R
n=φ
1/6
ks
Equation 2-15
ks
where, in SAM,
ks = effective surface roughness, ft
φ R/ks = 0.0342 for natural channels,
where ks = d50
= 0.0342 for velocity and stone size calculations in riprap design,
where ks = d90
= 0.038 for discharge capacity calculations in riprap design,
where ks = d90
SAM uses different values for the Strickler function, depending on the
calculations to be made. Chow (1959) recommends the use of 0.0342 for natural
sediment, so that is the default value. In accordance with EM 1110-2-1601
(USACE 1991, 1994), 0.0342 is also used for velocity and stone size
calculations. Thus, the same Strickler function is used in specified panels for
cross-sections where some panels have riprap and others have natural sediment.
Also in accordance with EM 1110-2-1601, 0.038 is used for water surface
calculations after riprap has been calculated. In this case, n-values for all panels
in the cross section will use this constant whether riprap was prescribed for that
panel or not, possibly calculating some n-values incorrectly. Problems can be
avoided by using another roughness equation in panels without riprap.
Limerinos n-value Predictor
Limerinos (1970) developed an empirical relative roughness equation for
coarse, mobile bed streams using field data. He correlated n-values with
hydraulic radius and bed sediment size. The resulting equation is shown below.
0.0926 R1/6
n=
Equation 2-16
R
1.16 + 2.0 log10
d 84
where
d84 = the particle size, ft, for which 84% of the sediment mixture is finer.
Data ranged from 1.5 mm to 250 mm.
n = Manning's n value. Data ranged from .02 to 0.10.
R = Hydraulic radius, ft. Data ranged from 1 to 6 ft.
15
Chapter 2
Theoretical Basis for SAM.hyd Calculations
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