A spillway has unit discharge of $7.5 \mathrm{~m}^{3} / \mathrm{s} / \mathrm{m}$. The flow depth at the downstream horizontal apron is $0.5 \mathrm{~m}$. The tail water depth (in meters) required to form a hydraulic jump is $\_\_\_\_\_\_\_\_$. (rounded off to $2$ decimal places).