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The information provided here has been summarized and made available for your review.

MAAPNEXT WHITE PAPERS

Recommendation for Rainfall Durations in Harris County

Recommendation for Rainfall Patterns in Harris County

Recommendation for Rainfall Depths and Intensities in Harris County

Recommendation for Precipitation Depth-Area Reduction in Harris County

INFORMATION RESOURCES

Harris County Flood Education Mapping Tool

The floodplains shown on this mapping tool are those delineated on the FEMA effective Flood Insurance Rate Map (FIRM or floodplain map) for Harris County that was adopted in 2007. This mapping tool is not an effective FIRM. The effective FIRM is produced, maintained and published by FEMA and not by the Harris County Flood Control District. Please visit FEMA's Map Service Center to view the effective FIRM for Harris County. For an official floodplain determination, please contact an insurance agent or mortgage lender. For an official floodplain determination, please contact an insurance agent or mortgage lender.

VIEW THE MAPPING TOOL

Flood Elevation and Datum References

All elevation and flood depth information shown through MAAPnext are measured using a standard reference system called the North American Vertical Datum of 1988 (NAVD88) with Geoid 12B. This system provides a consistent “zero point,” ensuring that ground elevations and water levels are measured accurately and uniformly across Harris County.

These references help engineers, surveyors, and residents interpret flood risk data correctly and compare information across different locations. The data is intended for general flood risk awareness and informational purposes and should not replace certified elevation surveys or site-specific engineering data.

How should local studies reference MAAPnext methods?

Local engineers, developers, and planners are encouraged to reference MAAPnext data and modeling methods as the current countywide standard for flood risk assessment, once FEMA releases preliminary data.

When performing site-specific studies, MAAPnext can be cited as the foundational model for baseline conditions, with project-level refinements documented separately. References should note that MAAPnext modeling follows the methodologies outlined in:

Interim Guidelines and Criteria for Atlas 14 Implementation
FEMA’s Guidance for Flood Risk Analysis and Mapping Standards.

Using MAAPnext as a reference promotes consistency in floodplain management and ensures that local analyses align with countywide data and FEMA’s national standards.

How were calibration and validation performed?

Calibration and validation are critical steps that ensure the model results accurately reflect how water behaves in the real world. MAAPnext models were calibrated using rainfall, flow, and water-surface data from past storms recorded by the Flood Warning System and other regional sources. Once calibration produced close agreement between modeled and observed conditions, validation was performed using independent storm events not included in calibration. This two-step process confirmed that model results reliably predict flooding patterns across both mapped and unmapped areas of Harris County.

What datasets underpin the countywide models?

MAAPnext integrates a wide range of high-quality, publicly sourced, and locally collected datasets to build the countywide flood models.

Key data inputs include:

LiDAR Terrain Data (2018): High-resolution elevation data captured across Harris County at three-foot resolution—nine times more detailed than previous datasets.
Land Use and Impervious Cover (2018): Updated using recent aerial imagery to reflect current development and drainage conditions.
Rainfall Data (NOAA Atlas 14): Incorporates the most recent rainfall frequency estimates to model realistic storm scenarios.
Hydrologic and Hydraulic Modeling Software: Based on U.S. Army Corps of Engineers HEC-HMS and HEC-RAS systems, adapted for countywide use.
Streamflow and Elevation Records: Historical and real-time gage data from the Harris County Flood Warning System.

These datasets form the foundation for the hydrologic and hydraulic models that estimate flood depths, velocities, and extents for different rainfall events across the entire county.