Flooding & Floodplains

As of 02/21/2018

Tropical Storm Allison

When Tropical Storm Allison suddenly formed 80 miles off the coast of Galveston, Texas, on Tuesday, June 5, 2001, no one expected that, five days later, it would go on record as one of the most devastating rain events in the history of the United States.

> Tropical Storm Allison "Off the Charts" Report (English) PDF
> Tropical Storm Allison "Off the Charts" Report (Spanish) PDF

Neither historical data nor weather forecasts could adequately predict this extraordinary storm that, before leaving Texas, would dump as much as 80 percent of the area’s average annual rainfall over some Houston and Harris County neighborhoods, simultaneously affecting more than 2 million people. 

Downtown Looking South, Tropical Storm Allison, 2001.

When the local rains finally eased, Allison had left Harris County, with 22 fatalities, 95,000 damaged automobiles and trucks, 73,000 damaged residences, 30,000 residents in shelters, and more than $5 billion in property damage in its wake. 


One thing everyone in our area realized, without qualification, is that it does not take a “perfect storm” to produce a perfect flood maker. Allison’s slow and erratic progress – first moving inland to the north, then meandering back to the Gulf of Mexico – combined for a horrific one-two punch that dealt many localities in the Houston region a critical blow. After flooding about 1,000 residences during its initial pass through the area June 5-7, Allison returned June 8-9 to deliver its knockout shot.


At one point during this second pass, 28 inches of rain fell during a 12-hour period just northeast of downtown Houston. Such incredibly intense rainfall over heavily populated areas set Allison apart from other historical storms to hit Texas in the past century.

Buffalo Bayou Flooding in Downtown Houston During Tropical Storm Allison, 2001.

Originally a mere “disturbance” passing through the Yucatan Peninsula into the Gulf of Mexico, Tropical Storm Allison formed Tuesday, June 5 – 80 miles off the Southeast Texas coastline. That night, it made landfall west of Galveston, with sustained winds of 50 miles per hour.

Visible Satellite of Tropical Storm Allison, 2001.

Hovering over Harris County initially for four hours, it dumped as much as 12 inches of rain and flooded some 800 residences as it drifted slowly to the north. During the next day, Allison continued its inland track and eventually reached Lufkin on the morning of Thursday, June 7 where it appeared to weaken and stall. Even then, when the storm seemed to be moving away from Harris County, it still produced enough rainfall locally on June 7 to flood an additional 200 area residences.

House Flooding in Greens Bayou Watershed, Tropical Storm Allison, 2001.

Allison looped back to the southwest – drawing new moisture off the Gulf and re-intensifying. The storm’s previous heavy rains had saturated the ground and caused immediate, excessive runoff when Allison returned to deliver the knockout punch the evening of Friday, June 8 and morning of Saturday, June 9. Friday night, as much as 28 inches of rain fell in parts of Harris County – flooding thousands of residences, stranding thousands of cars on hundreds of roads, and prompting Texas Governor Rick Perry and then U.S. President George W. Bush to declare Harris County a disaster area. On Saturday, June 9, alone, units of the Coast Guard, the National Guard, and local emergency agencies rescued nearly 7,000 people. In addition to these documented rescues, individual citizens acting on their own rescued thousands of other flood victims. On the evening of Monday, June 11, some 30,000 Houston area residents sought refuge in 51 shelters countywide.

Downtown Tunnels in Pennzoil Building During Tropical Storm Allison, 2001.

Leaving Texas, Allison produced rainfall amounts ranging from 20 to nearly 30 inches over parts of southeastern Louisiana. The storm also brought heavy rains across the northern Gulf Coast, with amounts of 10 to 12 inches from Gulfport, Mississippi to Tallahassee, Florida. Areas of North Carolina received as many as 21 inches of rain while southeastern Pennsylvania recorded amounts of 8 to 10 inches – capping nearly two weeks of devastation.

There is no precise way to count the loss, hurt and plain frustration Harris County residents experienced as a result of Tropical Storm Allison. There is no accurate way to measure the 22 lives the storm claimed, the priceless possessions and precious mementos it ruined, and the subsequent worry and hardship it randomly cast upon so many families and business owners. Furthermore, there is no scientific method to calculate the sense of community loss from the dozens of neighborhoods Allison destroyed. Yet, to statistically place the damage Allison caused in Harris County into perspective, consider the following facts:

  • Total damages directly associated with Tropical Storm Allison are estimated to be more $5 billion in Harris County alone ranking it as the costliest tropical storm in US history.
  • Of the 73,000 flooded residences, some homes were completely destroyed, while more than 2,800 residences sustained what is termed as “substantial damage,” or damage that is 50 percent or greater than a structure’s pre-flood value, not including land.
  • Flooding in downtown Houston was responsible for tens of millions of dollars in damages to buildings, the tunnel system and related infrastructure, and parking garages – not to mention the displacement of many workers from their places of business and lost productivity.
  • Four hospitals in the Texas Medical Center (TMC) were closed temporarily because of flooding and damage to electric service equipment. Although this flooding did not cause loss of life at the TMC, it certainly made situations difficult for affected patients and healthcare providers. Also, of the county’s two level-one trauma centers, one was closed while the other was at times unreachable because of the flooding.
  • Approximately 95,000 vehicles sustained $450 million in damages in Harris County. The damages resulted from the flooding of vehicles at residences, in underground parking garages and along flooded roads and highways.
  • State and local highway facilities sustained approximately $5.5 million in damages. Impassable highways and major roads paralyzed many parts of the city throughout Allison.
  • About 200 Houston area schools and three major Houston college campuses sustained significant damage. Rice University and Texas Southern University experienced significant flood damage, while the University of Houston main campus was especially hard hit. Of the University of Houston’s 105 buildings, 90 sustained water damage – with 55 critically affected. The total damages to area schools was estimated at more than $250 million.
  • Damages to Harris County’s facilities reached approximately $40.5 million, including the Criminal Justice Center, which faced repairs and flood proofing costs of $19.6 million. Eleven other county buildings were also damaged.
  • The city of Houston spent more than $53 million to repair city-owned facilities and estimated its total damage figure to approach $80 million.