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Seiches
Tsunamis
Wind-generated waves
Changes in parameters
Changes in direction
Longshore transport
BEACH PROCESSES
HUMAN INTERVENTION IN BEACH PROCESSES
SEICHES = STANDING WAVES (harbors, bays, oceans)
Natural resonance period (T) -- depth and dimensions
If entering wave has same T, it is "reflected" in basin
Rise and fall of water levels
Surface currents
Winds, tides, and waves can cause seiches
TSUNAMIS
H = 1-2 m in open ocean
H --> 20+ m as they move onshore!
Energy focused by bottom topography and man-made barriers
Prediction of a tsunami (?) -- earthquake event
Large vertical displacement required
Many large quakes --> no tsunami
Reflected by topography of continental margin
ENCROACHMENT OF WIND-GENERATED WAVES
(1) Changes in wave characteristics -- "feel bottom"
in shallow water (D < L/2). "Deep-Water" --> "Shallow
Water" waves
C & L decrease
T remains constant
H increases (wave energy confined to smaller area)
When D < L/20, C & L controlled only by depth.
Top of wave advances faster than deep part
--> "breakers," "surf" when H/L > 1/7
(D ª H)
(2) Changes in wave direction -- refraction. Bending of wave
"fronts" (crests) and "rays" (^ to fronts,
direction of wave energy)
.... waves approach at oblique angle (not parallel)
.... irregular topography of sea floor and coast
Why does refraction occur? -- C of different parts of front changes
as waves advance through water of different depth.
Shallow waters:
C decreases a lot
Fronts (energy) focused toward shallows, e.g. headlands
Deeper waters:
C decreases less
Fronts (energy) bend away, e.g., embayments
Consequences
Erosion of headlands -- focus of wave energy
Deposition in bays -- little wave energy
Current parallel to coasts
(3) Longshore transport
Wave fronts and onshore "swash" -- oblique to beach
front
"Backswash" ^ to beach front
Resultant = longshore transport, water and sediment
Erosion & transport -- strong wave action
Deposition -- low wave energy.
spits, quiet bays
convergence in bays
--> rip currents moving water a sediment offshore
BEACH PROCESSES -- Natural
Beach = accumulation of sediments from rivers, headland erosion
Beach dynamics -- continuous sediment movement due to waves and
longshore currents.
Summer -- gentle waves, onshore transport & deposition
Winter -- storm waves, erosion and seaward transport
Longshore transport -- depends on wave energy
HUMAN INTERVENTION IN BEACH/COASTAL PROCESSES
(1) Dam coastal rivers [water supply, flood control, recreation]
Reduced sediment input to coasts
Longshore tranport continues
"Downstream" beaches are eroded
(2) "Groins" to minimize loss of beach sand
Upstream deposition -- current speed reduced
Downstream erosion -- current resumes and regains
its suspended load of sediment
(3) "Jetties" to protect harbors
Same pattern of deposition and erosion
(4) Breakwaters to protect harbors
Santa Monica CA
Reduce wave energy --> reduce longshore currents
--> deposition in harbor
Santa Barbara CA
Sand deposition requires continuous dredging
Erosion of downstream beaches as longshore
transport resumes
Intervention in response to a single problem or demand of society often leads to a chain of unforeseen consequences due to erosion or deposition.
(Detailed notes)
Coastal regions, where land and sea meet, are very dynamic places. Much of the "dynamism" of coasts is due to the encroachment of waves. As waves in the ocean come onshore, they create a variety of features -- erosion of cliffs, or headlands; formation of beaches; transport of sediment parallel to coasts, etc. Very large waves, such as storm waves and tsunamis, can be particularly destructive in coastal communities. But even more subtle effects, like the continuous longshore transport of sediment, can have undesirable consequences for coastal development. We should recognize that the effects of wave activity are natural phenomena; they are problems for humans only because we have chosen to inhabit dynamic coastal areas.
