


Geomorphic Assessment of Channel Systems
3.1.4.2 HEC6
HEC6 (USACE, 1993) is a onedimensional moveable boundary open channel flow
numerical model designed to simulate and predict changes in river profiles resulting from
scour and deposition over moderate time periods, typically years, although applications to
single flood events are possible. A continuous discharge record is partitioned into a series of
steady flows of variable discharge and duration. For each discharge, a water surface profile
is calculated, providing energy slope, velocity, depth, and other variables at each cross
section. Potential sediment transport rates are then computed at each section. These rates,
combined with the duration of the flow, permit a volumetric accounting of sediment within
each reach. The amount of scour or deposition at each section is then computed and the
cross section geometry is adjusted for the changing sediment volume. Computations then
proceed to the next flow in the sequence and the cycle is repeated using the updated cross
section geometry. Sediment calculations are performed by grain size fractions, allowing the
simulation of hydraulic sorting and armoring.
HEC6 is a powerful tool that allows the designer to estimate long term response of
the channel to a predicted series of water and sediment supply. The primary limitation is that
HEC6 is onedimensional, i.e., geometry is adjusted only in the vertical direction. Changes
in channel width or planform cannot be simulated.
3.1.4.3 Integration of Results
The final part of a geomorphic assessment of a channel system is accomplished by
integrating the information from all the available analyses. Analysis using each of the
geomorphic tools discussed previously may yield a verdict of aggradation, degradation, or
dynamic equilibrium with respect to the channel bed, and stable or unstable with respect to
the banks. Often the individual assessments produce contradictory results. For instance, the
field investigations might indicate that a channel reach is vertically stable, but the empirical
relationships and SAM results indicate that the channel should be degradational. In this case
you would have to assign a level of confidence to the various components based on the
reliability and availability of the data, and your own experience with each tool in order to
reconcile these contradictory results. Once again we come back to the fact there is no
"cookbook" answer, and that we must always incorporate sound judgement based on insight
and experience when making a geomorphic assessment.
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