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The IV Method & Class Overview

The IV Method & Class Overview. Streamflow Records Computation using ADVMs and Index-Velocity Methods Office of Surface Water. Class Introductions. Victor. Instructor Intro’s Victor Levesque, SC WSC Kevin Oberg, OSW Lars Soderqvist , FL WSC Molly Wood, ID WSC. Outline for Webinar.

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The IV Method & Class Overview

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  1. The IV Method & Class Overview Streamflow Records Computation using ADVMs and Index-Velocity Methods Office of Surface Water

  2. Class Introductions Victor • Instructor Intro’s • Victor Levesque, SC WSC • Kevin Oberg, OSW • Lars Soderqvist, FL WSC • Molly Wood, ID WSC

  3. Outline for Webinar

  4. Index Velocity Gages • USGS operates approx. 7,400 streamgaging stations in the U.S. • In 2011, approximately 470 of these were index velocity gages • Velocity data are being collected at other sites for other purposes

  5. Index-velocity Method • Index Velocity Method is a method for computing discharge where the velocity measured in some portion of a channel cross-section is used as an indicator or index of mean channel velocity. • It involves developing and maintaining • an index-velocity rating (relation between index velocity, Vi, and mean cross-sectional velocity, Vm,) • a stage-area rating • Once these 2 ratings are established, we are able to compute a discharge, Q, for every measured stage and index velocity using the equation Q = A x V

  6. Why Use the Index Velocity Method? • Variable backwater Photos Courtesy of Art Schmidt

  7. Why Use the Index Velocity Method? • Measuring unsteady flows (many kinds)

  8. Why Use the Index Velocity Method? • Bi-directional flow (tidal, wind-driven, etc.) Images from Kevin Oberg From: “Nutrient Loading at Card Sound Canal; a tidally driven canal that experiences highly stratified bi-directional flow conditions” Mark Zucker, Jeff Woods and Carrie L Boudreau

  9. Why Use the Index Velocity Method? • Loop rating? • Different stage-discharge relation for the rising and falling limbs of a hydrograph

  10. What if You are Unsure? • Consider operating site as both index-velocity and stage-discharge • After series of measurements, develop and evaluate both ratings • You can always fall back on the stage-discharge rating and remove the ADVM

  11. Definition of Terms Streamflow Records Computation using ADVMs and Index-Velocity Methods Office of Surface Water

  12. Instruments • Acoustic Doppler Velocity Meters (ADVMs) • Acoustic current meters that use the Doppler principle to measure water velocities. • They are simplified ADCPs • Models include: • SonTek Argonaut, Ott SLD, TRDI Channel Master, etc. • Commonly have horizontal orientation but can have vertical orientation

  13. Instruments a • Acoustic Velocity Meters (AVMs) • Acoustic current meters that use the time-of-travel principle to measure water velocities • Models include: • AFFRA, ORE • Measure velocities horizontally across a channel or pipe. • Rarely used anymore c

  14. Instrument Terminology • Acoustic frequency refers to the frequency of transmitted or received sound. Typical units are kilohertz (kHz) or megahertz (MHz). For example Argonaut SL 1,500 kHz (1.5 MHz). • Lower frequency = greater nominal range • Higher frequency = smaller nominal range; for vertically oriented ADVMs  shallower flow depths • Sampling rate is the frequency at which velocity measurements are made. Typical units are hertz (Hz). For example, a sampling rate of 10 Hz = 10 samples per second

  15. 15 m 30 m Instrument Terminology • Averaging Interval (AI) is the interval during which velocity measurements are averaged • Sample Interval (SI) is the interval at which velocities are recorded (unit value interval) • The SI is controlled by either the ADVM or a data logger SI AI AI = 7 min & SI = 15 min pings 7 m Time

  16. Instrument Terminology • Range-Averaged Cell • Cell Begin (CB) • Cell End (CE) • Multi-Cells • Blanking Distance (BD) • Cell Size (CS) • Number of cells (Ncells)

  17. Instrument Terminology • Signal Amplitude • The strength of sound energy returned to the ADVM while velocity measurements are being made, with units of counts and ranging from 0-255. • Affected by the concentration and size of scatterers in the water, nearby boundaries, and even obstacles within the stream. • Sometimes referred to as signal strength, backscatter, or RSSI

  18. Instrument Terminology • Instrument Noise • A combination of environmental and electronic noise level near which a reflected or returned acoustic signal cannot be distinguished by the instrument electronics. • Signal to Noise Ratio (SNR) • Ratio of signal amplitude to instrument noise

  19. Rating Terminology • Rating • Mathematical or other relation between 2 or more parameters (e.g. stage-discharge or stage-area ratings). • Calibration • Process by which a rating is defined, typically by measurements. Concurrent measurements of index velocity and discharge are used to calibrate (or define) an index velocity rating. Similarly, concurrent measurements of stage and cross-sectional area are used to define a stage-area rating.

  20. Rating Terminology • Standard Cross-Section • A cross section near an ADVM which is chosen for the development of the stage-area rating and which can be re-surveyed at regular intervals. • Stage-Area Rating • A unique relation between stage and area from the standard cross-section used to compute area for the Index-Velocity method

  21. Rating Terminology • Index Velocity (Vi) • Velocity measured by an instrument that is “index” of (surrogate for) the mean channel velocity • May consist of • ADVM velocity • AVM path velocity • Profiler velocity

  22. Rating Terminology • Measured-mean Velocity • The mean velocity in the channel at the standard cross-section, Vm or Vmean • Qm = measured discharge • A = cross-sectional area computed from the stage-area curve (based on standard cross-section); • Not obtained from the discharge measurement! • Must always be computed from the standard cross-section for the index-to-mean velocity relation to work • Vm = Qm/A = Vmean

  23. Rating Terminology • Index-Velocity Rating • Mathematical equation defining the relation between the index velocity (measured by an ADVM in a portion of a channel), Vi, and the measured-mean-channel velocity, Vm. • For example: Vm= Slope × Vi + Intercept • Developed by means of statistical regression analysis

  24. Don’t Confuse These Terms! • Index Velocity (Vi) • Velocity measured by an instrument that is “index” of (surrogate for) the mean channel velocity • Measured-mean velocity (Vm) • Mean velocity computed from measured discharge and the corresponding area derived from the standard cross-section • Estimated-mean velocity (Vm_est) • Mean velocity computed from the index-to-mean velocity regression equation(s) • Also called computed mean velocity (Vm_comp)

  25. Questions & Discussion

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