Abstract: An outflow regulator for a gravity-fed liquid outlet of a liquid reservoir comprising a restriction plate mountable within the liquid outlet for limiting a flow rate of an effluent. The restriction plate allows a lower passage for the effluent when a first level of liquid is reached within the liquid reservoir, and an upper passage for the effluent when a second level of liquid is reached. The regulator also comprises a flapgate pivotable between a closed position where the flapgate covers the lower passage of the restriction device and an open position away from the restriction plate. When the flapgate is in the closed position, the effluent is free to flow out though the upper passage. A float counterweight switches the flapgate between the opened position and closed position according to the liquid level in the reservoir.