Introduction to Earth Science

Chapter 16 - Ocean Processes & Shorelines

16.1
According to a study by the United Nations, about 40 percent of the people around the planet live within about 60 miles of a coastline. This chapter is a summary of oceanographic processes that ultimately influence our planet's regional climates and coastlines. Factors discussed include the nature of seawater, waves, currents, tides, and erosional processes that influence features along coastlines. Landforms along coastlines are influenced by both the erosion and deposition of sediments and by the movement of sediments along coastlines by wave and current actions. Sunset on the Pacific Ocean. Fig. 16-1. The Sun's energy ultimately influences almost all things oceanographic.
16.2

Distribution of Water On Earth

97.2% in the world ocean
2.15% frozen in glaciers and ice caps
0.62% in groundwater and soil moisture
0.02% in streams and lakes
0.001% as water vapor in the atmosphere

Seawater is the most abundant resource on Earth! Seawater has evolved to what it is over the billions of years that oceans have existed on Earth. Seawater has gained its salt (salinity) from erosion from land, volcanism, and rock-water interactions on the seafloor.
New land forming along the coast in Hawaii.
Figure 16-2. New land forming and eroding on a Hawaii volcano.
16.3

Components of Seawater

Seawater is composed of:
Water
Dissolved matter: solids and gas (as ions)
Suspended matter (dust and organic residues)

Figure 16-3 illustrates the elemental components of salts dissolved in seawater.
Ions in seawater
• Cl - 55%
• Na - 30.4%
• SO4 - 7.6%
• Mg - 3.9%
• Ca - 1.2%
• K - 1.1%
• all other dissolved components - <1 %

 
Seawater components
Fig. 16-3. Components of seawater.
16.4

Salinity

Salinity is a measure of the total amount of solid material (salts) dissolved in water, defined as:



Weight (mass) of salt
—————————— = Salinity
Weight (mass) of water


Units are described as:
% is part per hundred (pph)
is parts per thousand (ppt)


• Open ocean seawater ranges is 33 to 37 ppt.
Average is about 35 ‰ (ppt).
Salinity variations in the environment
Fig. 16-4.
Variations in salinity in the natural environment depends on the mixing of fresh water with seawater, or the evaporation and concentration of seawater.
16.5

Salinity is Measured by:
• electrical conductance (higher salinity = higher conductivity)
• density (higher salinity = higher density)

At room temperature, water can dissolve about 30% of its weight in salt (NaCl). Hot water can hold about 40%.

Fresh water (including drinking water) is typically less than 1 ‰ (ppt).
Average seawater is typically about 35 ‰ (ppt).
Water that contains salt content between fresh and seawater is called brackish.
A brine is water that is concentrated with higher levels of salt than seawater.

The mixing of fresh water from streams or from precipitation reduces salinity, whereas, evaporation concentrates seawater. When evaporation concentrates salty water to it saturation point, salt crystals will precipitate (Figures 16-3 to 16-6).

Examples:
• Spring 0.3 ‰ (ppt) (fresh)
• Tap water 0.7 ‰ (ppt) (fresh)
• Limit on agriculture irrigation - 2 ‰ (ppt)
• Baltic Sea 10 ‰ (ppt) (brackish)
• Black Sea - 18 ‰ (ppt) (brackish)
• Average Ocean -35 ‰ (ppt)
• Mediterranean Sea - 38 ‰ (ppt)
• Red Sea 42 ‰ (ppt) (hypersaline)
• Great Salt Lake 280 ‰ (ppt) (hypersaline)
• Dead Sea 330 ‰ (ppt) (hypersaline)
Salt evaportation ponds near the Dead Sea in Jordon.
Fig. 16-5.
Evaporation ponds constructed near the Dead Sea in Jordan are developed to manufacture salt.
16.6

Evaporation of seawater results in precipitation of mineral salts

As seawater evaporates in a restricted basin, seawater is concentrated becoming a brine. As evaporation proceeds various mineral salts will precipitate out in the reverse order of their solubility. Salty sedimentary deposits produced by evaporation are called evaporites. The first to precipitate is calcite (if not consumed by organisms first). Next come CaSO4 (gypsum and anhydrite varieties). This is followed by salt (NaCl) (mineral name: halite; rock name: rock salt). By volume, NaCl is the most abundant salt from seawater. The last to precipitate are potassium salts (sylvite: KCl and others) and magnesium salts (epson salt; MgSO4 and others). Various other trace salt compounds are concentrated in the last of the brine to evaporate. About 80 different salt minerals have been reportedly found in evaporite deposits.
Order of precipitation of salts from seawaterFig. 16-6. Order that salts precipitate from seawater as evaporation proceeds. Salts being concentrated by evaporation in Death Valley, California
Fig. 16-7. Salts forming from evaporation in Death Valley, California.
16.7

Formation of Sea Ice

Fresh water freezes at 32° Fahrenheit (0° Celsius). Seawater freezes at about 28.4° F (-2 ° C) because of the salt content. However, when seawater freezes, the ice that forms contains very little salt. Only the water part freezes, the remaining salt is concentrated as brine that separates from the sea ice. This process is very important for deep-sea circulation.
Sea ice forms in the Arctic and Antarctic region, expanding and thickening during winter months, and melting back during summer months (Figure 16-8). The volume and extent of sea ice is a grave concern because it is disappearing at a rapid rate—attributed to global warming and associated climate change. See a NASA video that shows the changing extent of sea ice from 1979 to 2016: Annual Arctic Sea Ice Minimum 1979-2016 with Area Graph
Sea Ice in the Arctic region on March 7, 2017
Fig. 16-8.
Extent of sea ice in the Arctic Ocean, March 7, 2017.
16.8

Relationship of Salinity, Density and Temperature

Circulation of deep ocean currents is driven by density differences in seawater controlled by the temperature and slight variations in salinity. Cold water is denser than warm water, and salty water is denser than freshwater. Assuming a closed system:

Temperature and Density: Inverse (as temperature increases, density decreases)
Salinity and Density: Proportional (as salinity increases, density increases)
Temperature and Salinity: None (as temperature changes, salinity remains the same)

Figures 16-9 and 16-10 illustrate variations in sea surface salinity and temperatures of the world's oceans.
Sea Surface Salinity
Fig. 16-9.
Sea Surface
Salinity
Average 2005
Average Sea Surface Temperature
Fig. 16-10. Sea Surface
Temperature
Average 2005
16.9

Animations of Satellite and Surface Oceans Data

These websites provide animations that provide stunning views of the Earth created from satellite sensor data. These data are the combined efforts of ocean and atmosphere scientists from many organizations.
NASA animations: Average Sea Surface Temperature, Salinity and Density
(This website has links to animations on a globe and a mercator map.) Animations include:
* Average Sea Surface Temperature (SST )
* Average Sea Surface Salinity (SSS)
* Average Sea Surface Density (SSD)
NASA Animations: (mercator) 1 year: December 2011 through December 2012 showing:
Global Sea Surface Salinity
Salinity and ocean circulation
Salinity and global seawater migration
NOAA animations: Sea Surface Temperature (World mercator map, last 6 months)
Monthly Isopycnal & Mixed-layer Ocean Climatology (MIMOC): Animations show pycnocline (temperature and salinity) data of different parts of the world.
The NASA animations provide views of how salinity, temperature, and density change over the course of a year. Sea Surface Salinity Average 2009
Fig. 16-11. Sea Surface Salinity
Average 2009
Sea Surface Temperature 2009
Fig. 16-12. Sea Surface Temperature
Average 2009
Sea Surface Density Average 2009
Fig. 16-13. Sea Surface Density
Average 2009
16.10

Salinity and Latitude

Figure 16-14 is a map of the globe comparing the rates of evaporation and precipitation. The map is a compilation of evaporation minus precipitation (E-P) values. The data basically shows the regions where there is a net gain of salinity created in surface waters by high evaporation rates. There is also a net loss of salinity where precipitation is higher than evaporation rates (Figure 16-15). In general:

• The tropics (equatorial region) is humid and cloudy, and receives much more rain than evaporates.
• The temperate regions receive less precipitation, so evaporation dominates.
• The polar regions have low evaporation rates relative to the amount of precipitation they receive.
Map showing the difference betwen average evaporation an precipitation daily, world
Fig. 16-14. Map of net evaporation minus precipitation (E-P) on oceans.
16.11
Variability of ocean salinity: Ocean salinity is stable at depth but can be highly variable at the surface. The upper surface layers of the ocean impacted by wave energy is a mixing zone. Simply this: the more waves, the more mixing. Freshwater is less dense than seawater and without mixing freshwater will float (stratify) on top of seawater.
Evaporation and Precipitation curves by latitude
Fig. 16-15. Evaporation and precipitation curves compared with latitude.
Factors that decrease salinity:
Factors that increase salinity:
Precipitation
Runoff
Melting icebergs/sea ice
Evaporation
Freezing sea ice

16.12

The "Cline Curves"

Changes in temperature, salinity, and density with depth

• A Thermocline is a steep temperature gradient in a body of water marked by a layer above and below which the water is at different temperatures (Figure 16-16).
• A Halocline is a vertical zone in the oceanic water column in which salinity changes rapidly with depth.
• A Pycnocline is a layer in an ocean or other body of water in which water density increases rapidly with depth (may be a combination of the effects of temperature, salinity, or sediment suspended in the water).


Thermoclines significantly influence biological productivity in ocean environments.

Thermoclines form from solar heating of the ocean surface.
• As surface waters warm, they become isolated because they are lower density than the colder water below.
• A blanket of warm surface waters will thicken with seasonal heating and may eventually reduce or prevent surface waters from mixing with more nutrient-rich colder waters at depth.
• Warm surface waters above the thermocline can become depleted of nutrients essential for biological productivity in the oceans.
• Regions with thick thermoclines support less sea life than colder water regions with little or no thermocline.

NOAA Animation: Annual changes of pycnocline depth

Clne curves
Fig. 16-16. The "cline curves" - illustration showing the relationship of thermoclines, haloclines, and pycnoclines and water depth.
16.13

The Mixing Zone (Surface Waters)

Uppermost water where mixing from currents make temperature, salinity, and density mostly constant. The mixing is a result of surface waters by wave action and wind-driven currents (Figure 16-17).

Vertical changes in temperature with latitude:
Polar regions have almost no thermocline.
Temperate regions have weak thermoclines (moderate in summer, less in winter).
Tropical regions have a strong thermocline.

Mixed Layer above the thermoline and deep water.
Fig. 16-17. Mixing Zone
16.14

The Coriolis Effect on Atmospheric and Ocean Circulation Systems

Heat from "insolation" (INcoming SOLar radiATION) is the driving force behind the fluid motion of the atmosphere and the oceans. However, the patterns of motion are also influenced by the forces created by the rotation of the Earth on its axis. Because air and seawater have mass, they maintain momentum when moving from a location of high pressure to low pressure. However, because the Earth is rotating, this rotation causes an observable deflection in both the fluid atmosphere and oceans (Figures 16-18).

Moving water (like wind) are influenced by the coriolis effect:
Moving water is deflected to the RIGHT in the Northern Hemisphere.
Moving water is deflected to the LEFT in the Southern Hemisphere.
• The greatest impact of the coriolis effect is near the poles (north and south).
• There is no coriolis effect along the equator.

The global atmospheric circulation system influences the movement of air masses in general "belts" that move air in rotating masses within zones around the planet (Figure 16-19). These relatively stationary wind belts impact the surface of the oceans, creating currents that circulate waters in the oceans, creating five large gyres (Figure 16-20) and lesser rotational currents in other regions (such as in Figure 16-21 for the Pacific Ocean).
Coriolis Effect
Fig. 16-18.
The coriolis effect is cause by the rotation of the Earth on its axis. This rotation causes air masses moving from high to low pressure to deflect.
16.15

11. The Global Atmosphere and Ocean Systems Are Linked

The global atmospheric circulation system influences the movement of air masses in general "belts" that move air in rotating masses within zones around the planet (Figure 16-19). These relatively stationary wind belts impact the surface of the oceans, creating currents that circulate waters in the oceans under the influence of coriolis effect, creating five large subtropical gyres encircling the major oceans basins (Figure 16-20).

Currents in the oceans include surface currents and deep currents:
surface currents are driven horizontally by effects of the wind.
deep currents are driven horizontally and vertically by differences in density
(density changes typically start near the surface).

Ocean circulation is also influenced by seawater temperature and density.
Warm water in the tropics flows in currents to polar regions where it cools and the formation of sea ice concentrates the salt in seawater, increasing its density so that it sinks.
Cold and salty water (concentrated by surface evaporation) sinks. Elsewhere seawater rises where it is displaced by colder and saltier water.

World wind zones
Fig. 16-19.
Global wind circulation patterns impact regional climates and drive the large surface currents in the global ocean circulation system.
16.16

12. Surface Currents

Surface Currents involve large masses of water moving horizontally on the surface (Figure 16-21 to 16-23).
Wind is the driving force behind surface currents, however:

The transfer of wind energy to water is not very efficient
(only about 2% energy transfer of “friction” between water and air).
Wind produces both waves and currents (more on waves below).

Surface currents occur in the mixing zone within and above the pycnocline (layer of rapidly changing density).
Effects of surface currents is to redistribute heat from equatorial to polar regions.

