Waves and Coastlines
I. Waves
A. Nature of (water) waves
1. Waves are the regular
oscillation in the surface of the water. Waves move laterally
(horizontally)
2. Waves are generated by wind
3. Waves involve the lateral
movement of energy, but not of matter. Water molecules and
particles move about under the water surface in circular
orbitals. Water waves can be compared to waves in a rope.
In shaking a rope, the wave travels down the rope, although a segment
of rope only moves up and down.
B. Parts of wave
1. crest
2. trough
3. wavelength
4. wave height
5. wave base (wave depth) equal
to 1/2 wavelenghth.
C. Types of waves
1. Deep water wave (sometimes
called "sea-sick waves" or waves of oscillation)
a.
Waves swell and subside
b.
Occurs when sea floor is below the wave base, in deeper water.
c.
Circular orbitals below the wave surface do not feel interference from
ocean floor
2. Shallow water wave (surf wave
or wave of translation)
a.
Column of circular orbitals collapses or trips over, forming a surf
wave, such as would be ideal for surfing.
b.
Occurs when sea floor rises up above the base level.
c.
Circular orbitals start to touch the ocean floor. Orbitals near
the wave base slow down on contact with ocean floor. Orbitals
higher in the column move at normal speed. As a result, the whole
orbital column trips over.
d.
Waves that pass from deep water into shallower water slow down, causing
shorter wavelengths and greater wave heights.
II. Waves
on the coastline
A. Waves tend to approach a coastline at an
angle, but become more parallel the closer they get to the beach.
1. This results from bending of
the waves, called refraction
2. As waves
enter shallow
water they slow down, because the circular orbitals in the lower
portion of the wave start to feel the ocean floor
B. The approach of waves at an angle to the
coast generates longshore currents
1. Longshore currents are shallow
water currents that mostly move parallel to the shore.
a. Swimmers playing
in the surf (shallow water) zone at a beach may notice the longshore
current movement when they realize they've been slow drifting down the
beach after ~30-45 minutes.
b. Sand migrates
along a beach in a parallel direction to the longshore currents.
Sometimes the sand migration is observed to zig-zag down the beach.
2. Whereas longshore currents
move parallel to shore, rip
currents
move perpendicular to shore
C. Coastal features.
1. spit -- extension of sand that
grows out into a bay. Spits point in the down current direction
2. baymouth bar -- an extension
of sand that cuts across a bay, isolating it from the ocean.
3. tombolo --
4. barrier island --
D. What are some methods for stabilizing
coastlines from beach erosion?
1. Methods of hard stabilization
-- involve stone or concrete structures, typically
a.
groins
b.
jetties
c.
breakwater
d. sea
wall
2. Alternatives
a.
beach nourishment
b.
relocation
III. Beach/coastline classification
A. Seasonal classification
1. Summer beaches
are often wider and have finer sand
a. Result of a calmer environment, no storms, and longer
wavelengths
b. Sediment eroded in winter/storm season gets restored to beach
2. Winter beaches
are often narrower and have coarser sands
a. Result of storms, a more energetic environment, and shorter
wavelengths
b. Excess sediment often accumulates in the surf zone as a
longshore bar
B. Tectonic classification
1. Emergent
coastline
a. Results from land being uplifted or falling
sea level
b. Feastures include coastal cliffs, wavecut platform, and marine
terraces
2.
Submergent coastline
a. Results from land subsiding or rising sea level
b. Features include estuaries and barrier islands.