Flood protection for underground air vents

Apparatus allowing ventilation as usual through a ventilation shaft to an underground ventilation duct fluidly communicating through the ventilation shaft to a rectilinear atmospheric opening of the shaft and on threat of flooding operable to prevent downward flow of surface water into an underground ventilation duct, comprising:

• a four-sided box inclusive of sidewalls sized to internally fit in said shaft between said ventilation duct and said atmospheric opening defining a passage between top and bottom openings of the box for fluid communication of said ventilation duct up through said box to said atmospheric opening, said box further comprising flanges transverse to said sidewalls for overlaying a top of said shaft to hang said box in said shaft,
• a beam unobstructively horizontally centrally spanning across said passage and connected to opposed sidewalls of said box proximate said top opening
• one or more stops within and connected to said support proximate said bottom opening and not obstructing said passage,
• a hinge mounting member unobstructively horizontally centrally spanning across said passage and connected to opposed sidewalls of said box proximate said bottom opening,
• a plurality of hinge members each comprising a stationary member, a movable member and a hinge pin interconnecting the stationary and movable members, said stationary member being connected to said hinge mounting member,
• at least one pair of opposing panels, each panel of a said pair having a proximal and distal portion, said moveable hinge members attaching to said proximal portions of said panels for rotation of the paired panels in directions opposite each other from or to an upright home position not obstructing said passage, said paired panels in gravitational rotation falling from said upright home position to a lower passage closing position where further rotation is prevented by said one or more stops, each panel having a profile that closes said passage when said panels gravitationally rotate to said passage closing position, and
• a panel holder having a spring and moveable latch members spring biased to vertically extend said latch members below said beam to straddle said panels to hold the panels in said upright home position, said holder having a handle that extends above said beam, said handle being retractable upwardly to raise said latch members above said panels to release the panels and allow the panels to gravitationally rotate downwardly to said passage closing position, retraction of the extended latch members loading the spring to exert a force resisting retraction to re-extend said moveable latch members on release of said handle.