Residents might face mental health issues.

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 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.

“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 moved 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 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?’ ”

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.