Table 6. Graded Riprap Sizes.
Low Turbulence zones, placed in the dry, γs = 165 lbs/cuft
Layer DMAX1
D301 D501 D901
POROSITY2
in
ft
ft
ft
%
No.
1
9
0.37
0.43
0.53
38
2
12
0.48
0.58
0.70
38
3
15
0.61
0.73
0.88
38
4
18
0.73
0.88
1.06
38
5
21
0.85
1.03
1.23
38
6
24
0.97
1.17
1.40
38
7
27
1.10
1.32
1.59
38
8
30
1.22
1.46
1.77
38
9
33
1.34
1.61
1.94
38
10
36
1.46
1.75
2.11
38
11
42
1.70
2.05
2.47
38
12
48
1.95
2.34
2.82
38
13
54
2.19
2.63
3.17
38
Notes:
1
These values were taken from EM 1110-2-1601.
2
These values are estimated from one set of field data.
Riprap Size for a Given Discharge and Cross-Section
Shape
Riprap size is a more complicated calculation when water discharge and cross
section are given than it is when the flow velocity and depth are given because
n-value becomes a function of riprap size. The computational procedure in
SAM.hyd is as follows.
The computations begin with the unprotected channel. The bed sediment size
include hydraulic roughness equations and either n-values or ks values, as
required for normal depth computations. If the Strickler equation is selected for
hydraulic roughness, the Strickler coefficient is 0.034, which is the value for
natural sediment where ks = d50 (Chow, 1959). Normal depth is calculated using
the alpha method, and flow is distributed across the section. Stability of the bed
sediment is then calculated at each cross section coordinate using the distributed
velocity and the depth at that point. Shield's Diagram is used to test for particle
stability. The calculated shear stress is compared to the critical shear stress
determined from Shield's diagram.
32
Chapter 2
Theoretical Basis for SAM.hyd Calculations
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