Flash Floods

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There is a saying that flash floods cannot ocurr in a wash that has running water. Understanding the dynamics of a flash flood can explain this saying.

The fricton between water and water is much lower than between water and soil, rocks and plants. When it rains and runoff begins, in a dry wash the water is running over soil while in a wet wash with running water the runoff is faster because there is less friction to hold it back. Since the runoff is faster in a wet wash then shouldn't this be our concern? As the rain continues and more runoff occurs in a dry wash it is flowing over the initial runoff. The initial runoff travels slower than the later runoff which catches up with the initial runoff. This runoff passes over the top of the initial runoff and contacts the soil, slowing down. The water above is traveling faster than the water contacting the ground and a tumbling motion ensues that pulls the soil and rocks and everything into the vortex. The front of the flood travels slower than the runoff behind resulting in a wall of water building up and coming down the wash. The danger of a flash flood is due to both this wall of water that provides little warning and the amount of sediment and debris that is drawn into it by the turbulent action at the front of the flood.

There is an exception to the rule that flash floods cannot occur in a wash that has running water. If the amount of water running in the wash is small compared to the width of the runoff then the friction the runoff encounters is mostly water on soil with just a little water on water and so a flash flood can still occur. I have heard that sometimes there will be a small flow in the wash that warns of a flash flood coming and it is essentially of this nature where the flow behind is much wider than the small flow.

Flash flooding can occur many miles away from the spot where the rain fell. In the Grand Canyon, flash flooding is most common in the monsoon season of July and August when the intense build up of storm clouds often occurs in the late afternoons with heavy rains. While flash floods during late afternoons in the monsoon season are common in the Colorado Plateau, the steep nature of the Grand Canyon can make them especially dangerous. Hiking in the cooler morning and evening and finding a safe spot with shade to rest in the broiling afternoons during the monsoon season can reduce your chance of getting caught in a flash flood. Being aware of the possibility of flash floods, knowing what they are, and having a quick exit route during periods where there is a high probablity of flash flooding may reduce the likelihood of getting swept up in one.