An excessive build-up of liquid in adsorption columns or on the plates of a distillation column. It is due to high vapour flow rates up the column. In distillation columns this is caused by high heating rates in the reboiler. Compare weeping. The flooding point is a condition in a packed column such as an absorption column which receives a countercurrent flow of gas at the bottom and a liquid descending under gravity from the top where there is insufficient liquid hold-up in the packing for mass transfer to take place effectively. The liquid therefore descends to the bottom of the column without mass transfer. The rate of flow through the packing for effective mass transfer is controlled by the pressure drop across the packing material.