= Water depth
{title2=$d$}
<Water depth> is the local vertical distance from the bed to the free surface. If the bed has elevation $b(x,y)$ and the surface has elevation $\eta(x,y,t)$ in the same vertical coordinate, then $h=\eta-b$. The datum used to define the two elevations cancels from their difference.
Keeping the distinction matters in the <shallow water equations>: the horizontal <hydrostatic pressure> gradient depends on surface elevation, while the transported <volume (thermodynamics)> depends on depth. A sloping bed with a horizontal free surface can have a varying <water depth> without a horizontal gravitational driving term. Conversely, the same bed can support a flow driven by a sloping surface.
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