100
D85 (Filter ) >
= 20 mm
so
5
Therefore, with respect to the riprap, the filter must satisfy these requirements
2.5mm < D15 (Filter) < 20 mm
D50 (Filter)
> 5 mm
D85 (Filter)
> 20 mm
These riprap filter requirements along with those for the base material are shown in Figure
6.35. Any filter having sizes represented by the double cross-hatched area is satisfactory. For
example, a good filter could have these sizes:
D85 = 40 mm
D50 = 10 mm
D15 = 4 mm
Figure 6.35. Gradations of filter blanket for Problem 2 (after Anderson et al. 1968).
6.12 SOLVED PROBLEMS FOR STABILITY OF RIPRAP (ENGLISH)
6.12.1 PROBLEM 1 Stability of Particles Under Downslope Flow
^
Downslope flow over a plane bed inclined at an angle θ shown in Figure 6.36 is equivalent to
^
an oblique flow on a side slope with θ = θ and λ = 90. Then, according to Equation 6.4, β = 0,
and from Equation 6.6,
6.64
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