There are exactly two beaches on the Kalalau trail. The first, at Hanakapiai, is about 2 miles in from the road and is quite popular.
As my book says, the beaches are built up by the small waves of summer, and eroded by the thundering surf of winter. I believed the book, but didn't think I would see the process in action.
When I hiked in, the beach at Hanakapiai reached about 100 yards across, from a cave in the cliff to the right of the beach all the way to the cliff at the left. As I was hiking out, five days later, the beach had retreated about 100 feet, and all that was left was a thin strip about half as wide and half as long as the original.
The big waves were pounding the sand while I was there, so the beach may have shrunk more.
The beach at Kalalau never gets quite that small, but it did shrink as I was there. On the day I left, when waves retreated the wet part of the beach was over 50 feet long. I know because I was trying to wash my pots in the ocean, and had to either walk a long way to get to the water, or wait for a big wave to reach me.
The day I left, the westernmost part of the beach at Kalalau had a ledge of sand higher than my head. The waves had eroded over six feet of sand almost all the way back to the cliff. Many tons of sand washed out to leave this ledge behind.