An orifice plate is being designed for flow control at the outlet of a detention tank. The outlet pipe has a diameter of $450mm,$ slope of $0\mathrm{.}0018,$ and roughness $ks$ of $0\mathrm{.}6mm.$ The water level in the tank at the design condition varies between $1\mathrm{.}5$ and $1\mathrm{.}7m$ above the outlet invert, and the desired outflow is $100\frac{l}{s}.$ Select an appropriate orifice diameter. $($Assume orifice $C_d=0\mathrm{.}6$; check that the orifice will not be drowned.$)$

answers: $200mm$ not drown

Don't do asumption for orifice plate diameter.