Flow through riser

A two stage riser with low flow controlled with an orifice and high flow controlled with a weir.

$Q=\left\{{\begin{matrix}0,&Z\leq Z_{0}\\C_{w}L_{w}\left(Z-Z_{0}\right)^{1.5},&Z_{0}\leq Z\leq Z_{0}+H_{0}\\C_{d}A_{0}{\sqrt {2g\left(Z-Z_{0}-0.5H_{0}\right)}},&Z_{0}+H_{0}\leq Z\leq Z_{1}\\C_{w}L_{w}\left(Z-Z_{1}\right)^{1.5}+C_{d}A_{0}{\sqrt {2g\left(Z-Z_{1}\right)}},&Z_{1}<Z\end{matrix}}\right.$