Moving Boat River Measurements using the StreamPro ADCP

1. Moving Boat River Measurements using the StreamPro ADCP Randy Marsden RD Instruments

2. ADCP Discharge Measurement • Q = VW x A • A = W x D • Need to measure: • water velocity • water depth • distance traveled (width)

3. Water Velocity Profile • Doppler Effect • Transmit acoustic energy into water • Measure frequency shift of reflected signal • Calculate velocity – Radar gun • Profiling • Measure water velocity at fixed intervals called depth cells • for StreamPro cell size from 2 – 10 cm • With upgrade 20cm cells

4. Bottom Tracking • Measure depth like a depth sounder • Use Doppler effect to measure velocity over the bottom – use separate bottom tracking pulse and processing • d = Vb x t • For StreamPro: t = 1 second

5. ADCP Discharge

6. Accuracy • Need: • Accurate water velocity • Accurate depth • Accurate bottom tracking • Accurate clock • Minimum extrapolation

7. StreamPro Introduction • Streams 20-250 cm deep (400cm with upgrade) • Velocities 0-2 m/s • Bluetooth comm’s up to 80 meter range • Software on Pocket PC

8. StreamPro ADCP • Small Transducer • 4-beam • x, y, z, and error velocities • Error velocity can be used as a data quality indicator and as a way to filter out ‘bad’ data from fish, etc. • Redundancy – three good beams still allows valid x, y, z velocity solution • Lower noise – 4 beams 15% lower noise than three beams, need 30% more pings with three beams to get same average noise level.

9. StreamPro ADCP • Short Blank – 3cm • Get data closer to surface, less extrapolation • High ping rate • Low noise with small cells • Data same format as Rio Grande • Playback using WinRiver for more detailed analysis

10. StreamPro ADCP • Broadband ADCP • RD Instruments invented Narrowband ADCP in 1981 • Limitations • Large cells • High noise – need lots of pings to get good data • Bottom Tracking problems – Rayleigh ‘fade’

11. StreamPro ADCP • RDI invented Broadband ADCP in 1988 – multiple patents • Noise 7-10 times lower than Narrowband • Narrowband requires 50 pings to have same averaged noise as Broadband • Smaller cells possible • Smaller blank possible • Less extrapolation at top and bottom of profile • Get closer to edges – measure more of the flow • End result - Accuracy

12. Broadband continued • More accurate and reliable bottom tracking • RDI Doppler Velocity Logs (DVL) are biggest product • Allows shallower beam angle – less affected by flow asymmetry • Lower power consumption for same results • More consistent backscatter - sediment

13. Example Data

14. StreamPro ADCP • Simplified System • Simple software • No cable – Bluetooth wireless • No Compass – not needed for discharge • No pitch/roll – not needed for discharge • Use standard AA alkaline or rechargeable batteries

15. StreamPro Operational Constraints • Water velocity: 0-2.0 m/s • Speed over ground: 0-1.5 m/sec • Moderate dynamics • Turn rate < 3 degrees/sec • Pitch roll rate < 3 degrees/sec

16. Operation Technique • Smooth motion • Slow acceleration and stopping at edges gives most consistent results • At least three minutes for discharge measurement, more time in turbulent or water speed < 10 cm/sec • Take longer at edges if V < 10cm/sec

17. Does it Matter?

18. Why the difference?

19. Smoother Motion Pays Off!

20. Tag ‘Loop’ with pulleys

21. Basic Procedure • Set up StreamPro, establish communications • Use default setup, determine maximum stream depth • Use setup page to select best cell size • Do stationary measurement • Determine start and stop positions – at least two good cells. Mark these positions • Measure edge distances accurately • Measure discharge – typically fourpasses

22. Deployment Method • To obtain reproducible discharge, need a reproducible and well controlled method • Move smoothly • Reproducible edge distances • Controlled path

23. Basic Procedure Continued • Transfer data and configuration files from Pocket PC to laptop or desktop – use cradle or SD card reader • Archive data and configuration files • Playback in WinRiver for more detailed analysis • Convert .xml files to w.000 files using ini2xml utility

24. Stationary Test • In high velocity water or high sediment load a ‘moving bed’ may bias bottom tracking • Appears as movement directly opposite the flow • Reduces measured water velocity when boat velocity is reference • Do stationary measurement in deepest part of stream for 10 minutes • Look at data in WinRiver to see if there is upstream movement • If average upstream velocity <1% of water velocity, then ignore it. Otherwise adjust discharge upward by ratio of (upstream boat speed)/(mean water speed)

25. Measure Discharge • Start the Measurement • Collect 10 or more samples at first edge • Accelerate slowly from edge • Move smoothly across stream • Brake slowly at far side to prevent overshooting that edge • Stop • Collect second edge data

26. Discharge Measurement Uncertainty • Contributions • Directly Measured Area • Extrapolated regions • Top • Bottom • Edges • Bottom Tracking

27. Directly Measured Area Systematic error: temperature – speed of sound 0.33% Speed of sound computed from temperature – let the instrument set for a few minutes before measuring discharge –stationary test ideal time for this Dominant uncertainty: water velocity, depends on cell size, turbulence. Obtain from data or theory. Decreases as square root of total number of cells measured – slower is better!

28. Top Layer uncertainty Lowest error for thinnest top layer: small cells, small blank reduce the error. So, use smallest reasonable cell size. The wizard does this for you. Going slow allows more averaging to determine best power law exponent.

29. Bottom Layer uncertainty Small cells reduces thickness of bottom layer Need to know exponent accurately Going slow is good!

30. Edge Uncertainty • Need to measure edge width accurately! • Getting as shallow as possible at edges reduces percentage of flow at edges – small blank, small cells minimizes • Fewer cells in shallow water at edge – more ensembles at edge is good! • Make best possible estimate of edge coefficient

31. Table 1: Discharge Measurement Error Source Table

32. Example Parameters

33. Example Uncertainties Rio Grande used smaller cells with lower noise, but StreamPro used smaller transducer depth, smaller blank, closer to edges. For this stream the systems are about equal. But in shallower water, the StreamPro will be better.

34. Questions??????