Gyres in the global ocean circulation system
Fig. 16-20.
Five large gyres circulate surface waters in the global oceans. These rotating subtropical gyres are influenced by the patterns of atmospheric winds and the coriolis effect.
16.17
Major ocean currents of the world

Fig. 16-21. Major surface currents of the oceans. See
Oceans Currents Map
(NOAA-NWS)
Map of the earth showing global ocean current patterns. .
Fig. 16-22.
Map of the Earth showing global ocean currents based on both satellite and surface data collection.
Current speeds (cm/second)
Fig. 16-23.
Speed of currents measured by drifting devices (annual average in cm/sec)
16.18
Mechanisms moving surface currents include:

Wind: major mechanism (result of atmospheric circulation patterns).
Solar heating: (direct heating by the sun) - a minor mechanism (influences surface waters but not water at depth).
Tides: (affect currents in coastal regions - tidal currents are discussed below).
Geography: locations of continents (and islands) influence direction and flow currents (acting as barriers to flow).

Subtropical gyres are large system of rotating ocean surface currents driven by global wind currents with the influence of Ekman Transport (see below) and continental geography (land masses restrict and deflect the flow of water currents).

16.19

13. Deep-Ocean Thermohaline Circulation

Ocean circulation is also influenced by seawater temperature and density. Cold and salty water (concentrated by surface evaporation) sinks. Elsewhere seawater rises where it is displaced by colder and saltier (denser) water (Figure 16-24). Warm water in the tropics flows in currents to polar regions where it cools. In the Arctic region, the formation of sea ice concentrates the salt in seawater, increasing its density so that it sinks. Cold, salty water sinks both in the Arctic and Antarctic regions, feeding deep ocean circulation. The differences in temperature and salinity (or overall density) is the driving force behind deep-ocean thermohaline circulation.

The deep ocean basins have slow moving currents (compared with the surface waters exposed to atmospheric winds. As currents move about the globe, evaporation increases salinity. Increased salinity combined with cooling increases seawater density, allowing affected seawater to sink into the deep ocean. The movement of surface waters downward supplies oxygen to the seabed, assisting in the decay of organic matter. The deep, slow-moving water picks up nutrients from the seafloor and from decaying organic particles sinking through the water column. In locations where deep-water upwells to the surface, these nutrients supply the ingredients for phytoplankton blooms, providing food for the food chain.

Animations: global perspectives of ocean currents based on salinity and temperature
Surface Salinity (annual) (NASA)
Perpetual Ocean (2005-2007) [NASA] global ocean circulation time lapse - YouTube video.
22 Years Sea Surface Temperature 1985-2007 [NOAA Polar satellite data] YouTube video
Worldwide Sea Surface Temperature simulation 2008 YouTube video

Thermohailine Ciruclation
Fig. 16-24
. Thermohaline Circulation: cold and salty ocean water is dense and sinks, warm water stays at the surface until it cools and "dries" (increasing salinity) before it can sink.
16.20

14. Sea Ice and Thermohaline Circulation

Glaciers flowing into the ocean contribute large amounts of iceberg and sea ice to the polar ocean regions (Figure 16-25). However, sea ice also forms where very cold air is in contact with the ocean surface. Currents in the upper sea (mixing zone) can inhibit the formation of sea ice. Water is most dense slightly above the freezing point and tends to sink whereas ice floats. Once sea ice starts to form the salt is either expelled back into the seawater and some is concentrated in microscopic pockets trapped in the sea ice. Antarctic sea ice is typically 1 to 2 meters (3 to 6 feet) whereas most of sea ice in the Arctic is 2 to 3 meters (6 to 9 feet) thick. However, in some Arctic regions sea ice can grow to 4 to 5 meters (12 to 15 feet) thick. The formation of sea increases the salinity of the seawater, and the combination of the increased salinity and cold water results in the formation of dense water that sinks into the deep ocean, driving the thermohaline circulation through the world’s deep ocean basins.

Arctic Sea Ice
time lapse from 1987-2009 (NOAA/NSIDC satellite data YouTube video)

Origin of glaciers, icebergs and sea ice
Fig. 16-25. Origin of glaciers, icebergs, and sea ice. Sheets of sea ice form and melt back with the seasons.
16.21

15. The Coriolis Effect, Ekman Spiral, and Ekman Transport

The coriolis effect has a large influence on the movement of both surface water and deeper water. However, wind-driven currents move fastest near the ocean surface and diminish with depth. The difference in rate of movement results in a rotational process called Ekman transport.

Early sailors traveling in regions where icebergs are common noticed that the icebergs moved in a different direction than the wind (causing alarm as the icebergs were cutting across the paths of ships moving down wind) (Figure 16-26).

Walfrid Ekman (1874-1954, a Swedish physicist) resolved the problem of why wind currents and water currents were not the same. The force of wind affect surface water molecules, which in turn, “drag” (by “friction”) deeper layers of water molecules below them.

The deeper below the surface, the slower the water moves compared to the water layer above it.

Surface movement ceases at a depth of about 100 meters (330 feet).

As noted above, both surface water and deeper water is deflected by the coriolis effect.
—90° to the right in the Northern Hemisphere
—90° to the left in the Southern Hemisphere.

Depth is important: Each successively deeper layer of water moves more slowly to the right (or left), creating a spiral effect (called the Ekman Spiral, Figure 16-27). Because the deeper layers of water move more slowly than the shallower layers, they tend to “twist around” and flow opposite to the surface current. Net result is that net transport in surface currents is 90° from wind.

This twisting character of ocean surface waters is called the Ekman spiral. The impact of the Ekman Spiral is enhanced where geographic features create barriers to the movement of water. Ekman transport is the net motion of a fluid (seawater) as the result of a balance between the coriolis effect and turbulent drag forces (within surface waters and geographic features (shoreline and seabed).
Iceberg and a NOAA ship near Antarctica
Fig. 16-26.
Sailors of ships noticed that icebergs move in a different direction than the wind.
Ekman spiral
Fig. 16-27.
The Ekman spiral illustrated. The wind may move surface waters in the direction of the wind, but the coriolis effect impacts deeper waters, creating an overall migration of the upper waters in a deflected direction.
16.22

16. Boundary Currents

Boundary currents currents associated with gyres flow around the periphery of an ocean basin.
Boundary currents are well illustrated in Figures 16-21 to 16-23.

• Boundary currents are ocean currents with dynamics determined by the presence of a coastline.

Two distinct categories of boundary currents include:
western boundary currents
eastern boundary currents.

16.23

17. Western Intensification of Boundary Currents

Wind blows westward along the inter tropical convergence zone at the equator, causing western intensification (see Figures 16-21 to 16-23). The combination of winds, coriolis effect, and geography result in western intensification:

• Wind blowing across the oceans "mounds" water on the western side of ocean basins-up to 2 m.
• The mounding of water is caused by converging equatorial flow and surface winds.
• The coriolis effect is most intense in polar regions, so current flowing eastward near the poles is more dissipated than currents flowing westward at the equator.
• The higher side of a "mound" is on the western side of the ocean basins, having a steeper "slope" and are therefore faster moving currents.

Western boundary currents (WBC) are fast (km/hr), narrow (<100 km), and deep (up to 2 km)
Examples: Gulf Stream, Brazil, Kuroshio, E. Australian, Agulhas.
WBCs form along the "warm, wet" west side of ocean basins.

Eastern boundary currents (EBC) are slow (km/day), wide (>1000 km), and shallow (<1/2 km)
Examples: Canary, California, Benguela, Peru (The California Current is illustrated in Figure 16-28).
EBCs form along the "cool, dry" east side of ocean basins.
Pacific Circulation System
Fig. 16-28. The California Current is an eastern boundary current; part of the Northern Pacific Gyre. The Kuroshio Current near Japan is a western boundary current.
16.24

18. Meandering and Eddy Currents

Gyres and boundary currents are large scale, but are also complex. The combination of currents, winds, tides, and geography with the impact of the coriolis effect create complex and often changing patterns in surface currents. Boundary currents change constantly (called meandering) producing spinning cone-shaped masses of water—spinning off of larger boundary currents.
Rotation caused by the coriolis effect create swirling eddies and meandering movements in surface currents (see Figures 16-21 to 16-23).

Satellite temperature data of the ocean surface reveals the spreading and mixing of surface waters as currents move from one region to another, gaining intensity and dispersing energy as they move (Figure 16-29). Under the influence of the coriolis effect, currents move, meander, and locally spiral into "rings" that may persist for weeks, months, even years. The temperature data reveals large spinning eddies and ring in portions of the ocean basins along the margins of major currents.
Eddie currents in the South Atlantic Ocean
Fig. 16-29. Eddies and meandering currents in the South Atlantic Ocean (as viewed from satellite ocean temperature data).
16.25

19. The Gulf Stream Current

The Gulf Stream is a fast moving ocean current (Figure 16-30).
• The North Equatorial Current moves east across the Atlantic Ocean in the Northern Hemisphere.
• This flow splits into the Antilles Current (east of the West Indies) and the Caribbean Current (around the Gulf of Mexico).
• These currents merge into the Florida Current. (~30-50 miles wide, moving 2-6 mph, a mile deep).
• Along the East Coast, the Gulf Stream experiences "western intensification."
• North of Cape Hatteras (NC) the current moves away from the coast and gradually looses much of its intensity (by meandering) producing numerous warm and cold core rings.
• The Gulf Stream gradually merges eastward with the water of the Sargasso Sea, the rotating center of the North Atlantic Gyre (named for floating marine alga (seaweed) called Sargassum that accumulates in the stagnant waters.
• For comparison, the volume of water moved by the Gulf Stream is about 100 times all the world's rivers combined!
Gulf Stream

Fig. 16-30. The Gulf Stream is the world's largest ocean current (revealed here by water temperature patterns)
16.26

Antarctic Circumpolar Current

• The Antarctic Circumpolar Current is the only current to completely encircle Earth (Figure 16-31).
• The current moves more water than any other current.
• The current is in a region of the world with intense winds and wave action.
• The region has lots of upwelling - very "rich" ocean basin (nutrients for plankton; food for higher-level feeders)

Antarctic Circumpolar Current (NOAA website); also see an animation of the changes in the mixing zone by seasons: Antarctic Circumpolar Current (NOAA)
Antarctic Circumpolar Current Mixing Zone data
Fig. 16-31. Antarctic circumpolar current revealed by mixing zone depths.
16.27

Climate Effects of Ocean Currents

• Cold water offshore results in dry condition on land (example: California, Figure 16-34).
• Warm water offshore results in more humid condition on land (example: Florida).
• Depends on seasonal wind patterns and water temperatures.
• Depends also on regional geography along coastal regions.

California climates based on geography and air flow
Fig. 16-32.
California's climate and geographic factors.
16.28

Upwelling and Downwelling

Upwelling is the vertical movement of cold, nutrient-rich water from deep water to the surface, resulting in "high productivity" (plankton growth).
• Can bring cold, nutrient-rich water to the surface (photic zone) unless thermocline is strong and prevents it.
Nutrients are not food but act like a fertilizer.
• Upwelling water rich in nutrients feeds phytoplankton, the base of the food chain.

Downwelling
is the vertical movement of surface water downward in water column. Regions where downwelling is occurring typically have low biological productivity.
• Downwelling takes dissolved oxygen down where it is consumed by the decay organic matter.
Equatorial Upwelling
Fig. 16-33.
Equatorial upwelling involves the Trade Winds blowing across the equator and the coriolis effect taking over as diverging currents move away from the equator.
16.29

Where Upwelling Occurs:

Diverging surface waters occur where surface waters are moving away from an area on the ocean surface.
Equatorial upwelling occurs where SE trade wind blow across the equator (Figure 16-33); Ekman transport forces surface water movement to the south (south of the Equator), and to the north (north of the Equator). Upwelling of deep ocean waters is most intense in equatorial regions.
Coastal upwelling occurs where wind blowing along a coastline is influenced by Ekman current moving surface waters offshore. Winds blowing along coastlines or off the land pull surface waters away from the coast—pulling deeper water up to replace surface waters. Locations where coastal upwelling occurs around the world are shown in Figure 16-34.
• Other locations where upwelling occurs include around underwater obstructions (guyots) or sharp bends in coastlines.

NOAA animation: Coastal Upwelling
Regions of coastal upwelling around the world
Fig. 16-34. Regions of the world where coastal upwelling occurs.
16.30

Ekman Transport Influence on Coastal Upwelling and Downwelling Along the California Coast

Coastal upwelling is influenced by coastal geometry, wind directions, and the influence of the coriolis effect (Ekman transport). Figures 16-35 and 16-36 illustrate how the direction of wind movement determines how coastal upwelling and downwelling takes place in the Northern Hemisphere (such as in California). Figure 16-37 shows regions of coastal upwelling along the California continental margin—revealed ocean-surface temperature imagery. Upwelling water along the coastline is colder than waters farther offshore.
Coastal upwelling
Fig. 16-35. Coastal upwelling (example of California)
Coastal downwelling
Fig. 16-36. Coastal downwelling (wind reversed)
16.31

Large Cycles in Ocean Climate Variability

The ocean/atmosphere systems display cyclic changes beyond annual seasonal changes. Longer-term cycles are also taking place. Changes happening in one region can gradually impact other regions on multi-year to decade cycles (example: cycles in coastal upwelling on North America's West Coast, Figure 16-38). Even longer-term cycles are influenced by extraterrestrial pattern changes in the orbit and rotation of the Earth relative to the Sun over time. These changes impact the distribution of precipitation and influence the warming or cooling of climates over multi-year periods, and changes in sea level over time linked to the accumulation and melting of continental glaciers.
California ocean themperatures 2000
Fig. 16-37. Upwelling offshore of California revealed by ocean surface temperatures.
Upwelling activity by year in the California Current
Fig. 16-38. Cycles of upwelling on North America's West Coast influenced by ENSO.
16.32

El Niño/Southern Oscillation (ENSO)

El Niño/Southern Oscillation (ENSO) in the Pacific Ocean [also called El Niño-La Niña Cycles] is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific. El Niño/Southern Oscillation (ENSO) is perhaps the most important ocean-atmosphere interaction phenomenon to cause cyclic global climate variability. Here's how the ENSO cycle works: ENSO involves the interactions of ocean currents, ocean temperatures, and atmospheric effects, over time.

