High Quality Content by WIKIPEDIA articles! The Thames Barrier is the world's second largest movable flood barrier. It is a flood control structure located downstream of central London. The barrier's purpose is to prevent London from being flooded by an exceptionally high tide moving up from the sea, often exacerbated by a storm surge. It only needs to be raised for the duration of the high tide; at ebb tide it can be lowered to release upstream water that backs up behind it.