Table 3.5. Nonscour Velocities for Soils (Modified from a report by Keown et al. 1977).
Approximate Nonscour Velocities
Kind of Soil
Grain Dimensions
(feet per second)
(mm)
(ft)
Mean Depth
1.3 ft
3.3 ft
6.6 ft
9.8 ft
For Noncohesive Soils
Boulders
>256
>0.840
15.1
16.7
19.0
20.3
Large cobbles
256-128
0.840-0.420
11.8
13.4
15.4
16.4
Small cobbles
128-64
0.420-0.210
7.5
8.9
10.2
11.2
Very coarse gravel
64-32
0.210-0.105
5.2
6.2
7.2
8.2
Coarse gravel
32-16
0.105-0.0525
4.1
4.7
5.4
6.1
Medium gravel
16-8.0
0.0525-0.0262
3.3
3.7
4.1
4.6
Fine gravel
8.0-4.0
0.0262-0.0131
2.6
3.0
3.3
3.8
Very fine gravel
4.0-2.0
0.0131-0.00656
2.2
2.5
2.8
3.1
Very coarse sand
2.0-1.0
0.00656-0.00328
1.8
2.1
2.4
2.7
Coarse sand
1.0-0.50
0.00328-0.00164
1.5
1.8
2.1
2.3
Medium sand
0.50-0.25
0.00164-0.000820
1.2
1.5
1.8
2.0
Fine sand
0.25-0.125
0.000820-0.000410
.98
1.3
1.6
1.8
For Compact Cohesive Soils
Sandy loam (heavy)
3.3
3.9
4.6
4.9
Sandy loam (light)
3.1
3.9
4.6
4.9
Loess soils in the
2.6
3.3
3.9
4.3
Conditions of finished
Settlement
3.5.7 Figures for Determining Critical Shear Stress or Velocity
Figure 3.18 shows the relationship between critical shear stress and mean diameter as
determined and/or recommended by different investigators for different soil types. The
difference between investigators is possibly due to the effects of cohesion, when present,
causing the particles to aggregate and not act as individual particles.
Figure 3.18. Critical shear stress as a function of grain diameter (after Lane 1953).
3.44
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