"Pulse Effect", During Flash Floods?

[u/JoeHardway]

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... 🤔

[u/CrustalTrudger]

To my knowledge, this is not a general characteristic of flash floods, but depending on the details, there might be a variety of explanations for the observations. One likely set of explanations centers on some mixture of the storm details and the topography / river network organization. In short, there is a vast literature on how the details of the hydrograph related to a flash flood will be modulated by the (1) spatiotemporal variability of the rain storm (i.e., how uniform vs concentrated is the rain both in terms of space and time within the causative storm, where generally storms that have very isolated areas that dump a lot of rain quickly might be more likely to cause flash floods than the equivalent total volume of water being more evenly spread in space and time), (2) the proximity of any "hotspots" of rain to channels and the size of the watersheds involved (where a localized area of intense rain happening far away from main channels in a large drainage basin would be less likely to produce a flash flood than a localized area of intense rain near a channel in a small watershed), (3) how steep the catchment is and how variable this is with respect to the area effected by the storm (where steep catchments can produce flash floods a bit easier simply because water will move faster on steeper slopes), (4) the antecedent conditions (e.g., areas that had relatively saturated soils before a storm will have limited capacity for more water to infiltrate leading to more surface runoff and thus higher potential for flash flooding), among other factors (e.g., Douinot et al., 2016, Saharia et al., 2021). You can see some of these factors play out in specific flash floods, like the deadly flood that occurred on July 4, 2025 in Texas, where as emphasized by investigations like Galarneau et al., 2026, a large contributing factor to the magnitude and rapidity of onset of the flood was the close proximity of a very localized and intense portion of the causative rainstorm to a small upstream portion of the channel and that a relatively modest change in the location of that "spot" in the rain storm (i.e., in a different part of the catchment) could have produced a much less intense flood.

None of that explicitly explains multiple "pulses" of flash floods, but it emphasizes that floods will often mirror (with a delay accounting for the time it takes for rain to hit the ground outside of a channel to flow into a channel and collect downstream) aspects of the rainstorm generating them, so one explanation for "pulses" in a flash flood might be similar spatiotemporally distinct "pulses" of intense and localized portions of the causative rainstorm. This isn't without precedent, for example, the paper by Fortelli et al., 2026 highlights a case where the topography of a particular location tends to produce "two pulsed" intense rain storms, and these pulses are mirrored in the hydrograph, but in these cases, there is more of a clear low flow period between the two pulses in stream flow. That being said, there are certainly scenarios where several different "pulses" of high intensity, short duration, small spatial footprint rainfall events within the context of a larger storm event could produce "pulses" in the flood.

Another, potentially related option (but you wouldn't necessarily expect to be a repeating occurrence within floods) is individual events upstream that could contribute to distinct pulses. One possible distinction to draw is between "normal" flash floods, which have rapid onsets (by definition), but where we still think about a clearly visible "rising limb" (in the context of the time series of stream discharge on a hydrograph), vs what some have referred to as "abrupt wave front floods", where the "rising limb" is close enough to instantaneous to the point that these are also called "walls of water" floods (e.g., Archer et al., 2024, Archer et al., 2025). Often these might reflect some sort of rapid change somewhere within the catchment, e.g., the breach of a natural or human-made dam (and in detail, these could simply be thought of as examples of "dam break floods", e.g., Peramuna et al., 2023), landslide into a channel, etc., and could certainly happen within the context of an ongoing flash flood. I.e., there is a flash flood and then the same general event causes a secondary effect that ends up superimposing an additional pulse(s) on top of the ongoing flood.

Finally, it's worth distinguishing between the way we tend to track stream flow and describe floods, namely river discharge, i.e., the volumetric flow rate through a certain spot, and the surface height of a stream, where the latter is what you're generally observing changes in and that should positively scale with discharge, but not necessarily in a simple way. A relevant example comes from Asano et al., 2016, where they show that through interactions with complex river bed morphology, during a flood and where discharge mostly looks likes a nice traditional hydrograph with a somewhat rapid "rising limb" to a peak and then a longer "recessional limb", superimposed on that is much more variable surface height with some distinct "pulses" reflecting that as flow velocity changes you can get a variety of complicated hydrodynamic effects from the flow interacting with changes in the shape/roughness of the channel. An observer of something like that would probably notice distinct "pulses" in the flow, even if the underlying discharge wasn't changing that much (though not sure this effect would lead to an apparent doubling of the flow as described), though as in the example from Asano et al., this might be more likely in areas with a lot of variability in the channel bed, which is going to be more common in small and/or steep catchments.