9568

• UCREFRP Catalog Number: 9568
• Author: Resource Consultants Inc.
• Title: Sediment Transport Studies of the Little Snake, Yampa, and Green River Systems.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

Wash.load

Bed Load

Suspended Bed
Material Load

Composed of particle sizes finer
than those found in appreciable
quantities in the bed. Washload
moves in suspension and is pro-
vided by available bank and
watershed supply.

Composed of particle sizes
typically found in the bed that
move by surface creep, sliding,
saltation or rolling within the
bed layer.

Composed of particles
typically found in the.
bed that remain in
suspension during trans-
port.

Bed Materia I
Load

Note The term "suspended load" is used
when referring to the sum of the
"wash load" and "suspended bed
material load" components. There-
fore, an alternate definition of
total sediment load is the sum
of the suspended load and bed
load.

Total Sediment
load

Figure 5.1 Definition of sediment load components.

Although the Simons and Senturk method for defining wash load is descriptive of the physical
process, it is not easy to apply in the field, since the d1D size can vary spatially from reach to reach and
temporally with the rising and falling of discharge in the channel. Therefore, the definition of wash load as
that portion of the suspended sediment load of sizes in the silt/clay range will generally be used in this study.
However, it must be understood that the true wash load for the lower reaches of the Yampa River can also
consist of sand sized and fme gravel material.

5.2 Sediment Rating Curves

b 5.2.1 Development of Sediment Rating Curves. Sediment rating curves in the form of Os =
aO (Os = sediment discharge, 0 = water discharge, a and b are coefficients) are useful for a variety of
sediment transport analyses. Rating curves are typically derived based on measured water and sediment
discharge data. Hence, the sediment transport predicted by such relationships is considered indicative of
sediment supply, and not necessarily sediment transport capacity. Only for alluvial channel reaches (e.g.,
purely sandbed streams) where sediment supply equals transport capacity will the measured rating curve be
indicative of the transport capacity of the channel.

Rating curves are typically derived from measured water and sediment discharge data by linear
regression of the log transformed data. However, least squares regression using log transformations
introduces a statistical bias that typically results in underestimation of sediment load, with the degree of
underestimation increasing with the degree of scatter about the rating curve (Ferguson, 1986).

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