Abstract: A closure apparatus for covering a portal is disclosed herein. In various aspects, the closure apparatus includes a curtain assembly having one or more curtains and a spindle assembly mounted superjacent a portal on an exterior of the portal. The spindle assembly is operably engaged with the curtain assembly to extend the curtain assembly to cover the portal and retract the curtain assembly to uncover the portal. Channels disposed on lateral sides of the portal engage the curtain assembly when the curtain assembly is extended to cover the portal. Front channel members, which are pivotably connected to corresponding adjacent supports, pivot to release the curtain assembly from engagement with the channels upon application of sufficient force to the curtain assembly to allow egress or ingress through the portal when the closure apparatus is not operational, for example, during loss of power. Related methods of operation of the closure apparatus are also disclosed.