1 + sin (20 + 32.8)
η′ = 0.68
= 0.61
2
From Equation 6.3,
cos 18.5 tan 40
S.F. =
= 1.02
0.61 tan 40 + sin 18.5 cos 32.8
The procedure is repeated with increasing stone size until S.F. = 1.5. A riprap size of 9 inches
would give a S.F. slightly over 1.5.
fps and an angle with the horizontal λ of 0 degrees. Determine the size of riprap to protect
the toe of the abutment.
The riprap size is determined either by the same iterative procedure used in a or by using
Equations 6.11, 6.12, and 6.16.
Using the latter method:
Equation 6.11
Sm = tanφ/tanθ = tan 40/tan 18.5 = 2.51
Equation 6.12
Sm - S.F.2
(2.5)2 - 1.5 2
2
cos θ =
η=
cos 18.5 = 0.40
1.5 x 2.5 2
S.F. S 2
m
Riprap size is obtained from Equation 6.16
0.3 Vr2
0.3 x (9.45)2
=
=
= 1.26 ft
Dm
(S s - 1) g η (2.65 - 1) x 32.2 x 0.4
This is the size recommended.
6.13 SOLVED PROBLEMS FOR FILTER DESIGN (ENGLISH)
The requirements for a gravel filter are given in Section 6.7. The gradation of a filter should be
such that
D50 (Filter )
< 40
(6.24)
D50 (Base)
6.70
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