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Flood Insurance Rate Maps

Flood Insurance Rate Maps: What they are. What they are not.
Many of our citizens know that there are Flood Insurance Rate Maps for our area published by the Federal Emergency Management Agency (FEMA). And many, including those who use them on a regular basis, have a fundamental misunderstanding of these maps. Sometimes flooding occurs outside of the mapped, or regulatory floodplain. Does this mean that the maps are inaccurate? Not at all. The maps define the "regulatory floodplain," and other information, based on the "estimated" flooding from an assumed amount of rainfall. It could always rain more, and history tells us that it sometimes does. Furthermore, the maps only define flooding that occurs when a creek or bayou becomes overwhelmed. They do not define flooding when an area receives extraordinarily intense rainfall and is not able to drain quickly enough through street or roadside drainage systems. This was the case for many areas across the county during Tropical Storm Allison.

The regulatory floodplains are estimates of the potential for flooding. Analysis after past flood events has indicated that the estimates are, for the most part, relatively accurate. However, these estimates are only as good as the technical data on which they are based. So, there is some generally accepted range of uncertainty in these estimates. In other words, the floodplain maps are accurate, but only to a point. They provide a reasonable depiction of higher-risk flood areas along the primary bayous and creeks.

Image depicting an example of some features of a FEMA Flood Insurance Rate Map. Note: Not taken from an actual FIRM.
Example of some features of a FEMA Flood Insurance Rate Map. Note: Not taken from an actual FIRM.

Our Understanding is Always Improving
Over time, our knowledge of where the floodplain is has improved. And occasionally, FEMA Flood Insurance Rate Maps are changed. This is typically the result of large scale flood damage reduction projects or a new study that uses more current and modern data and technologies to estimate flooding and define the regulatory floodplain. New studies can result in smaller or larger 1% (100-year) regulatory floodplains in a given area because the floodplains are more clearly defined by using the new data and technology.

So, the maps change, but do the floodplains really change? Most often, the answer is that the floodplain did not change unless a large flood damage reduction project is built. It is our understanding of the floodplain that has actually changed.

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Flood Insurance Rate Maps | En espaņol

Know Your Flood Risk
In the aftermath of Tropical Storm Allison, FEMA and the District began an initiative called the Tropical Storm Allison Recovery Project (TSARP). A major portion of the project was the development of brand new FEMA Flood Insurance Rate Maps for all of Harris County.

FEMA and the 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 streams and bayous throughout the county's approximate 1,700 square mile area, including 22 watersheds and 35 communities. Conventional land surveying to develop the topography on a regional basis is costly and time consuming. To overcome these challenges, FEMA and the District implemented a state-of-the-art technology for mapping the ground's surface known as LiDAR, short for Light Detection And Ranging. LiDAR uses a laser device mounted to an airplane to accurately take ground measurements and develop a map of the surface.

The new Flood Insurance Rate Maps developed through TSARP will help residents & business owners, government officials and numerous other Harris County stakeholders become better prepared for the future because they will have a better understanding of the flood risks associated with the major flooding sources in the county.

Learn More About Tropical Storm Allison Recovery Project.

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