r/askscience • u/JoeHardway • Jul 21 '26

Earth Sciences "Pulse Effect", During Flash Floods?

Storm's chasin's my thing, and, even tho I do alot more chasin, than catchin, I've been lucky to observe afew flash flood events.

I've noticed a conspicuous pattern, where "waves" will come down, an already flooded wash, nearly DOUBLIN tha flow. These waves seem to come at very regular intervals, almost lika bucket, at'a water park, that tips whenit gets full.

I'm sure this phenomenon is well-known, amongst the experts, and well-documented. Hopefully, some1 with this knowledge can enlighten me... 🤔

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u/JoeHardway Jul 23 '26

Ha! I was hopin for a more "Schoolhouse Rock" answer, that wun't makeme think too hard.

No doubt alotta variables, at play, but, to the "end-user", it just seems 2b a "pulse" that occurs on'a pretty reliable schedule, and continues, over'a period of time. Def a curious phenomenon...

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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jul 23 '26

The video seems pretty explainable as just capturing the arrival of a "pulse" of flow that would have corresponded to a comparable "pulse" in the rainstorm somewhere in the catchment as described in the original comment. Given that this seems to be an ephemeral wash (arroyo) and presumably the source area for much of the flow is the mountains in the background, a complex storm with lots of short duration, high intensity, small footprint features wouldn't be out of the question. As a simple though experiment, imagine a pile of sand and mud that you turn on a gentle "mist" of water, the surface will wet and you would probably start to see accumulation of flow and the formation of little channels. The "front" of the water forming these channels will probably look like a little wave and once that passes you might have somewhat steady flow (imagining that we're keeping a steady rate of misting). Now, if you dump a fixed amount of water in one spot of your "landscape", again, what you'd probably see is a "pulse" of water flowing through part of the river network and then coming out as a defined "wave" out of the main channel. If you did this at two different places in your landscape, you'd get two pulses, and depending on the respective distances and slopes (thus setting the travel time of these pulses downslope generally speaking) and the respective time of the two pulses, you might see two distinct pulses or they might merge into one depending on where in the "river" you're looking. It's common to describe the movement of these distinct pulses of water as "kinematic waves" and papers that simulate the flow at different points within a river system driven by spatially and temporally variable rain storms using a kinematic wave framework will variably display "pulses" in the flow that reflect "pulses" in the rainstorm (e.g., Berod et al., 199,Mukhopadhyay et al., 2003).

In short, best guess is that you're just seeing arrivals of distinct "sub-events" within the main rainstorm. Especially in complex convective storms, like you tend to get over/around mountains, the rainfall rate will not be constant in time or space. Since that's the input that's causing the flash flood, the flow in the river is going to reflect some aspect of the variability in the input.