Fundamentals of Engineering Design
to particle size, the dominant sediment transport mode can be subdivided into bedload, mixed load, and
suspended load.
Suspended load is dominant for values of the shear velocity to fall velocity ratio greater than 2.5.
For a hypothetical range of particle sizes from 1 mm to 0.063 mm, slope from 0.0005 to 0.001, and depth
from 0.015 m to 1.52 m, the range of values for the shear velocity to fall velocity ratio varies from a
minimum of 2.6 to a maximum of 379. Therefore, for most conditions occurring within the DEC streams,
the dominant transport mechanism is suspended load, which implies that the suspended sand discharge
closely approximates the total bed material load. There is close agreement between the observed
suspended sand discharge and the Brownlie procedure for total bed material load in sand channels. The
dominance of suspended sand transport in the DEC streams is substantiated by the Guy et al. (1966) and
Julien (1995) analysis; therefore, the suspended sediment discharge was used in this investigation as the
total bed material load for sites at which the data are available. The Brownlie procedure will be used to
compute total bed material load in the absence of measured data. Nash (1994) assumed that the bedload
contribution is relatively insignificant as compared to total load. Others have also made this assumption
(e.g., Benson and Thomas, 1966; Biedenharn et al., 1987; and Dunne, 1979).
5.1.5
WATERSHED HYDROLOGY
The hydrology of a river reach is defined by the recorded flow data, flow duration, bankfull
discharge, and annual hydrograph shape. The mean annual flow of a river gives an indication of the size of
a stream and is readily available from discharge records. Based on the mean annual flow, a stream is
classified as being small if the mean annual discharge is less than 10,000 cfs (283 m3/s) and the bankfull
discharge is less than 50,000 cfs (1,416 m3/s).
Bankfull discharge and effective discharge are extensively discussed in this manual (Section 5.3.1).
The effective discharge should be computed and utilized in design calculations.
The shape of the annual hydrograph depends on the route in which runoff flows to a stream and
the storage characteristics of the basin. The hydrograph can be considered an indicator of the sediment
yield from the drainage basin and a reflection of the climate in which the stream reach is located. Rainfall
intensity, number of precipitation events, and seasonal distribution of precipitation are factors that affect the
sediment yield and are reflected in the hydrograph. Man-made structures such as channel diversions and
reservoirs can have a serious effect on flow rate and storage capacity. These effects are reflected in the
shape of the hydrograph and the skewness of the discharge. The land use in the region will also affect the
shape of the hydrograph. While it is difficult to exactly quantify these effects from the hydrograph, the
shape, nonetheless, can give a good insight into basin characteristics.
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