrier islands off the North Carolina Coast. 42
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Humans try to protect coastlines, but generally end up causing more erosion. 44
Groins (jetties) manmade structures designed to limit longshore drift. 45
The Story of the Cape Hatteras Lighthouse: Outer Banks of North Carolina When completed in 1870, the Cape Hatteras lighthouse was located a safe 1,500 feet from the ocean. Even then, however, storm-driven tides completely washed over Hatteras Island, eroding sand from the ocean side of the island and depositing it on the sound side. By 1970, this process, which has caused the gradual westward migration of the Outer Banks for at least the past 10,000 years, left the lighthouse just 120 feet from the ocean s edge and almost certain destruction. 46
The Cape Hatteras lighthouse was moved in 1999. http://www.nps.gov/caha/learn/hi storyculture/movingthelighthouse.htm 47
1930s: Coast Guard installed the first sheetpile "groins" (walls built perpendicular to the shore) to try to protect the tower. 1936: They let the lighthouse get washed over by the ocean and just moved its light to a skeleton steel tower 1960s and 1970s: various attempts to "stabilize" the coast included beach nourishment and three new groins installed north of the lighthouse. 1980: A severe storm in 1980 accentuated the island's westward movement washing away the foundation of the first (1803) lighthouse, which had been 600 feet south of the existing lighthouse. In 1803, that lighthouse had been one mile from the shoreline. In 1980, the National Park Service began a three-year process for long-term protection: - Relocation was considered but quickly discounted as impractical. - The option finally selected was a concrete and steel seawall revetment. - The technology to move large structures improved. 48
The National Academy of Science and North Carolina State University Investigated. NCSU independently reviewed the National Academy of Sciences report and then issued its own report, Saving the Cape Hatteras Lighthouse from the Sea, in January 1997. It not only endorsed the National Academy of Sciences findings, but also recommended that the National Park Service proceed as soon as possible with its present plans to obtain the financial resources necessary to preserve the lighthouse by moving it. Funding was appropriated by Congress to move the lighthouse. 49
Mangroves: nature s way to reduce erosion. Stopped here on 3/23 50
Deltas 51
Mississippi Delta Delta sediment deposits at river mouth. 52
Deltas can only form if sediment deposit is greater than removal. 53
Atolls Kayangel Atoll in Palau, in the tropical Pacific. 54
Atoll formation Fringing reef corals that cling to land. Barrier reef as land subsides or erodes away, lagoon separates reef and land. Atoll - When land completely subsides, ring-shaped island is left. 55
Moorea 56
Great Barrier Reef, Australia Largest barrier reef 57
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Estuaries 59
Types of Estuaries Estuaries water surrounded by land, exposed to both fresh and salt water. Very productive regions. 60
Types of Estuaries a. Drowned river mouth sea level rise floods inland rivers. Chesapeake Bay (largest estuary in the U.S.) 61
Another drowned river valley: Sydney Harbour, Australia Types of Estuaries 62
Types of Estuaries b. Fjords valleys cut by glaciers. Alaska Puget Sound, WA (second largest estuary in the U.S.) 63
In fact, the movement of glaciers have carved many water bodies and islands: example 1 64
In fact, the movement of glaciers have carved many water bodies and islands: example 2 Martha s Vineyard terminal moraine. 65
Types of Estuaries c. Bar-built http://dep.state.fl.us/coastal/habitats/estuaries.htm 66
Where is this and what is it called? Miami Beach Barrier Island 67
Types of Estuaries d. Tectonic A characteristically straight fault coast at Tomales Bay, California. Point Reyes is visible to the left (west); the city of San Francisco is out of view to the south. The San Andreas Fault trace disappears below sea level in the bay (arrow); the straight sides of the bay closely parallel the submerged fault. 68
The southeastern coast of the US, ~18,000 years ago Question for next class: What are the primary factors that will influence the coastline in Miami, over the coming years and decades? 69
Most important vocabulary terms & processes 1. Coastal processes are impacted by waves (primarily), volcanoes, sea levels, river transport, and tectonic activity. 2. Difference between erosion and deposition 3. Shapes of high energy coasts, compared to low energy coasts. 4. Longshore drift, longshore current and longshore transport 5. Ways that humans have modified coastlines, compared to natural reinforcement groin, seawall, beach nourishment vs. protection from mangrove forests, coral reefs, etc 6. Deltas, estuaries, atolls, moraine 70