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Index-Velocity Rating Development

Index-Velocity Rating Development. Streamflow Record Computation using ADVMs and Index Velocity Methods Office of Surface Water. Goals for This Presentation. Provide an overview of the next steps in rating development: Make Q measurements over range of conditions

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Index-Velocity Rating Development

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  1. Index-Velocity Rating Development Streamflow Record Computation using ADVMs and Index Velocity Methods Office of Surface Water

  2. Goals for This Presentation • Provide an overview of the next steps in rating development: • Make Q measurements over range of conditions • Develop a rating “workbook” • Organize Q measurement data • Synchronize stage and velocity data with Q measurements • Plot data • Review plots before proceeding with rating analysis

  3. Generalized Steps for Index Rating Development

  4. Step 1: Make Discharge Measurements

  5. How Many Q Measurements? • Rule of thumb: • Need minimum 10 measurements per explanatory variable used in the regression • Example: In a simple relation between Vi and Vmean, Vi is the only explanatory variable, so need a minimum of 10 measurements before developing a rating • But……measurements need to be over a range of hydrologic conditions!!

  6. Measurement Considerations • Strive to collect measurements over the complete range of discharge • All ratings have uncertainties outside the range of discharge measurements

  7. Avoid Acoustic Interference

  8. Proper Q Measurement Techniques • Follow guidelines in: • ADCP Measurements: T&M 3A-22 by Mueller and Wagner at: http://pubs.usgs.gov/tm/3a22/ • General Q Measurements: T&M 3A-8 by Turnipseedand Sauer at: http://pubs.usgs.gov/tm/tm3-a8/ • OSW Memoranda at: http://water.usgs.gov/osw/pubs/techmemos.html

  9. Q Measurements • Do a good job in the field! • Your rating will reflect the quality of the collected data • A rating error can be related to field data collection procedures • Make the best discharge measurements possible Is this a good idea?

  10. Reminder

  11. Steady vs. Unsteady Flow Review: • Steady flow – negligible change in stage, velocity, or discharge • Unsteady flow – continuous change in stage, velocity, or discharge

  12. Unsteady Flow • Exposure time rule (minimum 12 minutes) applies to steady flow; not to unsteady flow! • For unsteady flow, if at all possible, collect reciprocal transects to average together to make a measurement • In highly unsteady flow, it may be necessary to use a single transect as a measurement • Subjective determination!

  13. Unsteady Flow • One example is tidally-affected flow • Tidal cycle measurements: 7 to 13 hours • How would you group transects in this example?

  14. Unsteady Flow • With very rapidly changing flows, compromises may need to be made – for example, running ADCP transects faster than recommended • This can result in reduced data quality • The record from such sites may be poor 2009 flood on the Red River at Grand Forks, ND

  15. Data Collection For Rating Development

  16. Data Collection For Rating Development OR….. • Record data to the ADVM internal recorder • Disables SDI-12 communication withdata logger • Can manually input regular sampling interval to data base

  17. Reminder • Check and, if needed, sync clocks in DCP, ADCP, and ADVM • Within 1-2 seconds Correct Time: www.time.gov http://tf.nist.gov/

  18. Step 2: Develop a Rating Workbook

  19. What Do I Compile in the Workbook?

  20. Step 3: Organize Data • One example as a starting point…

  21. Compute Vmean • For every Q measurement, compute Vmean: Vmean = Q / Rated Area from Standard X-Section NOTVmean output in your ADCP software

  22. Rating Workbook Template • We have developed a template • Let’s do a demo…..

  23. Rating Workbook Demo

  24. Step 4: Synchronizing Data • Inaccurate synchronization of stage and velocity data with Q measurements can give you a headache when developing your rating!

  25. Synchronizing 1-Minute Data

  26. Example • Measurement on 6/1/12 • Start: 12:10:15 • End: 12:25:26 • What is the time of the FIRST velocity/stage data point that should be used in the average? • What is the time of the LAST velocity/stage data point that should be used in the average?

  27. Synchronizing 15-Minute Data • If have Vi and/or Stage data at intervals other than 1 minute (most commonly 15 minutes)…. • Interpolate to the mid-time of the Q measurement

  28. Example • Measurement on 6/1/12 • Start: 12:10:15 • End: 12:25:26 • What is the measurement mid-time? • What is the approx. interpolated stage at the mid-time?

  29. Rating Workbook Template • The provided template automatically synchronizes GH and Vi data to your measurements • You have to add GH and Vi data and enter averaging interval/measurement period

  30. Rating Workbook Template • Document your analysis in the workbook!!! • Otherwise you will forget your logic, and others who check your work will be confused!

  31. Data Synchronized With Q Measurements • Data synchronized • Now what?

  32. Rating Development

  33. Step 5: Plot Data

  34. Scatter Plots • In Excel, highlight x and y data and go to Insert-Scatter (under Charts) • Let’s do some scatter plots together in Excel….

  35. Multi-cell Plots • Look for which cells or combination of cells have the best relation with Vmean • Also look for any abnormalities – some cells (or the range-averaged cell) should be excluded from the rating

  36. Multi-cell Plots • The best relation with Vmean may be an average of multiple cells’ Vx • Experiment with and plot different explanatory variables!

  37. Is Stage Related to Vmean?

  38. Mean vertical velocity Stage vsVmean A relatively “flat” velocity profile --change in position of mean velocity with respect to the instrument is insignificant Note that for the flatter profile, measuring any place between positions A and B, the difference between measured and mean velocity could be negligible – whereas the difference is significant with the more curved profile A B

  39. Outliers • If you see outliers in your plots, first check that all date and time synchronizations are correct! • Also check raw data files • Beams obstructed? • Changing cell end? • Affected by biofouling? • Don’t exclude a measurement unless you can document a reason it is erroneous

  40. Step 6: Review Plots and Document Initial Thoughts

  41. Index-Velocity Rating Analysis

  42. Rating Development • Ratings can be based on single or multiple parameters: • Velocity • Stage • Velocity (x,y) and stage • Uncommon: Velocity (x,y), stage, and Velocity (x,y)2

  43. Rating Development • For bi-directional flow sites (i.e. tidal), the rating might need to be separated into positive andnegative flow directions (compound rating) • Velocity • Stage • Velocity and stage • Velocity (x,y), stage, and Velocity (x,y)2 • Wind? • Specific Conductance?

  44. Rating Development • The rating should: • Be the best fit of the field data • Make hydraulic sense for the site • Be expressible as one or more mathematical equations

  45. Rating Types • Simple Linear • 1 explanatory variable • Find best relation with Vmean • Compound • 2 or more simple linear regressions • Group data based on slope changes • Multiple Linear • 2 or more explanatory variables

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