Many Texans without flood insurance as Harvey keeps hammering Gulf Coast

Damage could potentially surge into the tens of billions of dollars.

— -- Thousands of Texans still getting pounded by Hurricane Harvey could find themselves financially devastated as total damage costs to homes, property and businesses potentially surge into the tens of billions of dollars in the wake of the storm.

Harvey, which has so far shuttered critical oil and gas refineries by its rain and riverine flooding, has the potential to cost as much as $42 billion, which would put it "in the top five" most costly storms ever, Chuck Watson, director of research and development at Enki Research, told ABC News.

The second most expensive storm came in 2012 when Superstorm Sandy hit the Northeast, causing $75 billion in damage and in which over 200 people died.

Back in 2001, Tropical Storm Allison ravaged the same Gulf region as Harvey and led to 55 deaths; but it caused a conservative $12 billion in damage.

Much of Harvey's price tag will come from stricken homeowners looking to rebuild after the storm's gale-force winds and torrential rains subside.

Sadly, most homeowners insurance policies do not cover flood damage.

While in many communities, flood insurance can be purchased through the federally backed National Flood Insurance Program, or NFIP, only 15 to 20 percent of Corpus Christi and Houston homeowners are insured for flooding, according to Mark Hanna, spokesman for the Insurance Council of Texas, a trade group that represents Texas insurers.

But he stressed that the total will grow because it's just "the tip of the iceberg."

Also, approximately 60 percent of Harvey’s windstorm damage will be covered by a quasi-state insurance agency called the Texas Windstorm Insurance Association, Hanna told ABC News.

That means that the lion's share of Texans hit by the wettest storm in recent memory may end up relying on government insurance.

The National Flood Insurance Program is the war chest intended to offset the exorbitant costs of flood damage.

The program was established in 1968 as a failsafe measure and, according to its website, has insured more than 5 million homes in 22,000 flood-prone communities nationwide.

Yet outside those specified high-risk areas, many homeowners go without flood insurance, experts told ABC News.

Last year, the average NFIP flood policy cost about $700 a year and the average claim was more than $31,000, according to the NFIP.

Right now, on the books, there are some 373,000 NFIP flood insurance policies in the 18 Texas counties impacted by Harvey, according to a review by the Pew Charitable Trusts.

Paying these expected claims could significantly drive up the $24 billion of debt the NFIP currently carries.

And Congress is now on the clock to reauthorize the program when it's slated to expire on Sept. 30.

But on Monday during a press conference alongside Finland President Sauli Niinistö, Trump was confident that an aid package would pass through the House and Senate to help pad the dwindling $3 billion that's been allocated for disaster relief by FEMA.

"The real number, which will be many billions of dollars, will go through Congress," he said.

Beyond the NFIP, FEMA and the Small Business Administration are two other government entities that could come through to provide no-interest or low-interest loans when Texans rebuild, said Hanna.