A damming device for forming a liquid-damming protective bank comprises at least one first board (1), in the active state laying, intended to be urged by the liquid against the surface and thereby anchoring the device, as well as at least a second board (2), in the active state up-right, intended to dam the liquid. There are liquid-draining means (5) on the bottom side of the anchoring board (1). Devices (9, 9') act between the two boards (1, 2) with the purpose of inhibiting tilting of the damming board (2) from the active position in the direction towards the dry the side, when liquid simultaneously affects the anchoring board (1) with a vertical force and the damming board (2) with a horizontal force.