The Thames Barrier is the world’s second largest movable flood barrier (after the Oosterscheldekering in the Netherlands) and is located downstream of central London. Its purpose is to prevent London from being flooded by exceptionally high tides and storm surges moving up from the sea. It needs to be raised (closed) only during high tide; at ebb tide it can be lowered to release the water that backs up behind it. Its northern bank is in Silvertown in the London Borough of Newham and its southern bank is in the New Charlton area of Charlton in the London Borough of Greenwich. Tone mapped in Photomatix.