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phdoofus t1_jai9fmw wrote

Reply to comment by coffeeeedrinker in When a river floods after a rainstorm, does it flow slower or faster? by Chiraqiian

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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