What causes flash flooding to happen? An in-depth explainer
Catastrophic and significant flash flooding has rocked places like Wheeling, West Virginia; central Texas, central North Carolina, Ruidoso, New Mexico; and Chicago, all within the last month.
All events but one have been directly tied to intense thunderstorms and not tropical cyclones. This may leave many wondering why the flooding has been so severe and what some of the factors are behind flash flooding in general.
Physical factors of flash flooding
Flash flooding is more complicated than simply "it rained a lot". There are physical processes leading up to a flash flood that make certain areas more vulnerable even before the rain begins.
Soil type and depth
The soil type and depth of a region determine how fast and how much rainwater can infiltrate from the surface down into the ground before runoff and flash flooding become a concern.
Sandy soils are comprised of larger pores or pockets. There are fewer of these large pores, but they typically allow for the rapid infiltration of water. Silt soil has a medium pore size and a moderate number of pores that allow for moderate infiltration.
Clay soil, on the other hand, has the greatest number of pores, but these pores are very small, allowing for slow and pore infiltration. Clay soil can hold a lot of water if the rain rates occur slowly and over a long period of time, but heavy rain rates are problematic for this soil type. Soil profiles that are shallower and moist going into an extreme precipitation event are also more prone to flash flooding as there is less availability for an excessive amount of water to be held within the pores.
Land use and coverage
Urban areas with a high concentration of roads, houses, and buildings and little natural greenspace have a substantially lower threshold to start a flash flood than vegetated or rural areas.
High volumes of water simply cannot penetrate through the concrete and altered natural landscape induced by urban development, so the water simply runs and floods, especially if drainage systems are old and not able to keep up.
Terrain and channelization
Finally, terrain and channelization influence the flow rate of water and duration of a flash flood should one occur. A channel is simply just a path, natural or manmade, for the water to take.
If there are more streams or channels in an area, this means there are more paths for water to take and more of a flash flood risk.
Straighter, smoother streams flow more violently than those that meander due to a reduction in friction. Areas with steeper terrain can see water accelerate downhill at a much faster rate causing a more destructive flash flood than flatter, open areas.
Atmospheric factors for flash flooding
After we consider the physical features on Earth's surface, now we can consider how the atmosphere plays a role in extreme precipitation and flash flooding.
Precipitable water
One common theme of all of the recent major events across the country is that they were characterized by extremely high or record breaking amounts of moisture in the atmosphere, slow-moving systems and extreme rainfall rates.
In meteorology, the value we use to assess how much moisture is in a large depth of the atmosphere is called precipitable water. Precipitable water is the measure of moisture in the air between two different levels or heights in the atmosphere that could be precipitated out. While this doesn't guarantee a certain amount of precipitation will occur at the surface, it does tell us the potential for heavy rain.
Warm rain vs cold rain processes
The temperature profile of the atmosphere also plays a role in the processes that result in rain reaching the surface. We have two types of clouds: cold and warm clouds.
Cold clouds are those clouds that occur and exist in air that is below freezing. This means a mix of ice and supercooled water exists inside the clouds.
Cold clouds are also more susceptible to pulling in dry air from the surrounding environment, which leads to stronger outflow of cooler air, which can cause an updraft to weaken and stop precipitation in a faster time.
The raindrops that result from cold clouds are larger and there are fewer per volume, which leads to less efficient or lighter rain rates despite bigger drops. Cold clouds are common over most of the mid and high latitudes.
In warm clouds, most of the cloud forms and exists in air that is above freezing, which means there are more liquid droplets than ice in the clouds. This leads to a greater number of rain drops that precipitate out of the cloud, which in turn leads to heavier rain rates.
Warm clouds also entrain less dry air, which means they have weaker outflow and can last longer over an area producing more rain.
Warm clouds are typically found in the tropics, but we can see warm cloud processes make their way deeper into the Continental US during the spring and summer months when the atmosphere warms up with the change in seasons.
Climate change implications
As Earth's atmosphere and oceans continue to warm thanks to human-induced climate change, there is a growing concern that extreme precipitation and flash flood events will become more frequent in addition to severe droughts.
There is math that proves that a warmer atmosphere holds more water vapor. The equation that proves this is called the Clausius-Clapeyron, which simply states that as the temperature of the atmosphere increases, so does its capacity to evaporate and hold more water vapor.
We may end up seeing more warm cloud processes dominate in a warmer atmosphere, and as mentioned above, these warm cloud processes are responsible for some of the most intense and efficient rainfall rates. This means droughts will become amplified and more severe in drier regions, but when it rains, the rainfall will be more intense and lead to more frequent flood disasters like we've seen in the last few weeks, months, and years.
