Weir ventilation

    When water flowing over a rectangular weir with no end contraction i.e.suppressed weir,the nappe touches the side wall of channel due to which air is trapped in the space between side wall falling nappe,weir and bottom of the channel.this air is carried by the flowing water there by reducing the pressure below the nappe which may become negative or below atmospheric pressure.

  # Further withdrawal of air increase the vacuum pressure causing the discharge will be (20-30)% more than that given by formula .

  #Hence for the calculation of discharge through a weir with no end contraction the pressure below the lower nappe should be equal to atmospheric pressure,so that there will be no extra withdrawal of water and discharge is obtained by using the formula.this in other to apply the formula and obtained the discharge through a suppressed weir atmospheric pressure has to be maintained. For this holes are made through the channel walls at the place just below the nappe and connected with the pipe to a atmosphere.this process is called weir ventilation.