NCEES 2000 Sample Exam transpo afternoon #518 (open channel problem)

[Windgate]

Don't know if I can post the whole problem because it might violate a copyright law, but if somebody who owns the sample test could take a look at the problem/solution and see what they think.

#518 Transportation afternoon, Open channel problem.

Nothing in the problem states the shape of the channel. The solution treats it as a rectangular channel, why? Couldn't it also be assumed to be circular, trapezoidal, or v-shaped?

 

[Venpr]

I haven't read the problem, but you have to think: If you data giving have diameter it is a circular, if have slope angles on the sides is a trapezoidal shape, if it is single angule is triangular, other waise it'll be rectangular. no more choices.

 

[CaltransPEHopeful]

You can post the whole problem and here it is:

A proposed storm sewer will have a slope of 0.20%. The design flow for the line has been determined to be 16 cfs. Assume steady, uniform flow and a Manning's roughness coefficient of 0.012 that is constant for all depths of flow. The minimum circular pipe size (in.) that will accomadate the design flow is most nearly:

30

24

18

12

I believe the last sentence does state the shape of the channel by the use of the word circular. Also, to reiterate what Venpr said, the presence of the diameter, D, in the problem statement would be indicative of a circular channel. And last but not least my dear Windgate, don't forget that a closed conduit flowing partially full is treated as an open channel.

Best of luck

 

[jeb6294]

The minimum circular pipe size (in.) that will accomadate the design flow is most nearly:

As I've said before, watch out for questions like these on the exam because you can bet there will probably be a few on there just like this. It's a pretty straightforward question and you'll breeze through in no time and, let's say, you come out with a sure fire answer of 19-in diam (I didn't bother solving it, I'm just using this question as an example). You're happy...slam dunk question and it only took you a minute or so to solve. Problem is you pick answer 'C' because that's the question is asking for the one most nearly, right? Wrong. You just lost out on an easy one because the first part says they want the minimum size that will flow 16cfs. Doesn't matter if it's 0.5-in or a foot, if it's more than 18 inches you need to pick the next size up.

 

[gecress]

CalTrans quoted the wrong problem.

The problem based on the 2000 NCEES copy that I have is:

Transportation Afternoon 518: A storm drain from a large shopping mall parking lot passes to a lake detention basin. Thr flow is 39 cfs with a flow depth to be equal to one-half the width. The channel width is 3.9 feet. The velocity of flow in the channel (fps) is most nearly: (A) 3.2 (B) 4.1 © 5.3 (D) 6.2

SOLN: 518. Determine the rectangular channel's flow velocity. Flow area = 3.9 x 1.9 = 7.4 ft^2. V=Q/A = 39/7.4 = 5.3 fps.

The correct answer is C.

Venpr's comments indicate that the problem statement describes a rectangle.