Current meter is used for measuring the stream velocity. Stream has trapezoidal cross section with an upper edge of $30$m and bottom edge of $10$m$.$ The Current meter recorded $5$ rev$/$s for a stream velocity is $20$ m$/$s; $15$ rev$/$s for a stream velocity of $50$ m$/$s$.$ When the discharge through the stream is $4920$ m$^3/$s$,$ the current meter has recorded $12$ rev$/$s$.$ Then determine the depth of water at that particular section