FLASH FLOODING IN KARSTIC TERRAINS IN SOUTH-CENTRAL TEXAS

Convective storms from the Gulf of Mexico that stall along the Balcones Escarpment and the karstic Edwards Formation limestone terrain that underlies this area in south-central Texas provide a physiographic-meteorological juxtaposition that leads to such a proliferation of flash floods that this area is referred to as Flash Flood Alley. The orographic effect of the Balcones Escarpment is sufficiently effective in stalling storms coming from the Gulf of Mexico that half of the most intense precipitation events on record worldwide are observed in south-central Texas. The severity of flash flooding is exacerbated when surface runoff is intensified due to thin soil coverage and karstic landforms (e.g., sinkholes, conduits, other karstic preferential pathways) that offer limited surface retention to impede rapid runoff and induce infiltration. Over 800 people have died from flash flooding related incidents in Flash Flood Alley during the past half century, many as a result of attempting to cross low-water crossings during flooding events. Future tragedy can be avoided, or at least reduced, by better understanding the cause and effect of flash flooding in a karstic terrain so that focal areas and points of highest risk can be identified and mitigation and safety measures can be improved. Of particular interest is the development of a warning network that correlates lag times and hydraulic response with precipitation location and intensity so that improved and refined designation of dangerous low-water-crossings is possible. In this way, the chance of missing a critical low-water-crossing is reduced without having to impose blanket, wide-spread warnings which tend to de-sensitize the public to flooding dangers.