98
Effects of Waves and Currents
Waves and currents leave their imprint on the sea floor in many ways, as depositional and erosional features. Familiar examples are wave forms on the sediment
surface, from the smallest ripple marks to large submarine dune fields. Others are
bedding structures within the sediment, from beach laminations to thick graded
layers, and also the grain size of the sediment, from muddy lagoonal deposits to the
highly sorted on wave-washed beaches. Scourmarks, channels, and clean-swept
banks and submarine plateaus are well-known products of erosion.
Just how effective are waves and currents as sculptors on the ocean floor? And
what are the clues to be studied if we wish to reconstruct the wave and current
regimes from the geologic record?
4.1 Sediment Transport
4.1.1 Role of Grain Size. The details of the relationships between water motion and
sediment response are by no means clear, despite considerable study. One problem is
the complicated feedback between water motion at the interface, and the changing
character of the sediment surface. It is difficult to predict what will happen in one
situation, from studying another, because grain size distributions, porosity, and cohesiveness of the sediment show large variability.
Perhaps the most basic question is, how strong does a curent have to be to move
sediment?
It is reasonable to expect that the coarser grains need more of a push than finer
ones (fig. 4.2). Pebbles with a diameter of 10 mm do not start to move until the
average velocity of the current near the sea floor is near 2 m/s. Grains of 1 mm size
move at 0.5 m/s. We can readily conclude, with a view to the geologic record, that
fine grains will be moved more often than coarse ones in a downstream direction, as
strong currents are less frequent than weak ones. Hence, grain size will normally
decrease in the downstream (or down-current) direction. This is a first clue to direction of transport, a clue which can be applied along the coast, on the shelf, or even in
the deep sea.
However, this simple and obvious relationship between grain size and water velocity is valid only down to sizes of 0.1 to 0.2 mm. When grain sizes decrease below
this value, water velocities may have to increase again to initiate erosion (Fig. 4.2).
Why should this be so?
When settled on the floor, the very fine sediments tend to produce a smooth
surface, which reduces turbulence at the interface and thus the opportunity for impact
of fast water particles on the sediment particles. More importantly, the smaller the
grains, the larger the surface area available for grain-to-grain cohesion, which can
form a strong bond after compaction. In addition, organic matter tends to settle out
with these fines and may provide for increased cohesion, by bacterial growth. Thus,
fine-grained sediment resists erosion more than coarse sediment. Another conclusion
follows: the sand grains between 0.1 and 0.2 mm are rather mobile on the sea floor;
they are the nomads among the sediment grains. They move easily at current velocities of only a little over 0.3 m/s. They are the sands that travel farthest, and they are
commonly found, therefore, on intertidal flats far removed from the source.
Effects of Waves and Currents
Waves and currents leave their imprint on the sea floor in many ways, as depositional and erosional features. Familiar examples are wave forms on the sediment
surface, from the smallest ripple marks to large submarine dune fields. Others are
bedding structures within the sediment, from beach laminations to thick graded
layers, and also the grain size of the sediment, from muddy lagoonal deposits to the
highly sorted on wave-washed beaches. Scourmarks, channels, and clean-swept
banks and submarine plateaus are well-known products of erosion.
Just how effective are waves and currents as sculptors on the ocean floor? And
what are the clues to be studied if we wish to reconstruct the wave and current
regimes from the geologic record?
4.1 Sediment Transport
4.1.1 Role of Grain Size. The details of the relationships between water motion and
sediment response are by no means clear, despite considerable study. One problem is
the complicated feedback between water motion at the interface, and the changing
character of the sediment surface. It is difficult to predict what will happen in one
situation, from studying another, because grain size distributions, porosity, and cohesiveness of the sediment show large variability.
Perhaps the most basic question is, how strong does a curent have to be to move
sediment?
It is reasonable to expect that the coarser grains need more of a push than finer
ones (fig. 4.2). Pebbles with a diameter of 10 mm do not start to move until the
average velocity of the current near the sea floor is near 2 m/s. Grains of 1 mm size
move at 0.5 m/s. We can readily conclude, with a view to the geologic record, that
fine grains will be moved more often than coarse ones in a downstream direction, as
strong currents are less frequent than weak ones. Hence, grain size will normally
decrease in the downstream (or down-current) direction. This is a first clue to direction of transport, a clue which can be applied along the coast, on the shelf, or even in
the deep sea.
However, this simple and obvious relationship between grain size and water velocity is valid only down to sizes of 0.1 to 0.2 mm. When grain sizes decrease below
this value, water velocities may have to increase again to initiate erosion (Fig. 4.2).
Why should this be so?
When settled on the floor, the very fine sediments tend to produce a smooth
surface, which reduces turbulence at the interface and thus the opportunity for impact
of fast water particles on the sediment particles. More importantly, the smaller the
grains, the larger the surface area available for grain-to-grain cohesion, which can
form a strong bond after compaction. In addition, organic matter tends to settle out
with these fines and may provide for increased cohesion, by bacterial growth. Thus,
fine-grained sediment resists erosion more than coarse sediment. Another conclusion
follows: the sand grains between 0.1 and 0.2 mm are rather mobile on the sea floor;
they are the nomads among the sediment grains. They move easily at current velocities of only a little over 0.3 m/s. They are the sands that travel farthest, and they are
commonly found, therefore, on intertidal flats far removed from the source.