Fundamentals of Engineering Design
The computation process begins by partitioning a continuous flow record into a series of steady
flows with variable discharges and durations, i.e., composing a discharge hydrograph. Starting with the first
flow in the hydrograph, a water surface profile is calculated. The water surface profile provides information
for each cross section, such as the energy slope, velocity, depth, etc.
Potential sediment transport rates are then calculated at each cross section. Combining the
sediment transport rates with the duration of flow gives a volumetric summary of sediment within each
reach. Sediment calculations use grain size fractions which allow the simulation of hydraulic sorting and
armoring. The amount of scour or deposition at each cross section is then computed and the cross section
geometry is adjusted accordingly. The computations move to the next flow in the hydrograph and the cycle
is repeated using the updated geometry (USACE, 1993).
Geometry data are represented by cross sections comprised of station-elevation coordinates,
distances between cross sections, and Manning's n-values. The movable bed portion of each cross section
and the depth of sediment material in the bed are also defined. HEC-6 raises or lowers cross section
elevations to show deposition or scour. Horizontal locations of the channel banks are considered fixed.
Floodplains on both sides of the channel are considered to have fixed ground locations but can be moved
vertically if within the movable bed limits specified by the user. Left and right overbank stations are defined
in the geometry data, as well as any ineffective flow areas or containment of flow by levees (USACE,
1993).
The one-dimensional energy equation is solved by the standard step method and used to compute
the water surface profiles for each flow in the hydrograph. Downstream water surface elevations must be
determined for each discharge in the hydrograph by either a rating curve specified by the user, or a time
dependent water surface elevation.
Sediment data includes the fluid and sediment properties, inflowing sediment load, and the gradation
of the stream bed material. Sediment transport rates may be calculated for grain sizes up to 2,048 mm.
Particle sizes larger than 2,048 mm existing in the bed material are used for sorting computations but are
not transported. Sediment transport functions used to calculate the bed material load are specified by the
user. Numerous sediment transport functions available in HEC-6 are available (USACE, 1993).
Thomas (1996) developed a HEC-6 simulation of Hotopha Creek, one of the DEC streams. The
results of that investigation indicated that a reduction in sediment yield of 16% resulted from the construction
of a series of grade control structures along Hotopha Creek.
During a 30-year simulation of the Hotopha Creek watershed, the results suggested that channel
degradation may resume downstream of several drop structures because of the success of those structures
in halting upstream erosion. The advantage of long-term simulation to check grade control and other
erosion prevention features is readily evident. When the goal of a project is to reduce sediment yield, and
the project is successful, the channel reaches downstream of the project will be susceptible to degradation.
HEC-6 modeling of the complete channel system in a watershed allows channel spacial and temporal
response to be predicted.
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