UPDATE 12-Thousands driven from homes, 7 dead, as Harvey hammers Houston

(Corrects location of Harvey to 100 miles from Houston, not 40 miles, in paragraph 14)

* 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

By Peter Henderson and Mica Rosenberg

HOUSTON, Aug 28 (Reuters) - Floodwaters from Tropical Storm Harvey, which has already killed at least seven people in Texas and was expected to drive tens of thousands from their homes, will likely rise in the coming days, officials warned on Monday as heavy rains continued to pound the U.S. Gulf Coast.

National Guard troops, police officers, rescue workers and civilians raced in helicopters, boats and special high-water trucks to rescue the hundreds of people still stranded in and around Houston, the nation's fourth-largest city.

The storm was the most powerful hurricane to strike Texas in more than 50 years when it came ashore on Friday near Corpus Christi, 220 miles (354 km) south of Houston and the worst was far from over as the National Weather Service issued numerous flood warnings across the region.

U.S. President Donald Trump plans to go to Texas on Tuesday to survey the damage and may in the future 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.

Harvey has killed at least six people in Harris County, where Houston is located, said Tricia Bentley, a spokeswoman for the county coroner's office, including a man who died in a house fire on Friday night and an elderly woman driving through flooded streets on the city's west side the next day.

A 60-year-old woman died in neighboring Montgomery County when a tree fell on her trailer home while she slept, the local medical examiner said on Twitter.

Both of Houston's major airports were shut down, along with most major highways, rail lines and a hospital, where patients were evacuated over the weekend. As of Monday evening 267,000 Texans were left without power in the southeast corner of the state.

The Harris County Office of Homeland Security and Emergency Management issued a "shelter in place" warning to residents of La Porte and Shoreacres, about 25 miles (40 km) east of Houston, after a chemical leak was caused by a ruptured pipeline.

As stunned families surveyed the wreckage of destroyed homes and roads flooded or clogged with debris, Texas Governor Greg Abbott warned Houstonians to brace for a long recovery.