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 Vt2

0.3 x (2.88)2

=

=

= 0.38 m

Dm

(S s - 1) g η (2.65 - 1) x 9.81x 0.4

This is the size recommended.

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)

D15 (Filter )

5<

< 40

(6.25)

D15 (Base)

D15 (Filter )

<5

(6.26)

D85 (Base)

The properties of the riprap and base material are given in Table 6.4. Determine if filter is

needed between the riprap and the base material.

Table 6.4. Sizes of Materials.

Base Material (Sand)

Riprap (Gravel)

D85 = 1.50 mm

D85 = 24 mm

D50 = 0.75 mm

D50 = 12 mm

D15 = 0.38 mm

D15 = 6 mm

In accordance with the recommended sizes for filters:

D50 (Riprap)

12

=

= 16

D50 (Base)

0.75

6.61

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