Crews dredged the beach in Kailua Wednesday to drain Kaelepulu Stream, adding more sand to a huge mound that's been sitting on the shoreline for months.
The city says it opens the mouth of the stream on a regular basis for mostly safety reasons.
University of Hawaii geology professor Charles "Chip" Fletcher said winds and currents push sand from the Lanikai end of the beach and pile it up in the area.
"In the past, episodes of erosion, a year or two in duration, would be followed by episodes of sand return back to the beach park. But this particular episode of erosion isn't ending," Fletcher said.
"We don't know why the erosion has occurred so dramatically. But many of us think that the reason it's happening more dramatically now is because we've had at least a hundred years or more of slow sea-level rise."
The city says there are plans to move the sand and use it to replenish eroded areas.
"It'll happen within the month. The timing will be based on low tide so that we can get the most sand in the right place without it washing away again," said Ross Sasamura, director of facility maintenance for the city and county of Honolulu.
"This process, it's called back passing sand. If we could back pass sand from the stream mouth to the boat ramp every couple of months, like quarterly for instance, it's a very good way to counteract the erosion. It's a good way to manage the erosion problem," Fletcher added.
Another nearby mound of sand along Lihiwai Road is material dredged upstream more than two weeks ago.
The city says it will not use any of that sand to replenish the beach.