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Friedumb t1_jbg5y8d wrote

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

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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