Algorithms for discharge calculation when velocity is zero

[Erica]

Hey Folks,
I recently uploaded a flow measurement from my AquaCalc. I am trying to manually calculate the discharge within the Aquacalc's .csv file (well I did convert it to .xls), just so I can understand how the AquaCalc does it. For this particular measurment, all velocities were calculated at 0.6 depth. As far as I can tell the AquaCalc is using the algorithms from Rantz et al. 1982 Vol. 1, pages 80-82. However, there is one exception, and that's when the velocity is zero at a particular observation that is mid-channel, not on the edge. Rantz et al does not address this situation. When the velocity is zero, the AquaCalc shows an area that is about, but not exactly half of the area that would be calculated with Rantz. Also, it must be taking the average velocity of the station measurments that were just prior to and just after the station with the zero velocity measurment. And so it reports an incremental flow that is greater than zero, even though the velocity was zero at that point. What I just described above is my best approximation in order to manually reproduce what the Aquacalc is doing. Could you please describe to me the method that the AquaCalc uses to calculate flow at a mid-channel station where velocity was measured as zero?
Sincerely,
Erica
P.S. I like the new forum.

[Greg Ruszovan JBSI]

Erica,

You are correct, the AquaCalc uses the standard USGS mid-section methodology, and should not produce a velocity in a vertical / sub-section where there is none.

I see that you have an AquaCalc 5000. What is the firmware version of your AquaCalc?

I've attached a sample AquaCalc 5000 Advanced measurement

AquaCalc 5000 Advanced Zero Velocity Test.csv

AquaCalc 5000 sample test for zero velocity

(2.73 KiB) Downloaded 14 times

done with a test meter here at JBS (variations in velocity are expected with the test equipment). It demonstrates;

1. a zero velocity observation with a stream depth at distance 4 where the Measure button was NOT pressed
2. a zero velocity observation with a stream depth at distance 6 where the Measure button WAS pressed
3. a zero velocity observation with a zero stream depth and no measurement at distance 10.

I am unable to reproduce your error. All velocities are zero and no discharge is associated with the observations.

Note that with the AquaCalc 5000, braided streams (streams with multiple channels) must be handled differently. If you enter a zero depth to mark the edge of a channel, you must follow that with another "empty" observation with zero depth. This way you mark the edges of the "island" for the AquaCalc. (See the AquaCalc 5000 manual "Measuring in Braided Streams".)

Please attach the raw output file from your downloaded Transect and I will review it further. I think we will be able to figure out what happened and how to prevent it in the future.

[Erica]

Hey Greg,
I had have an old Aquacalc 5000 and the firmware was from 1999 for this particular calculation. Since then, Ron sent me new firmware. It's good to know that when velocity is zero, the flow for that sub-section really is zero. It's simpler, and I'm not crazy. Whew!
Erica