PE Exam morning question

[cantaloup]

Design the most efficient flow rectangular shaped cross-section open channel, maximum velocity is 2.00 ft/sec , given flow of 200.00 cfs , Manning value is 0.013. What is the nearest value of slope?

A/ 0.07

B/ 0.09

C/ 0.12

D/ 0.15

 

[cantaloup]

Sorry, the answer choice should be:

A/ 0.00007

B/ 0.00009

C/ 0.00012

D/ 0.00015

 

[3gorgesdam]

I dont have an idea what most efficient rectangular shape means to R value. I came up with S=0.0003/R^4/3.

I made a guess on the S slope assuming R =0.5

S=0.0007

 

[JasonT33]

Most efficient rectangular channel is one having the depth equal to half the width

 

[JasonT33]

Q = VA = 200 = 2*A

A = 100 =width^2/2

width is approximately 14

perimeter is 28

R = 3.5

plug into mannings

S = 0.000057

 

[cantaloup]

Attached is the answer file. This is just an example only , to show if you know how to solve it and the solve it correctly. It would be harder if the question asked for traperzoid channel instead of rectangular.

PE_question_open_channel_flow_1.pdf

 

[JasonT33]

Hmmm for US units, v = (1.49/n) * R^2/3 * SQRT(S)

Why are you using 1 instead of 1.49?

 

[JasonT33]

with the correct formula, the velocity for answer C is almost 3 ft/s

double check and let me know

 

[JasonT33]

where did you get this answer from? I dont think its right... procedure is right, but the chezy-manning is wrong.

The closest slope is answer A. Even answer A gives you a velocity higher than 2 ft/s. The slope that gives the correct velocity is the one I gave above (0.000057)

 

[EnviroMe]

I agree with Jason. You forgot the 1.49 (k) in Manning's Equation. The answer should be 0.000057. If you were using metric system, then k=1.

 

[cantaloup]

OOPS, sory for the incorrect formula, I still have metric things in my mind. This type of question is to help you to freshen up your mind. You never know what you'll encounter in the exam. The more types of problem you look at the better you are (for PM session)

Thanks for all your inputs.

 

[owillis28]

OOPS, sory for the incorrect formula, I still have metric things in my mind. This type of question is to help you to freshen up your mind. You never know what you'll encounter in the exam. The more types of problem you look at the better you are (for PM session)Thanks for all your inputs.

Can anyone download the pdf from an earlier reply to this post? I went through this problem when it was first posted but can't get it to work. How did JasonT33 get 14' for his 'A' and 28 for his perimeter? Did he use an arbitrary number? Please explain

 

[Road Guy]

here it is, when we swithed from "the old board" a lot of the attachments didnt survive..