FLOOD PROTECTION FOR UNDERGROUND AIR VENTS

An assembly that fits within a ventilation shaft open to atmosphere and leading from underground ventilation ducts such as to subways prevents surface waters from flooding the underground ducts while maintaining the ventilation. The assembly fits in the ventilation shaft and houses a buoyant float in a chamber surrounding a passageway in fluid communication with the ventilation shaft. A grate covers the passageway. An outside wall of the chamber adjacent the ventilation shaft is open at top to atmosphere and allows flooding water into the chamber, buoying the float upward in the chamber, elevating the passageway above the flooding water. The chamber suitably has means for drainage and admission of flush water to flush flood water debris from the chamber through the drainage.