Pacific Ocean currents involved with ENSO (see Figure 16-5)

• West moving winds at the Equator help to drive the two Pacific Subtropical Gyres (North and South).

• In the North Pacific Subtropical Gyre, the western-intensified Kuroshio Current moves up the Asian seaboard (warming China, Japan), flows east with the North Pacific Current, then south as the California Current along the west coast of North America.

• In the South Pacific Subtropical Gyre, the western intensified East Australian Current moves south and merges with the Antarctic Circumpolar Current, the completes the gyre as the Peru Current (flowing northward along the west coast of South America).

ENSO Ocean Temperature Effects

ENSO Cycles are influenced by ocean surface temperatures throughout the Equatorial Pacific Ocean region. During the "El Niño" periods, ocean surface temperatures are much warmer than the "La Niña" periods. This is a reflection of the amount of cloud cover (deflecting incoming solar radiation) and winds driving cold upwelling currents to the ocean surface in the equatorial region. During "El Niño" periods, the "Pacific Warm Pool" grows larger and more intense in the Eastern Pacific region (Figure 16-39).

ENSO Weather Effects

• The rising warm-moist air in the western Pacific contrasts with the cool sinking air along South America, resulting in the "Walker Cell" (an unstable equatorial air circulation pattern region in the Pacific Ocean)(Figure 16-40). The Walker Cell operates perpendicular (East to West, not north to south like the Hadley, Farrell, and Polar circulation cells) because of temperature contrasts on opposite sides of the Pacific Basin along the equator.
Ocean temperatures during El Nino/La Nina events
Fig. 16-39. Ocean surface temperatures reveal the changing patterns and regional extent of the "Pacific warm pool" associated with El Niño-La Niña Cycles.
Enso
Fig. 16-40. Changes in the Walker Cell wind currents affect ocean surface temperatures which impact the thickness and extent of the thermocline (which impacts upwelling).
16.33
Under "normal year" ENSO conditions (which is rare) cool water conditions persist along the west coast of South America (Peru) (Figure 16-41):
• Trade winds blow to the west allow waters to upwell along the west coast of South America (some of the most productive waters in the world).
• West-moving winds drive surface currents westward across the Pacific Ocean where they heat up creating the "Pacific Warm Pool"- a thick thermocline in the western Pacific Ocean.

Under "El Niño" (the warm phase of ENSO) wind intensity of the Walker Cell circulation is diminished (Figure 16-42). El Niño is associated with high air pressure in the western Pacific and low air pressure in the eastern Pacific.

"La Niña" (the cool phase of ENSO) is associated with below average surface water temperatures and high air pressures in the eastern Pacific and low air pressures in western Pacific (Figure 16-43). Air circulation in the Walker Cell is intensified.

Normal thermocline in the Equatorial Pacific
Fig. 16-41. "Normal" Thermocline and weather pattern under somewhat rare "normal" conditions in the equatorial Pacific region.

Fig. 16-42. "El Niño"Thermocline and weather during strong El Niño conditions in the equatorial Pacific region; coastal upwelling near South America is diminished.
Equatorial thermocline during
Fig. 16-43. "La Niña"
Thermocline and weather during La Niña conditions in the equatorial Pacific region; coastal upwelling near South America is strengthened.
El Niño/La Niña Global Climate Impacts—NOAA videos, websites, animations

Warm El Niño Southern Oscillation (ENSO)- Episodes in the Tropical Pacific

El Niño/La Niña Explained (YouTube video).
NOAA's El Niño website.
El Niño/La Niña 1997-1998 (NOAA) - Shows sea surface temperature time lapse.
Global Sea Surface Temperature time lapse showing El Niño/La Niña (NOAA)
16.34

Impacts of ENSO Cycles

During El Niño - the "Walker Cell" circulation pattern is very week, and warm surface waters move in to and shut down upwelling in the Peru region (and causing both warm and wet conditions on land), and a collapse of fisheries offshore (associated with economic and ecological catastrophe). The warm conditions arrive around Christmas, so El Niño refers to the Christ Child in Peruvian weather.

During La Niña - the "Walker Cell" circulation intensifies, increasing greater cooling and more upwelling along the coast, enhancing ocean productivity, but drought on land in South America.

These fluctuating cycles of ocean surface water temperatures influence climate factors (warm/wet or cool/dry) conditions around the entire Pacific Basin, if not the entire world.

El Niño year
• High and Low pressures reverse
• Winds are slack or blow against the Equatorial Current
• Mounds warm water on eastern side of Pacific Basin
• Creates nutrient poor conditions. A temperate thermocline replaced with a tropical thermocline, this prevents mixing of deep cold nutrient rich water because of the buoyancy of extra warm surface water.

Monitoring for El Niño is conducted by:
• Studies of wind speed and direction on the Equatorial regions
• High and Low pressure systems on the Equatorial regions.
• Water temperature changes on Equatorial regions, mainly warming on east side of Pacific Basin
• Water heights ("mounding") along the Equator.

16.35

ENSO Impacts on Coastal California

During El Niño periods, California's coastal ocean waters are warmer, and a more well-developed thermocline hinders coastal upwelling. This reduces the nutrient supply for sea life, so marine specie either adapt and migrate elsewhere, or in many cases, loose populations due to competition for limited food resources. Southern California typically gets heavier winter rainy periods because the southern tropical jet stream move north from the Central America region. As a result, Southern California gets more tropical moisture which can translate to increased rainfall if conditions are right.

During La Niña periods, California's coastal ocean waters are cooler, only a weak thermocline can develop. As a result, there is stronger and well developed coastal upwelling. As a result, more food is available, and marine life flourishes in coastal waters. Colder waters offshore translate to drier conditions on land.

16.46

Waves

In oceanography, waves are:
• Short-term changes in sea level.
• A wave is energy moving through water.

Waves are generated by a "disturbing force" - something that transmits energy into a fluid medium (such as wind blowing on water). A pebble hitting a puddle generates a splash that creates ripples (tiny waves) that propagate away from the source (Figure 16-45). The ripples grow smaller as they move away from the splash (source) until they diffuse away with increasing distance, or when it encounters the edge of the puddle.
Wave crashing on shore
Fig. 16-44. A wave crashing onshore releases energy.
16.37
Wind is the disturbing force for waves in the ocean and large bodies of water. Waves are also generated by earthquakes, landslides, and volcanic eruptions (producing tsunamis), and tides are produced by gravitational interactions between the Earth, Moon and Sun.

Types of waves

Wind Waves
Tsunamis
Gravity Tides
• Height = range from small ripples up to 60 feet (sometimes higher)
• Speed = 10 – 75 mph
• Periods = 5 – 25 sec.
• Height = open ocean less than 2 feet;
-- onshore up to 300 feet
• Speed = jetliner speeds 400-500 mph
• Wavelength = 100’s of kilometers
• Periods = minutes
Height = up to 50 feet plus
Period 12 ½ to 25 hours

(discussed in Chapter 10)

See: BIGGEST WAVE in the World surfed 100ft at Carlos Burle Portugal (YouTube video)
Splash with ripples
Fig. 16-45. A splash is an example of a disturbing force creating waves.
16.38

Terms Used To Describe Waves

Crest - the highest part of a passing linear wave
Trough - the lowest part of a passing linear wave
16.

- Wavelength (L) = Distance between waves
- Period (T) = Time between passing waves
- Height (h) = Height from crest to trough (same as "amplitude")
- Water depth (d) = Average water depth - determines wave behavior

Characteristics of Waves

Ocean waves are created by wind blowing over water. The distance between two wave crests or two wave troughs is called the wavelength (Figure 16-46) . Wave height is a measure of wave amplitude. The period of a wave is the time interval between passing wave crests (completing one cycle) and are measured as wave crests pass a stationary point (such as a buoy or pole on a pier)(Figure 16-47). The greater the wave period, typically the higher the wave breaks as it approaches the shore.
Wavelength
Fig. 16-46. Wavelength and wave height of wave cycles.
Swells approaching a coastline
Fig. 16-47. Ocean swell waves approaching a shoreline arrive at cyclic intervals called a period.
16.39

Factors In the Formation of Wind Waves

Ocean wave intensity reflects characteristics of wind speed, wind duration, and fetch (the distance the wind has traveled over open water). Wind energy is gradually transferred to the waves forming on a body of water, causing waves to absorb energy and grow in amplitude and period over distance and time (Figure 16-48).

Fetch
Fig. 16-48.
Waves energy depends on wind speed, wind duration, and fetch.
16.40

Sea and Swell

Ocean swell refers to series of ocean surface waves that were not generated by the local wind. Ocean swell waves often have a long wavelength. Swell can develop on lakes and bays, but their size varies with the size of the water body and wave intensity. Swells are generated by storms over the open ocean. Certain regions of the world are more prone to producing large long-period waves. The southern ocean around Antarctica is a region of persistent strong winds blowing over a region with "infinite fetch" (south of South America, Africa, and Australia continents). Large wave with long periods are generated by large and powerful tropical storms, and storms in the North Pacific also produce similar waves, typically during seasons when storms occur in these different regions (Figure 16-49).

• The term "sea" refers to an area where wind waves are generated, mixed period and wavelengths. Seas are typically a chaotic jumble of waves of many different sizes (wave heights, wavelengths, and periods). Figure 16-50 illustrates the relationship of sea and swell.

• The term "fully developed sea" refers to the maximum size waves can grow given a certain fetch, wind speed and duration associated with storm winds.

Ocean swell refers to series of ocean surface waves that were not generated by the local wind. Swells are generated by storms over the open ocean. Swell also refers to an increase in wave height due to a distant storm . Ocean swell waves often have a long wavelength. Swell can develop on lakes and bays, but their size varies with the size of the water body and wave intensity. As waves move out and away from the storm center, they sort themselves out into groups of similar speeds and wavelengths. This produces the smooth undulating ocean surface called a swell. Swells may travel thousands of kilometers from the storm center until they strike shore (the ocean waves illustrated in Figure 16-47 are typical of swell from a distance source).
Origin of ocean swells in the southern oceans
Fig. 16-49.
Most ocean swells originate in the southern oceans where strong winds combine with unlimited fetch.
Sea and swell associated with a storm over open water.
Fig. 16-50.
Sea and swell associated with a storm over open water.
16.41

Wave Speed and Wave Energy

Wave speed is a function of wavelength and wave period, and is related to the wind velocity where the waves form.

Wave speed (c) is the distance the wave travels divided by the time it takes to travel that distance. Wave speed is determined by dividing the wavelength (L) by the wave period (T). [c = L/T]. Wave period is the average of how many seconds pass between a series of wave crests moving past a stationary object in the water, such as a post on a pier or a buoy.

What is important is the combination of the wave height and wave period. Wave period is directly related to the speed the wave is traveling. The longer the period, the faster the wave, and the more energy in contains.
The greater the period the faster the wave moves (Figure 16-51). Also, the greater the period, typically the higher the wave breaks as it approaches the shore.  
Wave speed compared with period
Fig. 16-51. Comparison of wavelength to wave speed and wave period.
16.42

Wave Base, Wave Orbits and Orbital Depth

Passing waves create a circular current in the water. This is revealed by the orbit-like motion of particles in the water (Figure 16-52). The orbital motion of a wave is greatest at the surface and diminishes with depth. Orbital depth is the depth to which the orbital motion of the wave energy can be felt. Orbital depth is equal to half of the wavelength. At the sea surface, orbital diameter is equal to wave height. As depth increases, less wave energy can be felt. The orbital depth is the depth where zero wave energy remains. For example, if a wave at the surface has a height of 4 meters and a wavelength of 48 m, then the depth where no motion from the wave 2 or 24 meters.

Deep-Water Waves and Shallow-Water Waves

The depth of the water determines the character of wave behaviors (Figure 16-53).

Deep-water waves are waves passing through water greater than half of its wavelength. Deep-water waves are waves of oscillation. A wave of oscillation is a wave in the open ocean where movement in the water below a passing wave is in a vertical circular motion. A wave of oscillation is a wave in the open ocean where movement in the water below a passing wave is in a vertical circular motion (in open, deep water).

