Skip to Main Content

Abstract

Beaches and bars have been formed during experiments conducted in a 46-foot wave tank at the Sedimentation Laboratory of the U. S. Geological Survey in Denver. By changing one variable factor at a time, elements responsible for major differences in primary structure and in shape of sand body have been determined. These elements are—differences in slope of sand floor (expressed in terms of water depth), intensity of wave action, and supply of sand. Stages in the growth of the bars and beaches were marked with dark layers of magnetite, and cross sections were preserved on masonite boards coated with liquid rubber, thus recording cross-stratification patterns and sand-body shapes.

Longshore bars are produced at the point of wave break. In very shallow water an emergent bar commonly forms; in somewhat deeper water a submarine bar is built; and in still deeper water no bar forms. Increase in intensity of waves tends to build a bar toward, and even onto, the beach. Weaker waves build bars upward to form barriers, with lagoons to shoreward. Abundant sand furnished on the seaward side of a growing bar simulates conditions caused by some longshore and rip currents, and causes gentle seaward-dipping beds to form. In contrast, a limited sand supply results in growth of bars that characteristically have shoreward-dipping strata of steeper angle.

Beach strata normally dip seaward at low angles from the crest to a point below water level. Offshore, the seaward extensions of these gently dipping beds include fore-set beds with relatively high angles which form a shoreface terrace. The sand body comprised of both sets of bedding builds outward if a large supply of sand is furnished. In shallow water, however, or at moderate depth where waves are strong, the period of beach growth is limited by the deposition of longshore bars which eliminate wave action as they grow into barriers and form lagoons. Under conditions in which no bar is built, growth of the beach and shoreface terrace is controlled by the amount of sand available; the proportion of top-set to fore-set beds is determined by the strength of waves.

You do not currently have access to this chapter.

Figures & Tables

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal