Environmental Engineering Reference
In-Depth Information
Center's open-channel hydrodynamic model, the
River Analysis System (HEC-RAS), making use of
the robust, existing hydrodynamic capabilities of
RAS and providing helpful user interfaces for one-
dimensional modelling of sediment transport.
The initial version with a sediment routing
capability (HEC-RAS 4.0) uses one-dimensional,
cross-section-averaged hydraulic parameters ob-
tained from RAS's hydraulic engine to compute
sediment transport rates and update the channel
geometry based on sediment continuity calcula-
tions. The hydraulic computations are explicitly
coupled with calculations of sediment transport,
erosion, deposition, bed mixing and cross-section-
al change. The result is a continuous simulation of
cross-sectional change as sedimentation processes
respond to the inflowing water and sediment
hydrographs.
HEC-RAS/SIAM does not update the hydrau-
lics or channel geometry to reflect any scour or
deposition predicted in a sediment reach, meaning
that its results indicate only the short-term trend
of morphological change due to a sediment imbal-
ance. An outcome of this is a tendency to some-
what over-represent erosion and deposition
compared to values actually observed in the field.
Also, as a reach-based model, HEC-RAS/SIAM
uses reach-averaged parameters and produces
reach-averaged results; it yields no information
on the distribution of erosion or siltation within
a reach. Consequently, the impacts of local scour
or deposition are not simulated.
Finally, in its current form, HEC-RAS/SIAM
implicitly assumes that the channel is alluvial:
that is, it is free to adjust to scour driven by an
excess of bed material transport capacity com-
pared to the supply from upstream and local
sources. Consequently, users must identify any
non-alluvial sediment reaches, where scour is
prohibited by naturally erosion-resistant materi-
als or artificial stabilization encountered in the
bed or banks. This may be achieved based on
stream reconnaissance (Thorne 1998) coupled
with bed material sampling to identify areas of
erosion-resistant substrate and locate artificial bed
and bank protection structures.
Computational methods
Hydrodynamics
Flow specification for sediment transport compu-
tations currently follows the 'quasi-unsteady'
flow approach used in HEC-6. An event or period
of record is approximated by computing a series of
steady flow profiles. HEC-RAS uses each steady
flow profile to develop transport parameters for
each cross-section. Durations are assigned to each
profile to define the temporal extent of the asso-
ciated hydrodynamics and to route sediment
movement. Usually, however, bathymetry up-
dates are required more frequently than the flow
increment duration, so a computational time step
must also be specified. Channel geometry and a
new steady flow profile are computed at the be-
ginning of each computational time step, even if
the flow remains unchanged (Fig. 5.10).
Hydraulic Engineering Centre, River Analysis
System (HEC-RAS) Version 4.0
Development and basis
In 1976, the US Army Corps of Engineers devel-
oped themobile boundarymodel HEC-6 (USArmy
Corps of Engineers 1993; Thomas 1994). This DOS
program has remained an industry standard
throughout the USA even while other popular
HEC hydrological and hydraulic models (e.g.
HEC-1, HEC-2 andUNET - unsteady flow through
an open channel Network) have been superseded
by more powerful and user-friendly products (e.g.
HEC-HMS and HEC-RAS). Recently, however,
most of the capabilities available in HEC-6 have
been incorporated into Hydrologic Engineering
Transport calculations and cross-section
updating
The seven transport functions currently available
in HEC-RAS are: Ackers and White (1973),
Engelund and Hansen (1967), Laursen (1958),
Meyer-Peter and Muller (1948), Toffaleti (1968),
