From available data on the stage-discharge relation at the stream gaging station (near
Wiggins, Colorado) it is estimated that the Manning roughness coefficient, n, during the design
floods is about 0.023. This value of Manning's n gives a computed mean flow velocity of 18.7
ft/s (5.7 m/s) in Bijou Creek for the extreme flood of June 1965 [discharge 466,000 ft3/s (13,200
m3/s)].
10.1.2 Level 2 - Quantitative Engineering Analysis
Hydrologic Analysis. The hydrologic analysis identified three design floods and corresponding
water surface elevations with return periods of 50, 100, and 200 years. Bank protection will be
specified for each of the design floods for five channel design alternatives. The design floods
were estimated using the Gumbel Method of frequency analysis and all of the available
hydrologic data. The design of riprap bank protection will consider the design flood discharge,
superelevation in the bend, bedform height, local scour, bed material size, and stability of
riprap materials.
The historical records show that there were two extreme floods 282,900 cfs (8,012 m3/s) and
466,000 cfs (13,200 m3/s) observed at the streamflow gaging station of Bijou Creek near
Wiggins, Colorado. Historic stream flow data were plotted in Figure 10.5 using Gumbel
probability paper. From this figure, the floods with various return periods can be interpolated or
extrapolated. The estimated design floods with return periods of 100 and 200 years are
respectively 62,000 cfs (1,756 m3/s) and 72,500 cfs (2,053 m3/s) .
Moveable Bed Hydraulic Analysis.
The information on hydraulic conditions needed for
designing bank protection includes the normal depth of flow, the cross sectional area of flow,
the mean flow velocity, the Froude number, the bedform height, the local scour depth, the
superelevation of the flow in the bend, the local depth, and the local velocity. Moreover,
different design alternatives will result in different hydraulic conditions. In this study, five design
alternatives were proposed; the first alternative was to protect the existing outer bank with
riprap (Figure 10.6), the second alternative was to realign the bend to its plan geometry before
1965 and to protect the outer bank with riprap (Figure 10.7), and the third alternative was to
realign the bend to that of a mild bend relative to the existing channel alignment and to protect
the outer bank with riprap (Figure 10.8). The fourth alternative was to determine the necessary
buffer strip distance between the railroad and the present north river bank if bank protection is
not utilized The fifth alternative was to use rock riprap spurs to protect the existing river bank
by realigning the channel (Figure 10.9).
As shown in Figure 10.9, the fifth alternative, to construct a series of rock riprapped spurs to
guide the flow away from the existing north bank, has the same flow alignment as the second
alternative (Figure 10.7).
A summary of computed hydraulic conditions in the first three basic alternatives is given in
Table 10.1. The data necessary to determine riprap size are included for three values of radius
of curvature, rc, of the bend.
The following normal depth-discharge relation is determined by using Manning's equation,
using the slope of energy gradient S = 0.00252, and Manning's roughness coefficient n =
0.023.
Q = 395.02 y n.32
2
In this relation Q is the design discharge and yn is the normal depth of flow measured from the
thalweg level.
10.5
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