10.1.1 Level 1 - Reconnaissance and Geomorphic Analysis
A Level 1 field study was conducted to obtain morphologic data for the Level 2 hydraulic
analysis. The information utilized in the analysis includes the representative cross sections of
the channel, the representative bed material size, and the energy slope. With these data, the
flow resistance coefficient was estimated.
The representative cross section in the vicinity of the study site was developed by considering
five selected cross sections established during the field study. Of these, two cross sections
were similar in shape and are considered representative of the cross sections of the study
reach. The dimensions of a representative cross section were established by averaging these
two cross sections. The results of this analysis are shown in Figures 10.2 and 10.3.
Figure 10.2 shows the relationship between cross sectional areas and the flow depth as
follows:
A = 242.6 y1.52
n
in which A is the cross sectional area and yn is the normal depth of flow measured from the
thalweg level.
Figure 10.3 gives the following relationship between the wetted perimeter and the flow depth.
P = 682.0 y n.31
0
The average top width of channel as estimated by field survey and topographic maps is 1,700
ft (518 m).
The results of sieve analyses of the bed material samples taken at three cross sections are
shown in Figure 10.4. The average median bed material size, D50, is 0.45 mm. The average
D16 size is 0.22 mm, and the average D84 size is 0.91 mm. The bank material has nearly the
same distribution as that of the bed material, but contains lenses of silt and clay. The bank is
highly stratified and can be easily eroded.
The energy slope may be less than the channel bed slope. However, for a safer design, it is
assumed that the energy slope is equal to the channel bed slope. The average channel bed
slope from field surveys is 0.00252 ft/ft (m/m). This slope is adopted as the design energy
slope.
The floods of June 1965 in South Platte River Basin, Bijou Creek were in upper regime, having
antidunes with breaking waves. Similarly, computations show that the bed forms of Bijou
Creek (D50 = 0.45 mm) will be antidunes or standing waves during floods. Resistance to flow
associated with antidunes depends on how often the antidunes form, the area of the reach
which they occupy, and the violence and frequency of their breaking. If the antidunes do not
break, resistance to flow is about the same as for a plane sand bed, and the discharge
acceleration) ranges from 14 to 23 (Manning's coefficient, n, is about 0.017 to 0.027 for the
flow depths being considered). The acceleration and deceleration of the flow through the
nonbreaking antidunes (frequently called standing waves) causes resistance to flow to be
slightly more than that for flow over a plane bed. If many antidunes break, resistance to flow
can be very large because the breaking waves dissipate a considerable amount of energy.
With breaking waves, C/(g)1/2 may range from 10 to 20, and Manning's coefficient n ranges
from about 0.019 to 0.038 for the flow depths being considered.
10.2
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