Radian measure

A circular tunnel has radius $4\,\text{m}$. During heavy rain, water rises to a depth of $1.5\,\text{m}$ from the lowest point of the tunnel.

Find the angle, in radians, subtended at the centre by the water surface at the edges of the water. (Take the water surface as a chord at depth $1.5\,\text{m}$.)