Seiches are "standing waves" that can occur in harbors, bays, partially enclosed seas, and even ocean basins. We experience a seiche as a periodic rise and fall of water levels around the margins of a basin and periodic flow of surface currents, sometimes rather strong. All "basins" have a natural resonance period that is determined by the depth and dimensions (width, length) of the basin. If an entering wave has the same period, it will be reflected in the basin and a standing wave will be set up. Waves from the open ocean and tides can cause a coastal basin to "seiche." Winds that pile-up water on one side of a basin (or a lake) can have the same effect.
Tsunamis are very long ocean waves generated by submarine earthquakes. In the open ocean, a tsunami is inperceptible because its height (usually 1-2 m) is very small compared to its wavelength (100-200 km). But as a tsunami moves through shallower and shallower water, its height is amplified -- some tsunamis are more than 20 m high! The energy of tsunami waves can also be focused by bottom topography and man-made barriers Waves of this height can be -- and have been -- devastating to coastal communities.
An obviously important concern for coastal and island communities is the ability to predict whether an earthquake at sea will produce and potentially damaging tsunami. Unfortunately, we can't make such predictions with any degree of accuracy. Why not? Because not all large earthquakes produce a tsunami. Marine geologists are finding that large vertical displacements of the sea floor are required to produce a tsunami. Such displacements can sometime occur with a minor quake, or not at all in a major quake. We usually don't know about vertical displacements until well after a quake -- too late for a "tsunami alert." In addition, tsunami waves can be reflected by bottom topography and the continental margin, thus sparing coastal communities of their devastating effects.
As wind-generated, deep-water waves from the open ocean move on shore, their characteristics (wave parameters) and direction change. In addition, encroaching waves produce currents that move water and sediments parallel to coasts.
Changes in wave parameters . . occur because the orbital motion
of deep-water waves begin to "feel the bottom" as they
move through shallower and shallower water. Most (but not all)
of the parameters become increasingly controlled by water depth:
C and L decrease
T remains constant (remember that the period of a wave does not
change!)
H increases because wave energy is confined to a smaller area
(as L decreases)
Waves become "shallow-water" waves when D < L/20.
C and L become entirely controlled by depth.
As waves move further onshore, the top of the wave advances more
rapidly than the deep portion because of friction with the sea
floor.
Waves become unstable and break to form surf when H/L > 1/7.
This occurs when the depth of water is approximately equal to
wave height. As waves break and strike the coast (or run up a
beach), their energy is dissipated.
Changes in wave direction: refraction of waves. As waves move
onshore, the wave fronts tend to bend, or be difracted. This happens
because different parts of a wave front are advancing through
waters of different depth. Because depth controls velocity, those
parts moving in shallow water travel slower than those parts in
deep water. Refraction occurs whenever waves advance on a coast
at an oblique angle (i.e., not parallel to the coast).
Refraction also occurs as waves move over an irregular sea-floor
topography (with shallow and deep regions) onto an irregular coastline
(with headlands, or clffs, and bays). In this case, refraction
focuses wave fronts and wave energy on headlands, thus eroding
them. In addition, refraction results in the divergence of wave
energy from bays.
Longshore transport . . is the consequence of waves advancing on a coastline at an oblique angle (almost always the case). Waves run-up a beach at an angle -- this is termed wave "swash." But water runs back down the beach ("backswash") at right angles to the beach front, i.e., directly downslope by gravity. The resultant "imbalance" in direction of water motion produces a longshore current parallel to the beach. Longshore currents are capable of carrying suspended sediments -- and remember that the amount of sediment transported is directly proportional to current speed. So, in places where wave energy is diminished -- by either natural or man-made effects -- longshore-current speed decreases and sediment is deposited. Longshore currents that converge in embayments produce rip currents, strong seaward-flowing currents that can carry sediment offshore.