Shallow-water waves are waves that are interacting with the seabed in depths less than half it wavelength. Shallow-water waves are called waves of transition because they change character as the move shoreward and dissipate their energy interacting with the seabed onto the shore. A wave of translation is a tumbling wave that continues onshore after it crests and breaks when entering a shallow coastal setting. Breakers then turn into a wave of translation and is called "surf." When the wave runs up on the beach and then retreats it is called "swash." (Figure 16-54).

Type of wave Define Orbit Speed
Deep water wave (wave of oscillation) d > L/2 Circle L/T
Shallow water wave (wave of transition) d < L/20 Elliptical √gd
Where...
• L/2 to L/20 is a transitional wave
• Speed of a shallow water wave is dependent on water depth
• G = gravitational constant 9.8 m/s2

Breakers: Why do waves break near shore?

Wind waves change as they approach the shore:
• As a wave approaches shallow water its begins to transform when it's orbital depth comes in contact with the seabed (when d < L/2).
• The friction caused by waves interacting with the seabed causes waves to slow down as the move onshore.
• The friction of the seabed begins to slow the bottom of the wave; whereas the top of the wave does not slow as quickly..
• Circular motion within the wave becomes interrupted and becomes elliptical.
• As waves approach the beach, their wavelengths (L) and velocity decrease. However the period (T) stays the same. The shortening of the wavelength results in an increase in wave height as it moves into shallow water.

• A wave breaks when the water depth (d) is about the same as the wave height (h). Where a wave curls over on itself is called a breaker. When a wave breaks the rising back of the waves moves up and over the slower moving front of a wave approaching the shore (Figure 16-56).
Breakers then turn into a moving turbulent front called "surf" that moves onto the beach.
• When the dying wave runs up on the beach and then retreats it is called "swash."

In a wave passing through the open ocean a water molecule in the water will move in a circular motion parallel to the direction the wave is moving. However, as a wave approaches the shore, its internal circular motion begins to impact the seabed causing the wave to drag along the bottom and slow down, shortening the wavelength (and wave period), but increasing the wave amplitude. Ocean waves typically "break" where the water depth is about one half of its wavelength or when the slope of the wave approaches a steepness ratio of 1 to 7 (feet or meters).

Example: If a moving wave has a height of one foot and a length from crest to crest of 8 feet, then the ratio is 1:8 and this wave is not going to break. However, if the height is 1 foot and the length decreases to 6 feet, then the ratio is 1:6, then the wave has now become steep enough that the crest topples and the wave breaks.
Wave oscillation
Fig. 16-52.
Orbital oscillations in deep and shallow waves.



Waves
Fig. 16-53.
Waves of oscillation, breakers, and waves of transition moving onto the beach.


Waves of transition approaching the beach
Fig. 16-55.
Waves of transition build up, break, and become surf before ending on the beach as swash.


Wave cresting during a high swell in the Atlantic, Puerto Rico
Fig. 16-56. A breaking wave (with surfer in Puerto Rico)
16.43

Slope of the seabed/beach creates different kinds of "Breakers"

There are three types of breaking waves: spilling breakers, plunging breakers, and surging breakers. Breakers may be one or a combination of these types.

Gentle slopes produce spilling breakers (Figure 16-57). Spilling breakers begin far from shore and take a relatively longer time to reach the beach. The breaking crest slides down the front of the wave in a flurry of foam as the wave moves shoreward. Spilling breakers give surfers a long slow ride.

Moderate slopes
produce plunging breakers (Figure 16-58). Plunging breakers build up rapidly into a steeply leaning crest. The crest curls further forward of the rest of the wave before crashing down in the surf zone. Plunging breakers are dangerous because the crash into shallow water.

Steep slopes produce surging breakers (Figures 16-59 and 16-60). Surging breakers occur where waves slam directly on the shoreline. With no gentle slope the waves surge onto a steep beach, producing no tumbling surf. Surging breakers also create huge splashes on a rocky cliff shoreline.
Spilling breakers at Torrey Pines Beach
Fig. 16-57. Spilling breakers at Torrey Pines Beach, CA.
Plunging breaker
Fig. 16-58. Plunging breaker (threatens a boat).
Surging waves on a Hawaii black sand beach
Fig. 16-59. Surging breaker on a narrow Hawaii beach.
Waves crashing on sea cliff at Point Reyes Headlands
Fig. 16-60. Surging wave crashing on seacliffs
16.44

How Waves Form

When the wind starts to blow, the surface of a water body will go through a progression as waves form and intensify. When the wind starts to blow, the ocean surface will change from calm (mirror-like) conditions to form capillary waves (ripples), chop, wavelets, to waves (each with increasing wavelengths, wave heights, and wave periods). Smaller wave features can form on existing larger wave features, adding to the complexity of the water's surface.

Ripples (Capillary Waves)

Capillary waves are very small waves with wavelengths less than 1.7 cm or 0.68 inches (Figure 16-61). The formation of capillary waves is influenced by both the effects of surface tension and gravity. The ruffling of the water’s surface due to pressure variations of the wind on the water. This creates stress on the water and results in tiny short wavelength waves called ripples. Ripples are often called capillary waves. The motion of a ripple is governed by surface tension. They are the first waves to form when the wind blows over the surface of the water and are created by the friction of wind and the surface tension of the water. These tiny little waves increase the surface area of the sea surface and if the wind continues to blow, the size of the wave will increase in size and become a wind wave.

Chop

Chop refers to small waves causing the ocean surface to be rough. Ripples and small wavelets form and move independently of large waves moving through an area, creating rough and irregular wave patterns (Figure 16-62). Over time and distance waves tend to consolidate to become more continuous rows of waves moving in parallel.
capillary ripples forming around calm patches on Lake Hodges
Fig. 16-61.
Capillary wave (ripples) forming next to a calm area (Lake Hodges, CA)
Ripples merge into wavelets in choppy water
Fig. 16-62. Chop: with increasing fetch, ripples merge to become wavelets in choppy surface water conditions.
16.45

Beaufort Wind Force Scale (Wind Velocity, Wave Height, and Sea Conditions)

The Beaufort wind force scale relates wind speed (velocity) to observed conditions at sea (including wave height) or impact of features on land. It is a numbered scale from 0 to 12 to describe sea conditions and wave size. The Beaufort Scale was developed by Rear Admiral Sir Francis Beaufort 1774-1857, an officer in Britain's Royal Navy). Zero 0 on the Beaufort scale represents the calmest of seas (the water is so smooth that it looks like glass). A 12 on the Beaufort scale represents hurricane force waves (Figure 16-21).
Beaufort scale
Figure 16-63. Beaufort Wind Force Scale for sea conditions (and on land).
16.46

Wave Interference Patterns

Wave interference occurs where waves from different sources collide (Figures 16-64 and 16-65).

Constructive wave interference occurs where waves come together in phase or crest meets another crest (or trough meets another trough) .

Destructive wave interference: Waves come together out of phase or crest meets a trough.
Wave interference
Fig. 16-64. Examples of constructive and destructive wave interference patterns.
Ripples merge into wavelets in choppy water
Fig. 16-65. Interference patterns created by winds gusts blowing from different directions.
16.47

Rogue Waves

Rogue waves are large, unpredictable, and dangerous. Rogue waves (also called 'extreme storm waves') are those waves which are greater than twice the size of surrounding waves. They often come unexpectedly from directions other than prevailing wind and waves. Many reports of extreme storm waves describe them sudden "walls of water." They are often steep-sided and associated with unusually deep troughs. Some rogue waves are a result of constructive interference of swells traveling at different speeds and directions. As these swells pass through one another, their crests, troughs, and wavelengths sometimes coincide and reinforce each other. This process produces large, towering waves that quickly form and disappear. If the swells are traveling roughly in the same direction, these massive waves may last for several minutes before subsiding. Rogue waves can also form when storm swells move against a strong current, resulting in a shortening of the wavelength and increasing it’s amplitude. Large rouge wave of this kind are frequently experienced in the Gulf Stream and Agulhas currents (Figure 16-66).
Rouge wate
Fig. 16-66.
This 60 foot rogue wave threatened a ship in the Gulf Stream near Charleston, South Carolina.
16.48

Waves Refraction, Diffraction, and Reflection

Waves can bend when they encounter obstacles or changes on the sea floor.

Refraction involves bending. Wave refraction starts when wave base starts to interact with the sea bed and slow the waves down, causing them to bend toward shore. Refraction occurs when wave swells approach the beach at an angle (Figure 16-67).

Diffraction involves spreading (or dispersion) of wave energy. Wave diffraction refers to various phenomena which occur when a wave encounters an obstacle or change in geometry of the seabed. For example waves are diffracted when they when they pass an island, or when they pass a point or other structure, such as a jetty at the mouth of a harbor (Figure 16-68).

Reflection (bouncing) involves crashing into a solid surface (such as a seawall or cliff) and reflecting back to sea. Reflection can result in standing waves—waves that move back and forth (oscillate) in a vertical position waves strike an obstruction head-on and then are reflected backwards in the direction they came from.

Wave refraction near the seashore
Fig. 16-67. Wave refraction as waves approach the beach at an angle.
Wave diffraction
Fig. 16-68. Wave diffraction around offshore obstruction on waves nearshore
16.49

Longshore Currents and Longshore Drift

A longshore current is a current that flows parallel to the shore within the zone of breaking waves. Longshore currents develop when waves approach a beach at an angle (Figure 16-69). When waves approach a beach, they slow down. If the waves approach a beach at an angle, the slowdown near the beach will cause the line of a wave crest to "bend" in a process called wave refraction. Some of the energy from the waves approaching the beach at an angle create currents that move parallel to the beach (moving downwind). Longshore drift is the process by which sediments (sand and gravel) move along a beach shoreline, caused by currents created by waves approaching the shore at an oblique angle. The waves create a movement of water close to the beach called a longshore current. Longshore drift is the movement of sediments along a coast by waves that approach at an angle to the shore but then the swash recedes directly away from it. The water in a longshore current flows up onto the beach, and then back into the ocean in a “sheet-like” formation. As this sheet of water moves on and off the beach, it can transport beach sediment back out to sea. Objects floating in the longshore current move in a zigzag pattern up and down the beach as it moves down current.

Longshore current and longshore drift
Fig. 16-69. Longshore currents and longshore drift are caused by waves approaching the beach at an oblique angle.

16.50

Rip Currents and Rip Tides

rip currents a current that flows away from the coast (Figures 16-70 and 16-71). Rip current, also commonly referred to simply as a "rip," is a strong channel of water flowing seaward from near the shore, typically through the surf line. Rip currents tend to form when wave swells approach directly onto the beach, causing water to bunch up and then spill seaward in locations along the beach. Rip currents form when wave break strongly in one direction, but weakly in another. In the surf zone, breaking waves produce currents that flow both along the shore and out to sea. Rip currents typical form on beaches with a sand bar and channel system in the nearshore area. A rip current forms as a narrow fast-moving current of water moving in an offshore direction. Obstructions in the water can also deflect current offshore. Rip current vary in size and speed (up to 6 miles an hour, or faster than an olympic swimmer). Rip currents move offshore and dissipate beyond the breaker zone. If caught in a rip current, swim parallel to shore to leave the current before heading for shore.

A rip current is different than a rip tide, which is current associated with the swift movement of tidal water through inlets and the mouths of estuaries, embayments, and harbors caused by the rise and fall of tides.

Rip Currents
Fig. 16-70. Formation of rip current. Rip currents are common when wave approach in a line parallel to the beach. The bigger the waves, the stronger the rip currents.

Rip Currents
Fig. 16-71. Rip current can vary with size and intensity depending of waves and shore geometry. Rip current are revealed by frothy bubbles streaming offshore.

16.51

Surfer's Guide to Wave Forecasting for San Diego County

San Diego County "Swell Window"

• The compass bearing window that we can receive swell from is between 180° and 340° (Figure 16-72). Waves are weak on the “edges” of this window. The best part of our window is really between 200° and 300° degrees because the waves simply have to bend too much to be received on our coastline if they are outside of that range.

• North San Diego County is better for S + SW Swells
• South San Diego County is better for N + NW Swells
• Everyone loves a West Swell!


Swells affection San Diego region
Fig. 16-72. The "swell window" for San Diego County is roughly between 180° and 340° (with North being 360°).
16.52

Summer Swells Affecting San Diego County

• Large storms (largest on Earth) generate swell between Antarctica and New Zealand (Figure 16-72).
• Storm track is always west to east.
• Initial angle is 210 degrees, then moves towards 180 degrees and out of our swell window.

• Timing is 9-10 days to San Diego from storms position. Equatorial Buoy is about ½ way in between.

• What determines whether a swell is received in 9 or 10 days? Wave speed/period!
• The Largest surf is in northern San Diego County comes from the south.

Winter Swells Affecting San Diego County

• Winter swells are largest swells on average
• Generated in the Gulf of Alaska, most begin off of the Kuril Islands
• Storm track is generally from West to East
• Early season swell is usually more Northerly, N or NNW direction or about 320 to 340 degrees.
• Later season storms drop farther south and give us a more westerly swell direction from about 280 to 300 degrees. We also get more rain from these storms.

• Timing is about 1 day from the offshore (way) California or Oregon Buoy. From Point Conception (Harvest Buoy) its about 6 hours. Harvest Buoy is best to get swell direction.

