Critical Depth vs Normal Depth?

[rascal22]

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 .

 

[John QPE]

It is my understanding that normal depth only applies to uniform flow, and occurs along uniform channels where the depth of flow remains constant.

 

[Pavilion]

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.



-Sam

 

[John QPE]

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

 

[ASchmidt]

For lulz, is the answer in 103. above C - 0.43'?

 

[K19]

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.

 

[ptatohed]

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.

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

 

[rascal22]

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