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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

Index-Velocity Rating Development

Streamflow Record Computation using ADVMs and Index Velocity Methods

Office of Surface Water

goals for this presentation
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
how many q measurements
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!!
measurement considerations
Measurement Considerations
  • Strive to collect measurements over the complete range of discharge
    • All ratings have uncertainties outside the range of discharge measurements
proper q measurement techniques
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
q measurements
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?

steady vs unsteady flow
Steady vs. Unsteady Flow

Review:

  • Steady flow – negligible change in stage, velocity, or discharge
  • Unsteady flow – continuous change in stage, velocity, or discharge
unsteady flow
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!
unsteady flow1
Unsteady Flow
  • One example is tidally-affected flow
  • Tidal cycle measurements: 7 to 13 hours
  • How would you group transects in this example?
unsteady flow2
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

data collection for rating development1
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
reminder1
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/

step 3 organize data
Step 3: Organize Data
  • One example as a starting point…
compute vmean
Compute Vmean
  • For every Q measurement, compute Vmean:

Vmean = Q / Rated Area from Standard X-Section

NOTVmean output in your ADCP software

rating workbook template
Rating Workbook Template
  • We have developed a template
  • Let’s do a demo…..
step 4 synchronizing data
Step 4: Synchronizing Data
  • Inaccurate synchronization of stage and velocity data with Q measurements can give you a headache when developing your rating!
example
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?
synchronizing 15 minute data
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
example1
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?
rating workbook template1
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
rating workbook template2
Rating Workbook Template
  • Document your analysis in the workbook!!!
  • Otherwise you will forget your logic, and others who check your work will be confused!
data synchronized with q measurements
Data Synchronized With Q Measurements
  • Data synchronized
  • Now what?
scatter plots
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….
multi cell plots
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
multi cell plots1
Multi-cell Plots
  • The best relation with Vmean may be an average of multiple cells’ Vx
  • Experiment with and plot different explanatory variables!
stage vs v mean

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

outliers
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
rating development1
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
rating development2
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?
rating development3
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
rating types
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