• Partial swell blockage occurs in the Southern California Bight from wave shadows created from the Channel Islands.
• Surf largest in Southern San Diego County and Northern Baja. We also can get colder water and upwelling conditions.
Circumpolar current around Antarctica
Fig. 16-73. The Antarctic Circumpolar wind belt is the source of most swells. Why?
NOAA bouy 51002
Fig. 16-74. Buoys like this one are used to measure wave speed, height, and direction.
16.53

Locally Generated Swell In San Diego Region

We have a number of locally generated swells that come from smaller storms in the Pacific Northwest.
• These storms produce surf that has a shorter period (between 6 and 10 seconds) because the storms are not very large.
• The swell angle is very steep from the NNW around 320 to 340 degrees.
• We often get upwelling associated with these storms as well.

Hurricane Swell In San Diego Region

During the late summer and early fall we can get swell from hurricanes that form off of the coast of mainland Mexico.
• The wave periods generated from these storms is usually between 10 and 14 seconds.
• The key identifying waves from these storms is the angle. The swell angle begins from the S or SSE between 160 and 180 degrees. The angle increases with time as the storm moves up the coast and either onshore or out to sea towards Hawaii.
Beacons_Beach looking south
Fig. 16-75. A southwest swell coming in from the southwest at Beacons Beach, Encinitas, San Diego Co.
 
16.54

Tsunamis

A tsunami is a very long and/or high sea wave or coastal serge of water caused by an earthquake or other disturbance. Tsunamis get their name from Japan (where they are fairly common): "Tsu"[ harbor], "nami" [wave].

Tsunamis are caused by displacement of the earth's crust under an ocean or body of water of any size. They can also be generated by earthquakes, volcanic explosions, underwater landslides, even asteroid impacts. When the solid earth moves, the water above it also moves with it (Figures 16-76 and 16-77). Tsunamis are the result of both the initial shock waves and the following motion of the water readjusting to a stable pool (sea level). Tsunamis can travel great distances throughout the world's ocean. Their energy is dissipated when they approach shorelines where they come onshore as a great surge of water, with or without a massive "tidal wave" crashing onshore. Although most tsunamis are small (barely detectable), some modern tsunamis have reached inland elevations many hundreds of feet above sea level.

Tsunami Characteristics:
• Tsunamis are usually less than 2 feet in the open ocean.
• In deep ocean, tsunami wavelengths are long, commonly 100’s of miles.
• Tsunamis always behave like shallow water waves ( d < L/20) because no ocean deep enough!
• Undetectable by ships in open ocean because wavelengths are so long (slow rise and fall as wave passes).
• Open ocean tsunami velocity is 400 – 500 mph. So about 4 – 5 hours from Alaska to San Diego (or Hawaii).
• Wave stacks up on continental shelf, about ½ of the time a trough arrives first (sea recedes from shore).
• Waves 30 – 100 ft are common – locally run-up can be higher.
• Highest is thought to be +300 ft., 66 million years ago from asteroid collision in the Gulf of Mexico.

Tsunami diagram
Fig. 16-76. How a tsunami is generated by an earthquake.
Tsunami surge
Fig. 16-77. Tsunamis move onshore more as a surge than just a wave.
16.55

Impact of Tsunamis in Modern World History

Major Tsunami Events Cause and Effects Damages
Sumatra, Indonesia,
26 December 2004
The 9.1 magnitude earthquake offshore of Sumatra. The fault zone that caused the tsunami was roughly 1300 km long, vertically displacing the sea floor by several meters. Tsunami was as tall as 50 m, reaching 5 km inland. Many billions in damage, estimated 230,000 people killed.
North Pacific Coast, Japan,
11 March 2011
Tsunami was spawned by an 9.0 magnitude earthquake. Many coastal communities were destroyed and the Fukushima Daiichi nuclear power plant was damaged, releasing radiation 10m-high waves swept over the east coast of Japan, killing more than 18,000 people, Most expensive disaster in history: ~$235 billion.
Lisbon, Portugal,
1 November 1755
A magnitude 8.5 earthquake produced a series of three huge waves that struck various towns along the west coast of Portugal and Spain. Tsunami was up to 30 m high in some places The earthquake and tsunami killed an estimated 60,000 people in the Portugal, Spain, and Morocco.
Krakatoa, Indonesia,
27 August 1883
This tsunami event was caused by explosive eruptions of the Krakatoa caldera volcano in the Sunda Strait between the islands of Java and Sumatra. Multiple waves as high as 37 m. The event killed about 40,000 people in total; however, about 2,000 deaths were from the volcanic eruptions.
Enshunada Sea, Japan, 20 September 1498 An earthquake estimated about magnitude 8.3, caused tsunami waves along the coasts of Izu, Kii, Mikawa, Sagami, and Surugu (Japan). Coastal communities were washed away; estimated 31,000 people were killed.
Nankaido, Japan,
28 October 1707
A magnitude 8.4 earthquake caused tsunamis as high as 25 m that swept onto the Pacific coasts of Kyushyu, Shikoku and Honshin. About 30,000 buildings were damaged and about 30,000 people were killed.

Sanriku, Japan,
15 June 1896

An estimated magnitude 7.6 earthquake off the coast of Sanriku, Japan generated a tsunami reported to have reached a height of 38.2 m. 11,000 homes destroyed and 22,000 people killed in Japan; 4,000 also killed in China.
Northern Chile,
13 August 1868
Earthquakes estimated at magnitude 8.5, off the coast of Africa, Peru (now Chile). Tsunamis affected entire Pacific Rim; waves reported up to 21 m high over two and three days. Estimated 25,000 deaths and an $300 million in damages caused by the tsunamis and earthquakes along Peru-Chile coasts.
Ryuku Islands, Japan,
24 April 1771
A magnitude 7.4 earthquake produced a tsunami that damaged coastal communities on Ishigaki and Miyako Islands and others in the region. Tsunamis were 11 to 15 m high. Tsunami destroyed 3,137 homes and about12,000 people were killed.
Ise Bay, Japan,
18 January 1586
An earthquake that caused a tsunami estimated to be about magnitude 8.2. The tsunamis rose to a height of 6 m. Earthquake and following fired destroyed most of a city. 8000 people were killed.

Simple tsunami origin animation (NOAA) - How tsunamis form from an earthquake.
Three Chile Tsunamis animation (PTWC) - Breakout animations of tsunamis of different intensities.
Drawback before arrival of 2004 tsunamis in Sri Lanka
Fig. 16-78. Drawback from tsunami in Sri Lanka exposed about 150 meters before the tsunamis arrived from 2004 earthquake.
tsunami 2010 Japan
Fig. 16-79. Tsunamis arriving on coast of Thailand, 2004
2004 Tsunami coming onshore in ThailandFig. 16-80. 2004 Tsunami coming onshore in Thailand
Whirlpool caused by tsunami in Japan, 2011
Fig. 16-81. Giant whirlpool caused by the Japan, 2011 tsunamis (note the boat for scale).
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Tsunami sources
Fig. 16-82. Map showing locations of tsunami-generating earthquakes
Tsunami travel time from 1960 earthquake in Chili
Fig. 16-83. Map of tsunami travel times generated by magnitude 9.5 earthquake Chili, 22 May, 1960.
Tsunami travel time from 2011 earthquake in Japan
Fig. 16-84. Map of tsunami travel time from magnitude 9.0 earthquake in northern Japan, 11 March 2011
Tsunami amplitude map of Japan earthquake, 2011
Fig. 16-85. Maximum wave amplitude of tsunami from northern Japan, 11 March 2011

Tsunami Damage from the 2004 Sumatra tsunami
Banda Aceh before 2004 tsunami
Fig. 16-86. Banda Aceh, Indonesia before 2004 tsunami
Banda Aceh after 2004 earthquakeFig. 16-87. Banda Aceh, Indonesia after 2004 tsunami Only a brick mosque survived tsunami damage in Banda Aceh, 2004 tsunamiFig. 16-88. Mosque survived 2004 tsunami in Banda Aceh, Sumatra Tsunami 2004 runup
Fig. 16-89. Vegetation stripped from hillsides by run up: Banda Aceh, Sumatra tsunami 2004

Examples of Tsunami Damage
Tsunami damage on Sri Lanka, 2004
Fig. 16-90. The Sumatra 2004 earthquake caused damage in Sri Lanka 2,000 miles away.
Damage from a 1993 tsunami in Japan
Fig. 16-91. Damage from the Tsunami of July 12, 1993, a magnitude 7.6 in the Sea of Japan near Hokkaido. The tsunami was 32 meters high on Okishuri, Island.
Tsunami damage from the Alaska 1964 tsunami
Fig. 16-92. Tsunami damage in Kodiak, Alaska from the March 27,1964 earthquake (magnitude 9.2)
Tsunami damage from the 2011 earthquake in Japan
Fig. 16-93. March 11, 2011 Tohoku Japan earthquake and tsunami

Tsunami Warning System
Tsunami Dart Alert System tsunami buoy
Fig. 16-94 & 95. DART Buoy and DART tsunami warning system (left).
Pacific Tsunami Warning_system
Fig. 16-96. Location of tsunami warning system buoys around Pacific and Atlantic basins.
Everything you ever wanted to know about tsunamis, and more:

Pacific Tsunami Warning Center website.

Learn about: Tsunami Paleogeohistory of San Diego County

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What are tides and what causes them?

Tides are one of the most reliable and predictable phenomena in the world. Tides are cause by the gravitation pull of extraterrestrial objects, the Sun and Moon being the most significant tidal forces on planet Earth (Figure 16-97). Tidal forces can affect crustal rocks and especially water (oceans and great lakes). Water will flow in the direction of gravitational pull. However, because the earth is rotating, this gravitational pull is constantly changing causing daily tide cycles.

Tides are very long-period waves that move through the oceans in response to gravitational forces exerted on the oceans by the Moon and Sun. Both the solid Earth and the oceans are impacted by tidal forces, but oceans can move because they are fluid. Tidal forces create "bulges"on the ocean surface (Figure 16-97). The largest tidal effect is from the Moon due to its proximity to Earth; a smaller tidal effect is from the Sun. The sun's gravitational pull on the Earth is about half (~44%) of the moon's gravitational pull.

Tides are consistently predictable because the rotation of the Earth is a consistent 24 hours (a solar day). Tides are influenced by a lunar day (a consistent 24 hours 50 minutes). Tides advance 50 minutes each day. This is because the Moon rises 50 minutes later each day.

Tides arise in the oceans and move toward the coastlines where they appear as the daily rise and fall of the ocean surface. Large lakes can have tides, but they are small because of the comparatively small volume of water.

A tidal range is the difference in height between the highest high water (HHW) and the lowest low water (LLW) (Figure 16-98). Tidal ranges vary from region to region, influenced by the geography of coastlines.

A tidal current is a horizontal flow of water that accompanies the rising and falling of the tides. Tidal currents can be strong on shallow continental shelves and coastlines with restricting geography (such as in bays, inlets, narrow straits, lagoons, and estuaries). Tidal currents are relatively weak in the open ocean.



Tidal bulge from the gravitaional attraction of Earth, Moon, and Sun
Fig. 16-97. Tidal bulges from the gravitational attraction of Earth, Moon, and Sun.

Tidal Ranges
Fig. 16-98. Tidal range
is the distance between average highest and lowest tides.


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Spring Tides and Neap Tides

Tides are periodic short term changes in the elevation of the ocean surface caused to the gravitational attraction of the Moon and Sun, AND the rotational motion (inertia) of the of the Earth. The gravitational pull of the Moon is slightly stronger than the Sun. However, sometimes the gravitational forces of the Sun and Moon join together to make higher tides (Figure 16-99).

Phases of the Moon and tides

The gravitational pull of the Moon is slightly stronger than the Sun. However, sometimes the gravitational forces of the Sun and Moon join together to make higher tides.

* During full moon or new moon phases, the gravitational forces of the Sun and Moon are maximized, producing very large ranges of tidal highs and lows called spring tides (Figures 16-99 and 16-100).

* During full moon or new moon phases, the gravitational forces of the Sun and Moon are maximized, producing very large ranges of tidal highs and lows called spring tides. A spring tide is the exceptionally high and low tides that occur at the time of the new moon or the full moon when the Sun, Moon, and Earth are approximately aligned.

* During the quarter moon phases, the gravitational forces of the Sun and Moon are at their minimum, producing very small ranges of tidal highs and lows (neap tides). A neap tide is the lowest level of high tide; a tide that occurs when the difference between high and low tide is least. Neap tide comes twice a month, in the first and third quarters of the moon.

Tidal ranges vary considerably around the world and are influenced by factors including shoreline and continental shelf geometries, latitude, size of the body of water, and other factors.

Neap and Spring Tides illustrated (NOAA animation)

spring and neap tides
Fig. 16-99.
Spring and neap tides are related to the orientation of the Earth, Moon, and Sun (note polar orientation in this view).

Monthly tidal cycle showing spring and neap tides.
Fig. 16-100.
Monthly tidal cycle showing spring tides and neap tides.
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The Effects of Elliptical Orbits of Earth and Moon On Tides

It take the Earth 365.242 days for the Earth to orbit the Sun. The Moon completes one orbit around the Earth in 27.3 days (called the sidereal month). However, due to the Earth's motion around the Sun it has not finished a full cycle until it reaches the point in its orbit where the Sun is in the same position (29.53 days) - this is the time from one full moon to the next.

