06-11-2008, 04:27 PM | #1 | ||
Death Herald
Join Date: Nov 2000
Location: Le stelle la notte sono grandi e luminose nel cuore profondo del Texas
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Ok math people (conversion problem)
I'm having a heck of a time trying to figure something out. I know I have some needed information missing, but not sure how I should approach it.
If you have the "cubic feet per second" (cps) flow rate of a river, how do you convert this into miles per hour for the flow of the river? I'm guessing at a minimum you'd need the average width of the river channel, and the average water depth. Any ideas for a formula to use?
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06-11-2008, 04:31 PM | #2 |
Coordinator
Join Date: Aug 2001
Location: Buffalo, NY
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The speed of the river is going to change depending on all those items you listed so the question is are you looking for the speed of the river at a certain point on the river or ar you looking for an average speed for the river from one point to another?
If the latter is the case simply ignor the cubic notation and take your data for Feet/sec and convert to miles/hour. This won't be precise of course but will give you a general idea of the velocity that you're looking for. |
06-11-2008, 04:51 PM | #3 |
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I must leave this thread before my head explodes! Gah...too much math!!!!
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06-11-2008, 04:59 PM | #4 |
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Join Date: Dec 2004
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Answer: It's flowing faster than it looks.
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06-11-2008, 05:14 PM | #5 |
Death Herald
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Location: Le stelle la notte sono grandi e luminose nel cuore profondo del Texas
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Yeah, we are looking for the average speed between two points. I know the answer won't be exact, but a ballpark guesstimate will help.
I think I might have it figured out what you were saying, RendeR. If the rate is 200cfs, and the avg. width of the river channel is 50 feet, and the average depth is 2 feet, that means 2 feet of distance was measured by the sensor (50x2x2 is 200). So you are saying take the 2f/s and convert that to mph. Which would be 1.36 mph.
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06-11-2008, 05:16 PM | #6 |
Coordinator
Join Date: Oct 2000
Location: Big Ten Country
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I'm not sure how you can do the conversion -- a cubic foot is a measurement of volume, and a mile is a measurement of length.
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06-11-2008, 05:20 PM | #7 | |
Death Herald
Join Date: Nov 2000
Location: Le stelle la notte sono grandi e luminose nel cuore profondo del Texas
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Quote:
True, but I think you can derive the length portion out of the cubic foot (l x w x h) to get the part needed, if you know the width and depth of the river.
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06-11-2008, 05:20 PM | #8 |
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06-11-2008, 05:42 PM | #9 |
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Yeah, just take a 1ftx1ftx1ft cube, your flow-rate = your speed.
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06-11-2008, 05:46 PM | #10 |
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Join Date: Jan 2001
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That is an invalid comparison.
1 cubic foot per second through a drinking straw would produce a much faster flow than 1 cubic foot per second through a sewer pipe which is faster still than 1 cubic foot per second down a river. Pass is correct - one is a rate of volume, the other is a rate of speed, and you cannot compare the two. |
06-11-2008, 05:48 PM | #11 |
Pro Starter
Join Date: Jan 2001
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No, because - as stated above - the length of the cubic foot is irrelevant in terms of cubic feet per second. One is a measure of volume, the other is of distance. Apples and oranges.
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06-11-2008, 06:03 PM | #12 |
Death Herald
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I understand what you are saying, but I guess what I'm missing is that there is a common component between the two measurements, one of the measure of feet (f/sec and f³/sec).
To use my assumptions from above on the 50 foot wide 2 foot deep river that has a 200cfm flow, the speed of the water would definitely be faster if another river, also with a 200cfm flow was 25 foot wide and 2 foot deep. To get 200cfm there, the distance portion would be 4 feet (25x2x4 =200), and the speed would be 2.72mph. If the river was 1 foot wide and 1 foot deep (the straw example) with a 200cfm flow, then 200 feet would be the distance measured, and the speed would be ~135 mph.
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06-11-2008, 07:05 PM | #13 |
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I seem to remember a ridiculous exercise like this from Physics one time and was wondering what one had to do with the other.
SI
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06-11-2008, 08:41 PM | #14 |
College Starter
Join Date: Oct 2000
Location: Berkeley
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If you know the area of a cross-section of the river (ie, the average depth and width) you should be able to convert between the two easily enough.
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06-11-2008, 09:00 PM | #15 | |
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Join Date: Mar 2005
Location: Lynchburg, VA
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From the Army Water Measurement Manual.
Quote:
So the velocity V is Q / A. Or in other words, what Daimyo and cartman said. |
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06-12-2008, 06:15 AM | #16 |
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Dude, just tie your beer tube to your tube and then you won't lose it no matter how fast the river is going. Geez...
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06-12-2008, 07:34 AM | #17 |
Pro Starter
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Confirming what others said... V = Q/A (and A = HW for the longitudinal velocity).
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