NANAKULI — Casual observers and daily commuters who pass over Ulehawa Stream on Farrington Highway frequently see heavy equipment digging sand out of the concrete channel at the mouth of the stream that empties into the Pacific Ocean.

Sometimes the sand mound dwarfs not only people but vehicles.

"Are they doing that again?" wondered Makaha resident Mike Targgart not long ago as he drove by the channel on his way to Honolulu. "Didn't I see them doing the same thing a few months ago?"

Correct. On both counts.

"It's perpetual," said Kenneth Silva, who has spent much his life in and around the stream. "Two or three times a year — sometimes four — they remove sand from there. It's been going on all our lives."

It's a modern version of an age-old ritual, according to those familiar with the history of the stream — similar to sand buildups in channels at Waimea on O'ahu's North Shore and Ka'elepulu Stream in Kailua, whether due to the vagaries of nature or manmade changes.

Folks such as longtime Wai'anae Coast resident William Aila say for eons Ulehawa Stream worked its way down the mountain to the sea, only to be stymied by the buildup of beach sands. Over time, heavy rains would swell the stream to the point that it would break through, wash away the sand and open up the channel. Then, the whole process would begin anew.

The process continued well into the 20th century, said Aila, the Wai'anae harbormaster. In the 1960s and early 1970s — as part of a public-works flood control project — a concrete channel was built at the mouth of the stream. But instead of keeping the channel open, it exacerbated the sand buildup, according to Aila.

"We've tried to outsmart Mother Nature, and we're losing that battle," Aila said. "The natural order of things would be that the stream would be backed up behind the sands. That area would be called a muliwai — an earthen pond that's blocked by sand. "This has been going on since before the first Hawaiian ever set foot on O'ahu," he said. At Ulehawa, wave action is, in part, causing the sand to slide back into the concrete channel, Aila said.

Craig Nishimura, director and chief engineer for the city's Facility Maintenance Department, said the concrete construction at the Ulehawa Stream was undertaken in an effort to keep the channel open to prevent flooding. However, he said "the area is just so dry that there's not enough water in the drainage system to keep the thing clear. So, it is just pushing in from the ocean rather than pushing out from the land."

For the past three years, Silva, 49, has lived in a tent on the beach next to Ulehawa Channel with his companion, Tammy Mahuka. Sometimes waves wash sand in from the Wai'anae side of the channel, the two say. Other times it washes in from the opposite direction. Occasionally, if it rains long and hard enough, the stream builds up and pushes all the sand back out to the sea, and the channel stays clear for several months.

Several years ago, the concrete channel was reconstructed, said Silva and Mahuka. But, as ever, the sands returned — along with city work crews and heavy equipment.

"They have to remove the sand," explained Mahuka, 46. "If they don't, the water (on the mountain side of the channel) becomes stagnant. It turns real nasty."

At one time, bulldozers removed the sand. Later, clamshell and dragline equipment were brought in. These days, it's long-reach excavators. Sand heaps 20 feet high and 50 feet long are piled next to the channel to dry. City crews return dry sand to Ulehawa Beach. And eventually it works its way back into the channel.

Nishimura said the sand removal at Ulehawa Channel is done by the department's own personnel on a work-order basis. It is budgeted as part of the department's overall maintenance costs. It would be difficult to itemize the specific costs attributed to removing sand from Ulehawa Stream, he said.

But one thing is certain: The sand removal there will continue because the city is obligated to keep the channel clear.

"And the sand just keeps coming back, no matter what they do," Nishimura said.