Deriving a Water Height Acceleration Threshold for Improved Flash Flood Alerts in the Upper Guadalupe River

Flash flooding along the Guadalupe River (Texas, USA) on 4 July 2025 caused more than 130 fatalities, highlighting a tragic pattern of casualties that occur despite flood alerts and community action. This pattern persists because county-scale warnings can be poorly aligned with watershed boundaries, rely on cell phone coverage, and often arrive noncoincident with flood surges. To address these challenges, a more rigorous criterion for flood alerts should be established. It should have clear meaning to residents in specific watersheds and be met before flooding is underway so that earlier action can be taken. In this study, stream gage data from an October 2018 flood were used to derive a water height acceleration threshold (WHAT), which tracked initial surges of floodwaters that resulted in flash flooding along the upper Guadalupe River. This threshold was applied to a more severe July 2025 flood to determine when theoretical gage-specific alerts could have been issued for the greatest efficacy. All gages, even those that were damaged and stopped providing data during the July 2025 flood, exceeded their WHAT before substantial flooding occurred at their locations. Upstream gages would have provided more notice (5.5 h) ahead of major downstream flooding as well as response time (1.16–2.25 h) at those upstream locations themselves. We posit that the WHAT framework can be integrated into flood alerting procedures and emphasize the importance of gage location in implementation.