Replaying ADCP Data

1. Replaying ADCP Data Getting the most from your time on the river

2. Typical Data Review • Play back the stationary test • Check for bottom track motion • Look at velocity profile – decide on correct extrapolation method • Set filter parameters

3. Data Review • Replay all of the discharge measurements • Hit F12 – examine the summary table • Remove aborted or bad transects • Look for any parameter that has high COV • COV = (standard deviation)/Average • If COV for discharge is <5%, you are done

4. Playback Stationary test Slight upstream Boat motion

5. Calculate Bottom Track Bias Divide Distance MG by Time to get apparent upstream velocity Vbias = 0.76/221 = 0.0034 m/s Divide by Water Speed %bias = 0.0034/0.580 = 0.59% So bottom track bias is less than 1% Probably safe to ignore

6. Look at velocity profile • Average entire stationary file • Look at Velocity magnitude and direction profile Velocity is smooth and Direction is very constant with depth. Nice data. Could consider using 3-point Extrapolation for top layer

7. Set filter parameters • Look at Velocity Tabular view for several ensembles • About 5% of error velocities are > 0.2m/s • Set that as filter to clean up data

8. Setting Thresholds • Hit F3 key • In Threshholds WT is for water, BT is for Bottom track • Enter 0.2 for WT Error Vel

9. Thresholds Now 7% of bins Are flagged as bad Data filtered out

10. How much can be ‘bad’ • As much as 40% bad may be OK. As long as the ‘bad’ data is not all in one area of the river. • But don’t set threshold too tight without a good reason. Usually only 1-10% of data will be get marked bad if you are using good threshold settings • If you are happy with your thresholds, then ‘freeze’ the value so it applies to the rest of the files as you play them back

11. Playback the rest of the files Use next file button To step through the files Threshold being applied

12. Look at summary table • After playing back all of the files, look at the Discharge History table – hit F12 Oops! The stationary test Is the culprit. Measurements marked red are more than 5% different from the average

13. Remove the ‘trouble’ file • Click on the filename and delete that from the table – much better now Notice the very low COV! The user was moving SLOW and SMOOTH! He also used 3 minutes for each measurement.

14. Extrapolation • Extrapolation methods • Top • Power – default, good most places • 3-point – use if top of profile curves different than rest of profile, also under ice • Constant – not recommended for rivers • Bottom • Power – default, good most places • Constant – not recommended for rivers • No-slip – use if there is bidirectional flow, also under ice

15. Playback the data with 3-point at top • Hit F3 to open configuration dialogue • Select Discharge tab • Set top Discharge Method to 3-pt. Slope • Replay the files

16. Results with 3-point Extrapolation Was 5.962 using power law extrapolation, So not much difference – only 1.3%. Choice is up to the user. In this case, 3-poit is probably more correct because of shape of the top of the profile. Might be because of wind or the geometry of the channel – this was an Irrigation canal.

17. Use good procedure! • The data above shows that taking at least three minutes to cross the channel, moving slow, and moving smooth will produce very accurate and low variance discharge results. Noisy discharge results are most often caused by going too fast and not being ‘smooth’.