TSARP Overview > Flood Insurance Rate Maps (FIRMs): What they are. What they are not.

Many of our citizens know that there are FEMA Flood Insurance Rate Maps for our area. And many, including those who use them on a regular basis, have a fundamental misunderstanding of these maps. Here's some information about what these maps really are, and what they are not. A new two page quick reference sheet has also been created to help you understand and read a FEMA Flood Insurance Rate Map, click here to download the quick reference sheet.

The FIRMs are published by FEMA to establish flood insurance rates and to assist local communities in regulating development activities. They are not depictions of which specific areas are and are not subject to flooding. In fact, the official name is "Flood Insurance Rate Map," not floodplain map.

Does this mean that the maps are inaccurate? No, it does not. 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.

As mentioned, 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

Over time, our knowledge of where the floodplain is has improved. And occasionally, FEMA FIRMs 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% chance 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. It is our understanding of the floodplain that has actually changed.