A horizontal bridge is built across a river. A student standing on the bridge throws a small ball vertically upwards with a velocity $4 \mathrm{~m} \mathrm{~s}^{-1}$. The ball strikes the water surface after $4 \mathrm{~s}$. The height of bridge above water surface is (Take $g=10 \mathrm{~m} \mathrm{~s}^{-2}$ )