Critical Depth vs Normal Depth?

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rascal22

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This might be a silly question, but on my practice exam I cannot differentiate when I need to solve for critical depth vs normal depth, and I'm wasting time. Can anyone outline when to use one vs the other for me? For example these two questions:

(Critical Depth) The figure shows a well-maintained, straight roadside drainage ditch with short vegetation cover (n =0.030). The ditch is to carry a maximum flow of 3 cfs over a reach of 1 ,200 feet and a gradual
vertical drop of 1.2 feet. The triangular ditch is to be constructed with side slopes of 4:1 (H: V) in sandy loam soil. If the flow is 2 cfs, the depth (ft) is most nearly...

and

(Normal Depth) Assume a 12-in. concrete (n = 0.013) trunk sewer has a slope of 0.004. The minimum depth (in.) required to maintain a self-cleansing velocity of 2 fps is:

Thank you in advance. I'm hoping this is a silly questions and I'm overlooking something simple .
 
It is my understanding that normal depth only applies to uniform flow, and occurs along uniform channels where the depth of flow remains constant.

 
The first question was taken from the NCEES Env. Eng. Morning Sample Question #103. It asked specifically asked for critical depth, that's why you solve for it; otherwise, it would be a normal depth question.

ENVSample.png

-Sam

 
This is what I was thinking, but I wasn't positive. The problem statement should specify what they are looking for.

 
That's the answer I got too ;)

It's a tricky problem in that all the givens point to solving for normal depth using Manning - enough red herring to make a fine stew.  At least they weren't so cruel as to include the normal depth solution in the answer choices.

 
K19 said:
That's the answer I got too ;)

It's a tricky problem in that all the givens point to solving for normal depth using Manning - enough red herring to make a fine stew.  At least they weren't so cruel as to include the normal depth solution in the answer choices.
Click to expand...


I think I read somewhere that NCEES doesn't use red herrings any more. 

 
Thanks all!  That was the answer a professor gave me too (use normal depth unless specifically asked otherwise).  

 

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