The device (100) has a conduit (1) that is arranged to ensure fluidic communication between an interior space and an evacuation passage (2). The conduit presents a conical interior section (12) whose surface ascends in a water flow direction in the conduit. A shutter (3) is placed in the conduit and is controlled so as to be moved inside the conduit according to the water level in the interior space. The displacement of the shutter controls leakage flow through the evacuation passage irrespective of water in the interior space between two levels. An independent claim is also included for a water retention system.
