Table 1.
Potential factors influencing human safety during flash flood events
| Potential influencing factor | Data source | Rationale |
|---|---|---|
| Flash flood event duration, (hour) |
NOAA storm events database | Rapid onset of flash flooding generates surprising and swift moving water. Longer event duration increases the amount of runoff and risk exposure. |
| Precipitation during Event, (inch) | Prism daily spatial climate dataset | Greater precipitation increases runoff amount and velocity. |
| Census tract average ground elevation *, (feet) | National elevation dataset | Flash floods are usually characterized by raging torrents after heavy rains that rapidly flood into lower-elevation areas. |
| Median age of built structures in the census tract in the event year, (year) | American community survey | The physical and natural environments of communities change over time. Older neighborhoods might have older infrastructure, but newer neighborhoods might have been built in high flood risk areas. |
| Census tract population density *, (people/sqmi) | American community survey | Densely populated areas, usually urban areas, tend to have highly developed land, more impervious surfaces, and higher runoff. Also, higher population density leads to greater number of people being exposed to the event. |
| Time of flood event (Binary variable: Night Event = 1 and Day Event = 0) | NOAA storm events database | Visibility affects people’s ability to react to flood warnings, making nighttime flash flood events more dangerous than daytime events. |
| Type of triggering storm (Binary variable: Tropical Storm or Hurricane = 1, Other Storm Types = 0) | NOAA storm events database | Tropical storms and hurricanes tend to produce greater rainfall and stronger wind than thunderstorms and affect wider areas, they also offer warning time and potential evacuation affecting regular population movement patterns. |
| Vehicle-related incident (Binary variable: Vehicle-Related incident = 1, Non-Vehicle-Related Incident = 0) | NOAA storm events database | Flash flood fatalities and injuries tend to be related to vehicles, such as attempting to drive through floodwater, low water crossing, and flooded bridges. |
| Flash flood alley (Binary variable: Event located in Flash Flood Alley = 1, Event located outside Flash Flood Alley = 0) | NOAA storm events database | Past research indicates that communities in the Flash Flood Alley suffered a high number of fatalities and injuries in flash flood events. |
| Census tract developed impervious surface *, (%) | USGS national land cover database | During heavy rainfall, impervious surfaces reduce the amount of water that infiltrates into the ground and therefore increase the severity of flash floods. |
| Census tract bridge density *, (count/sq.mi) | TxDOT bridge inventory dataset | Flooded bridges increase the risk of human harm to pedestrians and vehicles. |
| Census tract low water crossing density *, (count/sq.mi) | Texas natural resources information system inventory of low water crossings | Drivers may underestimate the risk of driving through low water crossings during storms. As in bridges, flooded water crossings increase the risk of human harm to both pedestrians and vehicles. |
| Census tract road density *, (mi/sq.mi) | TxDOT roadway inventory dataset | Greater road density increases impervious surface area and could lead to more vehicle-related flash flood injuries and fatalities. |
| Census tract average ground slope *, (degree) | National hydrography dataset | Areas with steep ground slope could increase the runoff velocity, increasing the risk of water moving swiftly across communities. |
| Soil type (runoff potential: A– low; B– moderately low; C– moderately high; D- high) | National hydrography dataset | Different soil types have different abilities to absorb water and correspond to different levels of runoff potential during flooding. |
*
The year of the influencing factor is the closest or most recent available data from the sources