The River Plym, which flows off Dartmoor to the north-east, forms a smaller estuary to the east of the city called Cattewater. Plymouth Sound is protected from the sea by the Plymouth Breakwater, in use since <answer>1814</answer>. In the Sound is Drake's Island which is seen from Plymouth Hoe, a flat public area on top of limestone cliffs. The Unitary Authority of Plymouth is 79.84 square kilometres (30.83 sq mi). The topography rises from sea level to a height, at Roborough, of about 509 feet (155 m) above Ordnance Datum (AOD).
When did the Plymouth Breakwater open?
1814