However, both the Earth and the Moon have orbits that are slightly elliptical (not circular). This has an influence on the intensity of tide cycles (Figure 16-101).

Perigee is when the Moon is closest to the Earth.
Apogee the Moon the farthest from the Earth.

Perihelion is when Earth is closest to the Sun (in early January).
Aphelion Earth is farthest from the Sun it is called (in early July).

Because the Moon has a greater influence on tides, the highest tides happen at perigee when there is a a full or new moon. This happens a couple times a year and are called king tides. King tides occur when the Earth, Moon and Sun are aligned at perigee and perihelion, resulting in the largest tidal ranges seen over the course of a year.
Apogee and Perigee of the moon's orbit and perihelion and aphelion of the Earth's orbit.
Fig. 16-101.
Effects of elliptical orbits.

Tidal curves for diurnal, semi-diurnal, and mixed tides
Fig. 16-102.
Tidal curves for diurnal, semi-diurnal, and mixed tides.
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Types of Tidal Cycles

If the Earth were a perfect sphere with no continents, all parts of the planet would have two equally proportioned low and high tides every lunar day as the Earth rotates. However, the large continental land masses block the westward movement of the tidal bulges. This blocking of the tidal bulges results in the development of complex tidal patterns within each ocean basin. As a result, different parts of ocean basins have different types of tides (Figures 116-102 and 16-103).

Diurnal Tides—a region where there is only one high tide and one low tide each lunar day. For example, the Gulf of Mexico has diurnal tides.
Semidiurnal Tides—a region that experience 2 high tides and two low tides of approximately equal size each lunar day. For example, the Atlantic Coast of North America has semidiurnal tides.
Mixed Semidiurnal Tides—a region where the two high tides and two low tides differ in height. For example, West Coast of the North America (including here in San Diego) has mixed semidiurnal tides.

Map of world showing locations of different types of tides
Fig. 16-103.
Map of the world showing the regions affected by semidiurnal, diurnal, and mixed tides.
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Regional Tidal Variations

Tidal ranges vary considerably around the world and are influenced by factors including shoreline and continental shelf geometries, latitude, size of the body of water, and other factors. For instance, the equatorial regions have very minimal tides compared with higher latitudes. Like all ocean currents, tidal currents are influenced the influence of the coriolis effect. Ebb and flood currents influenced by the coriolis effect create circular flow patterns in large bays.
Tides of the Bay of Fundy
Fig. 16-104.
Tides at the Bay of Fundy, Maine and Canada, are the largest in the world with spring tide ranges more than 50 feet!
The Bay of Fundy has the highest tidal range in the world! (Figure 16-104). Check out:
Bay of Fundy Tides, New Brunswick and Nova Scotia YouTube video
Fall and rise of the tide in the Bay of Fundy - Time Lapse YouTube video
Bay of Fundy Tide, Time-lapse, Fundy National Park YouTube video

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Tidal Currents

An incoming tide along a coast is called a flood current; an outgoing tide is called an ebb current. The strongest currents usually occur near the time of the highest and lowest tides. The tidal currents are typically weakest midway between the flood and ebb currents and are called slack tides.

Daily tides move vast quantities of water along coastlines, filling in and emptying coastal bays and estuaries, flushing out stagnant waters, and moving nutrients in and out. The ebb and flood tides cause rivers in delta regions to reverse their flow directions and bring in seawater to mix with freshwater (creating brackish waters).

The speed of tidal currents can reach up to several miles per hour.

Tides at Mont St.-Michel
Fig. 16-105. Tides and tidal flats at Mont Saint-Michel, France, a region with a high tidal range. Left shows flood (during high tide); right shows ebb (during low tide).
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What is a "Tidal Wave?"

"Tidal wave" is a term often confused with the term "tsunami." They are different.
Tsunamis are seismic sea wave formed by rapid displacement of the seafloor, such as by earthquakes, volcanic explosions, landslides, etc.). Tsunamis are not related to tides. Tsunamis are generally unpredictable, especially close to the source of the disturbance, with only minutes to hours to warn large coastal populations.

A tidal wave is a large wave associated with a tidal bore. A tidal bore is a surging flow of a large about of water moving with the incoming tide that funnels a large amount of water into a river mouth or a narrow bay (Figure 16-106). Tidal bore can produces sizable waves that move inland along rivers and estuaries (they are "surges" of water that can behave like a tsunami). Tidal bore characteristics are often predictable, but can be influenced by storm surges and high sea waves causing potentially hazardous conditions.
Tidal bore near Truro, Nova Scotia
Fig. 16-106. A tidal bore moving up a tidal estuary near Truro, Nova Scotia on the Bay of Fundy. A tidal bore is associated with the surge of an incoming ebb tide.
Sea cliff at Del Mar Dog Beach
Fig. 16-107.
High tide combined with storm waves can cause intense erosion at the base of sea cliffs, such as illustrated here at the Del Mar Dog Beach, CA.
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Check out these tidal bore videos:
Tidal bore surfing on the "Bono" waves, Kampar River, Indonesia
(YouTube video)
Tidal bores surge in Qiantang River(YouTube video)
Tidal Bore Surfing "Seven Ghosts" in Indonesia (YouTube video)
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Storm Surge and Storm Tides

A storm surge is a wind-driven current of water that piles water into shallow coastal areas and onshore areas with low coastal elevation. A storm surge is a buildup of water created by winds associated with large storms where wind moves water into coastal areas that have no place to drain away.

Storm surges are typically associated with large low pressure tropical cyclones (hurricanes and typhoons) and strong extra-tropical storms that move into shallow neritic zone environments, and often have enhanced effects where coastal geography, such as a shallow bay or estuary, that cause water to accumulate. Storm surge effect are most catastrophic when they occur in association with high tide, and are often the cause of the greatest death & destruction associated with large storms.

A storm tide is when a storm surge coincides with a regular high tide. The effects of storm tides adds to the catastrophic effects of storms associated with cyclones on coastal settings (Figures 16-108 and 16-109).

Fortunately, storm tides can be forecasted in association with large storms.
Storm surge and storm tide
Fig. 16-108. Storm surge associated with a cyclone.
Storm surge and storm tide
Fig. 16-109. Additive effects of storm surge with high tide.
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Subdivisions of the Intertidal Zone

The intertidal zone is the region where land surface is intermittently exposed between the lowest-low water and the highest-high water. The intertidal zone is between the subtidal and supratidal zones (Figure 16-110). Tidal ranges influence the distribution of sediments and the habitats occupied by plants and animals.

The subtidal zone is the submerged region lying below the low-tide mark but still shallow and close to shore.

The supratidal zone is the typically vegetation-free "splash or spray" zone above the high water line where back-beach dunes accumulate.

A wrackline is an accumulation of shell material and debris that typically marks the location of the last high tide cycle on a beach or after a storm surge (Figure 16-111).
Coastal Depositional Environments
Fig. 16-110.
Coastal environments within the intertidal zone extend from offshore bars to inland estuaries and bays.
A wrackline on Plumb Beach in Jamaica Bay, New York
Fig. 16-111.
A wrackline consisting of most shell material, pebbles, and flotsam along Plumb Beach, Jamaica Bay, Brooklyn, NY
   
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What is Sea Level?

"Sea level” is generally used to refer to mean sea level (MSL). A common accepted definition of mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location.

Sea level is an average level for the surface of one or more of Earth's oceans from which heights such as elevations may be measured. However, sea level varies for place to place due to gravitational differences in the solid earth, and variations in sea water characteristics (water density) and atmospheric pressure effects. For instance, Figure 16-112 shows topography of the ocean surface one specific day, however, it is constantly changing day by day, season to season. MSL is a standardized geodetic reference point for geographic locations.

Sea level height map on a particular day (departure from mean sea level)
Fig. 16-112. Sea level height map on a particular day (departure from mean sea level).
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Mean Sea Level (MSL) is not really level...

* Sea levels are different for each ocean basin. Sea level is about 20 cm higher on the Pacific side of North America than the Atlantic due to the water being less dense (on average) than on the Pacific side. Variations in sea level are due to the prevailing weather and ocean conditions.

* Differences in MSL are also related to the gravity variation cause by different densities rocks in the lithosphere and depth of the ocean basins (Figure 16-113). For instance mid-ocean ridges (MORs) tend to be low gravity areas.

Sea level is influenced gravitational acceleration. A boat on sea level region near the North Pole in the Arctic Ocean has the highest gravitational acceleration of the planet: 9.8337 m/s2. The lowest is on Mount Huascaran in Peru on the equator where the gravitational acceleration is only 9.7639 m/s2 (a difference of 0.7% - which you would not feel).
Gravity acceleration map of the globe showing highs in red and lows in blue
Fig. 16-113. Gravity map of the Earth exaggerated: highs are red, lows are blue.
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Changes of the Sea Level Over Time

Sea levels are constantly changing around the globe. Long-term trends in sea-level rise are linked to global climate change. Sea level changes are primarily due to the melting and freezing of the icecaps due to global temperature changes. Sea level change is also due to the expansion and contraction of the total water mass due to global temperature changes. Figure 16-114 illustrates the dramatic rise in sea level over the past 20,000 years—estimated at about 120 meters (400 feet)! Figure 16-115 shows how much sea level has risen since detailed global record have been kept (starting around 1900).

Figure 16-116 show that in most places around the coastline of North America sea level is rising, however, in some places sea level is falling. In northeastern North America the land is rising due to glacial rebound (an isostatic adjustment caused by the melting of the great Laurentide continental glacier). In Alaska and other part of the West Coast, tectonic forces are pushing up coastal regions, some of these were rapid adjustments associated with massive earthquakes.
Sea level rise since the end of the last ice age.
Fig. 16-114. Sea level changes of the past 20,000 years (Late Pleistocene and Holocene)
Global average sea level rise over the past century.
Fig. 16-115. Average global sea-level rise 1900 to 2010.
Sea Level Trends around North America
Fig. 16-116. Sea level changes around North America.
What is Sea Level? YouTube video explaining the geodesy of defining sea level.
Sea Level Trends (NOAA website linked to data used in Figure 16-14).
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Coastlines and Coastal Processes

>Coastlines are a dynamic interface between land and sea. Coastlines preserve evidence of many process from the past, going back hundreds, thousands, even millions of years. Coastlines are shaped by an ongoing series of processes involving daily wind and wave action, tides, occasional storms and superstorms, earthquakes, and massive tsunamis. Coastlines reflect process of their origin including erosion of bedrock features, and are influenced by regional geology, geography, and climate.

Understanding coastline dynamics is important considering that about 75% of the worlds megacities are on coastlines. According to the United Nations. presently about 40% of the world’s population lives within 100 kilometers of the coast, with hundreds of millions living in low-lying coastal areas (below about 10 meters elevation).

Wave erosion is persistent and intense
, especially when storm waves combine with high tides. As a result, coastal landforms are generally delicate, and short-lived features. The sediment supply to coasts are offset by erosion rates along shorelines. Sediment supply is influenced by climate factors and geography, and can vary significantly from place to place, season to season, and by isolated events, such as changes caused by a massive superstorm (Figure 16-118).

New York City
Fig. 16-117. New York, the largest coastal city in North America. More than 12 million people in the US live in regions within 3 meters above current sea level.

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Classifications of Coastlines and Shoreline Features

Three different classification schemes of coastlines include:
a. Primary or Secondary Coastlines
b. Active or Passive Margins
c. Emergent or Submergent Coasts


Note below that characteristics of each classification scheme overlap and complement each other.

hurricane katrina
Fig. 16-118. Hurricane Katrina, North America's most expensive disaster, wiped out an estimated 328 square miles of coastal land along the Gulf of Mexico.

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Primary and Secondary Coastlines

Primary: Young coasts formed by terrestrial influences, and have not been significantly altered by marine processes. Processes including active faulting, volcanism, recent glaciation, and rapid sea level rise or fall create young coastlines.

Secondary:
Coasts that have been significantly changed by marine processes after sea level has stabilized. Secondary coastlines are dominated by shoreline erosion and deposition processes.

Jade Beach
Fig. 16-119. Jade Beach, California, a primary coast along a tectonically active coastal mountain range.

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Primary Coasts - 5 Types

Ria Coasts: Drowned river valleys caused by a rise in sea level.
Examples: Chesapeake Bay (Figure 16-120).

Glacial Coasts: Coastlines influenced by recent glacial activity such as glacial cut “U shaped” valleys called “fjords.”
Examples: Norway, British Columbia, Alaska, Hudson Valley, New England region, Long Island (Figure 16-121).

Deltaic Coasts:
Coastlines associated with active river and delta systems.
Examples: Mississippi and Nile Rivers (Figure 16-122).

Volcanic Coasts
: Coastlines associated with recent or active volcanoes (mostly basaltic or andesitic volcanoes).
Examples: Hawaii, Aleutian Islands, Japan, Philippines, Indonesia (Figure 16-123).

Fault/tectonic Coasts:
Coastlines associated with major active fault systems along continental margins
Example: San Andreas fault going off shore at San Francisco (Figure 16-124).

Chesapeake and Delaware Bays and the Delmarva Peninsula
Fig. 16-120. Ria Coast: Chesapeake and Delaware Bays (estuaries), and the Delmarva Peninsula. Sea-level rise has back filled river valleys draining into the Atlantic Ocean. Ridges on land became peninsulas.

