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. 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. 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.

Description

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 satisfy

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

with mean depth $h$ and width $b$ (of the channel), while ${\sqrt[{4}]{h}}$ is the fourth root of $h.$ Consequently, 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 follow from the geometrical optics approximation to the linear wave propagation.

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

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

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

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

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.

Derivation of Green's law

Wave equation for an open channel Starting point are the linearized one-dimensional Saint-Venant equations describing 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 open-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 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 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 ${\frac {\mathrm {d} x}{\mathrm {d} \tau }}={\sqrt {g\,h}},$   so   $\tau =\int _{x_{0}}^{x}{\frac {1}{\sqrt {g\,h(\mu s)}}}\;\mathrm {d} s$ 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 $\psi (x)$ 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): ${\begin{aligned}u(x,t)&=g^{\tfrac {1}{2}}\;b^{-{\tfrac {1}{2}}}(x)\;h^{-{\tfrac {3}{4}}}(x)\;F\left(t-\int _{x_{0}}^{x}{\frac {1}{\sqrt {g\,h(s)}}}\;\mathrm {d} s\right)\\&+\mu \,{\tfrac {1}{2}}\,g\,b^{-{\tfrac {1}{2}}}(x)\;h^{-{\tfrac {1}{4}}}(x)\;{\frac {(b\,{\sqrt {h}})'}{b\,{\sqrt {h}}}}\,\Phi (x,t)+{\frac {Q}{b(x)\,h(x)}}+{\mathcal {O}}\left(\mu ^{2}\right),\\&\qquad {\text{with}}\qquad \Phi (x,t)=\int _{t_{0}}^{t}F\left(\sigma -\int _{x_{0}}^{x}{\frac {1}{\sqrt {g\,h(s)}}}\;\mathrm {d} s\right)\;\mathrm {d} \sigma \end{aligned}}$ 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 $V\,b^{\tfrac {1}{2}}\,h^{\tfrac {3}{4}}={\text{constant}}.$ 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.

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

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, vol. 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., doi:10.1029/2008JC005027 {{citation}}: Unknown parameter |nopp= ignored (|no-pp= suggested) (help)
• 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. (eds.), International Handbook of Earthquake and Engineering Seismology, International Geophysics, vol. 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)