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Overview of Index Ratings Process

Overview of Index Ratings Process. Streamflow Records Computation using ADVMs and Index-Velocity Methods Office of Surface Water. Goals for this Presentation. Index-velocity Method. Channel area is computed at a specific, selected location using a stage-area rating

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Overview of Index Ratings Process

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  1. Overview of Index Ratings Process Streamflow Records Computation using ADVMs and Index-Velocity Methods Office of Surface Water

  2. Goals for this Presentation

  3. Index-velocity Method • Channel area is computed at a specific, selected location using a stage-area rating • Mean-channel velocity is computed from the index velocity measured by an instrument using an index-to-mean velocity rating

  4. Generalized Steps for Index Rating Development

  5. Stage and Area • Flow area changes with stage • A stage-area rating must be developed for a selected “standard” cross-section to compute flow area for a range of stages • Standard cross section should be surveyed • Stage-area rating is developed from survey data

  6. Standard Cross Section • Q1 = Q2 = Q3 • Area is always computed at location 1! 1 2 3 1 = standard cross-section 2 = wading measurement section 3 = bridge measurement section

  7. Stage-Area Rating Development

  8. Index Ratings • ADVMs measure a portion of the channel (Vi); mean channel velocity is not measured • Thus we need to relate the index velocity,Vi,(ADVM measured velocity) to the measured-mean channel velocity (from discharge measurements)

  9. Index Rating Development

  10. Index Rating Flow Chart Overview

  11. Graphical Data Analysis

  12. Simple Linear Rating Example Rating Example Rating

  13. Compound Linear Ratings Plot data Hydraulic basis for compound rating? A Vm B Example Rating Vi

  14. Compound Linear Ratings High end Low end Final Rating w/ Transitions

  15. Multiple Linear Ratings Data plots Residual plots Change in flow distribution with stage? Significant variation in stage?

  16. Multiple Linear Ratings Residual plots Vm = 0.82*Vi – 0.15(Stage*Vi) + 0.16

  17. Computing Continuous Discharge • For every stage, an area (A) is computed from the stage-area rating • For every index velocity (Vi) a mean cross-sectional velocity (Vcomp) is computed from the index rating • For every time step, discharge is computed using the equation Q = Vcomp*A (unit values of Q) • The discharge record is published (e.g. mean daily flows or tidally-filtered flows)

  18. Questions?

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