Computation of Discharge Using the Index-Velocity Method in Tidally Affected Areas

By Catherine A. Ruhl and Michael R. Simpson



Scientific Investigations Report 2005-5004—ONLINE ONLY

Sacramento, California 2005

Prepared in cooperation with the
Interagency Ecological Program

Complete accessible text of report (2.8 MB PDF)

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Computation of a discharge time-series in a tidally affected area is a two-step process. First, the cross-sectional area is computed on the basis of measured water levels and the mean cross-sectional velocity is computed on the basis of the measured index velocity. Then discharge is calculated as the product of the area and mean velocity. Daily mean discharge is computed as the daily average of the low-pass filtered discharge. The Sacramento-San Joaquin River Delta and San Francisco Bay, California, is an area that is strongly influenced by the tides, and therefore is used as an example of how this methodology is used.




Purpose and Scope


Principles of Operation

Ultrasonic Velocity Meters

Acoustic Doppler Velocity Meters

Point Velocity

Single Bin



Calculating Area on the Basis of Stage

Calculating Mean Velocity on the Basis of the Index Velocity

Calculating Discharg

Calculating Daily Flow



Appendix A. Sample Computation of the Cross-Sectional Area Relationship

Appendix B. Sample Computation of the Mean Velocity

Appendix C. Sample Discharge Calculation

Appendix D. Calculation of Daily Discharge

Appendix E. High-Quality Data and Examples of Common Mistakes

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Water Resources of California

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