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In support of developing the FFRA mapping data, a hazard called Flash Flood Hazard Index (FFHI) was developed. FFHI provides a quantitative measure of the likelihood of flash flood occurrence. This index combines terrain, geomorphic, hydrologic, land cover, and meteorological factors into a single score to quantify flash flood potential. A threshold for FFHI was estimated based on literature review and sensitivity analyses. It was determined that a value greater than 0.525 may pose moderate to high hazard to people outdoors. A provisional FFRA was identified by combining areas above the FFHI and locations where people outdoors may be exposed to flooding in a 0.2 percent annual chance flood.The provisional FFRA map is a conservative estimate intended to encourage local jurisdictions to incorporate site-specific data, field observations, historical experience, and community knowledge to further refine siren placement within the identified FFRA. Absence from the mapped FFRA map should not be interpreted as absence of risk. Results are dependent on the quality, resolution, and recency of available datasets, and are generalized at a 1 kilometer scale. Final decisions to determine FFRAs require input from local officials and emergency managers with local knowledge, field observations, and site-specific considerations to request revisions to the map. Once input is received and FFRAs are finalized communities must install, maintain, and operate sirens that comply with TWC §16.502 (c, f, h, & j).For more information, please refer to Technical Memorandum: Identification of Flash Flood-Prone Areas that warrant Outdoor Warning Sirens, specifically sections 6-7 and Appendix E.", "AntialiasingMode": "Fast", "Title": "Provisional Flash Flood Risk Areas Map", "Author": "", "TextAntialiasingMode": "Force", "Subject": "Identification of flash flood-prone areas that may warrant outdoor warning sirens." }, "maxImageWidth": 4096, "maxRecordCount": 86516, "description": "Following the severe flash flooding in Central Texas in July of 2025, the 89th Texas Legislature passed Senate Bill 3, establishing requirements for outdoor warning sirens in designated flash flood-prone areas within the 30 counties included in the disaster declaration, and Senate Bill 5, providing funding through a state-administered grant program to assist local governments with the cost of installing outdoor warning sirens required under SB3.The Texas Water Development Board developed the Flash Flood Risk Areas (FFRA) map viewer to support community officials with identifying where sirens may be warranted within their communities. 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Following the severe flash flooding in Central Texas in July of 2025, the 89th Texas Legislature passed Senate Bill 3, establishing requirements for outdoor warning sirens in designated flash flood-prone areas within the 30 counties included in the disaster declaration, and Senate Bill 5, providing funding through a state-administered grant program to assist local governments with the cost of installing outdoor warning sirens required under SB3.<\/span><\/p>

The Texas Water Development Board developed the Flash Flood Risk Areas (FFRA) map viewer to support community officials with identifying where sirens may be warranted within their communities. This document describes the data included in the map and how to interpret the Flash Flood Risk Areas map. <\/span><\/p>

In support of developing the FFRA mapping data, a hazard called Flash Flood Hazard Index (FFHI) was developed. FFHI provides a quantitative measure of the likelihood of flash flood occurrence. This index combines terrain, geomorphic, hydrologic, land cover, and meteorological factors into a single score to quantify flash flood potential. A threshold for FFHI was estimated based on literature review and sensitivity analyses. It was determined that a value greater than 0.525 may pose moderate to high hazard to people outdoors. A provisional FFRA was identified by combining areas above the FFHI and locations where people outdoors may be exposed to flooding in a 0.2 percent annual chance flood.<\/span><\/p>

The provisional FFRA map is a conservative estimate intended to encourage local jurisdictions to incorporate site-specific data, field observations, historical experience, and community knowledge to further refine siren placement within the identified FFRA. Absence from the mapped FFRA map should not be interpreted as absence of risk. Results are dependent on the quality, resolution, and recency of available datasets, and are generalized at a 1 kilometer scale. Final decisions to determine FFRAs require input from local officials and emergency managers with local knowledge, field observations, and site-specific considerations to request revisions to the map. Once input is received and FFRAs are finalized communities must install, maintain, and operate sirens that comply with TWC §16.502 (c, f, h, & j).<\/span><\/p>

For more information, please refer to Technical Memorandum: Identification of Flash Flood-Prone Areas that warrant Outdoor Warning Sirens, specifically sections 6-7 and Appendix E.<\/span><\/p><\/div><\/div><\/div>", "copyrightText": "Texas Water Development Board", "supportsTimeRelation": true }