= Solution
For a cylinder of <radius> $R_c$ in unbounded two-dimensional incompressible <potential flow>, a <velocity potential> is $\varphi=u(r+R_c^2/r)\cos\theta$. It gives
$$
v_r=u(1-R_c^2/r^2)\cos\theta,\qquad
v_\theta=-u(1+R_c^2/r^2)\sin\theta.
$$
On the surface $v_r=0$. The points nearest the banks are the sides perpendicular to the stream, $\theta=\pi/2,3\pi/2$, hence \b[the <speed> is $2u$ at each]. This is the inviscid outer-flow <speed>; a real no-slip <boundary layer> has zero <velocity> at the material surface. The small-radius/wide-river assumption neglects bank effects.
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