A storm drain system is provided to prevent rising water from flowing substantially vertically out of a storm drain inlet. The storm drain system comprises a seat generally horizontally mounted within a chamber and a moveable buoyant door assembly responsive to rising water in the chamber to float upwardly to engage the seat to thereby prevent water flowing upwardly out of the storm drain inlet. A support frame may be mounted along a flow passageway leading to the storm drain gating such that the seat and the buoyant door assembly is mounted to or integral with the support frame. In an embodiment, one or more pivot or hinge mountings secure the buoyant door assembly pivotally with respect to the drain support.