The Federal Emergency Management Agency has now released official Preliminary Flood Insurance Rate Maps as part of the Tropical Storm Allison Recovery Project (TSARP). These preliminary maps represents the most current understanding of the 1% (100 year) and 0.2% (500 year) floodplains and floodways for all of Harris County using the latest engineering methods and technology.
TSARP represents an entirely new study of flooding potential, not an update of old information. As such, it is not appropriate to characterize mapped floodplain changes as an increase or decrease in flood risk -- it is simply a new understanding of our flood risk.
A number of technical differences in the new study approach make direct comparisons to the old study inappropriate. For example, the detail of the ground surface defined by LiDAR is unprecedented and represents a significant difference. Another difference is the use of new (larger) rainfall values based on additional years of rainfall records.
These and other differences make it impossible to attribute any change in our understanding of flood risk to an individual cause. Also, each watershed is unique and possesses independent flooding conditions; information released from one watershed should not be used to make generalizations about another.
Some of these new engineering methods and advanced technologies are outlined below, and by clicking on a watershed you can learn more about specific issues that are unique to that watershed.
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Current FEMA Floodplain Maps |
FEMA Preliminary Flood Insurance Rate Maps |
What does this mean for the new maps? |
| Shape of the Ground Surface |
Used 5-foot contour maps developed by the U.S. Geological Survey from aerial photography and supplemented by ground surveys (cross sections) of the streams. |
Used a Digital Elevation Model with elevations on a 15-foot grid developed for the TSARP project using LiDAR, a new technology developed by NASA. Ground surveys (cross sections) of the streams supplemented and verified the LiDAR information and were used in the computer simulations. This technology also resulted in 2-foot contour maps for the entire county. |
This accounts for most of the differences between the current FIRMs and the FEMA Preliminary FIRMs. A much more accurate and detailed representation of the ground surface allows for more accurate computer modeling of flooding, and for a more accurate map delineating the areas of highest risk of flooding from the streams. |
| Elevation Data (Relative to sea level and adjusted for subsidence) |
Mostly based on elevations defined in 1973. Subsidence since 1973 is generally not accounted for in the mapping (although a few of the watersheds do have more current elevation information). |
Elevations based on the ground surface defined in 2001, including subsidence since 1973. Benchmarks have been established at more than 1,700 locations (typically at bridges) by the TSARP project. |
This effect is most noticeable in coastal areas where subsidence directly impacts the depth and extent of flooding related to tidal surge associated with hurricanes. In inland areas, regional subsidence does not significantly affect the carrying capacity of the streams or the amount of water that runs off of the land. |
| Observed Storm and Flood Data |
Data was only available for about 20 stream gages and a slightly larger number of rain gages. |
One of the most extensive urban stream and rain gage networks in the nation was established by the Harris County Flood Control District, beginning in the early 1980s. Information was used from over 50 stream gages and 150 rainfall gages which included more than twenty additional years of data. In addition, rainfall data based on Doppler Radar, a relatively new technology, provides detailed rainfall data from more recent events. Furthermore, significantly more field data has been collected for more recent flood events, such as high water marks at most bridges (in addition to those with stream gages). |
The availability of more real storm and flood data allows for more confidence in the calibration and testing of the computer simulation models used to estimate flood risk, and ultimately more accurate models and floodplain maps. Ten different storm events (most of which occurred in the past 20 years) were used to calibrate and test the computer simulation models created under TSARP. |
| Rainfall Data |
Maps were based on a theoretical 1% probability rainfall of approximately12.5 inches in 24 hours. This rainfall amount was determined by using a nationwide 1959 National Weather Service Report, TP-40, based upon rainfall data from the first half of the century. Based on the fact that large storms are less likely to cover large areas, the study included an adjustment factor that would reduce the rainfall amount based on the drainage area upstream of the analysis points. |
The new maps are based on theoretical 1 percent probability rainfall of approximately 13 inches of rain in 24 hours. These rainfall amounts were determined from a 1998 U.S. Geological Survey report based upon rainfall data collected in Texas as recent as 1995. This rainfall amount was not reduced for larger drainage areas. |
A slightly larger 1 percent probability rainfall, based on more recent observed data, will produce more runoff and consequently a larger 1 percent probability floodplains. |
