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<br />Changing Bed Forms
<br />
<br />Resistance to flow can vary greatly in sand
<br />channels because the bed material is unstable and may
<br />take on different configurations or bedforms
<br />throughout a single period of flow. The type of bedform
<br />is a function of many components that include flow
<br />velocity, grain size, shear stress, temperature, and other
<br />variables (Aldridge and Garrett, 1973), The magnitude
<br />of Manning's roughness coefficient may relate directly
<br />to the type of bed form that is manifested, which makes
<br />accurate assessment of n values in sand channels a
<br />difficult and complex procedure,
<br />On the basis of data obtained with the aid of
<br />laboratory flumes, Simons and Richardson (1966)
<br />proposed that median grain size and stream power can
<br />be used to determine the flow regime and the type of
<br />bed form that will develop in sand channels, Stream
<br />power is determined from the equation:
<br />
<br />SP = 62RSV,
<br />
<br />where
<br />
<br />SP = stream power, in foot'pounds per second
<br />per square foot;
<br />
<br />62 = specific weight of water, in pounds per
<br />cubic foot;
<br />
<br />R =
<br />S =
<br />V =
<br />
<br />hydraulic radius, in feet;
<br />water, surface slope, in feet per foot; and
<br />mean velocity, in feet per second,
<br />
<br />(8)
<br />
<br />Other investigators have modified results
<br />presented by Simons and Richardson (1966, p. J24,
<br />fig. 28) in order to develop practical criteria for
<br />determining Manning's roughness coefficient in sand,
<br />dominated streams (Benson and Dalrymple, 1967). As
<br />shown in figure 3, flow in sand channels can be
<br />classified as either lower'regime or upper-regime flow
<br />separated by a transition zone,
<br />In lower,regime flow (fig, 3), the bed may have
<br />a plane surface with little or no movement of sand or
<br />small uniform waves (ripples), or large irregular waves
<br />(dunes) that are formed by sediment moving
<br />downstream, Water, surface undulations manifested in
<br />lower,regime flow generally are out of phase with the
<br />bed surface, The fact that the water surface is out of
<br />phase with the bed surface is a positive indication that
<br />the flow is tranquil or subcritical (Froude number < 1;
<br />Simons and Richardson, 1966, p. J9),
<br />The bed configuration in the transition zone can
<br />be erratic and may range from that typical of the lower-
<br />flow regime to that typical of the upper, flow regime
<br />depending mainly on antecedent conditions (Simons
<br />and Richardson, 1966, p, JlI), Resistance to flow and
<br />sediment transport also have the same variability as the
<br />
<br />U)
<br />0
<br />Z
<br />:;)f- 2
<br />00
<br />"-0
<br />fCLL
<br />Ow
<br />00:
<br />LL<( 1
<br />z:;)
<br />-0 0,8
<br />sU)
<br />0)0: 0,6
<br />Cl;w
<br />N"-
<br />~o
<br />o:Z 004
<br />wO
<br />SO
<br />O~
<br />"-0:
<br />2W 0,2
<br /><("-
<br />w
<br />0:
<br />f-
<br />U)
<br />
<br />
<br />4
<br />
<br />0,6 0,8
<br />MEDIAN GRAIN SIZE, IN MILLIMETERS
<br />
<br />1,2
<br />
<br />Figure 3. Relation of stream power and median grain size to form of bed roughness, (Modified from
<br />Benson and Dalrymple, 1967,)
<br />
<br />Assumptions and Limitations 9
<br />
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