Green's law

Propagation of shoaling long waves, showing the variation of wavelength and wave height with decreasing water depth.

In fluid dynamics, Green's law describes the evolution of non-breaking surface gravity waves propagating in shallow water of gradually varying depth and width. The law is named after George Green. In its simplest form, for wavefronts and depth contours parallel to each other (and the coast), it states:

H_{1}\,\cdot \,{\sqrt[{4}]{h_{1}}}=H_{2}\,\cdot \,{\sqrt[{4}]{h_{2}}}

\left(H_{1}\right)^{4}\,\cdot \,h_{1}=\left(H_{2}\right)^{4}\,\cdot \,h_{2},

where $H_{1}$ and $H_{2}$ are the wave heights at two different locations – 1 and 2 respectively – where the wave passes, and $h_{1}$ and $h_{2}$ are the mean water depths at the same two locations.

Green's law is often used in coastal engineering for the modelling of long shoaling waves on a beach, with "long" meaning wavelengths in excess of about twenty times the mean water depth.^[1] Tsunamis shoal (change their height) in accordance with this law, as they propagate – governed by refraction and diffraction – through the ocean and up the continental shelf. Very close to (and running up) the coast nonlinear effects become important and Green's law no longer applies.^[2]^[3]

Description

Convergence of wave rays (reduction of width

b

) at Mavericks, California, producing high surfing waves. The red lines are the wave rays; the blue lines are the wavefronts. The distances between neighboring wave rays vary towards the coast because of refraction by bathymetry (depth variations). The distance between wavefronts reduces towards the coast because of wave shoaling (decreasing depth

h

According to this law, which is based on linearized shallow water equations, the spatial variations of the wave height $H$ (twice the amplitude $a$ for sine waves, equal to the amplitude for a solitary wave) for travelling waves in water of mean depth $h$ and width $b$ (in case of an open channel) satisfy^[4]^[5]

H\,{\sqrt {b}}\,{\sqrt[{4}]{h}}={\text{constant}},

where ${\sqrt[{4}]{h}}$ is the fourth root of $h.$ For wave perpendicular wave propagation towards a coast, take $b$ a constant, say 1 metre or yard. Consequently, when considering two cross sections of an open channel, labeled 1 and 2, the wave height in section 2 is:

H_{2}={\sqrt {\frac {b_{1}}{b_{2}}}}\;{\sqrt[{4}]{\frac {h_{1}}{h_{2}}}}\;H_{1},

with the subscripts 1 and 2 denoting quantities in the associated cross section. So, when the depth has decreased by a factor sixteen, the waves become twice as high. And the wave height doubles after the channel width has gradually been reduced by a factor four.

For refracting long waves in the ocean or near the coast, the width $b$ can be interpreted as the distance between wave rays. The rays (and the changes in spacing between them) follow from the geometrical optics approximation to the linear wave propagation.^[6] In case of straight parallel depth contours this simplifies to the use of Snell's law.^[7]

Green published his results in 1838,^[8] based on a method – the Liouville–Green method – which would evolve into what is now known as the WKB approximation. Green's law also corresponds to constancy of the mean horizontal wave energy flux for long waves:^[4]^[5]

b\,{\sqrt {gh}}\,{\tfrac {1}{8}}\rho gH^{2}={\text{constant}},

where ${\sqrt {gh}}$ is the group speed (equal to the phase speed in shallow water), ${\tfrac {1}{8}}\rho gH^{2}={\tfrac {1}{2}}\rho ga^{2}$ is the mean wave energy density integrated over depth and per unit of horizontal area, $g$ is the gravitational acceleration and $\rho$ is the water density.

Wavelength and period

Further, from Green's analysis, the wavelength $\lambda$ of the wave shortens during shoaling into shallow water, with^[4]^[8]

{\frac {\lambda }{\sqrt {g\,h}}}={\text{constant}}

along a wave ray. The oscillation period of shoaling waves does not change, according to Green's linear theory.

