Iashia Nelson cradled her 4-year-old son, shielding him from the pounding rain as she and close to 30 others huddled together on a rooftop, praying they would be rescued.

In the eight hours they waited, without any food or water, Nelson, 36, said she witnessed at least four people die.

She saw a woman and child trapped in a car, honking their horn in panic until the water engulfed them. She saw another woman, clinging to a tree, lose her grip and be swept away by the rapids. She saw three men attempt to resuscitate an elderly woman who had drowned.

The images are seared into her mind, emerging whenever she closes her eyes. It’s why she’s hardly slept since her family moved to a downtown Houston shelter on Monday.

She’s worried about what comes next for them. She’s worried about the emotional toll on her three sons, who also saw bodies floating in the flooded streets.

Parents Shannon Hayes and Tony Jenkins are grappling with how to move forward after they were forced to abandon their flooded home and find shelter at George R. Brown Convention Center on Aug. 29. (Jerome Bailey Jr.,Monica Akhtar/The Washington Post)

“I’m going to need some counseling. I’m really going to have to talk with someone,” she said. “I’ve been keeping my mind sane because my children are still looking to me, and if I start falling, they are going to fall too.”

In the immediate aftermath of a crisis like Hurricane Harvey, the priority is survival.

But after the volunteers and emergency workers leave and the outpouring of support subsides, the survivors are left with haunting memories and the daunting task of rebuilding their lives — stresses that can take a toll on mental health.

Nelson knows this pain well. She lost all of her worldly possessions in Hurricane Katrina.

She and her family relocated to Houston to rebuild their life. Having to go through this a second time — something she never thought she’d face again — is too much to bear without professional help.

Ronald C. Kessler, a professor at Harvard Medical School, studied the long-term psychological effects on Hurricane Katrina victims. Humans are resilient, and many do recover from their immediate grief, but he found the number of people reporting post-traumatic stress, depression and suicidal ideation increased as time passed and life didn’t resume normalcy.

“Most of the mental health problems we found, most of the long term issues after Katrina had to do with things like trying to go back to my house and electricity is still out 12 months later,” Kessler said. “There was hope in the short term, the community pulls together, politicians give speeches and then reality sets in … that kind of nagging, dragging feeling leads to depression and despair. People give up hope: ‘Is it ever going to be better?’”

Drivers faced long lines at gas stations in Texas as people rushed to fill up ahead of the Labor Day weekend and amid reports of gasoline shortages following Hurricane Harvey. (Taylor Turner/The Washington Post)

After Katrina, Diana Meyers, a nurse affiliated with a local church, said victims overwhelmingly identified mental health services as a key need.

People struggling with the trauma of the storm were then retraumatized when they started to navigate how to restart their lives. There were frustrating dealings with contractors and insurance companies. There were fears about finding employment. There was shame in seeking out government benefits like food stamps and Medicaid after never needing subsidies before.

Meyers, whose own house in New Orleans flooded, experienced mild depression after Katrina and for a short time took medication for it. Most of her family also lived in New Orleans. With each suffering their own hardships, they couldn’t lean on one another for support.

Meyers remembers standing in line for food and crying in disbelief she was the one who needed the help and not the one doing the helping.

“All my thoughts were just darker thoughts, I wasn’t having a lot of positive thoughts at that point,” Meyers said. “I couldn’t focus.”

In Houston, droves of mental health professionals have fanned out to various shelters to volunteer. At the George R. Brown Convention Center in downtown Houston, a temporary hospital has erected a separate mental health section.

The volunteers are also encouraged to walk the halls and check in with people to see how they are processing the trauma.

Because children are especially vulnerable, they’ve set up a play area to allow kids a fun release for their pent-up energy and anxiety.

“Having that early, early support is very important,” said Susan Fordice, president and chief executive of Mental Health America of Greater Houston. “I think it’s important that people get back to some sense of normalcy and routine, and I realize what an incredible challenge that is going to be. What has happened here is unprecedented, it’s going to take a lot, and we’re going to have to really rally the mental health community and be present in many different places to provide support.”

In addition to concerns about the trauma triggering new mental health conditions, there is also a serious risk to people with existing mental illness.

Many evacuees left without their medications and will have gone days without them. The state’s mental health system was already underfunded before the storm — Texas regularly ranks low in per capita funding for mental health services.

Andrea Sawyer-Gray, a 33-year-old nurse in Houston who left for Dallas just as the storm started, watched the horrors unfold from afar and worried about her fellow residents’ mental health. She reached out to Talkspace, an online therapy company, and suggested it open up a support group for Houston victims facilitated by therapists.

Since the group launched on Thursday, more than 150 people have taken advantage of the free resource, sharing intimate issues such as the inability to calm their anxious minds or control their emotions.

“The donations, the things coming in are to replace our material things, but there’s nothing that can replace our peace of mind,” Sawyer-Gray said.

Jean Rhodes, a psychology professor at the University of Massachusetts, said there are other actionable things that could be done to ease people’s distress.

Rhodes has been studying a group of low-income New Orleans residents since just before Hurricane Katrina and has tracked their behavioral health.

Nearly five years after Katrina, one-third of Rhodes’s sample had high enough stress to indicate the presence of a diagnosable mental illness.

*Through her work, Rhodes has concluded that, early on, people need familiar systems of support, so she has recommended moving people to shelters near their neighborhoods so there is a sense of community.

She is also an advocate for allowing people to keep their pets in the shelters because of the tremendous comfort they provide.

Rhodes’s long research also showed a promising trend. Many Katrina survivors reported “post-traumatic growth.”

Even if they had experienced mental distress, they simultaneously expressed feelings of gratitude and a renewed appreciation for life.

Nelson is trying to keep some of that perspective.

While stranded on the rooftop, she prayed for God to save them, and he did.

Now, being together with her family in the shelter — about 16 of them in total — has eased some of the pain.

“I’m hurt that I lost everything,” she said, “but I’m grateful that I’m here and that I have my family.”