For the purpose of discharge measurement, the width of rectangular channel is reduced gradually from 3m to 2m and the floor is raised by 0.3m at a given section. When the approach depth of flow is 2m, what rate of flow will be indicated by a drop of 0.15m in the water surface elevation at the contracted section.

$\begin{aligned} E_{1}=E_{2}+\Delta z \Rightarrow & 2+\frac{v^{2}}{2 \times 9.81}=1.55+\frac{v^{2}}{2 \times 9.81}+\Delta z \ & \Rightarrow 2+\frac{Q^{2}}{19.62 \times 3^{2} \times 2^{2}}=1.55+\frac{Q^{2}}{19.82 \times 2^{2} \times 1}+.3 \ & \Rightarrow 0.15 =Q^{2}\left(\frac{-1}{9 \times 4}+\frac{1}{4 \times 155}\right) \ & \Rightarrow \quad Q=6.21 \mathrm{~m}^{3} / \mathrm{s} \end{aligned}$