Compositing compound channels and overbanks
Variable roughness and depth across the cross-section introduce turbulence
into the flow which results in additional energy loss due to the momentum
transfer. Existing theory is currently inadequate to properly quantify the
magnitude of these losses. However, it is important that these losses be
recognized and considered when choosing a compositing technique.
Compositing by the equal velocity and total force methods can overemphasize
the contribution to roughness of channel subareas in cases where homogeneous
flow conditions do not exist, such as on irregular overbanks. The same effect can
occur with the conveyance method in SAM when only a single subsection is
considered. James and Brown (1977) reported that without adjustments to either
the resistance coefficient or the hydraulic radius, using composite hydraulic
parameters in the Manning or Chezy equations did not accurately predict the
stage-discharge relation in a channel-floodplain configuration when there were
shallow depths on the floodplain--1.0 < Y/D < 1.4, where Y is water depth in the
channel and D is the bank height. However, the effects of geometry seemed to
disappear at the higher stages, i.e., for Y/D > 1.4, when it no longer became
necessary to make any correction to the basic equations. Figure 2.7 summarizes
that finding, stressing that the conveyance (separate-channels) method is the best
choice if Y/D falls within the 1.0 to 1.4 range and that another method should be
used if Y/D > 1.4.
Figure 2.7. Comparison of measured data and theoretical methods.
Effective Hydraulic Parameters for Sediment Transport
The problem of obtaining representative hydraulic parameters is critical when
making sediment transport calculations involving complex cross sections. The
velocity, depth, width and slope are needed for subsections having similar
hydraulic properties. This requirement leads to a compositing technique that
26
Chapter 2
Theoretical Basis for SAM.hyd Calculations
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