where
V
=
velocity, feet per second
R
=
hydraulic radius, feet
S
=
slope, feet per feet
n
=
Manning's roughness coefficient
Hydraulic Roughness Equations Hydraulic roughness can be prescribed directly
with n-values (option 0) or it can be related to the physical properties of the cross
section by another hydraulic roughness equation. Available roughness equations
are given in Table 1.
0. Manning's Equation
1. Keulegan equations
2. Strickler Equation
3. Limerinos Equation
4. Brownlie Bed Roughness Equations
5-9. Five Soil Conservation Service equations for grass lined
channels
Table 1. Hydraulic Roughness Options in SAM.hyd.
Note that as each channel problem is unique, no one predictor should be
considered as "best."
Effective Surface Roughness Height, ks
A value for ks is required when the Keulegan equations or Strickler equation
is specified. For the design of concrete channels the suggested values for ks are
shown in EM 1110-2-1601 (USACE 1991, 1994). For the case of channels in
natural materials, there are no tables of generally accepted ks values as there are
for Manning's n-values. Moreover, there is no generally accepted technique for
measuring this property geometrically. Therefore, unless a specific value of ks is
known, it is recommended that the hydraulic roughness be prescribed with n-
values or by another analytical method. When sufficient data are available --
discharge, area, hydraulic radius, and slope -- ks can be calculated and then used
to calculate hydraulic parameters for additional discharges.
Relative Roughness
Relative roughness refers to the ratio of the effective surface roughness
height, ks, to the hydraulic radius, R. The relative roughness parameter is R/ks.
When this parameter is less than 3, which indicates a very rough surface, the
logarithmic velocity distribution theory breaks down. For this reason, SAM will
10
Chapter 2
Theoretical Basis for SAM.hyd Calculations
Previous Page
