* 30,000 people expected to seek shelter - U.S. officials
* Authorities rush to rescue marooned residents
* Flooding expected to peak Wednesday or Thursday in Houston
* Trump to visit Texas on Tuesday (Updates death toll, details on storm, rainfall, paragraphs 1, 5-7, 10, 16-17, 21)
HOUSTON, Aug 28 (Reuters) - Floodwaters from Tropical Storm Harvey are likely to rise as more torrential rain pounds the U.S. Gulf Coast, where at least eight people have already been killed in Texas and tens of thousands driven from their homes, officials said on Monday.
Thousands of National Guard troops, police officers, rescue workers and civilians raced in helicopters, boats and special high-water trucks to rescue the hundreds stranded in the catastrophic storm that has crippled Houston, the nation's fourth-largest city.
Harvey has already dumped more rain in the past few days than some affected areas normally see in a year.
The storm was the most powerful hurricane to strike Texas in more than 50 years when it hit land on Friday near Corpus Christi, 220 miles (354 km) southwest of Houston.
The worst is far from over because the slow-moving storm will continue to dump rain over the next few days in an area hit by "unprecedented" flooding, the National Weather Service said.
"Additional heavy rainfall overnight is expected to worsen the flood situation in southeastern Texas and southwestern Louisiana," the National Hurricane Center said.
Forecasts show that some spots in and around Houston could see an additional 12 inches (30 cm) of rain on Tuesday, bringing the total rainfall from Harvey to about 50 inches (127 cm) in parts of the city's metro area.
U.S. President Donald Trump plans to go to Texas on Tuesday to survey the damage and may also visit Louisiana, where the storm is now dumping rain.
Trump, facing the biggest U.S. natural disaster since he took office in January, has signed disaster proclamations for Texas and Louisiana, triggering federal relief efforts.
Among the most recent fatalities from the storm was a family that included two adults and four children who were believed to have drowned after the van they were in was swept away by floodwaters in Houston, authorities said on Monday.
In scenes evoking the aftermath of Hurricane Katrina in 2005, police and Coast Guard teams have each rescued more than 3,000 people, plucking many from rooftops by helicopter, as they urged the hundreds more believed to be marooned in flooded houses to hang towels or sheets outside to alert rescuers.
Regina Costilla, 48, said she and her 16-year-old son had been rescued from their home by a good Samaritan with a boat. She worried until she was reunited with her husband and dog, who had been left behind because they did not fit into the boat.
"I'm not complaining, we're alive," she said.
Schools and office buildings were closed throughout the metropolitan area, home to 6.8 million people, as chest-high water filled some neighborhoods in the low-lying city.
The Federal Emergency Management Agency director Brock Long estimated that 30,000 people would eventually be housed temporarily in shelters.
"A DIFFERENT NORMAL"
Both of Houston's major airports were shut, along with most major highways, rail lines and a hospital, where patients were evacuated over the weekend. More than a quarter of a million customers in the region were without power by Monday evening, utilities said.
The Brazos River was forecast to crest at a record high in the next two days about 30 miles (50 kms) southwest of Houston, forcing the mandatory evacuation of about 50,000 people in Fort Bend County, where officials described the predicted deluge as the worst in at least eight centuries.
Rising river and reservoir levels also forced evacuations in the counties of Brazoria and Galveston, near Houston.
As stunned families surveyed destroyed homes and roads flooded or clogged with debris, Texas Governor Greg Abbott warned Houstonians to brace for a long recovery.
"We need to recognize this is going to be a new and different normal for this entire region," Abbott said.
Harvey's center was in the Gulf of Mexico about 105 miles (170 km) south of Houston and forecast to arc slowly toward the city through Wednesday, adding more rain to areas already inundated.
The storm was expected to linger over Texas' Gulf Coast for the next few days, dropping another 10 to 20 inches (25 to 50 cm) of rain, with threats of flooding extending into Louisiana.
The U.S. Army Corps of Engineers said on Monday it was releasing water from the nearby Addicks and Barker reservoirs into Buffalo Bayou, Houston's primary body of water, to prevent dangerous buildups.
In Rockport, National Guard troops distributed water to residents as utility crews worked to restore power, amid reports of sporadic looting.
Resident Savannah White, 20, welcomed the president's visit because she said Houston needed help.
"I'm glad to see him coming down here because it's in such bad condition," White said. "Pretty much destroyed, there's nothing left standing."
Houston did not order an evacuation due to concerns about putting its 2.3 million residents on the street, causing chaos on the roads that could be more deadly than the storm, Mayor Sylvester Turner said.
Gasoline futures hit their highest in two years as Harvey knocked out about 13 percent of total U.S. refining capacity, based on company reports and Reuters estimates.
The United States' second-largest refinery, in Baytown, was shut down, and the largest refinery, in Port Arthur, was expected to make a final decision on shutdown on Tuesday.
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, Wall Street analysts say.
(Additional reporting by Peter Henderson, Mica Rosenberg, Erwin Seba, Nick Oxford and Ruthy Munoz in Houston, Andy Sullivan in Rockport, Texas and Steve Holland in Washington; Writing by Scott Malone, Dan Whitcomb and Jon Herskovitz; Editing by Tom Brown, Clarence Fernandez and Paul Tait)