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The forms of bed roughness which are molded from the bed material by the flow in alluvial channels are broadly classified by their shape and their effect on flow resistance and by the mode of bedload transport associated with the bed form. The most common forms of bed roughness include ripples, dunes, a transition roughness as the dunes change to a plane or flat bed, plane bed, standing waves, and antidunes. The forms of bed roughness change from ripples to dunes and ultimately to antidunes as the tractive force is increased.
The physical relationships between form of bed roughness and the variables upon which form roughness depends are extremely complex. The independent variables are interrelated so that it is impossible to completely isolate the effect of a single independent variable on form roughness in a flume or stream. Important interrelated variables include: characteristics of bed material such as physical size, fall diameter and gradation; the temperature or fluid viscosity; the concentration of very fine sediment; and the depth as well as other flow variables. Methods of predicting forms of bed roughness from known characteristics of the flow and sediment are inexact, but as a first approximation, the bed roughness is related to stream power and median fall diameter of bed material.
Different types of cross-bedding are associated with the various roughness elements, based on equilibrium flow conditions in recirculating flumes. The tendency for all flows to meander, even in straight channels, complicates both the forms of bed roughness and the types of cross-bedding. This tendency to meander is reinforced by the large bars which exist adjacent to one bank and then the other on the beds of alluvial channels. These bars may be of such small amplitude 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 th se large bars, but the roughness forms change dramatically as the depth, velocity, and direction of flow change over the bar as deposition and development of the bars progress.
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