Derivation

Green derived his shoaling law for water waves by use of what is now known as the Liouville–Green method, applicable to gradual variations in depth $h$ and width $b$ along the path of wave propagation.^[9]

Derivation of Green's law

Wave equation for an open channel

Starting point are the linearized one-dimensional Saint-Venant equations for an open channel with a rectangular cross section (vertical side walls). These equations describe the evolution of a wave with free surface elevation $\eta (x,t)$ and horizontal flow velocity $u(x,t),$ with $x$ the horizontal coordinate along the channel axis and $t$ the time:

{\begin{aligned}&b\,{\frac {\partial \eta }{\partial t}}+{\frac {\partial (b\,h\,u)}{\partial x}}=0,\\&{\frac {\partial u}{\partial t}}+g\,{\frac {\partial \eta }{\partial x}}=0,\end{aligned}}

where $g$ is the gravity of Earth (taken as a constant), $h$ is the mean water depth, $b$ is the channel width and $\partial /\partial t$ and $\partial/\partial x$ are denoting partial derivatives with respect to space and time. The slow variation of width and depth with distance $x$ along the channel axis is brought into account by denoting them as $b(\mu x)$ and $h(\mu x),$ where $\mu$ is a small parameter: $\mu \ll 1.$ The above two equations can be combined into one wave equation for the surface elevation:

${\frac {\partial ^{2}\eta }{\partial t^{2}}}-{\frac {g}{b}}\,{\frac {\partial }{\partial x}}\left(b\,h\,{\frac {\partial \eta }{\partial x}}\right)=0,$ and with the velocity following from $u=-g\int {\frac {\partial \eta }{\partial x}}\;\mathrm {d} t.$

(1)

In the Liouville–Green method, the approach is to convert the above wave equation with non-homogeneous coefficients into a homogeneous one (neglecting some small remainders in terms of $\mu$ ).

Transformation to the wave phase as independent variable

The next step is to apply a coordinate transformation, introducing the travel time (or wave phase) $\tau$ given by

{\frac {\mathrm {d} x}{\mathrm {d} \tau }}={\sqrt {g\,h}},

\tau =\int _{x_{0}}^{x}{\frac {1}{\sqrt {g\,h(\mu s)}}}\;\mathrm {d} s

and $x$ are related through the celerity $c={\sqrt {gh}}.$ Introducing the slow variable $X=\mu x$ and denoting derivatives of $b(X)$ and $h(X)$ with respect to $T$ with a prime, e.g. $b'=\mathrm {d} b/\mathrm {d} X,$ the $x$ -derivatives in the wave equation, Eq. (1), become:

{\begin{aligned}&{\frac {\partial \eta }{\partial x}}={\frac {\mathrm {d} x}{\mathrm {d} \tau }}\,{\frac {\partial \eta }{\partial \tau }}={\frac {1}{\sqrt {g\,h}}}\,{\frac {\partial \eta }{\partial \tau }}\qquad {\text{and}}\\&{\frac {\partial }{\partial x}}\left(b\,h\,{\frac {\partial \eta }{\partial x}}\right)=b\,h\,{\frac {\partial ^{2}\eta }{\partial x^{2}}}+\mu \left(b'\,h+b\,h'\right)\,{\frac {\partial \eta }{\partial x}}={\frac {b}{g}}\,{\frac {\partial ^{2}\eta }{\partial \tau ^{2}}}+\mu \,{\frac {b'\,h+{\tfrac {1}{2}}\,b\,h'}{\sqrt {gh}}}\,{\frac {\partial \eta }{\partial \tau }}.\end{aligned}}

Now the wave equation (1) transforms into:

${\frac {\partial ^{2}\eta }{\partial t^{2}}}-{\frac {\partial ^{2}\eta }{\partial \tau ^{2}}}-\mu \,{\sqrt {g\,h}}\,\left({\frac {b'}{b}}+{\tfrac {1}{2}}\,{\frac {h'}{h}}\right)\,{\frac {\partial \eta }{\partial \tau }}=0.$

(2)

The next step is transform the equation in such a way that only deviations from homogeneity in the second order of approximation remain, i.e. proportional to $\mu ^{2}.$

Further transformation towards homogeneity

The homogeneous wave equation (i.e. Eq. (2) when $\mu$ is zero) has solutions $\eta =F(t\pm \tau )$ for travelling waves of permanent form propagating in either the negative or positive $x$ -direction. Considering waves propagating in the positive $x$ -direction, Green proposes an approximate solution:

$\eta (\tau ,t)=\Theta (X)\,F(t-\tau ).$

(3)

