Recent Marine Sediments

The conditions of transport of detritus by moving water constitute one of the most vital problems confronting students of sedimentation. This article by Hjulstrom summarizes the main aspects of present knowledge of the problem as applied to rivers. Many of the relations for river water apply to ocean water; but the sea differs materially from rivers in at least three ways: the large masses of water involved, the slowness with which the water moves, and the effect of tides. Notwithstanding the fact that these three phenomena modify the picture as portrayed by Hjulstrom for rivers, the information he presents is of great value to students of sedimentation. This article is a summary of a longer article, which in itself for the most part also is a summary. Consequently in this abstract it seems futile to attempt to present more than a few comments about the main features he describes. Water moves in two main ways—by laminar flow and by turbulent flow. In laminar flow the water travels in parallel bands; in turbulent flow it moves in pulsations in a variable way. The velocity of water in laminar flow is always low; in turbulent flow it may be low or high. There are several types of turbulent flow, each of which depends mainly upon the velocity. The transport of detritus by moving water is affected by many factors, the most influential of which is the velocity of the water. Particles are transported individually or collectively. Transport of individual particles is in four types: sliding, rolling, saltation (jumping), and suspension. The first three are along the bottom and the fourth is in the water. With increasing velocity the mode of transport ordinarily passes successively through these four states of transport. The particles also may move collectively, that is in masses. In this way are formed such features as ripples, bars, and banks. The laws governing the different kinds of transportation are complicated. For particles larger than sand (0.5 millimeter) the size of particles that can be put in motion increases as the velocity of the water becomes greater; but for smaller particles the minimum velocity that is required in order to bring them into suspension does not decrease as the particles become smaller; instead it increases. Thus it is easier to move sand off the bottom than silt. Once a particle is in motion it continues to be transported until the velocity of the water decreases to a certain speed. This minimum transporting velocity for particles of sand size or larger seems to be about 30 per cent less than the velocity needed to remove the particles from the bottom; but for progressively smaller particles, the minimum transporting velocity becomes increasingly less in proportion to the velocity required to make the particles go into suspension. (Editor’s abstract.)