(b) Gravel Size Bed Material
Given depth y is 12 ft and bed material size D50 Is 3.1 mm = 0.0102 ft, what is the critical
velocity Vc?
Vc = 11.25 y1/ 6 D1/ 3 = 11.25 121/ 6 x 0.01021/ 3 = 3.70 ft / s
(c) Cobble size Bed Material
Given depth y is 12 ft and bed material size D50 is 128 mm = 0.42 ft, what is the critical
velocity Vc?
Vc = 11.25 y1/ 6 D1/ 3 = 11.25 x 121/ 6 x 0.421/ 3 = 12.75 ft / s
Critical Size for Beginning of Bed Material Movement
Given depth y is 12 ft and velocity V is 2.0 ft/s, 4 ft/s, 8.0 ft/s, 12 ft/s, and 16 ft/s, respectively.
Determine the critical bed material size D50.
V = 2.0 ft/s
0.0007 V3
(0.0007) 2.0 3
Dc =
=
= 0.00162 ft
(0.49 mm)
1/ 2
121/ 2
y
V = 4.0 ft/s
0.0007 V3
(0.0007) 4.0 3
Dc =
=
= 0.0129 ft
(3.93 mm)
1/ 2
121/ 2
y
V = 8 ft/s
0.0007 V3
(0.0007) 8.0 3
Dc =
=
= 0.104 ft
(31.7 mm)
1/ 2
121/ 2
y
V = 12 ft/s
0.0007 V3
(0.0007) 12.0 3
Dc =
=
= 0.349 ft
(106 mm)
1/ 2
1/ 2
12
y
V = 16 ft/s
0.0007 V3
(0.0007) 16.0 3
Dc =
=
= 0.828 ft
(252 mm)
1/ 2
121/ 2
y
3.65
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