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Cap5_COASTAL PROCESSES_ DEAN_e_Dalrymple

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because the wavelength appears in
the argument of the hyperbolic tangent. Thismeans that iterative numericalmethods,
such as Newton–Raphson methods, or approximate means are used to solve for
wavelength; see, for example, Eckart (1951), Nielsen (1983), or Newman (1990).
One convenient approximate method is due to Fenton and McKee (1989), which
has a maximum error less than 1.7 percent, which is well within most design criteria.
Their relationship is
L = L0
{
tanh
[(
2π
√
(h/g)/T
)3/2]}2/3 (5.4)
EXAMPLE
A wave train is observed to have a wave period of 5 s, and the water depth is
3.05 m. What is the wavelength L?
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5.2 WATER WAVE MECHANICS 91
First, we calculate the deep water wavelength L0 = gT 2/2π = 9.81 (5)2/
6.283 = 39.03 m. The exact solution of Eq. (5.3) is 25.10 m. The approxima-
tion of Eq. (5.4) gives 25.48 m, which is in error by 1.49 percent.
The waveform discussed above is a progressive wave, propagating in the positive
x direction with speed C . Near vertical walls, where the incident waves are reflected
from the wall, there is a superposition of waves, which can be illustrated by simply
adding the waveform for a wave traveling in the positive x direction to one going in
the opposite direction:
η(x, t) = H
2
cos(kx + σ t)
The resulting standing wave is
η(x, t) = H cos kx cos σ t,
which has an amplitude equal to twice the amplitude of the incident wave. By sub-
tracting, instead of adding, we have a standing wave with different phases involving
sines rather than cosines. Examining the equation for the standing wave, we find that
the water surface displacement is always a maximum at values of kx equal to nπ ,
where n = 0, 1, 2, . . . . These points are referred to as antinodalpositions.Nodes (zero
water surface displacement) occur at values of kx = (2n − 1)π/2, for n = 1, 2, 3, . . . ,
or x = L/4, 3L/4, . . . .
Under the progressivewave,Eq. (5.1), thewater particlesmove in elliptical orbits,
which can be decomposed into the horizontal and vertical velocity components u and
w as follows:
u(x, z, t) = Hσ
2
cosh k(h + z)
sinh kh
cos(kx − σ t) (5.5)
w(x, z, t) = Hσ
2
sinh k(h + z)
sinh kh
sin(kx − σ t) (5.6)
At thehorizontal seabed, the vertical velocity is zero, and thehorizontal velocity is
u(x,−h, t) = ub cos(kx − σ t),
where
ub = Hσ2 sinh kh = Aσ
The parameter A is half the orbital excursion of the water particle over the wave
period and has been shown to be related to the size of sand ripples formed on the
bottom.
The associated pressure within the wave is given by
p(x, z, t) = −ρgz + ρg H
2
cosh k(h + z)
cosh kh
cos(kx − σ t)
= −ρgz + ρg
(
cosh k(h + z)
cosh kh
)
η(x, t) (5.7)
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92 WAVES AND WAVE-INDUCED HYDRODYNAMICS
Table 5.1 Asymptotic Forms of the
Hyperbolic Functions
Large kh Small kh
Function ( >π) (<π/10)
cosh kh 12 e
kh 1
sinh kh 12 e
kh kh
tanh kh 1 kh
in which the first term on the right-hand side is the hydrostatic pressure component,
and the second is the dynamic (wave-induced) pressure component. The dynamic
pressure is largest under thewave crest, for thewater column is higher at that location
and at a minimum under the wave trough.
Finally, the propagation speed of the wave energy turns out to be different than
the propagation speed of the waveform owing to the dispersive nature of the waves.
This is expressed as the group velocity Cg, which is defined as
Cg = nC,
where
n = 1
2
(
1+ 2kh
sinh 2kh
)
(5.8)
The rate at which the waves carry energy, or the energy flux per unit width, is defined
as F = ECg. Both Cg and n are functions of the water depth; n is 0.5 in deep water
and unity in shallow water.
