# Green's law

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:

$H_{1}\,\cdot \,{\sqrt[{4}]{h_{1}}}=H_{2}\,\cdot \,{\sqrt[{4}]{h_{2}}}$ or   $\left(H_{1}\right)^{4}\,\cdot \,h_{1}=\left(H_{2}\right)^{4}\,\cdot \,h_{2},$ 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. 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 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

$H\,{\sqrt {b}}\,{\sqrt[{4}]{h}}={\text{constant}},$ where ${\sqrt[{4}]{h}}$ is the fourth root of $h.$ 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 wave propagation perpendicular towards a straight coast with depth contours parallel to the coastline, take $b$ a constant, say 1 metre or yard.

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. In case of straight parallel depth contours this simplifies to the use of Snell's law.

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 also 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 (and therefore also the frequency) 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.