Flash floods are a type of flood characterized by a rapid rise in water levels when a massive volume of water moves swiftly from high terrain to lower areas.
How Flash Floods Form
Flash floods occur when a massive volume of water is brought on by thunderstorms, hurricanes, or tropical storms, or due to a sudden melting of large amounts of snow and ice in the mountains. They can also occur when a dam breaks or when water is suddenly released from a dam at a rate of thousands of cubic meters per second (the volume required to create a flash flood also depends on the width and slope of the river downstream of the dam).
Flash floods are commonly observed in steep areas, such as at the foot of mountains.
Characteristics of Flash Floods
Since flash floods involve the movement of a massive volume of water from high to low areas at an increasing speed, their destructive power also escalates depending on the length, slope, and “smoothness” of the path it follows (areas like mountains and hills without vegetation experience flash floods more frequently due to the lack of barriers to stop the flow of water). They can cause extremely severe damage to the areas they traverse. With high speed and large volume, flash floods can sweep away homes, trees, and nearly everything in their path.
Flash floods are frequently seen in steep areas, such as at the foot of mountains or in valleys. Despite their intensity and high destructive potential, flash floods typically do not last longer than six hours.
When the drainage path of a flash flood is obstructed (due to levees or large structures, even if they do not completely block the flow), the massive volume of water moving at high speed can rebound, creating a cycle before it can continue flowing, which also causes the water level to rise more quickly and become more dangerous. This rebound of water collides with the incoming flow, creating numerous whirlpools that can submerge everything; these whirlpools can also form beneath the surface, pulling in everything around them. Thus, even individuals wearing life jackets can be submerged in such floods (this phenomenon is most visible when flash floods invade urban areas or densely populated residential zones), making rescue efforts extremely challenging.
Along with flash floods, there can also be numerous landslides and mudslides, along with debris they carry, making flash floods even more dangerous as the moving mass includes more than just water.
Flash floods can occur over a wide area, but the larger the flood area, the less destructive the impact will be, as the volume of water gets dispersed rather than concentrated, which reduces damage.
Flash floods do not occur in flat areas or areas with large rivers. In flat regions, there is little to no slope for water to flow, causing water to lose speed and only leading to flooding rather than washing away anything. In areas with large rivers, similar to flat regions, the river will absorb and regulate the excess water; if it overflows, it will cause regular floods rather than flash floods, as the water moves much more slowly than in flash floods.
Flash floods typically pose threats to small and medium rivers, but less so to large rivers.
Types of Flash Floods
Flash floods can be categorized into three types:
- Floods caused by local rain concentrated heavily in natural basins (with minimal human impact).
- Floods resulting from heavy rain in basins that have been significantly affected by human economic activities, destabilizing or disrupting the ecological balance of the basin (altering vegetation cover, flow regimes, retention capacity, or basin characteristics).
- Floods caused by the sudden release or bursting of stored water from dam failures or ice dams.
Impacts of Flash Floods
Flash floods often pose threats to small and medium rivers, but less so to large rivers. Investigations into basins that have experienced flash floods indicate that these floods typically recur approximately every 30 years. However, in many locations, flash floods have occurred consecutively due to severe environmental degradation in these basins. Flash floods are a complex, diverse issue that is deeply local in nature.
In just 1 to 6 hours of destruction, the force of flash flood waters is extremely dangerous. When flash floods occur, transportation is interrupted, as accidents are more likely at such times. Notably, when a powerful current rushes down from above and encounters significant resistance, the massive volume of water moving at high speeds will rebound in a cycle before it can continue flowing, causing the water level to rise even more rapidly. The rebound water collides with the incoming flow, creating numerous whirlpools that suck in everything nearby, making it highly hazardous.
Water retention is one of the primary functions of soil. Rainwater that does not infiltrate the ground and instead flows over it is known as “runoff.” Suppose there is a torrential downpour in a certain area. When the soil becomes saturated, and it begins to rain, it can no longer absorb more liquid. Consequently, many terrifying runoff situations can arise, significantly increasing the risk of flash floods.
If the soil is too dry, it also poses a major issue as it reduces the ability to absorb water. Another important point is that not all types of soil absorb water equally. Artificial surfaces such as concrete and asphalt have a poor capacity for absorbing rainwater.
Another consideration is that driving through floodwaters is a bad idea. The U.S. Centers for Disease Control and Prevention reports that over 50% of flood-related deaths are attributed to individuals driving through moving water.
