${v^2} = {u^2} - 2gh$, ${\left( {10} \right)^2} = {u^2} - 2g\,\left( {\frac{H}{2}} \right) = \,{u^2} - 2g\,\frac{{{u^2}}}{{4g}}$ $⇒$ $\,{u^2} = 200$
$\therefore \,H = \frac{{{u^2}}}{{2g}} = \frac{{200}}{{2 \times 10}}$ $= 10\,m$
$OA, \,AB,\, BC,\, CD$
($g = 9.8\, m/s^{2}$)