Then

{\begin{aligned}{\frac {\partial ^{2}\eta }{\partial t^{2}}}&=\Theta \,{\frac {\mathrm {d} ^{2}F}{\mathrm {d} \tau ^{2}}},\\{\frac {\partial \eta }{\partial \tau }}&=\Theta \,{\frac {\mathrm {d} F}{\mathrm {d} \tau }}+\mu \,{\sqrt {g\,h}}\,\Theta '\,F\qquad {\text{and}}\\{\frac {\partial ^{2}\eta }{\partial \tau ^{2}}}&=\Theta \,{\frac {\mathrm {d} ^{2}F}{\mathrm {d} \tau ^{2}}}+2\,\mu \,{\sqrt {g\,h}}\,\Theta '\,{\frac {\mathrm {d} F}{\mathrm {d} \tau }}+\mu ^{2}\,g\,h\,\Theta ''\,F.\end{aligned}}

Now Eq. (2) becomes:

\mu \,{\sqrt {g\,h}}\,\left(2\,{\frac {\Theta '}{\Theta }}+{\frac {b'}{b}}+{\tfrac {1}{2}}\,{\frac {h'}{h}}\right)\,\Theta \,{\frac {\mathrm {d} F}{\mathrm {d} \tau }}+\mu ^{2}\,g\,h\,\left({\frac {\Theta ''}{\Theta '}}+{\frac {b'}{b}}+{\tfrac {1}{2}}\,{\frac {h'}{h}}\right)\,\Theta '\,F=0.

So the proposed solution in Eq. (3) satisfies Eq. (2), and thus also Eq. (1) apart from the above two terms proportional to $\mu$ and $\mu ^{2}$ , with $\mu \ll 1.$ The error in the solution can be made of order ${\mathcal {O}}(\mu ^{2})$ provided

2\,{\frac {\Theta '}{\Theta }}+{\frac {b'}{b}}+{\tfrac {1}{2}}\,{\frac {h'}{h}}=0.

This has the solution:

\Theta (X)=\alpha \,b^{-{\tfrac {1}{2}}}\,h^{-{\tfrac {1}{4}}}.

Using Eq. (3) and the transformation from $x$ to $\tau$ , the approximate solution for the surface elevation $\eta (x,t)$ is

$\eta (x,t)=b^{-{\tfrac {1}{2}}}(x)\;h^{-{\tfrac {1}{4}}}(x)\;F\left(t-\int _{x_{0}}^{x}{\frac {1}{\sqrt {g\,h(s)}}}\;\mathrm {d} s\right)+{\mathcal {O}}(\mu ^{2}),$

(4)

where the constant $\alpha$ has been set to zero, without loss of generality. Waves travelling in the negative $x$ -direction have the minus sign in the argument of function $F$ reversed to a plus sign. Since the theory is linear, solutions can be added because of the superposition principle.

Sinusoidal waves and Green's law

Waves varying sinusoidal in time, with period $T,$ are considered. That is

\eta (x,t)=a(x)\,\sin(\omega \,t-\phi (x))={\tfrac {1}{2}}\,H(x)\,\sin(\omega \,t-\phi (x)),

where $a$ is the amplitude, $H=2a$ is the wave height, $\omega =2\pi /T$ is the angular frequency and $\phi (x)$ is the wave phase. Consequently, also $F$ in Eq. (4) has to be a sine wave, e.g. $F=\beta \sin(\omega t-\phi (x))$ with $\beta$ a constant.

Applying these forms of $\eta$ and $F$ in Eq. (4) gives:

H\,b^{\tfrac {1}{2}}\,h^{\tfrac {1}{4}}=\beta ,

which is Green's law.

Flow velocity

The horizontal flow velocity in the $x$ -direction follows directly from substituting the solution for the surface elevation $\eta (x,t)$ from Eq. (4) into the expression for $u(x,t)$ in Eq. (1):^[10]

{\begin{aligned}u(x,t)&=g^{\tfrac {1}{2}}\;b^{-{\tfrac {1}{2}}}(x)\;h^{-{\tfrac {3}{4}}}(x)\;F\left(t-\int _{x_{0}}^{x}{\frac {1}{\sqrt {g\,h(s)}}}\;\mathrm {d} s\right)\\&+\mu \,{\tfrac {1}{2}}\,g\,b^{-{\tfrac {1}{2}}}(x)\;h^{-{\tfrac {1}{4}}}(x)\;{\frac {(b\,{\sqrt {h}})'}{b\,{\sqrt {h}}}}\,\Phi (x,t)+{\frac {Q}{b(x)\,h(x)}}+{\mathcal {O}}\left(\mu ^{2}\right),\\&\qquad {\text{with}}\qquad \Phi (x,t)=\int _{t_{0}}^{t}F\left(\sigma -\int _{x_{0}}^{x}{\frac {1}{\sqrt {g\,h(s)}}}\;\mathrm {d} s\right)\;\mathrm {d} \sigma \end{aligned}}

and $Q$ an additional constant discharge.

