Allison’s Aftermath

June 21, 2002
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LIDAR provides the silver lining for a storm cloud in Texas.



On June 5, 2001, Tropical Storm Allison barreled into southeast Texas from the Gulf of Mexico. After initially dumping heavy rain on Houston and the surrounding suburbs, the storm drifted away to the north. Allison then reversed course and returned to the Houston area a second time. By the end of the week, 38 inches of rain had fallen in some areas, and President George W. Bush issued a Presidential Disaster Declaration for 31 Texas counties and 25 Louisiana parishes.

In its wake, Tropical Storm Allison left behind the costliest urban flood in the nation’s history. The brunt of the damages occurred in Harris County, which includes the majority of the city of Houston. In total, there was an estimated $5 billion in damage in Harris County, where 73,000 dwellings and 95,000 vehicles were flooded. Allison resulted in 22 deaths in Harris County and 43 deaths nationwide.

While a storm of Allison’s magnitude is a rare occurrence, there is, of course, no guarantee that it won’t happen again. And even a less severe storm can cause significant damage. While area officials realize they can’t control the weather, they are determined to do all they can to lessen damages from future flooding that may occur. That’s why they turned to Light Detection and Ranging (LIDAR) technology to help them gather the data needed to meet that goal.

Full-featured data set of the entire storm-hit area of Harris County, Texas.

Disaster Declaration Frees Funds

In early 2000, the Federal Emergency Management Agency (FEMA) and the Harris County Flood Control District (HCFCD) entered into a Cooperative Technical Partners Agreement to combine resources toward identification of flood hazards. Just prior to the arrival of Tropical Storm Allison, the district, with assistance from all 34 Harris County floodplain administrators, completed a Flood Insurance Rate Map Needs Assessment. This assessment verified a growing need to modernize FEMA’s Flood Insurance Rate Maps (FIRMs) for Harris County. As a result of the Presidential Disaster Declaration, funds became available to study Allison and to expedite the remapping project to best aid in the recovery efforts and to minimize damage from such future events.

The purpose of this study is to critically analyze Allison and to develop technical products the community can use to further minimize impacts from flooding. One of the largest of its kind, the Tropical Storm Allison Recovery Project (TSARP) was launched in September 2001 and will be completed in March 2003. FEMA and HCFCD are sharing costs for the project, which will total approximately $20 million.

“We expect the study will have a long-term impact on lessening damages from future flooding events that may occur in Harris County,” says FEMA Federal Coordinating Officer Scott Wells.

Through TSARP, the two agencies are working together to develop, exchange, produce and disseminate updated flood hazard information. The results will be used to aid the local community in short- and long-term recovery from Tropical Storm Allison. Some of the major recovery activities that TSARP will support include flood hazard identification and risk assessment, floodplain management, damage assessment and public education.

A lattice cross section view of LIDAR data from a Harris County stream channel. The bright points on the lattice are actual cross section points taken from field surveys, which demonstrate the accuracy of the LIDAR data.

Officials Turn to LIDAR

Recognizing the immediate need for accurate and timely ground elevation data to support the remapping efforts for nearly 2,000 square miles of Harris County, officials looked to LIDAR technology to help them collect the necessary data. LIDAR technology is a cost-effective and efficient means of acquiring high-resolution elevation data for a large area in a short period of time. LIDAR systems collect millions of elevation data points per square mile to create a high-density stream of accurate digital information.

Airborne LIDAR systems use aircraft-mounted lasers that bounce infrared beams off the Earth’s surface and determine the time difference between the laser’s emission and the return of its reflected signal to the aircraft. In post-flight data processing, the LIDAR time interval measurements are converted to distance and referenced to the aircraft’s Global Positioning System (GPS), Inertial Measurement Unit (IMU, which measures the plane’s roll, pitch and yaw) and ground-based reference GPS stations.

Harris County lies along the southeast Texas gulf coast, where the topo-graphy is very flat, so accurate ground elevation data is essential. Using conventional survey methods to acquire accurate ground data over such a large area, especially with an extremely tight schedule, would be both cost- and time-prohibitive. LIDAR technology offers a viable alternative, by acquiring data within a reasonable budget. The flexible LIDAR technology can operate both day and night in a range of weather conditions and tree cover, assuring fast turnaround and delivery of map data.

After an extensive interview process, the TSARP project team chose TerraPoint LLC of The Woodlands, Texas, as the LIDAR provider for the project. TerraPoint was found to possess extensive experience in supplying LIDAR-based, Earth surface mapping data for floodplain management projects in the United States and abroad.

A solid color view of contours over the LIDAR bare earth points from the same Harris County stream channel as seen on page 15. This image shows how the two-foot contours line up with the processed LIDAR data.

Extremely Detailed Data

Unlike other LIDAR providers, TerraPoint builds and maintains its own custom-built LIDAR systems. The equipment produces four returns rather than the usual two data points from each laser pulse that other LIDAR companies offer. This results in more detailed terrain information and helps in distinguishing obstructions from the terrain.

