There are 3 locks on the barrage. Each lock is 40 metres long; two are 8 metres wide and one is 10.5 metres wide.  The locks allow boats to navigate between the Severn Estuary and Cardiff Bay. As the estuary has a high tidal range, the sector lock gates are up to 16 metres high to enable the boats to pass through at all stages of the tide.

The inner cills are at –0.5m OD but to minimise salt water intrusion beams may be placed on the cills to prevent as little estuary/saline water as possible from entering the Bay. Any estuary/saline water entering the Bay is contained in a sump and is then flushed out via a 1200mm gravity discharge pipe.

An outer harbour that can accommodate up to 200 boats has been constructed and is dredged to allow 24 hour access for boats.  Previously Cardiff Bay was tide dependant and boats were prevented from accessing the estuary at times of low tides. Each lock can accommodate up to 10 average size vessels and passage through the locks takes between 5 and 20 minutes.  Over 65,000 vessels have passed through the barrage locks between April 2000 and April 2004.

There are also three bascule bridges on the barrage, one for each lock.  These allow passage over the barrage structure when they are closed and the passage of vessels in and out of the Bay when they are open. 

The bridges each weigh 88 tonnes and work on a cantilever design using minimum energy to raise and lower the bridge decks.  The word bascule originates from the French for seesaw.

One lock operation at low tide can use up to 4 million litres of fresh water.