and 32.6, 12.2 and 5.2 are empirical coefficients determined from laboratory
experiments. These equations, when graphed, produce the Moody-type diagram
for open channel flow, Figure B-3, EM 1110-2-1601 (USACE 1991, 1994).
The Iwagaki relationship
Using experimental data obtained from many sources, Iwagaki (Chow, 1959)
found that the coefficients 12.2 and 5.2 in equations 2-3, 2-4, 2-5, and 2-6 varied
with Froude number. The results of his study disclosed that resistance to
turbulent flow in open channels becomes obviously larger with increase in the
Froude number. Iwagaki reasoned that this is due to the increased instability of
the free surface at high Froude numbers. The Iwagaki relationship is shown in
Figure 2.2.
Figure 2.2. The Iwagaki relationship (Chow, 1959), used with permission from McGraw-
Hill Book Company, Inc.
Keulegan's equations can be modified to incorporate Iwagaki's results.
(1) Chow (1959) presents Keulegan's equation for the average flow velocity
V in the following form
R
V = U * 6.25 + 5.75 log10
Equation 2-7
k
s
12
Chapter 2
Theoretical Basis for SAM.hyd Calculations
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