Note that – when the width $b$ and depth $h$ are not constants – the term proportional to $\Phi (x,t)$ implies an ${\mathcal {O}}(\mu )$ (small) phase difference between elevation $\eta$ and velocity $u$ .

For sinusoidal waves with velocity amplitude $V,$ the flow velocities shoal to leading order as^[8]

V\,b^{\tfrac {1}{2}}\,h^{\tfrac {3}{4}}={\text{constant}}.

This could have been anticipated since for a horizontal bed $V={\sqrt {g\,h}}\,(a/h)={\sqrt {g/h}}\,a$ with $a$ the wave amplitude.

Notes

↑ Dean & Dalrymple (1991, §3.4)
↑ Synolakis & Skjelbreia (1993)
↑ Synolakis (1991)
1 2 3 Lamb (1993, §185)
1 2 Dean & Dalrymple (1991, §5.3)
↑ Satake (2002)
↑ Dean & Dalrymple (1991, §4.8.2)
1 2 3 Green (1838)
↑ The derivation presented below is according to the line of reasoning as used by Lamb (1993, §169 & §185).
↑ Didenkulova, Pelinovsky & Soomere (2009)

References

Green

Green, G. (1838), "On the motion of waves in a variable canal of small depth and width", Transactions of the Cambridge Philosophical Society, 6: 457–462, Bibcode:1838TCaPS...6..457G

Others

Craik, A. D. D. (2004), "The origins of water wave theory", Annual Review of Fluid Mechanics, 36: 1–28, Bibcode:2004AnRFM..36....1C, doi:10.1146/annurev.fluid.36.050802.122118
Dean, R. G.; Dalrymple, R. A. (1991), Water wave mechanics for engineers and scientists, Advanced Series on Ocean Engineering, 2, World Scientific, ISBN 978-981-02-0420-4
Didenkulova, I.; Pelinovsky, E.; Soomere, T. (2009), "Long surface wave dynamics along a convex bottom", Journal of Geophysical Research, 114 (C7): C07006, 14 pp., Bibcode:2009JGRC..114.7006D, doi:10.1029/2008JC005027
Lamb, H. (1993), Hydrodynamics (6th ed.), Dover, ISBN 0-486-60256-7
Satake, K. (2002), "28 – Tsunamis", in Lee, W. H. K.; Kanamori, H.; Jennings, P. C.; Kisslinger, C., International Handbook of Earthquake and Engineering Seismology, International Geophysics, 81, Part A, Academic Press, pp. 437–451, ISBN 978-0-12-440652-0
Synolakis, C. E. (1991), "Tsunami runup on steep slopes: How good linear theory really is", Natural Hazards, 4 (2): 221–234, doi:10.1007/BF00162789
Synolakis, C. E.; Skjelbreia, J. E. (1993), "Evolution of maximum amplitude of solitary waves on plane beaches", Journal of Waterway, Port, Coastal and Ocean Engineering, 119 (3): 323–342, doi:10.1061/(ASCE)0733-950X(1993)119:3(323)

Physical oceanography

Waves	Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Fetch Gravity wave Green's law Infragravity wave Internal wave Kelvin wave Kinematic wave Longshore drift Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Sneaker wave Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunami megatsunami Undertow Ursell number Wave action Wave base Wave height Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow water one-dimensional Saint-Venant equations shallow water equations Wind wave model

Circulation	Atmospheric circulation Baroclinity Boundary current Coriolis effect Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño Southern Oscillation General Circulation Model Geostrophic current Global Ocean Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Maelstrom Modular ocean model Ocean dynamics Ocean gyre Princeton ocean model Rip current Subsurface currents Sverdrup balance Thermohaline circulation shutdown Upwelling Whirlpool World Ocean Circulation Experiment

Tides	Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline

Landforms	Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano

Plate tectonics	Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Vine–Matthews–Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Volcanic arc

Ocean zones	Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash

Sea level	Deep-ocean Assessment and Reporting of Tsunamis Future sea level Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea level rise World Geodetic System

Acoustics	Deep scattering layer Hydroacoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics

Satellites	Jason-1 Jason-2 (Ocean Surface Topography Mission) Jason-3

Related	Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean exploration Ocean observations Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas

Category Commons

This article is issued from Wikipedia - version of the 11/10/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.