Sediment Transport

Terrigenous sediments are typically introduced to the ocean by either fluvial (river), eolian (wind) or glacial transport. Their distribution in the marine environment is primarily a function of the interaction between the strength of waves and currents and the size of individual sediment grains. Although produced locally, carbonate sediments are also redistributed by these same processes.

Sediment particles respond to hydraulic forces such as shear and lift, whose effects are related to current speed, particle size, shape and density. While quantitatively modeling sediment transport can be very difficult, it can generally be thought of as moving water exerting both lift and drag on a sediment grain at rest. As the velocity of fluid flow over the bed increases, both lift and drag increase. At some point, the fluid exerts sufficient force to cause the grains to move. This fluid force must overcome both gravity and any friction exerted by adjacent grains. Once the force is great enough to overcome these, the grain begins to move, either along the bed or in the water column.

Transport in the ocean obeys the same hydraulic laws that apply in streams, but the forces available for transport are more varied and the motions are more complex. The remainder of this chapter focuses on the effects of these processes on marine sediments. Serious theoretical sedimentologists will argue that any discussion of sediment transport that is decipherable by the average geologist is, by definition, totally inadequate inasmuch as it ignores the necessity for things in nature to be complicated beyond our practical understanding. While this cynical statement may reflect the authors' poor grasp of things theoretical, the problem nevertheless remains of expressing these ideas in a form that can be grasped conceptually by the student who is interested more in the general relationships that occur in nature than in generating rigorous predictive solutions. We, therefore, beg the forgiveness of those that prefer shear stress to velocity and dimensionless grain number over sediment size and hope that our simplistic approach is informative to the student and minimally repulsive to the rest.

Sediment grains move in three modes . Grains in suspension move with the water mass in which they are contained. This is generally fine-grained material, but larger grains can be carried if the velocity of water motion is very great. Bed-load transport involves particles that are too heavy to be put into suspension and are moved along the bottom in a rolling or sliding motion. The layer of active transport is only a few grains thick. Intermediate between the two is a process called saltation in which the particles move in a series of jumps. A particle is thrown into suspension either by fluid turbulence or grain impact, and moves with the water until it falls again to the bottom. This is an important process in the movement of sand in both nearshore and shelf settings because wave action can periodically throw sediment into the water column to be moved by weaker, unidirectional currents to a new spot. Repetition of this with each wave can result in effective grain transport.

Grain Settling

Because grains in motion spend much of their time in the water column, it is important to understand the interactions between the two. In water, the rate at which a particle settles is a function of both grain and fluid properties (i.e., grain size, shape and density for the grain; density, viscosity, flow rate and turbulence for the water). In calm water, the settling rate of fine-grained sediment is adequately approximated by the Stokes Equation:

where:

V = fall velocity in cm/sec
g = gravity (980 cm/sec/sec)
D = grain diameter in centimeters
ds = density of the particle (g/cm3; or 2.65 for quartz)
dw = density of the fluid in the same units or 1.0 for water at 20o C
µw = viscosity of water; 1 x 10-3 at 20o C

For gravel sized particles, the impact formula is:

Fall velocity can be plotted for these two formulae and the results compared to empirical measurement. Gibbs, et al., 1971 While the theoretical lines plot consistently above the experimental data, it can still be seen that there is a consistent relationship between smaller grain size and declining fall velocity.

Transport in the Marine Environment

Unfortunately, the water in most marine systems is neither "ideal" nor motionless. Current-flow patterns can vary greatly from one environment to another. In estuaries, currents generated by tides dominate. While unidirectional at any one time, these currents reverse with each change of the tide. Net transport in a tidal system reflects the relative strengths and durations of seaward vs. landward currents. Once we move outside the protection of the estuary, wave-induced motion is added to the effect of tidal currents and net transport is the result of various combinations of unidirectional and oscillatory flow.

The orbital motion of passing waves produces oscillatory flow that decreases exponentially in its magnitude from the water surface toward the bottom. At a water depth roughly one half of the wave's length, the orbits begin to interact with the bottom, causing bed shear. The orbital motion becomes increasingly elliptical toward the bottom until the motion is transformed into back-and-forth oscillations with more and more energy transferred to the bed as water depth shoals. The more intense flow under the crest results in greater the transport.

The water depth to which sediment will be moved by waves is a function of particle size and the wave regime that exists. During fair weather, wave base (the depth to which wave-induced oscillatory flow occurs) is on the order of 10-15 meters. During storms, this can increase dramatically. Off the Texas coast, hurricane waves are capable of moving sediment anywhere across the continental shelf. Along the shelf edge west of Britain, the intense wave climate can stir fine sand at a depth of more than 180 meters for more than 20 percent of the year. Leeder, 1991

There is strong evidence that total amount of sediment transportation during a single storm can be much greater than the total for the rest of the year. On the north coast of St. Croix, sediment export from the reef at water depths of 15-30 meters is on the order of 6,300 kg per meter of shelf per year (kg/m-yr) during normal sea conditions. Hubbard, et al., 1986 Small annual storms with waves reaching 5 m in height can remove at least 4,400 kg of sediment from the same area in a matter of hours. In 1989, Hurricane Hugo removed in excess of 48,000 kg/m from these channels, an amount equal to nearly eight years of day-to-day sediment export. Hubbard, et al., 1991

Predicting Sediment Transport

Particles that are settling in a moving column of water will do so at rates that are slower than those in calmer water. Several useful diagrams have been constructed to approximate the transportability of sediments under different flow regimes. The Shields Diagram relates the initiation of sediment motion to shear developed along the bed. Unfortunately, shear stress is nearly as difficult to measure accurately as it is to explain quantitatively.

The shear-stress relationship can be used to predict the velocity one meter above the bed (U100) caused by waves of various heights and lengths (expressed as period, T). If we can then determine the current velocity that is needed to initiate or sustain sediment transport, we can then relate wave character to bed stability.

While somewhat dated, the Hjulstrom Diagram is still useful because current speed is more easily visualized than other factors now in vogue. The principle that is illustrated states that a sediment grain of given size will require a certain current velocity to pluck it from the bed. The flow needed to keep that grain in suspension is slightly less. In sand and coarser sediment, grain size is directly proportional to the velocity needed to either erode grains or keep them in suspension. In sediments finer than medium to fine sand, grain-to-grain adhesion and electrostatic charges increase the resistance of the bed to erosion. Clay and silt particles may be especially difficult to erode, and compaction and cohesion lead to the formation of a smooth surface that promotes laminar flow. It is difficult to make quantitative estimates of the threshold for moving cohesive particles of compacted fine silt or clay, but the velocities can exceed those necessary to move gravel-sized material. The best that can be said is that mud beds will be eroded at current velocities on the order of 20 to 30 cm/sec, provided the water content of the sediments exceeds 80 percent. Below that, the bed becomes increasingly resistant to erosion.

Whether we use the Hjulstrom Diagram to illustrate the general principle or the Shields Diagram to make precise predictions, the message to take from all of this is that sediment transport is a predictable phenomenon that follows well-defined physical laws and at least conceptually operates in a predictable manner. Using Figure 3.18b, we can say that medium sand (0.25 - 0.50 mm) will be suspended by currents on the order of 20-25 cm/sec; it will stay in suspension until flow drops below 15-18 cm/sec. In a wave dominated environment like the shoreface at a depth of 10 meters, sand suspension can be initiated by waves only one meter high with a period of 4-5 seconds. From a general geological perspective, this level of accuracy is quite adequate.