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Kenai_fjords National Park, Alaska
Fig. 16-121. Glacial Coast: Kenai Fjords National Park, Alaska

Nile River Delta from space
Fig. 16-122. Deltaic Coast: Nile River Delta

New land forming on Hawaii
Fig. 16-123. Volcanic Coast: Hawaii Volcanoes National Park, Hawaii

Thornton Beach where the San Andreas Fault runs out to sea.
Fig. 16-124. Tectonic Coast:
Thornton State Park, San Francisco, California

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Secondary Coasts

Secondary coasts are coastlines that have been significantly changed by marine processes after sea level has "stabilized" allowing "erosional" and/or "depositional" processes to dominate shaping of the landscape. However, to explain this better, we need to examine the other classifications of coastlines first.

Both primary and secondary coasts
are influenced by whether they are "active" or "passive" continental margins (the second method of "coast classification")(Figure 16-145).

Both primary and secondary coasts
are influenced by whether they are emergent or submergent coastlines (the third method of "coast classification" - discussed below). Passive margins tend to be submergent due to the ongoing rise in sea level (Figure 16-146). In contrast, active margins can be both "emergent or submergent" depending on local tectonic forces, such as caused by faulting.
Active versus passive continental margins
Fig. 16-145. "Active" versus "passive" margins.
Sea level trends
Fig. 16-146. Changing sea level trends.
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Coastlines on Active vs. Passive Continental Margins

In North America, the Pacific Coast is an "active continental margin" whereas the Atlantic Coast is a "passive continental margin" (Figures 16-147 and 16-148).

An active continental margin is a coastal region that is characterized by mountain-building activity including earthquakes, volcanic activity, and tectonic motion resulting from movement of tectonic plates. Active margins typically have a narrower and steeper continental shelf and slope. They can also be subsiding or uplifting. Active continental margins are also associated with subduction zones, often include a deep offshore trench. The Pacific Coast is an active margin that is characterized by narrow beach, steep cliffs, rugged coastlines with headlands and sea stacks (see features discussed below).

Passive continental margins occur where the transition between oceanic and continental crust which is not an active plate boundary. Passive margins are characterized by wide beaches, barrier islands, broad coastal plains. Offshore passive margins typically have a wider and flatter continental shelf and slope. They are usually subsiding. Examples of passive margins are the Atlantic and Gulf coastal regions which represent setting where thick accumulations of sedimentary materials have buried ancient rifted continental boundaries formed by the opening of the Atlantic Ocean basin.

North Carolina Outer Banks satellite view
Fig. 16-147. Passive margin: North Carolina's Outer Banks region showing coastal plain, rivers, tidal estuaries, lagoon, barrier islands, and shallow Atlantic continental shelf.

Satellite view of San Francisco and Monterey Bay region
Fig. 16-148. Active margin: San Francisco Bay and Monterey Bay region has actively rising coastal range mountains and sinking coastal basins.

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Emergent and Submergent Coasts

Another important factor in understanding shorelines is tectonic activity and the rise and fall of sea level.

Submergent coastlines display characteristics caused when sea level rises or the land sinks down. Submergent coastlines:
* Contain estuaries and barrier bars, and barrier island systems.
* Ridges that separate valleys that propel into the sea.
Example: East Coast (see Figure 16-147).

Emergent coastlines
display characteristics caused when sea level drops or the land rises (from tectonic uplift).
* Wave cut platforms and elevated marine terraces.

Example: West Coast California (Figure 16-149).

In some regions around the world, tectonic forces are pushing rocks up along coastal regions, mostly in regions associated with active continental margins. There areas are called emergent coasts and display features including sea cliffs and marine terraces (see below). Where sea level is rising faster than land is rising, or where coastal areas are sinking, it is called a submergent coast. Submergent coasts are associated with passive continental margins with wide coastal plains and continental shelves. Estuaries are associated with submergent coastlines formed when sea level rises and floods existing river valleys. Active margins can have both emergent and submergent coastlines in close proximity to each other.

Torrey Pines State Preserve, CA
Fig. 16-149. San Diego's coastline displays characteristics of both emergent and submergent coastlines, having both seacliffs, headlands and marine terraces (emergent), and bays and estuaries filling flooded river valleys (submergent). View south along the Coast Highway at Torrey Pines Nature Preserve, California.
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How do waves contribute to coastal erosion?

Shoreline erosion depends on several factors:
1) Amount of sediment to buffer land: If the sand supplied to a beach is less than the amount removed by shoreline erosion processes, the beach will retreat landward.
2) Amount of tectonic activity: Uplift along the coastline allows erosion to provide sediments to a coastline. If the coast is not rising, then shoreline will retreat landward.
3) Topography: Coastal uplands provide more sediments to beaches than flat coastal plain regions.
4) Composition of land: Hard bedrock (such as granite) is harder to erode than softer unconsolidated deposits.
5) Waves and weather: The greater the waves and storm-generated currents, the more material can be eroded.
6) Coastline configuration: Coasts facing prevailing storm waves are eroded faster than isolated bays and down-wind protected shorelines (Figure 16-150).

Coast erosion of headlands and bays
Fig. 16-150. Wave refraction focuses wave energy on headlands and deposits sand in quieter bay settings.

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Erosional features along emergent coastlines

There are a variety of erosional coastal landforms or features (typical of secondary coastlines on active continental margins):

Emergent coastlines typically have sea cliffs carved by wave and current action along the shoreline. The geometry of a coastline is largely a reflection of how some rocks along a coastline are more resistant to erosion. Waves erode the base of sea cliffs, often causing it to subside or fail (Figure 16-151).

Wave action carves a flat surface where they scour the seabed leading up to the beach and base of a sea cliff creating a wave-cut platform (Figure 16-152).

When sea level locally falls falls (such as from uplift of a regional earthquake) wave action scours out a new wave-cut platform, leaving remnants of the old seabed surfaces exposed as expose wave-cut bench (Figure 16-152) (also see marine terraces below).

Headlands are rocky shorelines that have resisted wave erosion more than surrounding areas, forming points or small peninsulas that jut seaward. Small sandy beaches typically occur in bays between headlands (Figure 16-153).

Sea stacks
are large rocky outcrops that have resisted wave erosion and stand offshore as the beach and sea cliff continues to erode landward (Figure 16-154).

A wave-cut bench is a flat bench-like platform of rock that form by wave erosion at the base of a an actively eroding sea cliff on an emergent coastline (Figure 16-155).

A sea cave is an underground passage or enclosed overhang carved into a sea cliff carved by focused wave action.

A sea arch is a natural rock arch caved by wave action (Figure 16-156).

Marine terraces are elevated step-like benches formed by the combined effects of long-term wave erosion during the rise and fall of sea level on an emergent coastline
Sea cliff at Del Mar Dog Beach, CA
Fig. 16-151. Sea cliffs rise above a wave-cut platform (with beach) at Del Mar Dog Beach, California.

Headlands and Bays along Point Reyes National Seashore.
Fig. 16-153
. Headlands and bays at Point Reyes National Seashore, California.
Wave-cut platform on Drakes Beach, Point Reyes National Seashore, CA
Fig. 16-152.
Wave-cut platform (covered with sand), a elevated wave-cut bench, and sea cliffs on Point Reyes National Seashore, California.

sea stacks at Olympia National Park
Fig. 16-154. Sea stacks along the coast at Olympic National Park, Washington.
Wilder Ranch State Park, wavecut bench and sea cave
Fig. 16-155. Wave-cut benches and a sea cave at Wilder Ranch State Park, Santa Cruz, CA
sea arch at Natural Bridges State Park, Santa Cruz
Fig. 16-156. A sea arch at Natural Bridges State Park, Santa Cruz, CA
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Elevated marine terraces on the California coastline

California preserves much evidence of geologic, geographic, and climatic changes caused by ice ages. During the last ice age, alpine glaciers and ice caps covered upland regions in the Sierra Nevada Range and Cascades volcanoes, but lower elevations were ice free (Figure 16-157). The formation of continental glaciers in North America and Europe caused sea level to fall almost 400 feet, causing the shoreline to migrate seaward as much as 10 to 70 miles westward of the current coastline in some locations. This rise and fall of sea level happened with each glaciation cycle (of which there were many through the ice ages of the Pleistocene Epoch). In places where the California coastline is slowly rising, each of the major glaciation cycles is preserved as a step-like bench, called a marine terrace. The formation of marine terraces is illustrated in Figure 16-158. Examples in northern and southern California re illustrated in Figures 16-159 and 16-160.
California glaciation
Fig. 16-157. California at the peak of the last ice age. Glaciers covered the higher mountains, lakes filled inland valleys, and a coastal plain was extended offshore.
Formation of marine terraces
Fig. 16-158. Formation of marine terraces. This example shows the formation of two terraces. At least seven major terrace levels are preserved in some areas along the California coast.
Davenport marine terraces
Fig. 16-159.
Marine terraces at Davenport, California
Marine terraces on San Clement Island, California
Fig. 16-160.
Step-like marine terraces on San Clement Island located offshore in southern California
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Depositional coastal landforms or features

Longshore drift contributes to the deposition of sediments in some locations. Landforms associated with deposition tend to change significantly over time.

Spits are ridges of sand projected from land into the bay (Figure 16-161).

A Bay-mouth bar is a sandbar that stretches across a bay, separating it from the ocean (Figure 16-162).

Barrier islands are ridges of sand islands that run parallel to the coast (Figure 16-163).

Spits: Rockaway Spit (NY) and Sandy Hook (NJ), NY)
Fig. 16-161. Spits: Rockaway Spit (on Long Island, NY) and Sandy Hook Spit (New Jersey project into outer New York Harbor and Raritan Bay.

Baymouth Bar at Bolinas Lagoon
Fig. 16-162. Bay-mouth bar: Bolinas lagoon has a baymouth bar composed of sand from the Point Reyes. Peninsula.

Barrier islands along Cape Cod's south shore, Massachusetts
Fig. 16-163. Nauset-Monomoy barrier islands along Cape Cod's south shore, Massachusetts.

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Seasonal Erosional Changes to a Beach Profile

During the winter, storm-wave energy is most intense. Waves wash up on the beach and erode sand, and transport it offshore to where wave-driven currents aren't so strong and the sand accumulates on offshore bars. Heavier materials (gravel and boulders) are concentrated on the beach (Figure 16-164).

During the summer, lower wave energy prevails, and the sand gradually migrates back onshore, gradually expanding the beach seaward (Figure 16-165).
ano nuevo
Fig. 16-164. Cove Beach at Año Nuevo State Park (California) in winter.
ano nuevo sandy beach
Fig. 16-165. Cove Beach at Año Nuevo State Park in summer.
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Tidal Deltas

Tidal currents (including rip tides) are strong erosional forces where they are restricted at the mouths of inlets and straights between bodies of water. One example is the narrow straights of the Verrazano Narrows (between Staten Island and Brooklyn on Long Island, New York) (Figures 16-166 and 16-167). Another example is the Golden Gate Narrows between San Francisco and Marin County in northern California (Figure 16-168). In both cases, the seabed has been scoured deeply in the narrow channels by the daily tidal flows (ebb and flood tides). Tidal flows redistribute sediments building submerged tidal deltas at opposite ends of the channel that need to be dredged frequently to mitigate hazards to shipping.

New York City waves, winds, currents, and longshore and tidal currentss
Fig. 16-166. Dominant winds, waves, and currents of the New York City region.
Bathymetry of the New York City region
Fig. 16-167. Bathymetry shows tidal current scour in the Verrazano Narrows.
Bathymetry of San Franciso bay
Fig. 16-168. Bathymetry shows tidal current scour in near the Golden Gate Bridge.
Tidal deltas are created on both ends of an inlet. Incoming flood tides push sediments through narrow passages where they are deposited on the inland side of the inlet where currents spread out and slow down. Another tidal delta forms when the ebb tidal flow scours the inlet area and deposits sediments in the quieter waters on the ocean side near the mouth of an inlet.
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Coral Reefs, Keys, and Atolls

Biogenous carbonate sediments can accumulate faster than sea level is rising. Skeletal reefs (including coral reefs) thrive in the surf zone, and are able to weather wave action, although they can be heavily damaged by superstorm wave energy. The sediments generated by wave erosion and bioerosion (critters eating critters) contribute to the buildup of carbonate islands (keys) and atolls associated with fringing reefs forming around extinct and eroding volcanic islands (Figures 16-169 to 16-171). Keys and reefs of the world experience exposure and erosion during low sea levels during the ice ages.

Carbonate depostional environments
Fig. 16-169. Landforms associated with carbonate depositional environments.

