(USA TODAY) -- Firefighters began a block-by-block search for survivors in some of the Houston's most devastated areas Thursday while fires at a Crosby chemical plant and a water crisis in Beaumont reflected the complicated and arduous process of returning some form of normalcy to the battered region.

Also Thursday, authorities ordered a mandatory evacuation for communities around the Barker reservoir near Houston amid fears of new flooding.

Some of the hardest hit areas of Houston have been awash in water for several days. Authorities were concerned that some residents might still be awaiting rescue.

"We'll be doing a block-by-block, door-by-door search of streets we believe have had three feet or more (of water) to make sure there are no people we've left behind,” said Richard Mann, the fire department’s executive assistant chief. “This will be a one- to two-week-long process to make sure we address all those areas that have been hit hardest."

In Crosby, 25 miles northeast of Houston, chemicals sparked fires and black smoke was reported at the Arkema Inc. chemical plant. The plant was flooded when more than 40 inches of rain fell in the area.

One Harris County sheriff's deputy was taken to hospital after inhaling fumes and 14 others drove themselves to the hospital as precaution, the sheriff's office said. The fumes were later determined to be a "non-toxic irritant."

In Beaumont, 100 miles east of Houston, flooding damage knocked out the main and secondary sources providing water to the city of more than 100,000 people.

"At this time there is no water supply for the city water system," the city said in a statement Thursday. "We will have to wait until the water levels from this historical flood recede before we can determine the extent of damage and make any needed repairs."

More than 33,000 people were in shelters Thursday, the American Red Cross said. The number increases only slightly from the previous day.

More than 13,000 people have been rescued from flooded homes. Gov. Greg Abbott said another 10,000 National Guard troops from across the nation would be joining the 14,000 already deployed in the region to provide security and aid in rescue efforts.

The storm has claimed dozens of lives, including six family members whose bodies were found Wednesday in a van that disappeared in high water three days earlier.

"We are sad to confirm we have retrieved six victims from a van that was submerged in Greens Bayou. #harvey," the Harris County Sheriff's Office tweeted.

Vice President Mike Pence, joined by Secretary of Labor Alexander Acosta, Secretary of Transportation Elaine Chao, Secretary of Energy Rick Perry, Secretary of Veterans Affairs David Shulkin, Acting Secretary of Homeland Security Elaine Duke were traveling to Texas Thursday to visit with those affected by Hurricane Harvey and assess the damage.

AccuWeather estimated Harvey's cost at $160 billion, making it the costliest natural disaster in U.S. history. AccuWeather president Joel Myers called Harvey a "1,000-year storm" and said parts of Houston will be uninhabitable for weeks or months.

Contributing: Doyle Rice, USA TODAY; KHOU-TV, Houston