“In a situation like this, where a large area must be surveyed in a short period of time, there’s no doubt that LIDAR is the way to go,” says TerraPoint President Dan Cotter. “A project that might take years to complete with conventional ground surveying methods instead takes only weeks.”

Ultimately, the information collected through TSARP will be used to define the natural flood hazard areas in Harris County associated with the annual 1 and 0.2 percent probability floods (otherwise known as the 100-year and 500-year floods, respectively). The 1 percent probability flood has a 1 percent chance of being equaled or exceeded in a specific location in any given year. Over a long period of time, a flood event would statistically occur on average once every 100 years.

FEMA disseminates flood hazard information on FIRMs through the National Flood Insurance Program (NFIP). The goal of the program is to provide the opportunity for homeowners and business owners to purchase flood insurance within participating communities. In order to participate, communities must agree to adhere to certain floodplain management requirements. The NFIP recognizes the 1 percent floodplain in these floodplain management activities. Furthermore, areas within the 1 percent floodplain are subjected to higher flood insurance rates. Accordingly, the FEMA FIRMs delineate the estimated 1 percent floodplain.

Central to TSARP is the collection of topographic data for more than 1,800 square miles of Harris County, including more than 1,200 miles of streams. That information will be used to produce digital elevation models and topographic maps, as well as to delineate the 1 percent and 0.2 percent floodplains in Harris County.

Cross section points and stream breaklines along White Oak Bayou are used when comparing LIDAR data to the survey points for checking accuracy and developing hydrologic models.

September 11th Delays

TerraPoint began acquiring LIDAR data for the Harris County recovery efforts in early September 2001. Federal flying restrictions that resulted from the September 11th terrorist attacks delayed TerraPoint’s work for several weeks. The company was able to complete its flights in November of 2001. TerraPoint then processed the LIDAR data and presented the TSARP team with two products: the raw LIDAR data (also known as the digital terrain model) and the digital elevation model (also known as the bare earth model). Features such as vegetation and buildings were removed in the digital elevation model, revealing an unobstructed surface that shows the true elevation of the land. TerraPoint also used data from the bare earth model to develop two-foot contour topography and stream centerlines, which engineers will review to determine the best methods for preventing damage from future flooding.

In addition to collecting elevation data with LIDAR technology, TSARP project officials mobilized nine land survey companies to acquire data for approximately 1,200 miles of stream channel, which included about 10,000 surveyed cross sections. Later this year, surveyors will also establish a new countywide monumentation network. This network will be at a NAVD88 (North American Vertical Datum 1988) datum, adjusted for subsidence to 2000. This effort will involve the re-leveling of 750 bench marks and the creation of 250 new bench marks.

After the LIDAR data was received from TerraPoint, Dodson & Associates Inc., a Houston-based hydrology and civil engineering firm that specializes in storm water management, performed a Quality Assurance and Quality Control (QA/QC) review. The goal of the study was a Root Mean Squared Error (RMSE) representing the difference of the actual elevation and the LIDAR determined elevation of 15 centimeters. This goal reflects FEMA’s specifications for LIDAR data. TerraPoint’s data was shown to have a RMSE of 11.6 centimeters, comfortably meeting the requirement.

Using the data obtained from the field survey and LIDAR, floodplain elevations will be estimated by constructing computer models to simulate the rainfall-runoff relationship (hydrologic model) and the runoff-flood elevation relationship (hydraulic model). This will include the modeling of 1,200 miles of waterways in all 22 major watersheds in Harris County. The computed flood elevations and LIDAR-based ground elevation will be compared to develop preliminary Digital Flood Insurance Rate Maps (DFIRMs).

Preliminary Maps in 2003

The development of the preliminary map products will be completed in March 2003. The preliminary DFIRMs and FIRMs will subsequently be made available to the local communities and the general public as part of FEMA’s review and appeal process. FEMA will publish the final maps in 2004.

Once the TSARP products are made available, federal, state and local officials, along with home and business owners, will be better equipped to make informed decisions about repairing, rebuilding, and/or relocating damaged structures, infrastructure and public facilities. Specific community benefits will include:

  • insight into rainfall and flooding from Tropical Storm Allison
  • increased understanding of flood risk in Harris County
  • countywide bench mark network at a current datum
  • two-foot contour countywide topographic maps
  • improved watershed models using LIDAR technology
  • digitally based products compatible with GIS
  • current computer models based on state-of-the-art software and current data

Tropical Storm Allison has taken a heavy physical, emotional and financial toll on the residents and businesses of Harris County, Texas. But the TSARP project does provide a new reason for optimism, despite the grim headlines from last summer. There’s no doubt that this study will have long-term impacts on lessening damages from future flooding events that may occur. And in that sense, TSARP truly represents the silver lining of the storm cloud for the people of Harris County, Texas.

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