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Book Chapter

Sedimentary Structures Generated by flow in Alluvial Channels

By
D. B. Simons
D. B. Simons
Fort Collins, Colorado
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E. V. Richardson
E. V. Richardson
Fort Collins, Colorado
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C. F. Nordin, Jr.
C. F. Nordin, Jr.
Fort Collins, Colorado
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Published:
January 01, 1965

Abstract

Several geometric forms or bed configurations are molded from the bed material by the flow in alluvial channels. These forms are broadly classified by their shape and effect on flow resistance and by the mode of bedload transport. The most common bed forms include ripples, dunes, a transition form as the dunes change to a plane or flat bed, plane bed, standing waves, and antidunes. The bed forms change from ripples to dunes and ultimately to antidunes as the tractive force is increased.

Important interrelated variables likely to affect bed forms include: characteristics of bed material such as physical size and sorting; the temperature or fluid viscosity; the concentration of very fine sediment; and the depth as well as velocity and slope. Methods of predicting bed forms from known characteristics of the flow and sediment are inexact, but as a first approximation, the bed form is related to stream power and median fall diameter of bed materia).

Different types of cross bedding are associated with the various bed forms established under equilibrium flow conditions in recirculating flumes. The tendency for all flows to meander, even in straight channels, complicates both the bed forms and the types of cross bedding. This tendency to meander is reinforced by the large bars (depositional structures) which exist adjacent first to one bank and then to the other on the beds of alluvial channels. These bars may be of such small amplitude as to go almost unnoticed in a given system, particularly when small width-depth ratios exist. However, if by reducing depth or by widening the channel, the width–depth ratio is increased, these large bars may develop to almost the full depth of the channel. The regular forms of bed roughness are in general superposed on these large bars, but the bed forms change dramatically as the depth, local slope, and direction of flow change over the bar as deposition and development of the bars progress.

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Contents

SEPM Special Publication

Primary Sedimentary Structures and Their Hydrodynamic Interpretation

Gerard V. Middleton
Gerard V. Middleton
McMaster Ulliversity
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SEPM Society for Sedimentary Geology
Volume
12
ISBN electronic:
9781565761421
Publication date:
January 01, 1965

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