I found this meme in a facebook group on engineering concepts:

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I selected A (steady and uniform). In selecting A I assumed the width of the river to remain constant along its path and no parameters change with time.

But why steady? Velocity doesn't change with time. It's steady.

But why uniform? We get the same volume across any cross section because width of stream doesn't change.

I think people who selected any other option are confusing laminar with turbulent flow. This has nothing to do with it if you made the correct assumptions.

Is my selection (A) and justification for it correct?

  • 1
    $\begingroup$ What was the result? $\endgroup$
    – Solar Mike
    Jul 21 '19 at 5:43
  • $\begingroup$ The definition of a flood is that it isn't doing what you assumed it is doing. $\endgroup$
    – Phil Sweet
    Jul 22 '19 at 1:45

I'm not sure if there is any further context to this meme, but I would have chosen B) Unsteady Uniform Flow. Since almost all real flows are unsteady and non-uniform, it is really an application of assumptions to characterize the flow as something simpler to analyze. I would assume a flood is highly transient, therefore, unsteady. There is probably not a constant flow source feeding the flood, rather a series of time-dependent, differently sized sources (streams, drainage basins, rain, etc). At the same time, the flow is turbulent. This means the velocity profile is relatively "flat" (http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/introvisc/node8.html), thus, justifying the uniform assumption. On a more macroscopic level, a flood is likely entirely turbulent through its domain, further justifying the assumption of relative uniformity. While the profile remains more or less flat, it will change in magnitude depending on the sources. On a microscopic level, the assumption of uniformity does not hold because of the turbulence (i.e. vorticies of varying sizes everywhere in the flow).

  • $\begingroup$ What about non-violent floods that slowly spread out over a large flood-plain, remain as a large lake for a few days, then slowly drain away? At any time, all one sees are calm waters, with no flow or turbulence. $\endgroup$ Jul 24 '19 at 13:01
  • $\begingroup$ Again, it's all about the assumptions. "Flood" is a general term as is "river" in the problem statement. Depending on where you are from, you may picture a flood differently than I do. My assumptions did not assume this scenario you described, but that does not mean your comment is not valid. It just means you assumed a different condition. I would argue that even rivers that appear slow are most likely turbulent. More likely than not, the surface roughness of the river bed (or flooded surface) trips turbulence at least to some degree. $\endgroup$
    – mechcad
    Jul 24 '19 at 13:18

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