= Half-contraction free streamline at a horizontal canal lip
{title2=$a=\tfrac12$}
A semi-infinite horizontal plate at unit height and a floor at zero give a <velocity potential> strip of height $a$ and an upper unit <semicircle> in the <complex velocity> $q$. Their matching <half-plane> maps give $(1+e^{\pi w/a})^{-1}=((q-1)/(q+1))^2$. Along the <fluid free boundary> put $q=e^{-i\theta}$, $-\pi<\theta<0$. Then $dz/d\theta=-(a/\pi)e^{i\theta}\cot(\theta/2)$ and $y=1-(a/\pi)(\theta+\pi+\sin\theta)$. Its downstream limit is $1-a$, which must equal the downstream depth $a$. Thus the contraction is exactly one half.
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