'Massive, massive cleanup' in store for Houston after Hurricane Harvey The big challenge is "how to spend long term big amounts of money."

 -- The floodwaters are receding from Houston a week after Hurricane Harvey slammed into southeastern Texas. But officials say there's a long road ahead for recovery, and Houston will be picking up the pieces for days, months and even years to come.

How long will recovery take?

Since making initial landfall as a Category 4 hurricane in southeastern Texas last Friday, Harvey has dumped an estimated total of 20 trillion gallons of rain on the Houston area.

The Texas Division of Emergency Management so far estimates a total of 185,149 homes in the Lone Star state have been damaged or destroyed by Harvey. Harris County officials estimate 30,000 to 40,000 homes have been destroyed in the Houston area alone.

Meanwhile, about 80 percent of Texans don't have flood insurance.

The geographic scope and scale of the disaster from Harvey in Texas will be "far larger" than Hurricane Katrina in 2005 and its aftermath, Texas Gov. Greg Abbott told ABC News.

"This is going to be a massive, massive cleanup process," Abbott said in an interview today with "Good Morning America" co-anchor Robin Roberts. "People need to understand this is not going to be a short-term project. This is going to be a multiyear project for Texas to be able to dig out of this catastrophe."

To put things into perspective, the Federal Emergency Management Agency (FEMA) had offices and help centers in New Orleans for more than a decade after Hurricane Katrina hit, according to a former senior FEMA official who served in the Obama administration.

About 325,000 people affected by Harvey and its aftermath have already registered for FEMA assistance, and $57 million has been distributed so far.

“This is not just a ‘Hey, what’s the problem’ photo op. The Trump administration is going to have to own this for the entire term, and the next president will, too,” the former senior FEMA official told ABC News. “It’s easy to get people to rally around immediate housing and shelters but it’s going to get hard when it comes to how to spend long term big amounts of money.”

A housing crisis

There was a quick and effective effort to evacuate residents and get them to shelters. But shelters are temporary and taking steps to return evacuees to their normal lives are crucial in the coming weeks.

Houston Mayor Sylvester Turner Thursday said the number of people in shelters is decreasing. Still, about 42,000 people stayed in shelters across the state Thursday night, according to the American Red Cross.

FEMA Administrator Brock Long told reporters Thursday that the agency will first try to move displaced residents from shelters into local hotels before helping them clean their flooded homes so they can return to them. FEMA has already placed more than 1,800 flood survivors in hotels, Long said.

The former senior FEMA official told ABC News that a flood brings more psychological challenges than other disasters because "everyone's stuff is still there but it's ruined."

Residents may think their homes or belongings can be saved but federal agents must intervene to say, "No, it's not safe," because of structural damage that is invisible to the untrained eye, the official said.

Hidden health risks

Although Houston's mayor Thursday said the city is now "mostly dry," officials and residents will face a number of potential health problems from the floodwater and whatever it left behind.

Floodwater is not just water. It's often contaminated with sewage, chemicals and debris, including glass and other sharp objects. That's why floodwaters and standing waters pose various risks, including infectious diseases, injuries and chemical hazards, according to the federal Centers for Disease Control and Prevention.

Meanwhile, Texas' heat, combined with the wet conditions in Houston, is an ideal environment for mold to grow quickly. The former senior FEMA official told ABC News a sober reality will hit Texans in the coming weeks when many homes and buildings still standing after Harvey must be torn down because of mold.

Secretary of Health and Human Services Tom Price declared a public health emergency in Texas last week in response to Hurricane Harvey, which puts additional medical personnel from across the country on high alert and ready to provide help in the Lone Star state.

The U.S. Department of Health and Human Services has deployed more than 500 personnel to assist local, state and federal officials responding to the storm and its aftermath. The department's medical teams cared for nearly 500 patients at three locations in Texas Thursday alone, Price said.

Getting kids back to school

It's September and school is back in session. But some Houston-area schools may not be able to reopen immediately because of storm damage or mold, which means students will need to attend classes elsewhere.

The former senior FEMA official warned that schools in areas outside the disaster zone may become overcrowded or lack sufficient resources to deal with the influx of new students, putting a strain on teachers and administrators.

Moreover, school provides some semblance of stability and routine in a child's life and it will be a critical part of recovery. Changing schools adds another layer of psychological challenges to children who survived Hurricane Harvey's wrath but witnessed the devastation, the official told ABC News.

Infrastructure damage could affect roadways

The storm surges during 2005’s Hurricane Katrina breached the dozens of levees protecting New Orleans and its below-sea-level neighborhoods. Although the flooding devastated communities, the former senior FEMA official said, it didn't have as much of an impact on the greater highway routes in and out of the Louisiana city, meaning vital goods and relief supplies could be trucked in soon after the storm.

It's unclear whether this will be the case with Harvey, the former official told ABC News. Images from Houston showed inundated streets, highways and overpasses, all of which will need to be inspected before heavy trucks carrying relief supplies can travel on them.

Houston's airports reopened Wednesday and air traffic is expected to normalize in a few days. But the city isn't a major hub for air cargo. So if trucks can't start driving into Houston soon, the former official said, the city of 6.5 million residents could face a major disruption to supply chains of basic necessities, like food and clean water.