FLOOD02752

• County: Statewide
• Basin: Statewide
• Title: HEC-6 Scour and Deposition in Rivers and Reservoirs
• Date: 3/1/1977
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

to describe the physical process is not completely developed. It
is possible, however, to extend open-channel hydraulics programs to
include some aspects of sedimentation and thereby increase the
engineer's capability, by use of digital modeling, to analyze a
much wider range of open-channel flow problems.

5. CAPABILITY OF COMPUTER PROGRAM

The movable boundary problem is simplified into one involving a
movable bed in a channel. The horizontal location of the channel
banks is considered fixed, and the flood plain on each side of
the channel is considered as having a fixed ground surface. This
is similar in concept to the movable bed hydraulic model. However,
if movement of the bed is expected in the flood plains, as in the
case of deposition in a reservoir, the entire width can be considered
as channel.

By entering a sequence of water discharges from a discharge hydrograph,
changes are calculated with respect to time and with respect to
distance along the model for each of the following: total sediment
load; volume and gradation of sediment that is deposited; armoring
of the bed surface; and the resulting bed elevation. In addition,
sediment outflow at the downstream end of the model is calculated.
The location and amount of material that has to be dredged is calculated
if desired.

Geometry of the physical system is represented by cross sections,
specified by coordinate points (stations and elevations), and the
distance between cross sections. Hydraulic roughness is measured
by Manning's n-values and can vary from cross section to cross
section. At each cross section n-values may vary vertically and
horizontally. The program raises or lowers cross section eleva-
tions to reflect deposition or scour.

The water discharge hydrograph is approximated by a sequence of
steady flow discharges each of which occurs for a specified number
of days. Water surface profiles are calculated by using the standard
step method to solve the energy equation. Friction loss is calculated
by Manning's equation, and expansion and contraction losses will be
included if the representative loss coefficients are specified. The
velocity distribution across each section is calculated unless this
option is suppressed.

It is necessary to specify the water surface elevation at the down-
stream end of the model to initiate water surface profile calculations.
In the case of a reservoir, the operating policy is utilized, but if
open river conditions exist, a stage-discharge rating curve is usually
specified for the downstream end of the model.

4

-,-:-::;"~':~- ;~~~~\\>""-;;"'-" "

'''''-- '4.~ ~~~.(~ _ ___]~ "";-".'>,, ~;>.j.:;,,,,-