Beaches are regions of coasts where sediments (usually sand
or coarser particles) are accumulating. The source of beach sediment
is rivers and the erosion of headlands. Beaches are particularly
dynamic environments, constantly changing in size and shape as
a result of wave action and longshore transport.
During the summer, waves tend to be gentle and transport sediment
landward. Beaches are at their widest during the summer.
During the winter, storms at sea produce larger waves that erode
beaches and transport the sediment seaward.
Longshore transport also alters the shape and profile of a beach
Where wave action is strong, sediment is eroded and transported
Where wave action is reduced, sediment is deposited.
Wave action and longshore transport can erode beaches and barrier
islands and be a hazard to boat harbors. Soceity has taken measures
to minimize those undesirable effects (with mixed results):
"Groins" to stop beach erosion, and "jetties"
to protect harbors. These barriers are constructed perpendicular
to the shore. Jetties do keep large waves out of harbors, and
groins are generally effective in "stabilizing" beaches.
In both cases, sediment is deposited as longshore currents slow
on the "upstream" side of a barrier. But when the current
resumes on the downstream side, erosion occurs.
"Breakwaters" to protect harbors. Breakwaters are constructed
more-or-less parallel to the coast to protect the area behind
them from waves. But because waves no longer reach the shore,
longshore currents slow and sediment is deposited. The harbor
of Santa Monica CA is being silted in by this combination of effects.
In the harbor at Santa Barbara CA, sediment is continuously deposited
beyond the downstream part of the breakwater. In order to keep
the harbor open, constant dredging is required.
Humans also interfere with natural beach processes by damming coastal rivers. This reduces the amount of sediment reaching the coast. Longshore transport tends to erode beaches in the downstream direction from the sediment-depleted river.
30-1. What are standing waves, or seiches? What factors determine
whether a standing wave will be set-up in a bay or harbor?
30-2. Tsunamis in the open ocean are hardly perceptible at the
surface. Yet they can be devastating when the come onshore. Why
is that ?
30-3. Why is it difficult to predict whether an earthquake will
produce a dangerous tsunami?
30-4. When wind-generated waves from the open ocean encroach on
a coast, the characteristics of the waves change. Briefly, why
does this occur?
30-5. Describe the changes in C, L, and H that occur. Does wave
period change? Explain your answer (see question 28-5).
30-6. Why do waves become unstable and break" to form surf
as they come onshore? Relative to wave height, at about what depth
do waves "break?"
30-7. Why are waves often bent (diffracted) as they come onshore?
Briefly describe the types of situations where diffraction occurs
and the relationship between velocity and depth that accounts
for diffraction.
30-8. Because of diffraction, where is wave energy concentrated
(and thus erosion occurs) as waves encroach on an irregular coastline?
30-9. Describe why waves approaching a straight coastline at an
oblique angle produce a longshore current (and transport of sediment)
parallel to the coast. A sketch would be helpful.
30-10 Sediment often accumulates along a coast where ever wave
action is blocked. Considering question 29-12, explain why this
happens.
30-11. What are rip currents? Where and how do they form?
30-12. What are the sources of sediment (sand, cobbles, etc.)
on a beach?
30-13. Beaches are a dynamic system, in which sediment is moving
continuously. Describe how seasonally changes in wave action along
a beach drives offshore and onshore movement of sediment. Thus,
at what season are beaches likely to be widest? How about narrowest?
30-14. Consider the following situation: Longshore currents are
eroding sand from a beach. In order to stop that erosion, obstacles
("groins") are constructed perpendicular to the beach
face to impede the current. Sketch this situation, showing the
direction of longshore transport and the distribution of sand
between two groins.
30-15. Apply the same principles to the distribution of sand that
would result when two jetties are constructed to protect the entrance
to a harbor. Relative to the direction of longshore transport,
where does deposition occur and where does erosion occur?
30-16. What are the consequences of damming a major river that
flows to a coast on the "budget" of beach sediment along
that coast?
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