(Adds details on water releases from reservoirs, quotes)
HOUSTON, Aug 29 (Reuters) - U.S. President Donald Trump headed to Texas on Tuesday to survey the response to devastating Tropical Storm Harvey, the first major natural disaster of his White House tenure, as officials in Houston struggled to manage the massive floodwaters.
The slow-moving storm has brought catastrophic flooding to Texas, killed at least nine people, led to mass evacuations and paralyzed Houston, the fourth most-populous U.S. city. Some 30,000 people were expected to seek emergency shelter as the flooding entered its fourth day.
Officials in Harris County, where Houston is located, said reservoirs built to handle drainage water were beginning to overflow on Tuesday. They released water to alleviate pressure on two dams, a move that would add to flooding along the Buffalo Bayou waterway that runs through the area.
"This is something we've never seen before," said Jeff Linder, a meteorologist with Harris County's flood control district. "We have uncertainty in how the water is going to react," when releases from the reservoirs hit overflowing drainage.
Officials encouraged residents in six neighborhoods around the reservoirs to evacuate before the water levels around their homes rise.
Harvey has roiled energy markets and wrought damage estimated to be in the billions of dollars, with rebuilding likely to last beyond Trump's current four-year term in office.
"Leaving now for Texas!" Trump said on Twitter on Tuesday morning before he left the White House to begin his trip.
Trump was scheduled to arrive in Corpus Christi, near where Harvey came ashore on Friday as the most powerful hurricane to strike Texas in more than 50 years. He will later go to the Texas capital, Austin.
While much of the damage in Houston has been rain-related, the storm's winds picked up overnight, bending street signs and tearing at metal fences in the downtown.
Much of the Houston metropolitan area, where 6.8 million people live, remained underwater on Tuesday, with some parts of the region recording more than 40 inches (1 m) of rain since the storm's arrival. Dangerous rescues went on through the night as police, firefighters and National Guard troops in helicopters, boats and trucks pulled stranded residents from flooded homes.
WAITING TO BE RESCUED
Officials believed about 1,000 households remained to be rescued, Houston Fire Chief Samuel Pena told ABC's "Good Morning America."
"We keep getting wave after wave after wave of rain and so that's not calming the situation," Pena said.
Forecasters drew comparisons to Hurricane Katrina, which lay waste to New Orleans and killed 1,800 people. Tuesday marked the 12th anniversary of Katrina hitting Louisiana.
The administration of then-President George W. Bush drew accusations that his response was slow and inadequate - criticism that dealt a serious blow to his presidency.
Some who fled the rising floodwaters in the Houston area found they had few options, as roads were washed out and emergency services overloaded.
"I'm still soaking wet and freezing cold and they are short on blankets," said Cheryl Whitely, who sat under a portico outside a Red Cross shelter with her children, mother and six animals. The group was kept outside because of their pets, Whitely said.
Before Harvey, the last Category 4 hurricane to make landfall in Texas was Carla in 1961. Its high winds and torrential rains destroyed about 1,900 homes and nearly 1,000 businesses, the National Weather Service said.
The U.N. weather agency on Tuesday said the high volume of rainfall produced by the storm is probably linked to climate change associated with global warming, which increases the amount of moisture in the atmosphere.
Among the most recent deaths from Harvey was a man who drowned on Monday night while trying to swim across flooded Houston-area roads, the Houston Chronicle quoted the Montgomery County Constable's Office as saying.
LURKING OFF THE COAST
The storm center was in the Gulf of Mexico about 115 miles (185 km) southeast of Houston on Monday morning. It was likely to remain just off the coast of Texas through Tuesday night before moving inland over the northwestern Gulf of Mexico on Wednesday, according to the U.S. National Hurricane Center.
Since coming ashore, Harvey has virtually stalled along the Texas coast, picking up warm water from the Gulf of Mexico and dumping torrential rain from San Antonio to Louisiana.
The Houston metro area has suffered some of the worst precipitation with certain areas expected to receive more than 50 inches (127 cm) of rain in a week, more than it typically receives for a year.
Harvey was expected to produce another 7 to 13 inches (18-33 cm) of rain through Thursday over parts of the upper Texas coast into southwestern Louisiana, the National Weather Service said.
Schools and office buildings were closed throughout the Houston metropolitan area.
About 9,000 people were packed into a Houston Red Cross shelter, with another in Austin, 160 miles (258 km) away expected to take in 7,000.
Hundreds of Houston-area roads were blocked by high water. The city's two main airports were shut.
The Gulf of Mexico is home to half of U.S. refining capacity. The reduction in supply led gasoline futures to hit their highest level in two years this week as Harvey knocked out about 13 percent of total U.S. refining capacity, based on company reports and Reuters estimates.
The floods could destroy as much as $20 billion in insured property, making the storm one of the costliest in history for U.S. insurers, according to Wall Street analysts.
(Additional reporting by Gary McWilliams, Ernest Scheyder and Erwin Seba in Houston, Andy Sullivan in Rockport, Texas, Jon Herskovitz in Austin, Steve Holland and Jeff Mason in Washington and David Gaffen in New York; Writing by Scott Malone and Jon Herskovitz; Editing by Chizu Nomiyama and Bill Trott)