Fundamentals of Engineering Design
The third basis for definition of sediment transport is by the sediment source. Bed material load
is the material that is found in appreciable quantities in the bed. Wash load is finer, and is not of a size
found in appreciable quantities in the bed.
5.1.4
MEASURED SEDIMENT DATA - RATING CURVE DEVELOPMENT
The sediment transport capacity of the river is determined by developing a sediment rating curve
which defines total sediment load (suspended and bedload) as a function of discharge. An example of the
approach used for developing a sediment rating curve is presented below for the DEC Project.
For the DEC Project, suspended sediment samples were collected in a consistent manner by the
USGS at each of the ten USGS gaging stations utilized in the present research. Observers collected single,
vertically integrated samples three days each week (Monday, Wednesday, and Friday) and supplemented
these data during selected storms. Each site is also equipped with a PS-69 automatic point sampler, which
is stage activated during storms. USGS personnel collected samples on a biweekly basis and during
selected storms, which may be either single or multiple, vertically integrated samples taken at several
sections across the stream (Rebich, 1993). The sampling procedures are described by Guy and Norman
(1970).
The effort by the USGS is significant, for example from July 1985 through September 1991, about
20,000 suspended-sediment samples were analyzed and reviewed, and data were stored in USGS
computer files. Sediment samples were then used to compute daily mean values of suspended sediment
concentration and sediment discharge according to standard USGS procedures (Porterfield, 1972).
Sediment rating curves were developed from the observed USGS data, with sediment discharge
as a function of water discharge. Figure 5.1 depicts the log-log (Qs = aQb) of total suspended
sediment discharge as a function of water discharge for Fannegusha Creek. Table 5.4 presents the
regression results for ten USGS gaging stations.
Table 5.5 provides similar regression data for the bed material portion of the measured sediment
discharge for the gages. Sand is the predominant bed material found in substantial quantities in the shifting
portions of the bed for the gage locations. Data were not available for the sand portion of the sediment
discharge at Otoucalofa Creek.
Figure 5.2 portrays the regression of the USGS sand fraction data, the measured USGS sand
fraction data, and a computed total sand discharge, which was computed using a HEC-2 computation of
the hydraulic characteristics of the backwater, and the SAM program (Thomas et al., 1994) using the
Brownlie (1981) sediment transport equation for the Abiaca Creek, Site No. 6. Close agreement is
apparent between the Brownlie computation of the total bed material load and the regression of the
observed USGS sand fraction regression. The SAM program is discussed in Section 5.3.6. Based on
data from Guy et al. (1966), Julien (1995) has subdivided the dominant mode of sediment transport into
three zones. Using a ratio of shear velocity to fall velocity, and the ratio of depth
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