Environmental Engineering Reference
In-Depth Information
partitioning coefficient. In-stream nutrient dynamics in SWAT are simulated using
the kinetic routines from the QUAL2E in-stream water quality model (Brown and
Barnwell
1987
).
SWAT allows detailed agricultural management practices to be simulated,
tracking planting, tillage, and fertilization operations and calculating resultant
plant growth with specific dates or with a heat unit scheduling approach during the
year. The plant growth component of SWAT utilizes routines for phenological
plant development based on plant-specific input parameters such as energy and
biomass conversion, temperature, water and nutrient constraints, canopy height
and root depth, and shape of the growth curve. A single plant growth module is
used in SWAT for simulating all crops and assessing removal of water and
nutrients from the root zone, transpiration, and biomass/yield production.
10.3.2 HEC-RAS Model
Few models provide the ability to couple river flow quantity with sediment and
water quality, and those that do are proprietary products that are both expensive as
well as difficult, if not impossible, to modify to suit local conditions. HEC-RAS is
a public domain model developed by the U.S. Army Corp of Engineers (USACE)
(
http://www.hec.usace.army.mil
)
and is widely used and accepted by the engi-
neering community and many regulatory agencies. The HEC-RAS model contains
1D river analysis components for: (1) hydraulic simulation; (2) movable boundary
sediment transport simulation; and (3) water quality analysis. A key element is that
all three components use a common geometric data representation and common
geometric and hydraulic computation routines. In addition to the three river
analysis components, the model contains several hydraulic design features that can
be invoked once the basic water surface profiles are computed, data storage and
management capabilities, graphics and reporting facilities.
The hydraulic simulation is the key computation engine. It performs 1D steady
and unsteady flow calculations on a network of natural or manmade open channels.
Hydraulic calculations are performed at each cross section to compute water
surface elevation, critical depth, energy grade elevation, and velocities. HEC-RAS
is able to perform mixed flow regime (subcritical, supercritical, hydraulic jumps,
and draw downs) calculations in the unsteady flow computations module. The
hydraulic calculations for cross-sections, bridges, culverts, and other hydraulic
structures that were developed for the steady flow component were incorporated
into the unsteady flow module. The model can handle a full network of channels, a
dendritic system, or a single river reach.
Sediment simulation in HEC-RAS utilizes one dimensional, cross-section
averaged, hydraulic properties from RAS's hydraulic engines to compute sediment
transport rates and update the channel geometry based on sediment continuity
calculations. The sediment module runs in the quasi-unsteady mode, i.e. it com-
putes the unsteady hydraulics as a series of steady state events. The sediment
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