= Saint-Venant dry-front condition
{c}
{title2=$h_f=0$}
= St. Venant front condition
{c}
{synonym}
The <shallow water equations> at a dry bed, with negligible ambient <mass density>, end in a vanishing-depth edge, rather than a finite-depth <gravity current> head. In a prismatic triangular channel the <Riemann invariant> $u+4\sqrt{g'h/2}$ gives the speed $2\sqrt{2g'h_0}$ after release of a resting semi-infinite layer of depth $h_0$. In a rectangular channel the corresponding speed is $2\sqrt{g'h_0}$; the channel geometry matters.
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