D-R-A-F-T
Sediment size classes for sand: SA ,SR, ST
The program requires that sediment sizes and/or their characteristics be specified.
For noncohesive sediment bed problems, input allows for multiple grain sizes on the
SA card, BUT AT PRESENT THE PROGRAM CONSIDERS ONLY ONE
EFFECTIVE GRAIN SIZE. (The input reflects some changes that are planned for
the SED2D WES program, but are not currently functional). The grain size specified
on the SA card is applied to every node in the mesh. Values at specific nodes may
be changed by use of the SR and ST cards. The ST card specifies grain sizes to be
used in noncohesive sediment transport equations and the SR card specifies the
effective grain size to be used in bed roughness calculations (Ackers-White transport
equations only). These two sizes will be the same only for plane beds in straight
channels. Bed forms and channel curvature introduce form roughness that causes the
roughness size to be larger than the size used for transport computations.
Note that the SA, SR, and ST cards constitute a cascading set of defaults. The SA
card should precede the SR and ST cards. If neither an SR nor ST card is present,
the grain size on the SA card will be used at all nodes for both transport and effective
roughness. If SR cards are present, they override the roughness size on the SA card
at those nodes specified. The ST cards override the transport size on the SA card at
every node specified on the ST card.
Two characteristic length parameters are requested on the SA card. CLDE is the
length factor for deposition. The default is a value of 1, corresponding to an average
settling depth equal to the water depth. For fine sediments that are distributed
throughout the water column, a value of 0.5 is recommended. For coarser sediments
in less turbulent flow, a smaller value is suggested. CLER is the length factor for
erosion. The default value of l0 is suggested, but more investigation is needed to
find the best value.
Settling velocity: WC cards
Settling velocities are specified on the WC cards. This settling velocity is an
effective fall velocity which goes up with grain size, goes down with increasing
turbulence, goes up with increasing aggregation (cohesive sediments), and goes up if
a too large value of CLDE is used. The best starting point for noncohesive
sediments are fall velocities for spherical particles of equal diameters. For data on
appropriate values for settling velocities see "Some Fundamentals of Particle Size
Analysis," 1957, Committee on Sedimentation, Interagency Committee on Water
Resources. For cohesive sediments, the settling velocity of particles can vary
enormously with sediment type, salinity, turbulence, and other chemical and physical
conditions. Laboratory or field tests are needed to define effective settling velocities.
Cohesive sediment characteristics: CC and CI
cards
The Figure below illustrates the relation between the various critical shear stresses
for cohesive sediments. These values must generally be determined by laboratory or
field experimentation, but published results for similar sediments can be used if
caution is exercised. Values specified on the CC card for critical shear stresses for
erosion and the erosion rate constant are overridden by those contained on the CI
cards. The CC card should precede the CI cards. The CI cards are used to assign
characteristics to various types of cohesive sediment bed layers. These
characteristics are assigned to existing bed layers as specified on the CL cards and to
new layers as they are deposited. Freshly deposited sediments are assigned a type 1
designation and increase to higher numbered types as the thickness of sediment
Run Control 57
Users Guide To SED2D-WES
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