Shorelines
Beaches and Coasts
• The coastal zone and beaches along the
continental margin are among the most
dynamic geologic setting on earth
• Human settlements are concentrated along
the coast and most of the world’s
population lives along the coast
• Human activities along the coast
oftentimes accelerate coastal erosion
processes
Nearshore Circulation
• Waves
• Wave Refraction
• Longshore Currents
• Rip Currents
Wave Amplitude
Still Water Line
Wave Morphology
Wave Refraction
• As waves approach the shore they will
bend or refract because of their interaction
with the seafloor
• As the wave enters shallow water it will
slow down as a consequence of friction or
drag with the seafloor; however the wave
portion in deep water still moves relatively
faster
• The wave front then turns or refracts
Longshore Drift
• Sediment movement along the coastline is
done by longshore currents and is termed
longshore drift
Rip Currents
• Rip currents are very strong currents that
flow out to sea in the surf zone
THE COASTAL ZONE
or Dune
supratidal intertidal
subtidal
Sources of Sand
Sand, Gravel, Pebble, Cobble
Quartz
Volcanic
Carbonate
Nearshore Features
Depositional Coasts
–
–
–
–
–
Dunes
Beach Face, Berms
Beach sediment
Barrier Islands
Spits, baymouth bars, tombolos
Erosional Coasts
–
–
–
Headlands
Sea Cliffs, Sea Stacks, Sea Arches and Sea
Caves
Wave-Cut marine Terraces
Coastal Development
• Coastal development is increasingly
important to the economic development of
many countries
• However coastal development often
interferes with natural processes resulting
in enhanced erosion
• Engineering solutions to coastal erosion
are often very costly and marginally
successful
Miami , FL
Developed
Barrier Island
One in every three people on
the planet now live within 100
kilometres of the sea
Two thirds of all the cities with
over 2.5 million inhabitants are
on the coast
In the US more than one-half
the nation's population lives
within 50 miles of the
coastline, but coastal areas
account for only 11 percent of
the nation's land area.
In recent years, 40 percent of
new commercial development
and 46 percent of new
residential development
happened near the coast.
Coastal Erosion
North Carolina Coast
After Huriicane Impact
Beach Nourishment
• Beach re-nourishment id one engineering
solution to coastal erosion
• It involves transfer of offshore sand or sand
from quarries to the coast are a cost of
millions of dollars
• Many tourist beaches in the USA are there
because of beach renourishment
Between 1995 and 2006, the Minerals Management Service provided
over 23 million cubic yards of OCS sand for 17 coastal projects. These
projects restored over 90 miles of the Nation’s coastline mainly in
Florida, Maryland and Virginia
Central Boca Raton Beach Renourishment Project
Geologists are aware that the level of the ocean surface is not fixed, but instead
a dynamic feature of our planet. Sea-level during the Last Glacial Maximum
(LGM, 20,000 years ago) was 125 - 130 m lower than it is at the present time.
Using digital elevation data, geologists can develop maps showing the continental margins
during episodes of lowered sea-level. The margin of the Gulf of Mexico and other parts of the
western Atlantic as they would have appeared 20,000 years ago during the sea-level
lowstand. Note that large areas of the continental shelf are exposed well above sea-level
Geologists can also model sea-level rise. Sea-level is 15 m higher than at present. Note the
dramatic change in the outline of the Gulf of Mexico as much of south Florida is inundated.
Also note that the Bahamas are completely submerged, and a narrow strip of land is all that
remains of Cuba.
Both Moon and Sun Cause Tides
DEFINITIONS
• Tidal day
– 24 hr 50 min
– Time between successive phases of moon over a
given point on the earth
• Tidal Period
– Time between two successive high or low tides
• Tidal Range
– Difference between highest and lowest tide levels
• Daily inequality
– Difference in height between successive high or
low tides
TIDES
• Periodic changes in sea level relative to land
along a coast
• Daily or Diurnal Tides
– One high and one low tide each day
• Semi-daily or Semidiurnal Tide
– Two high and two low tides of approximate equal
heights occur each day
• Mixed Tide
– Two high and two low tides of unequal heights
(HHW, LHW, HLW, LLW)
TIDAL CURRENTS
• Horizontal water movement caused by
tides
• Tides are like Shallow water waves
• Orbital motion of water is highly elliptical:
can be assumed to be to and from motion
• Flood tides when water moves in
• Ebb tide when water moves back
The Bay of Fundy
Nova Scotia, Canada
In the Bay of Fundy
the tidal range can
be up to 16m
TIDAL BORE