2.5
γ 1/ 2
V
= S f C s C v C T y
γ - γ (K gy)1/ 2
D30
(6.18)
s
1
where:
=
Riprap size, m, ft
D30
Sf
=
Safety factor, minimum 1.1
Cs
=
Stability coefficient for incipient failure, (D85/ D15 = 1.7 to 5.2)
=
0.30 for angular rock
=
0.36 for rounded rock
1/3
D85/D15
=
Cv
=
Vertical velocity distribution coefficient
=
1.0 for straight channels, inside of bends
=
1.283 0.2 log (R/W) for outside of bends, 1 for (R/W) > 26
=
1.25 downstream of concrete channels and at ends of dikes
R
=
Centerline radius of curvature of bend (main channel only) m, ft
W
=
Water-surface width at upstream end of bend (main channel only) m, ft
CT
=
Blanket thickness coefficient
=
1.0 for thickness = 1 D100 (max) or 1.5 or D50 (max), whichever is
greater
y
=
Local depth of flow, m, ft
3
3
γ
=
Unit weight of water, Kg/m , lb/ft
3
3
γs
=
Unit weight of rock, Kg/m , lb/ft
V
=
Local depth-average velocity, m/s, ft/s
Vss
=
Local depth average velocity on side slopes, m/s, ft/s
Vavg
=
Average velocity in the main channel, m/s, ft/s
K1
=
Side slope correction factor
2
2
g
=
Acceleration of gravity, 9.81 m/s , 32.2 ft/s
The use of D30 for the sizing of the riprap is somewhat controversial. Its use probably results
in an overall increase in the sizes of the riprap in the blanket compared to using the
traditional D50 approach. Maynord (1988) states "Stability tests conducted at a thickness of 1
X D100, which is the most commonly used thickness for bank protection, showed that
gradations ranging from uniform, to highly nonuniform exhibited the same stability if they had
the same D30." The Corps manual gives an approximate relationship between D50 and D30. It
1/3
is D50 = D30 (D85 / D15 ) .
The U.S. Army Corps of Engineers manual (1994a) states that the minimum safety factor
may have to be increased for the following conditions:
Impact forces from logs, uprooted trees, vessels, ice etc.
Natural variations in the quality of the rock used as riprap
Vandalism
Placement quality
Freeze-thaw of the riprap is anticipated
Precision of the determination of the hydraulic variables
6.26
Previous Page
