my solution. Didn't had time so posting later.
From the conservation of mass
.............(1)
But
as shown in the figure. Therefore, 
As every steamline is horizontal between 1 & 2, Bernoull's theorm becomes
= constant which gives
as 
As the difference in height of water column is
, therefore
.......(2)
From Bernoull's theorm between points 1 and 2 of a streamline


or
.............(3)
Using (1) in (3), we get 
Hence 

Rate if helpful.