Circ Basin

$\eta ={\begin{array}{cc}{\frac {Wx_{1}x_{2}}{2ghR}}&f_{c}=0\\{\frac {Wf_{c}}{Rgh\kappa }}\left[{\frac {R^{2}}{8}}+{\frac {1}{4}}\left({\frac {2\kappa x_{1}x_{2}}{f_{c}}}-x_{1}^{2}-x_{2}^{2}\right)\right]&f_{c}\neq 0\end{array}}$

(3)