The present invention provides for a dual port arrangement for a water seal chamber of a drainage device for channeling and restricting fluid flow through the water seal during normal operation of the device when the patient is breathing normally, while preventing the breach of the water seal during reverse flow conditions when the patient takes a sudden and deep inhalation. The dual port arrangement comprises a narrow side port and a low profile tunnel port defining an elongated conduit interposed between and in communication with both arms of the water seal chamber. The side port has a thin slot configuration that restricts fluid flow communication through the side port, and the back tunnel port defines a low profile, elongated conduit with openings that channel fluid flow between both arms of the water seal chamber. The elongated configuration of the conduit prevents a steep water gradient from falling below the upper portion of the conduit opening during reverse flow conditions.