Controllably, {HEAD ~ the chamber} may be filled either with air from an external source of pressurized air or may be allowed to flood through {TAIL ~ the channel} with water from the surrounding body of water 
allowed to flood through
Controllably , { HEAD ~ the chamber } may be filled either with air from an external source of pressurized air or may be allowed to flood through { TAIL ~ the channel } with water from the surrounding body of water
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH