Circ Basin

$\eta =\begin{array}{cc}\frac{W{x}_1{x}_2}{2ghR}& f_c=0\\ \frac{W{f}_c}{Rgh\kappa }[\frac{R^2}{8}+\frac{1}{4}(\frac{2\kappa x_1{x}_2}{f_c}-x_1^2-x_2^2)]& f_c\ne 0\end{array}$

(3)