Much of Waikiki's 'natural' beach was created as a tourist feature. It has required constant maintenance because the sand keeps disappearing.
This section of the beach is enclosed by a wall. The sand drifting from the shore toward the coral reefs and the sea collects by the wall.
Twice a year, the city pays for bulldozers to push the sand back onto the shoreline to maintain the 'natural' beach.
For more information on Waikiki's 'natural' beaches