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CrustalTrudger t1_jai1ytd wrote

It's an interesting question, but unfortunately one that's not going to have a definitive or single answer. The short version is that you expect a lot of variability in flow velocity during a flood where the velocity in any given area (or with depth) will depend on the discharge (i.e., the volume of water moving through the system) but also critically the "roughness" of the contacted area. For a bit of a treatment on this from a fluid dynamics perspective, you could consider things like Pang, 1998. Roughness always play a role in flow velocity in rivers, but compared to periods where the flow is constrained below "bankfull width", during floods roughness can vary a lot both from natural elements (i.e., as flow moves up the walls of the bank and potentially over natural/manmade levees) but also from flows encountering all sorts of "objects", i.e., trees, other vegetation, and a whole host of things in the built environment (roads, poles, houses, etc.). You can see some of the extreme variations in relationships between depth and velocity estimated from modeling flood flows in built environments in papers like Kreibich et al, 2009. Another aspect of this is that this actually something that's pretty challenging to measure because floods are not necessarily the easiest or safest time to go measure flow velocity (and it may often destroy or damage more autonomous instrumentation deployed to track flow velocity). This is discussed a bit in papers like Tauro et al., 2016, which is attempting to use image techniques to estimate flow velocities during floods. This highlights some of the challenges in empirical measures of flood velocities, but also demonstrates the significant spatial variability in velocities during floods (as estimated from their techniques).

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coffeeeedrinker t1_jai2tko wrote

Wouldn’t the slope of the region also play in a role in the velocity?

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phdoofus t1_jai9fmw wrote

Yes, if you look at the shallow water equations for river flow, the right hand side will basically be predominated by a) slope driving flow and b) friction resisting flow. The friction resisting flow will be determined by the water velocity, the characteristics of the bed material, and the water depth. As water depth increases (e.g. during a flood) the frictional forces decrease as h^(4/3) where h is water depth.

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coffeeeedrinker t1_jaicyi7 wrote

I see. So the characteristics of the bed material will factor in the role of sediments in river flows? Due to flooding, there should be an increase in loose sediments that the water would carry/get hindered by.

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formerlyanonymous_ t1_jairoz8 wrote

There is increases in sediment load with increased water depth and hydraulic gradient (2 of the 3 major components to shear stress at the channel bottom), but that suspended sediment is marginally going to affect water velocity, and in ways that are terribly difficult to quantify. Larger waterborne debris could have a larger effect as trees or boulders deposit temporarily causing local scour or eddy currents.

Bed characteristics such as sediment type can be an identifier of what roughness (ie friction) the water will face at the boundary. The scoured bottom, a function of grain size, may also have localized effects to velocity, similar to the debris mentioned above. Small pockets form holes or preferred paths along the bed.

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Friedumb t1_jbg5y8d wrote

Correct me if I am wrong, but the distance from the bed increases velocity due to lack of friction. As such a flooding river should expect higher velocity at the thalweg?

Im envisioning a lazy looking river with serious undertow.

Stream charecterization plays a role, I have always envisioned water finding the easiest path. Perhaps in a braided environment velocity could theoretically be decreased as the flow hops channels?

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formerlyanonymous_ t1_jbg8bfs wrote

You're correct for the most part. Turbulence mixing can create localized areas of high velocity, but the free surface (more specifically, just below the free surface) is the highest average flow due to lack of friction.

Normally if you're seeing large current, the top will be reflective of that, with chops, waves, or dune/anti-dune shape.

And stream characterization does play a role, however very difficult to define accurately. My comment above was more related to sediment characteristics, but very much a braided gravel bed is going to be different than a braided river delta than a singular sand bed.

As far as braided being lower, that depends a lot on topography. You may see those with lower depth but higher velocity. Those braids exist partially from the large tractive forces cutting paths through the floodplain. They typically have larger sediment (gravel, cobble, boulders) that may also add friction. Sand braided rivers typically have fewer paths because the braids can cut deeper more easily, creating a greater cross sectional area to balance the increase in flow.

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