Stage-Area Ratings

1. Stage-Area Ratings Streamflow Record Computation using ADVMs and Index Velocity Methods Office of Surface Water

2. Stage-Area Rating • Developed first because it is needed for development of the index velocity-mean velocity rating! Q = VA Q = Discharge (ft3/s) V = Mean channel velocity (ft/s) A = Channel cross-section Area (ft2)

3. Overview

4. Stage Area Relation – What Is It? • Channel cross-sectional area changes with stage • To compute area at every stage, we need a rating Floodplain Main Channel

5. Step 1: Select Standard Cross-Section • Guidelines: • Locate as close as possible to ADVM • Pick location that is not prone to large amounts of scour/fill and aquatic growth/decay.] • Mark and document location!!! • You will need to periodically re-survey the cross-section

6. Try to Avoid This! FLOW Bypass Channel Outflow Standard X-Section Gage and ADVM Bypass Channel Main Channel

7. Step 2: Survey Cross-Section • Survey perpendicular to primary flow direction • Channel + banks + floodplain • Measure depth and distance • Need to accurately define channel shape

8. Survey Tools • Level and stadia rod • Depth soundings • ADCP • Echosounder • Other: mapping-grade GPS, total station • Will likely be a combination…..

9. Survey Tools - Level and Stadia Rod • Needed for most dry land measurements • For in-channel survey, • Use a tagline for distance • Works well in shallow channels • Low boat traffic ideal

10. Survey Tools – Depth Soundings • Similar to Stadia rod method • Made with tagline or from a structure (bridge) • Can work in deeper channels • Need Stadia rod to measure dry land portion

11. Survey Tools – ADCP • Convenient for in-channel measurements • Most ADCPs have 4 diverging beams and are not designed for depth sounding • Can use vertical beam for M9 ADCPs • ADCPs will keep track of distance • When using an ADCP, • Use tagline if possible • Make transect straight across channel

12. Survey Tools – Echosounder • Provides accurate depth • Need to record distance with GPS, Total Station, or ADCP

13. Survey Tools – Echo Sounder Calibration • Draft • Speed of Sound • Bar check is required for hydrographic surveys • 2 per day • Adjust draft • Adjust scale • Repeat

14. Marking • Permanent rebar stakes • Other types of markers • Also measure and document location relative to a immobile object (e.g. 55’ downstream of gage house)

15. Data Collection–Example Data Format • Important to measure water level (record gage height) • Distance between soundings should vary as the rate of depth changes • Need enough resolution to capture changes in channel shape • Keep good notes!

16. Data Collection • Important items to remember • Cross section must be referenced to stage • Survey past the maximum expected stage

17. Step 3: Rating Development • AreaComp • USGS program written for the purpose of computing cross-sectional area • Can import *.CSV, WinRiver ASCII, SonTek*.mat files • Provides output for stage and area AreaComp2 available for general use: http://hydroacoustics.usgs.gov/indexvelocity/software.shtml/

18. AreaComp2

19. AreaComp2

20. AreaComp2

21. AreaComp2

22. AreaComp2

23. AreaComp2

24. AreaComp2 • After lunch, we’ll do a demo of AreaComp2 and a class exercise on stage-area ratings

25. Step 4: Implementing the Stage-Area Rating • Data table from AreaComp is used to enter the rating into ADAPS • Tip sheet in T&M Report, Appendix 6

26. Step 4, cont.: Implementing the Stage-Area Rating • Area-rating developed with AreaComp or AreaComp2 must be entered into your database as a first step towards computing continuous discharge ADAPS – a tabular rating with linear interpolation

27. Step 5: Rating Validation • Must routinely check for channel changes • Every year for first 3 years of gage operation • After an event or condition that would cause a change; e.g., major storm event • After first 3 years, re-survey every 3 years if: • Standard cross-section does not show substantial change • Rating validation indicates no substantial change • No deviations in index-velocity rating • Always survey the same standard cross-section!!!

28. How do I determine if there is a change? • See T&M Report, Appendix 5 • Compare new survey data to old survey data • Evaluate % changes : • Over range of stages measured since last survey • Relative to accuracy of survey • Relative to accuracy of discharge measurements

29. Rating Validation Example

30. After Flood Event Percent differences exceed uncertainties in survey and measurements Change needed!

31. After 1 Year, No Flood Event Percent differences do NOT exceed uncertainties in survey and measurements No change needed!

32. After 1 Year, No Flood Event Highest percent difference at low stage Look for divergence in index-velocity rating for measurements made at low stage to determine if change needed

33. Reminders • Always look for departures in your index-velocity rating, too • Index-velocity rating may also need to be revised since Vm = Q/Rated Area!!

34. Questions for Class • Do you have to make discharge measurements at your standard cross-section? • What are characteristics of a good standard cross-section? • Why is it important to mark the standard cross-section? • Which survey methods are you most likely to use for your sites? • How often must stage-area ratings be checked?

35. Questions?