ORANGE — Walking hurriedly under the Chapman Avenue bridge through jagged rocks and sand, Heidi Yrigoyen yelled across the Santa Ana River, warning her fellow homeless: “Bulldozers are here!”

“They coming and they serious this time,” said the 31-year-old, who has lived along the Santa Ana River Trail for three years.

Yrigoyen urged the homeless who have set up elaborate encampments and makeshift dwellings on the east side of the riverbed to move to the other side as OC Public Works on Wednesday began a six-month project to stockpile heavy boulders and material for flood control.

Heavy rains had delayed the start of the project earlier this week.

Before dump trucks came in with the first load of boulders Wednesday morning, a crew of about 10 maintenance workers helped the homeless separate their belongings and clear trash from the river bank along Chapman Avenue near Anita Street. Four or five people were asked to relocate their tents away from the project area the first morning, said Carrie Braun, public information officer for the county of Orange.

They were offered free transportation to homeless shelters in Santa Ana and Fullerton.

“Our health agency outreach and the sheriff’s homeless liaison team came out today and asked for voluntary compliance,” Braun said. “There have been signs posted since Jan. 25 informing those who are encamped in the area of the project. … They have offered them resources providing them an opportunity to stay in a shelter. Some decided to take that opportunity and some didn’t.”

Several hundred people have lined the eastern side of the riverbed with camps, adjacent to the 57 freeway and Angel Stadium. They will have to move.

The county will use the eastern stretch of the river’s banks to store 5,100 tons of riprap and sand. A larger fence will be built to protect the materials being stockpiled in case of future flooding. The project stretches from Orangewood Avenue in Anaheim to just south of the 22 freeway near Hesperian Way in Santa Ana.

“Flood control channels are not a safe place to live,“ said Braun, adding that outreach teams will regularly visit the area to assist the homeless as the project moves forward.

Fifteen to 20 advocates and protesters stood above the bridge holding signs, “Protect your neighbors from eviction” and “They have nowhere to go.” Some cars honked in support. Other motorists flipped them off, protesters said.

Orange County Sheriff Department deputies guarded the entrance gate to an access road as maintenance workers cleared the nearby encampment.

Sheriff deputies had to forcibly remove a protester, Mohammed Aly, after the Santa Ana attorney held on to a gate and fence in the construction area. Aly was arrested on suspicion of misdemeanor obstructing officers and trespassing, Sheriff’s Department Lt. Mark Stichter said.

“This is inhumane treatment of our homeless,” said Heidi Zimmermann, a longtime homeless advocate. “If we don’t help them, we’ll lose our humanity.”

Tammy Schuler, 41, was one of the homeless asked to relocate. Schuler said she’s cooperating with the county.

“I’m in the wrong. They are in the right,” she said. She has been homeless for six years. “I’m going to set up on the other side until I find something else.”

Rasheeda Dickerson, 40, came down to help Schuler and other friends move their belongings. Dickerson said she lived in the same riverbed for three years before finding help. She now lives with her fiancé in Orange.

“When I stayed here, it never used to be like this,” Dickerson said. “There are a lot more people here than when I was homeless. A lot of the homeless from Santa Ana and other cities have come here and set up.”

Carrying blankets and walking above the Chapman Avenue bridge, Sarah Watts, 37, made a preemptive move to the western side of the riverbed next to a Motel 6 and a bike trail where cyclists cruised. Watts said the county’s project is only going to make the homeless situation in the county worse. She’s lived in a tent along the riverbed for a year.

“They are pushing us out, but it’s not like we’re going to disappear,” she said. “We’re just going to move somewhere else.”

Staff Writer Scott Schwebke contributed to this story.

Contact the writer: 714-796-2443 or jpimentel@scng.com or follow on Twitter @OCDisney