Coral reefs and keys on an atoll in the Marshall Islands.
Fig. 16-170. Coral reefs and keys, Kwajalein Atoll, Marshall Islands, South Pacific Ocean

Mataiva Atoll
Fig. 16-171. Mataiva Atoll, Tuamotu Archipelago, South Pacific Ocean

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Coastal Littoral Cells, an example from San Diego

A coastal cell is a relatively self-contained "compartment" within which sediments circulate. A coastal cell contains a complete cycle of sedimentation including sources, transport paths, and sinks. In the San Diego area, the Oceanside Coastal Cell extends from Dana Point to La Jolla Canyon; some of the sand is lost to Carlsbad Canyon as well (Figure 16-172). Streams and cliff erosion provide sediments to the shore zone. The arrow on the map indicates the predominant longshore current direction (and the direction of the migration of beach sand along the coast). Most of the sand moves down the coast and eventually drains down La Jolla Canyon and is deposited as turbidity flow deposits on the La Jolla Canyon deep-sea fan in the San Diego Trough (Figure 16-173).
Oceanside Littoral Cell Compartment
Fig. 16-172. Map of the Oceanside littoral cell and Carlsbad and La Jolla Canyons offshore.
La Jolla Canyon
Fig. 16-173. Sediments move from shore down La Jolla Canyon to the San Diego Trough.
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Coastal Erosion Problems Related To the Oceanside Coast Cell

The dominant swell direction in northern San Diego County is from the northwest. This creates longshore currents that move sediments (longshore drift) from north to south along area beaches. The sand on northern San Diego County beaches are mostly derived from sediments derived from coastal erosion in the shallow nearshore, beach, and sea cliffs along the coast between Dana Point and Oceanside (much of it from along the undeveloped coast within Camp Pendleton north of Oceanside). In addition, large quantities of sandy sediments are contributed to beaches from streams (small rivers) that, during episodic floods, dump large amounts of fresh sediment into the nearshore environment, contributing about half of the sand supply to area beaches over time. The amount of sand from river sources is highly variable with the seasonal weather, year to year.

Large waves (swell) especially during high tides in stormy conditions can erode, transport, and deposit large quantities of sediments.

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Shoreline Erosion Problems and Human Impacts

Shoreline changes quickly with natural forces; they are not a stable landforms. Coastlines, especially on the East Coast and Gulf regions, are constantly changing, especially from the impacts of superstorms. These coastal regions are underlain by unconsolidated sediments that are easily eroded by strong currents. They remain relatively stable, as long as there is a new supply of sediment to replace materials eroded by longshore currents, tides, and storm waves. Figure 16-174 illustrates how much shorelines can change. In less than two centuries, Fire Island's eastern spit has grown about 5 miles longer. The sediments creating this new land came at the expense of coastal lands father east on Fire Island, making the island increasing narrower. Barrier Islands are prone to be breached by storm erosion, creating new inlets, and filling in others.

Fire Island-Robert Moses State Park
Fig. 16-174. Fire Island (on Long Island, NY) has steadily grown about 5 miles longer since 1825 by longshore drift (see Figure 16-170). Fire Island Inlet at the west end of Fire Island is scoured by rip tides, adding sediments to the tidal delta in Great South Bay.

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Many attempts have been made, often at great expense, to try to prevent the effects of erosion and deposition along coastlines. Common construction efforts include jetties, groins, and seawalls to protect harbors, infrastructure, and communities.


Prevailing wind and wave swell patterns and storm events affect shoreline erosion and deposition, changing shoreline geometry over time. Man-made structures designed to control wave and storm damage include seawalls, groins, and jetties. These features affect wave energy dispersion and longshore currents, modifying shoreline geometries. They must be designed for long-term stability or they may fail.

Great coastal storms can severely impact coastal communities and change shoreline geometries. Great storm can move as much sediment in a couple days what may take 100 years or more of "normal storms."Coastal erosion is a HUGE problem for humans intent on living along the shore. Coastal erosion is inevitable and unstoppable in the long term. Coastal environments are locations where people should probably be building parks and nature preserves, not megacities! All coastal communities are at some risk of potential disaster. The question isn't "if..." but the answers are in "how, where, and when!"

Man-made structures, such as groins, jetties, and seawalls, are designed to control erosion, but their construction often creates other problems. Groins and jetties trap sand moving along a beach with longshore drift. Whereas they may trap sand in one location, their construction shuts off the sand supply for beach areas down current. This scenario was well illustrated by a disaster involving the community Westhampton Beach on Long Island, New York where a wealthy community decided to have a groin field built to protect coastal homes. The groins constructed by the US Army Corp of Engineers successfully stopped the erosion, but down current of the groin field, it was the recipe for disaster which took place when a typical "nor'easter" storm hit in the winter of 1982 (Figure 16-175). The storm surge and waves eroded the beach, cutting a new inlet across the barrier island and hundreds of homes were either destroyed or heavily damaged. In the end, it was the tax payers in New York who had to pay many millions of dollars to clear up the damage for a relatively small population of coastal dwellers. This is a story has been generally repeated many times, but in many different locations.

The movement of the coastline is unstoppable. It is well illustrated along the Atlantic shore of Long Island. For example, coastal erosion is causing the shoreline to migrate landward, removing sediments from the increasingly narrow barrier island, but that sand is moving and accumulating in other locations. Since the construction of a lighthouse at the western end of Fire Island in 1825, the island has grown nearly 4 miles longer. built from sediments contributed by erosion from the eastern end of the barrier island (Figure 16-176). The sediments are accumulating at the entrance to Moriches Inlet, which is making dredging at the harbor entrance an unending expense. Eventually, the Fire Island barrier island will no longer exist as the landward erosion of the coastline continues. A similar situation exists farther west on Long Island in an area called the Rockaways. The end of the barrier island has grown nearly 2 miles since 1866 (Figure 16-177). The westward migration of the end of the barrier was temporarily halted with the construction of a jetty to protect the harbor entrance to Jamaica Bay, but the supply of sand filling in the harbor entrance continues unabated, and will require dredging to keep it open. As an aside, the community of Breezy Point constructed near the jetty was nearly completely destroyed by fire during Superstorm Sandy in 2013.

Billions have been spent trying to cope with the coastal erosion problems in California. A big problem that affects the California coastline is landsliding. In many areas, the coastline is extremely steep, and wave erosion is constantly gnawing away the at the base of the seacliffs along the coast. In many places, the rocks exposed along the shoreline consist of soft sedimentary deposits which easily erode. This make the development of prime real estate of oceanfront property a costly venture for everyone.

Figure 16-179
illustrates the expensive efforts to shore up eroding sea cliffs in Encinitas, California. Seawall construction only delays the inevitable erosional retreat of the seacliffs and visually impacts the view of the natural environment. Figure 16-180 shows the jetty constructed around the Oceanside Harbor and to Fort Pendleton. Although the jetty protects the harbor, it impacts the natural longshore drift of sand to the beaches of Oceanside south of the harbor.

When the beach washes away, the seacliffs are exposed to wave erosion, undercutting hillsides. The result can be catastrophic landsliding (Figures 16-181 and 16-182). Coastlines are not ideal locations for construction!
Westhampton Beach Disaster, 1982
Fig. 16-175. Construction of a poorly designed groin field led to the Westhampton Beach disaster of 1982. The groins shut off the natural sand supply to a coastal community next door.
Formation of Jones Beach
Fig. 16-176. Jones Beach and Robert Moses State Park, New York has grown by nearly 4 miles by sand accumulation by longshore drift. Erosion is making the island longer and narrower.
Rockaway Beach and Jamaica Bay
Fig. 16-177. Beach wracklines and historic changes to the coastal landscape of Rockaway Beach caused by longshore drift. Rockaway beach, located Queens, New York (seaward of New York Harbor), has grown nearly 2 miles since 1866, largely influences by construction of groins and a jetty.
Coastal dynamics of Sandy Hook, New Jersey
Fig. 16-178. Sandy Hook is a sand spit on the southern New Jersey side to the entrance to New York Harbor. The spit has changed its geometry continuously through historic times, and is now preserved as a national park. Many attempts have been made to control coastal erosion on Sandy Hook.
Sea wall in Encinitas, California
Fig. 16-179. Attempts to halt sea cliff erosion to protect coastal properties in Encinitas, California. Sea cliff erosion naturally provides a sand to the beach.
Oceanside Harbor, CA
Fig. 16-180. A jetty constructed around the entrance to Oceanside Harbor impacts longshore drift, requiring occasional dredging operations.
Thornton Beach cliffs, CA
Fig. 16-181. In 1905, a railroad line was constructed along the coast south of San Francisco, it was destroyed by coastal landsliding in the great 1906 earthquake. Today very little evidence of the railroad line is visible along the coast.
Daly City coastal landslide
Fig. 16-182. A massive coastal landslide took out many homes along the sea cliffs in Daly City, a coastal community near San Francisco. As coast erosion proceeds the entire mountain face is subject to landslides.
16.88

Jetties, Groins, Breakwaters, and Seawalls

Jetties are built at entrances to rivers and harbors. Their purpose is to protect properties from storm and wave damage, and to keep sand out of channels (so that there is no beach). Jetties require high maintenance costs to manage because they impede longshore drift (which is continues relentlessly). Most the costs are for dredging sand from one side, and moving it down current to replenish sand to community beaches. Loss of the sand supply makes down current areas susceptible to beach loss and coastal erosion (a major problem for Southern California's coastal communities, Figure 16-183).

Dana Point Harbor Jetty
Fig. 16-183. Dana Point Harbor and Jetty (Orange County, CA)

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Groins are built as barriers perpendicular to the beach in an attempt to stabilize shorelines. Their purpose is to trap sand migrating along the shore by longshore drift (Figure 16-184). Figure 16-185A is an aerial view of a wash-over fan created by a breach in Sandy Hook Spit (on the New Jersey side of New York City's Outer Harbor (also see Figure 16-169).The inlet formed when coastal storm waves and currents cut an inlet across the spit. Note the sand trapped on the left side of the groins (longshore drift is moving left to right). Figure 16-185B shows an accretionary prism of sand building up at the end of Sandy Hook Spit. Figure 16-186 shows the growth of Rockaway Spit on the north east side of New York's Outer Harbor. It has grown nearly 2 miles since the end of the Civil War (1866). The area has been heavily modified by construction of groins and a jetty to keep the inlet to Jamaica Bay accessible to boat navigation.

Lonshore Drift
Fig. 16-184. Groins are designed to trap migrating sand by impeding the flow of longshore drift.

Sandy Hook groins, washover fan, and accretionary prism.
Fig. 16-185 (A&B). Groins, a washover fan, and accretionary prism on Sandy Hook, New Jersey.

Sandy Hook groins
Fig. 16-186. Map showing the growth of Rockaway Spit impacted by construction of groins and a jetty.

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* Breakwaters are structures used to protect boats from large waves (jetties and groins are forms of "breakwaters").

* Seawalls are walls built to protect land structures from large waves and coastal erosion (Figure 16-179 shows an example of some seawalls used to stop cliff erosion).

* Rip Rap are piles of large boulders put on the beach or shoreline. They are cheap but take up beach space and are not as permanent as a seawall, and are unsightly and dangerous. However, they do create habitat for sea life that needs a hard substrate to live (Figure 16-187).

* Beach nourishment adds large amounts of sand to the beach to keep water away from land structures. Sand is dredged form harbor areas or mined from sand bars offshore and pumped onshore in slurries. The process is quite expensive.

Rip-rap was used in the construction of the breakwater (Oceanside Harbor).
Fig. 16-187. Rip-rap was used in the construction of the breakwater (Oceanside Harbor).

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The "Dam Problem"

Dams have been constructed on most of the small rivers and streams throughout upland regions of San Diego County. The intentions of dam construction were to store water (reservoirs) and to reduce flood damage in low-lying communities. The problem is that dams have largely shut off the supply of sand from rivers and streams to the shore. One of the largest dams is for Lake Hodges on the San Dieguito River near Escondido, California (Figure 16-188). Construction of highway and railroad bridges, dikes, and causeways also restrict the flow of sediment-bearing water, preventing the migration of sediment to the coast. As a result, less sand is finding its way to the shore, resulting in narrower beaches. Without the protection of well-developed beaches, erosion of the sea cliffs are progressively endangering homes and infrastructure along the coast.

Lake Hodges Dam
Fig. 16-188. Lake Hodges Dam has shut off the sand supply that used to feed beaches along the coast in San Diego County.

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Dams on rivers trap sediments that would otherwise find their way to ocean beaches. A classic example was the construction of a dam on Matilija Creek in Ventura County. The dam is currently being demolished in order to return the sediment flow to sensitive habitats along the river downstream, but also to return a sediment supply to the Ventura County coastline (Figure 16-189). Many other dams constructed in the 19th and 20th century are being removed for the same reasons.

Ventura River Delta
Fig. 16-189. The Ventura River (left) supplies massive amounts of sediments to the coast during infrequent floods, then persistent coastal erosion processes take over the action. Construction of the Matilija shut of much of the sediment supply to the coast. The dam is now being removed.

Coastal Dynamics—The Unending Saga

Sea-level rise due to global warming is a highly political topic of our times. The world's Scientific Community has been studying the changes happening around the world for many decades. Real-time observations show that the atmospheric temperatures are steadily rising along with the concentrations of greenhouse gases in the atmosphere. What is perhaps most alarming is that the rate of sea-level rise is accelerating as the ice caps melt and the oceans expand from increased warmth (there are many NASA and NOAA websites on these matters). However, the threat doesn't seem to fit within the interest span of politicians and corporations keen on making high profits from the extraction of coal, oil, and gas. sea-level rise will likely continue unabated until either we either consume all the economically available carbon-based resources, or human populations collaborate to change the fate we, collectively, are facing.

Chapter 16 Quiz Questions
http://gotbooks.miracosta.edu/earth_science/chapter16.html
7/20/2017