Hi all. This is my first time posting here.
I am currently in the process of studying for FE Civil exam and I am taking it in two months. I have come across practice problems that say:
"Use a reasonable value of the Manning's roughness coefficient" or "Use a reasonable value of the Hazen-Williams roughness coefficient"
I am currently working on two problems that utilize the same channel constructed of smooth concrete (one to solve for slope, the other to solve for flowrate... problems are independent of each other, meaning the givens are different in each). In the solution to the problems, they use n = 0.013 and C = 100.
In the NCEES Reference Manual, there is no table displaying Manning's roughness coefficients. There is a table displaying Hazen-Williams coefficients for several materials, but not "smooth concrete." They do have a value for "Concrete (regardless of age)", which is C = 130. Note that in the solution they used a smaller value.
My question is:
What do they mean by "reasonable value?"
And, how do I estimate what the value is, if I'm not given a table? Am I expected to memorize them? It's not a problem, if I need to. I just would like to know so that I do it.
When I worked the problems out, I used value of n = .01 and C = 130. My result for slope was waaaaay off (I got .0009, correct answer is .002). My result for Q was much closer to the correct answer (I got 1.0, correct answer was 0.8). This seems imprecise to me and I am afraid on the FE exam it will cost me.
Thank you for all your help.
I am currently in the process of studying for FE Civil exam and I am taking it in two months. I have come across practice problems that say:
"Use a reasonable value of the Manning's roughness coefficient" or "Use a reasonable value of the Hazen-Williams roughness coefficient"
I am currently working on two problems that utilize the same channel constructed of smooth concrete (one to solve for slope, the other to solve for flowrate... problems are independent of each other, meaning the givens are different in each). In the solution to the problems, they use n = 0.013 and C = 100.
In the NCEES Reference Manual, there is no table displaying Manning's roughness coefficients. There is a table displaying Hazen-Williams coefficients for several materials, but not "smooth concrete." They do have a value for "Concrete (regardless of age)", which is C = 130. Note that in the solution they used a smaller value.
My question is:
What do they mean by "reasonable value?"
And, how do I estimate what the value is, if I'm not given a table? Am I expected to memorize them? It's not a problem, if I need to. I just would like to know so that I do it.
When I worked the problems out, I used value of n = .01 and C = 130. My result for slope was waaaaay off (I got .0009, correct answer is .002). My result for Q was much closer to the correct answer (I got 1.0, correct answer was 0.8). This seems imprecise to me and I am afraid on the FE exam it will cost me.
Thank you for all your help.
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