Effects of Waves
103
The motion quickly decays with depth. At depths greater than one half of the wave
length there is virtually no motion. Thus, surface waves involve only the uppermost
part of the water column. Internal waves also exist at the thermocline, and at other
density discontinuities in the upper few 100 m. Such waves have been shown to
induce currents in submarine canyons. However, on the whole, their effects are
poorly known.
A surface wave running up onto the shore from deep water "feels bottom" when
the depth of water becomes less than one fourth of the wave length. At this point, the
circular motion of the water particles becomes more and more elliptical, and there is
a back-and-forth motion directly on the sea floor. In general, the maximum depth to
which sand is moved, the "wave base", is near 10 to 20 m. In exceptionally strong
storms, wave motions can reach considerably deeper. Ripple marks are seen on
shelves evep out to the shelf break. However, the relative importance of surface
waves, intemal waves, tides, and currents in producing such ripples is not always
clear. Recently, symmetrical ripples were discovered in fine sands of the outer shelf
off Oregon, at depths to 200 m, with crest-to-crest lengths of 10 to 20 cm, and with
the crests parallel to the coast. These oscillation ripples are thought to be produced by
winter storms on the exposed shelf. The measurement of ripples (Fig. 4.6) yields
clues as to the water motion which produced them.
The nature of the waves working the sea floor helps determine not only the
distribution of various types of ripples, but also the character of the sediment. Above
wave base, fine sediment is put into suspension periodically, and is transported away
by currents. It settles in quiet environments at greater depths. Below wave base, mud
can accumulate. The composition and the rate of production of bottom-living organisms changes greatly across the wave base, pres umably largely because of the change
in sediment character.
The rule "coarse sediment equals shallow water, fine sediment equals deep water"
derives from this general decrease of energy input to the sea floor with depth. However, it is the energy that is important, not the depth as such. Along any shore line, the
less exposed bays also tend to collect fine material, while coarse sediment characterizes the headlands.
4.2.2 Beach Processes. Nowhere are the effects of water motion more obvious than
on the beach (Figs. 4.7 to 4.11). Each uprushing wave moves the sand, leaving swash
marks at its upper limit and V -shaped backwash rills upon retreat. The balance
between erosion and deposition of beach materials produces a typical profile, featuring a berm and a foreshore (Figs. 4.7b and 4.9). The balance between erosion and
deposition shifts through time. After storms, in many cases, a considerable loss of
beach sand can be noticed. The sand is eroded and moves offshore. Typical for this
"dissipative" beach stage are high, steep waves with short periods.
How do beaches hold on to their sand? The answer to this question is that normal,
more gentle wave action with no prominent surf zone moves sand back inshore. The
resulting condition is called the "reflective" stage. Between two extremes, some
intermediate stages with or without longshore bars and throughs or bars transverse to
the beach may form.
103
The motion quickly decays with depth. At depths greater than one half of the wave
length there is virtually no motion. Thus, surface waves involve only the uppermost
part of the water column. Internal waves also exist at the thermocline, and at other
density discontinuities in the upper few 100 m. Such waves have been shown to
induce currents in submarine canyons. However, on the whole, their effects are
poorly known.
A surface wave running up onto the shore from deep water "feels bottom" when
the depth of water becomes less than one fourth of the wave length. At this point, the
circular motion of the water particles becomes more and more elliptical, and there is
a back-and-forth motion directly on the sea floor. In general, the maximum depth to
which sand is moved, the "wave base", is near 10 to 20 m. In exceptionally strong
storms, wave motions can reach considerably deeper. Ripple marks are seen on
shelves evep out to the shelf break. However, the relative importance of surface
waves, intemal waves, tides, and currents in producing such ripples is not always
clear. Recently, symmetrical ripples were discovered in fine sands of the outer shelf
off Oregon, at depths to 200 m, with crest-to-crest lengths of 10 to 20 cm, and with
the crests parallel to the coast. These oscillation ripples are thought to be produced by
winter storms on the exposed shelf. The measurement of ripples (Fig. 4.6) yields
clues as to the water motion which produced them.
The nature of the waves working the sea floor helps determine not only the
distribution of various types of ripples, but also the character of the sediment. Above
wave base, fine sediment is put into suspension periodically, and is transported away
by currents. It settles in quiet environments at greater depths. Below wave base, mud
can accumulate. The composition and the rate of production of bottom-living organisms changes greatly across the wave base, pres umably largely because of the change
in sediment character.
The rule "coarse sediment equals shallow water, fine sediment equals deep water"
derives from this general decrease of energy input to the sea floor with depth. However, it is the energy that is important, not the depth as such. Along any shore line, the
less exposed bays also tend to collect fine material, while coarse sediment characterizes the headlands.
4.2.2 Beach Processes. Nowhere are the effects of water motion more obvious than
on the beach (Figs. 4.7 to 4.11). Each uprushing wave moves the sand, leaving swash
marks at its upper limit and V -shaped backwash rills upon retreat. The balance
between erosion and deposition of beach materials produces a typical profile, featuring a berm and a foreshore (Figs. 4.7b and 4.9). The balance between erosion and
deposition shifts through time. After storms, in many cases, a considerable loss of
beach sand can be noticed. The sand is eroded and moves offshore. Typical for this
"dissipative" beach stage are high, steep waves with short periods.
How do beaches hold on to their sand? The answer to this question is that normal,
more gentle wave action with no prominent surf zone moves sand back inshore. The
resulting condition is called the "reflective" stage. Between two extremes, some
intermediate stages with or without longshore bars and throughs or bars transverse to
the beach may form.