A 2013 map of the estimated 100-year and 500-year floodplains for Harris County, where Houston is located. [Army Corps of Engineers, “National Economic Development: Flood Risk Management,” 2013.]

 

 

Accurate floodplain maps are needed to set risk-based insurance premiums and to effectively communicate hazard flood risk to families, communities, and other stakeholders.

The purpose of the mapping is to identify the Special Flood Hazard Areas, which are defined by the Federal Emergency Management Agency (FEMA) as “the land area covered by the floodwaters of the base flood.” The base flood is an alternate term for the 100-year flood. However, the 500-year flood is also routinely mapped as well.

Floodplain maps help a community make informed decisions about where to plan and allow development. During a disaster, they provide critical information for response planning, recovery, and rebuilding decisions.

Floodplain maps are modified when there are changes in population growth and development, and improved science including changes in climate and weather patterns. FEMA, state, local and tribal officials collect current and historic flood-related data, which includes hydrology, infrastructure, hydraulics, land use, and existing maps.

Source: Fema’s “What Goes Into A Flood Map” PDF

After the collected data is reviewed, it informs whether a floodplain map project is necessary. Sometimes in lieu of a floodplain map, state, local, and tribal officials may opt to undertake mitigation action to reduce the community’s risk of flood damage. Common types of mitigation projects include the elevation of infrastructure, construction of dams, levees or seawalls, or the control of soil erosion.

As you can see, floodplain mapping is vital but there is concern about the accuracy of the mapping process. Most flood maps are not maps of real floods, but maps of theoretical floods based on gross surveys and hydraulic models. Financially it is not viable to perform on the ground topographic survey to use for regional maps.

Light Detection and Ranging (LIDAR) is the most cost-effective method to collect elevation data, however, it can be inaccurate by more than a foot. For areas that are generally flat, this inaccuracy can drastically affect the associative flood map. The accuracy of the hydraulic models can also be called into question. As a result the flood mapping cannot be considered an accurate depiction that an area will flood, but more a way to determine areas that could probably flood.

Are you wondering where you can find the most up-to-date flood map information for your city? Visit FEMA’s Map Service Center (MSC) (at https://msc.fema.gov) or the local community map repository for the current, effective information.

If you have additional questions about how NFIP could affect your plans, please reach out to PRD Land Development at [email protected] today.

Additional Resource: https://www.fema.gov/media-library-data/1505152190101-dd8a8d8c7c472e0267adab827fdbbfec/What_is_a_Flood_Map_Harvey.pdf

Floodplain Mapping explained and why it is important