Torrential Flooding

Flooding, on an average, costs of $4.7 billion per event. Over the past 20 years the US has reported 4x the amount of billion-dollar flood disasters compared to 1980-2000. New research has shown that, as the baseline temperature annually increases, flooding events will become 8% “flashier” by the end of the century.

With the shift in stability of weather systems, the shift in location and timing of rainfall events will present unique hazards to all critical infrastructure sectors with minimal notice for preparation during a developing event.

What is torrential flooding?

Torrential flooding is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours which can cause more intense flash flood events. Atmospheric moisture has increased since 1976 which enabled the five 1-in-1000-year rainfall events in the US in 2022. Heavy rainfall events are likely to occur more often and in concentrated areas which could lead to more record-breaking events. The air is on average warmer and moister than it was prior to about 1970 and in turn has likely led to a 5-10% effect on precipitation and storms that is amplified in extreme downpours.

A flood is an overflow of water onto normally dry land. The inundation of a normally dry area caused by rising water in an existing waterway, such as a river, stream, or drainage ditch. Ponding of water at or near the point where the rain fell. Flooding is a longer-term event than flash flooding: it may last days or weeks.

A flash flood is a flood caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through riverbeds, urban streets, or mountain canyons and can occur within minutes or a few hours of excessive rainfall. They can also occur even if no rain has fallen, for instance after a levee or dam has failed as runoff over inundates rivers, or after a sudden release of water by a sudden break of an ice jam.

Flash flooding has increased by more than 10% in the Southwest accounting for the greatest increase in “flashiness” among hot spots, while storms in the Northeast are generating about 27% more moisture than a century ago.

Critical Infrastructure Impacts

Just one inch of floodwater can cause up to $25,000 in damage according to FEMA. 12 inches of flowing water can lift and carry away most vehicles, presenting a secondary threat to infrastructure in large floating debris.

Large floods can damage roadways, bridges, energy equipment, residential properties, agricultural sites, intake pipes, water infrastructure, and most physical infrastructure as soil erosion, debris flows, mud slides, rockslides, river flooding, and rapid wash flows present secondary threats made possible by increased precipitation. The damage from a single flood can last multiple seasons if the soil health is strongly affected by debris, contaminated water, higher levels of toxins or chloride, or by stripping nutrient filled topsoil.

• Pipelines and above and below the surface could be damaged by changing soil conditions causing pipes to bend and shift and rapid erosion exposing infrastructure to debris damage.
• Most commercial power facilities are located near natural waterways for cooling and have associated increased flood risks as heavier rainfall events result in more runoff.
• The World Wildlife Foundation (WWF) states by 2050, 1 in 5 existing hydropower dams will be in high flood risk areas, up from the current rate of 1 in 25 hydropower dams.
• Energy equipment in the direct path of water flow is subject to catastrophic damage due to debris and speed of water flow. Rising water can cause damage to control circuits and distribution panels.
• Flooding on farmlands can cause crop loss, contamination, soil erosion, equipment loss, debris deposition, and the spread of invasive species.
• Compounding threats with increased rates of drought-triggered subsidence can result in unexpected water pooling in areas which have become the low-lying elevation.