10.1.3 Riprap Design for Alternatives 1, 2, and 3
Side Slope of Riprap Bank. The side slope of the riprapped bank should be less than the angle
of repose of the bank material. Analysis shows that the angle of side slope should be at least
5 less than the angle of repose of the bank material. The bank material is medium sand with
some clay and silt with average D50 of about 0.35-0.45 mm. Its angle of repose is about 29
(see Chapter 6). A side slope 2.5:1 (horizontal to vertical distance) is utilized and the
corresponding angle of side slope is 21.75 (Figure 10.10).
Riprap Design. The size of riprap is determined using the method presented in Chapter 6. The
method developed by the Bureau of Reclamation (1958) to determine the maximum rock size
in a riprap mixture is used for comparison. The flow conditions required for designing riprap
sizes were discussed earlier and given in Table 10.1.
Using a stability factor of 1.3 and an angle of repose of riprap material of 41, the required
riprap sizes with a uniform gradation are determined. The results are shown in Table 10.3.
Table 10.3. Design of Riprap Sizes.
Riprap Mixture by
Maximum
Return Period
Stevens et al. (1974)
Design
Uniform Size
Size by
(yrs)
Discharge
Alternatives
by Stevens
USBR (1958)
Median
Maximum
(cfs)
et al. (1974)
(ft)
Diameter (ft)
Size (ft)
1
2.6
1.5
3.3
2.6
50
51,600
2
2.0
1.1
2.4
2.0
3
1.8
1.0
2.2
1.8
1
3.0
1.7
3.7
3.1
100
62,000
2
2.2
1.3
2.9
2.2
3
2.0
1.2
2.6
2.0
1
3.4
2.0
4.4
3.5
200
72,500
2
2.5
1.4
3.1
2.5
3
2.2
1.3
2.9
2.3
It is not acceptable to riprap a bank with uniform size rock, especially when large rock is
required. Therefore, it is necessary to design a riprap mixture that includes an adequate range
of sizes. In addition, the stability factor for determining the D50 of the riprap mixture is assumed
to be 1.1. According to Chapter 6, riprap gradation should follow a smooth size distribution
curve with gradation coefficient of about 1.3. Riprap sizes are given in Table 10.3.
The Bureau of Reclamation (1958) developed a figure to determine the maximum rock size in
a riprap mixture downstream from stilling basins. If the bottom velocity is assumed equal to the
reference velocity on the top of the rock, the maximum rock sizes in the riprap mixtures for
different design conditions can be estimated. The results utilizing both methods are given in
Table 10.3 for comparison. The sizes obtained by use of the Stevens method given in Chapter
6 are recommended.
10.11
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