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Page Title:
COMBINED CURRENT/WAVE SHEAR STRESS (cont.) |
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 Technical Note DOER-N1
April 1998
1 30h
k A
0.5
2
= 1n
- 1n
κ ek N
k N
fc
(9)
where kN is the Nikuradse roughness coefficient (assumed to be 0.015 ft), kA is the apparent
roughness (described below), κ is the von Karman constant (=0.4), and h is the water depth.
b. Compute fw and parameter J for pure waves (m=1):
2
β k N ωa
3
fw = 2
uwbm
(10)
uwbm mf w 0.5
J=
k N ωa 2
(11)
where uwbm is the maximum value (amplitude) of the oscillatory wave particle velocity at the top
determined constant (=0.0747). Now recompute fw using:
2βm
fw =
J
(12)
c. Keeping fc fixed, iterate to find σ, m, J, and fw using Equations 6, 7, 11, and 12.
d. Compute δw (wave boundary layer thickness), and kA (apparent roughness):
βJ
0.5
δw = r k N π
2
(13)
1
-κδw σ 2
k A = 30δw exp
βk N m
(14)
where r is an experimentally determined constant (=0.45).
e. Recompute fc from a.; this time the second term on the right-hand side will not be zero.
f. Repeat steps b. through e. until fc converges.
g. Calculate τbm, maximum bed shear stress, using Equation 4.
6
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