Volunteers rescue people from their flooded homes along Beamer Road in Houston on Sunday. (Jabin Botsford/The Washington Post)

Even as government officials vowed a powerful, forward-leaning­ response to the disaster that Hurricane Harvey has left in Texas, the situation on the ground remained chaotic and desperate on Monday, with millions of soaked residents resorting to their own resources and wits to survive.

Houston is notoriously flood-prone, but no one in the government had planned for a disaster quite like this. They had planned for calamities with elements of what has unfolded in the city and surrounding communities, but they didn’t war-game the inundation of thousands of square miles, including a huge, sprawling metropolis, from a storm that sticks around for days.

“You could not draw this forecast up. You could not dream this forecast up,” William B. “Brock” Long, administrator of the Federal Emergency Management Agency, said Monday.

Two of his predecessors and other former FEMA officials concurred. “I don’t remember an exercise that says, ‘Now we’re going to drop historic amounts of rain on you for three days.’ We never did anything like that,” former FEMA chief of staff Jason McNamara said.

Harvey has exposed anew how vulnerable the nation’s fourth-largest city is to the forces of nature and the failings of infrastructure — and how natural disasters have a way of revealing the fault lines in governmental operations.

Residents received mixed messages from the state capital and Houston’s City Hall about whether to evacuate in advance of Harvey’s landfall late Friday, when it was a Category 4 hurricane. Gov. Greg Abbott (R) urged people to leave voluntarily, but Houston’s mayor said it was better to shelter in place — a decision influenced by the harsh memory of the evacuation in advance of Hurricane Rita in 2005, when scores of people died in overheated cars stuck in highway traffic jams.

When asked Monday whether the city should have been evacuated, the governor said there was no point in thinking about past decisions. “We are where we are now,” he said.

Abbott announced that he had mobilized the entire Texas National Guard to assist with rescue-and-recovery efforts. Surrounded by local officials from the Corpus Christi area, he said Texans are united in their effort to recover from Harvey, and he rated the early federal response to the disaster as an “A plus.”

FEMA said Monday that it already had made major deployments of personnel and resources to Texas, including about 900 people in search-and-rescue teams, a million liters of water, a million meals, 20,520 tarps and 70 generators. The FEMA-coordinated response includes 100 ambulances and 15 air ambulances from San Antonio.

U.S. Coast Guard helicopters buzzed the city for much of the day and plucked stranded residents from rooftops. National Guard troops drove massive trucks through high water, looking for people in desperate straits. But professional first responders could not meet all the demands of people immersed in floodwater.

“Please don’t give up on us. None of us are going to give up,” Houston Police Chief Art Acevedo implored residents during a news conference.

Texas state Rep. Armando Walle (D), who represents a district on the north side of the Houston metropolitan area, put out a call on his Facebook page: “If you know of any families still needing to get rescued please let me know.” Within three hours, he had a legal pad with 25 houses listed.

“Government can’t do it all alone,” Walle said later. “We’ve had to have the county judge ask for boats from the public. The mayor has asked for boats. We’ve had people all over the state converge. The Cajun Navy is here from Louisiana in their airboats.”

Social media has proved to be the crucial piece of infrastructure for rescue operations.

“We are actually seeing situations where Bob down the street has a flat-bottom boat and Mary up the street has a canoe, and you put those resources together, hitch them together, and get everybody on the block the hell out of there,” said Matthew Seeger, a communications professor at Wayne State University in Detroit who advises government agencies.

“That may not sound significant, but when you think about where we were just 15 years ago when Katrina hit and we had to pass out fliers — in shelters, by boat, whatever we could do just to get some basic health and safety information out there. Not everyone has a transistor radio,” Seeger noted, “but almost everyone has access to Twitter.”

Retired Adm. Thad Allen, who played a key role in shoring up the initially weak federal response to Hurricane Katrina in 2005 and served as national incident commander for the 2010 Gulf of Mexico oil spill, said government agencies seem to be doing their best in a complex situation.

“I’m not sure how much more you can ask for,” he said. “If you have more total fires than you have firemen, the scale, complexity and scope becomes a risk aggregator.”

Allen also pointed out that every storm is different: “When you’ve seen one hurricane, you’ve seen one hurricane.”

W. Craig Fugate, who led FEMA during Barack Obama’s presidency, said the agency’s previous advance planning had been more concerned with “no-notice” disasters such as earthquakes. He said he pushed FEMA staff members to think outside the box. “You have to prepare for the things that haven’t happened, or you haven’t thought of that will exceed the daily capabilities,” he said.

Yet he acknowledged what is obvious in the wake of Harvey: “You can’t anticipate every scenario that might happen.”

Having learned lessons from Hurricane Katrina’s aftermath, FEMA positioned many of its personnel in advance of Harvey, as did the Red Cross and other organizations. But when the floodwater rose, Houstonians were still largely on their own. Some rode to shelters or hospitals in the back of city dump trucks, while others waded through chest-deep water.

A neighborhood in Friendswood, Tex., was so heavily flooded by Hurricane Harvey that some people are using boats to travel through it. (Zoeann Murphy/The Washington Post)

Leaders of the federal response to the Hurricane Harvey disaster had an early morning news conference Monday in Washington that was by turns a declaration of competence, an expression of grave concern and a plea for the public’s help.

“Under the president’s direction, we have made every resource available to respond to this historic storm,” said Elaine Duke, acting secretary of the Department of Homeland Security, who on Tuesday will accompany President Trump on a tour of storm-ravaged areas.

And that’s before the actual cleanup begins — which FEMA chief Long is warning will be “one of the largest recovery housing missions that the state has ever seen.”

Flooding persists as Harvey downgraded to tropical depression

Evacuees are taken Sunday to the George R. Brown Convention Center in Houston. (Nick Oxford/REUTERS)

McNamara, the former FEMA chief of staff, said Texas officials are overwhelmed by the task of saving lives.

“Very quickly, you’re going to get behind the curve on what comes next week,” he said. “The other thing? Schools. How do we get the schools open and kids back to school? If they can’t go back to school, the parents can’t go to work, and that creates another economic problem.”

Mary Lee Grant in Corpus Christi and Janell Ross in Washington contributed to this report.