To simplify equations involving hyperbolic functions in shallow and deep water,
asymptotic representations of the functions can be introduced for relative water
depths of h/L < 1/20 and h/L > 1/2, respectively. These asymptotes are shown in
Table 5.1.
5.2.1 OTHER WAVE THEORIES
TheAiry theory is called the linear theory because nonlinear terms in such equations
as the Bernoulli equation were omitted. The measure of the nonlinearity is generally
the wave steepness ka, and the properties of the linear theory involve ka to the first
power. For this text, the linear wave theory is sufficient; however, the tremendous
body of work on nonlinear wave theories is summarized here.
Higher order theories (including terms of order (ka)n , where n is the order of
the theory) have been developed for periodic and nonperiodic waves. For periodic
waves, for example, the Stokes theory (Stokes 1847, Fenton 1985, fifth order), and
the numerical StreamFunctionwave theory (Dean 1965,Dalrymple 1974) show that,
because of nonlinearity, larger waves travel faster and wave properties are usually
more pronounced at the crest than at the trough of the wave, causing, for instance,
the wave crests to be more peaked than linear waves. The Stream Function theory,
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5.2 WATER WAVE MECHANICS 93
which allows any order, is solved numerically, and the code is available from the
authors.
For variable water depths, including deepwater, approximations have beenmade
to the governing equations to permit solutions over complicated bathymetries. A
major step in this effort was the development of themild-slope equation by Berkhoff
(1972), which is valid for linear waves. Modifications for nonlinear waves and the
effects ofmean currentsweremade byBooij (1981) andKirby andDalrymple (1984).
In shallow water, the ratio of the water depth to the wavelength is very small
(kh < π/10 or, equivalently, as before, h/L < 1/20). Taking advantage of this, wave
theories have been developed for both periodic and nonperiodic waves. The earliest
was the solitarywave theory ofRussell (1844),whonoticed thesewaves being created
by horse-drawn barges in canals. The wave form is
η(x, t) = H sech2
(√
3H
4h3
(x − Ct)
)
(5.9)
This unusual wave decreases monotonically in height from its crest position in both
directions, approaching the still water level asymptotically. The speed of the wave is
C =
√
gh
(
1+ H
2h
)
(5.10)
In the very shallow water of the surf zone on a mildly sloping beach, waves behave
almost as solitary waves. Munk (1949) discusses this thoroughly.
For periodic shallow water waves, the analytic cnoidal wave theory is sometimes
used (Korteweg and deVries 1895). This theory has, as a long wave limit, the solitary
wave, and as a short wave limit, the linear wave theory.
Amore general theory that incorporates all of the shallow water wave theories is
derived from theBoussinesq equations (seeDingemans 1997 for adetailedderivation
of the various forms of the Boussinesq theory). For variable depth and propagation
in the x direction, equations for the depth averaged velocity u and the free surface
elevation can be written as (Peregrine 1967)
∂u
∂t
+ u ∂u
∂x
= −g ∂η
∂x
+ h
2
∂2
∂x2
(
h
∂u
∂t
)
− h
2
6
∂2
∂x2
(
∂u
∂t
)
(5.11)
∂η
∂t
+ ∂(h + η)u
∂x
= 0 (5.12)
For constant depth, the solitary wave and the cnoidal waves are solutions to
these equations. For variable depth and a two-dimensional problem, numerical so-
lutions by several techniques are available. Some of the more well-known include
that of Abbott, Petersen, and Skovgaard (1978), which is based on finite-difference
methods.
Several recent developments have extended the use of the Boussinesq equations
in coastal engineering. One of these is the modification of the equations to permit
their use in deeper water than theoretically justified. These extensions include those
of Madsen, Murray, and Sørenson (1991), Nwogu (1993), and Wei et al. (1995). The
second effort has been to include the effects of wave breaking so that the Boussinesq
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94