Tropical Storm Allison 2001

Tropical Storm Allison 2001


In the aftermath of Tropical Storm Allison, the Federal Emergency Management Agency (FEMA) and the Harris County Flood Control District began a multi-year initiative called the Tropical Storm Allison Recovery Project (TSARP) that comprehensively assessed the flood risks associated with the major flooding sources within Harris County.

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

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


In the aftermath of Tropical Storm Allison, the Federal Emergency Management Agency (FEMA) and the Harris County Flood Control District began a multi-year initiative called the Tropical Storm Allison Recovery Project (TSARP) that comprehensively assessed the flood risks associated with the major flooding sources within Harris County.

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

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

FEMA and the Flood Control District have worked closely together in the past to identify major flooding sources and associated flood risks within Harris County, but never on such a scale.

For TSARP, both agencies used innovative scientific techniques to determine the current flood risks posed by bayous and creeks throughout the county's approximate 1,700 square mile area, including 22 major watersheds and 35 communities. The information developed as a result of TSARP has been provided to the public, government officials and numerous other Harris County stakeholders, so that they can make informed, scientifically-based flood management decisions about the places in which they work and reside.


Each of Harris County's 35 communities is responsible for planning and regulating land development projects within its municipal boundaries. TSARP has provided access to more accurate flood risk information, and it allows community planners and engineers to make decisions based on up-to-date technology and conditions. This is critically important as the population of Harris County continues to grow and land development projects are considered, planned and constructed. In addition, the District has been able to use the TSARP products as new tools for planning and design of new flood damage reduction projects within Harris County.


FEMA's Flood Insurance Rate Map, commonly referred to as a FIRM or a floodplain map, shows which areas of the county are at a higher risk of flooding than others.

Periods of heavy rain regularly occur in Harris County, an area historically prone to flooding. As a result, it is very important that citizens possess the best possible knowledge of their flooding risks for their homes and businesses. The Flood Control District and FEMA believe that the citizens of Harris County want to and should know what these risks are so that they can plan accordingly.

During the late 1970s and early 1980s, the first comprehensive effort was undertaken to map the floodplains for major flooding sources in Harris County. Since that time, updates for particular areas have occurred to reflect changes in conditions resulting from things such as land development and the completion of flood damage reduction projects. These map updates were made with reference to the original data from the first comprehensive effort.

As a result of TSARP, not only were Harris County's floodplains entirely remapped, but brand new and more accurate data and computer models were created using important advances in science and technology.


TSARP was a massive undertaking involving extensive use of a sophisticated laser technology called Light Detection and Ranging, commonly referred to as LiDAR, to map the ground's topography.

Developed by NASA, this laser technology has already been successfully employed by FEMA on other important projects, including recovery efforts at Ground Zero after the 9-11 attacks.

The use of LiDAR for the Tropical Storm Allison Recovery Project involved directing millions of laser beams toward the ground from low-flying aircraft, and measuring the time it took for that light to bounce off the earth and back to the recording equipment on the plane. The measurable differences in the laser's bounce time represent different heights, or the relief, of the land.

The information generated by this laser technology, along with many other parameters, was used in computer models to estimate flood elevations. Flood elevations were then mapped on base topographic maps, also produced from the laser technology. This information became the basis of the new FEMA Flood Insurance Rate Map for Harris County.


FEMA's National Flood Insurance Program (NFIP) – of which every community in Harris County is a member – underwrites flood insurance for the entire nation. FEMA uses Flood Insurance Rate maps (FIRMs or floodplain maps) to help determine flood risk zones and associated rates for flood insurance policies.

For many, the new FEMA Flood Insurance Rate Map for Harris County had little or no impact on their flood zone designation and associated policy rate, and for some, their status actually improved. However, there were instances in which the maps showed new boundaries that required a homeowner to purchase flood insurance for the first time.

FEMA and the NFIP were mindful that such changes would be unwelcome. For that reason, the NFIP created a "Grandfather Rule" that allowed alternative rating rules for certain structures. Contact an insurance agent for more information.

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.

Photo: 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.