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LC and SMBA Updates. Office of Surface Water Hydroacoustics Webinar January 6 and 9, 2008 David S. Mueller. Overview. Brief description of loop method Common problems using loop method Discussion of LC updates Compass evaluation Bad bottom track warning
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LC and SMBA Updates Office of Surface Water Hydroacoustics Webinar January 6 and 9, 2008 David S. Mueller
Overview • Brief description of loop method • Common problems using loop method • Discussion of LC updates • Compass evaluation • Bad bottom track warning • Description of reason for no correction • Brief description of SMBA • Discussion of SMBA updates • Correcting discharge with SMBA • Using multiple moving-bed tests • Demonstration of SMBA • WinRiver II Version 2.04
Loop Method Flow Distance moved upstream DUP Vmb=DUP/Total Time 1. Establish a starting marker 2. Calibrate compass 3. Make a transect across and back to the starting marker 4. Compute the measured mbvel and the mean channel velocity (vmean). 5. Is a moving bed present? (mbvel >0.04 ft/s and mbvel/ vmean > 0.01) 6. Make typical measurement 7. Compute corrected discharge
LC (Loop Correction) • Automate processing of loop data • Compute corrected discharge, if a moving-bed is detected. • LC is the recommended method for making loop corrections. Manual corrections are discouraged.
Common Problems 1) Inadequate compass calibration 2) Invalid bottom track or lost ensembles 3) Misinterpretation of LC results
Compass Evaluation • Velocity direction varies between left to right and right to left portions of the loop. • Loop reversal occurs at the maximum distance made good (DMG)
Compass Evaluation Algorithm • Identify the ensembles before and after the maximum distance made good. • Compute the mean flow direction for the two halves of the loop. • Compute the difference in flow direction. • Compute the standard deviation (SD) of flow direction using all ensembles. • If the difference in flow direction is > 5 degrees, print warning that data indicate a potential inaccurate compass. • HOWEVER, if SD > 90 degrees, print warning the water velocities are too noisy to evaluate the compass.
Bottom Track Problems • The loop method relies on accurate bottom track. • Invalid bottom track or lost ensembles may result in an erroneous loop. • Invalid bottom track is the result of the ADCP being unable to measure a boat speed. • Lost ensembles are a communications issue that may be resolved with changes in data radios, serial ports, or laptop computers. • Warning criteria set at 5%. • NOTE: Valid bottom track is most critical during speed or heading changes. It is extremely important to maintain bottom track at the far edge while reversing course.
Misinterpretation of LC Results • Closure error was in downstream not upstream direction. • Error was over 1% of mean velocity, but moving-bed velocity was too low.
SMBA • SMBA was developed to allow analysis of stationary moving-bed tests. • Originally developed for the StreamPro • Can be used with the Rio Grande • SMBA was enhanced to provide a correction algorithm using one or more stationary moving bed tests. • The corrected discharge is computed by distributing the moving-bed velocity correction based on the near-bed velocity in each ensemble of the transects comprising a measurement.
Moving-Bed Velocities Swimming/Stationary Vmb=0.17 ft/s 3-Point/Stationary Vmb=0.002 ft/s
StreamPro Has a Moving-Bed • Can’t • Use GPS because you have no compass • Use the loop method because you have not compass • Can • Use section by section if you have the software • Compute the mid-section method by hand • Use one or more moving-bed tests to correct discharge by hand • Use one or more moving-bed tests and SMBA to correct discharge
Rio Grande Has a Moving-Bed • Use GPS • Good GPS • Good compass calibration • Use the Loop Method • Accurate bottom track • Good compass calibration • Use multiple stationary moving-bed tests and SMBA to apply the correction. • Accurate bottom track • Use the mid-section or section by section method • Poor bottom track
Bed velocity distribution Measured = Mean Bed Velocity Moving bed test
Multiple Moving Bed Tests Bed Velocity Moving bed test Moving bed test Moving bed test Moving bed test Moving bed test
Distributing the Correction • Compute velocity components 0.1*depth above bed for each ensemble using 1/6th power law. • Distribute the moving-bed velocity based on the near-bed velocity. • Compute the corrected velocities
Distributing the Correction – cont. • Compute the corrected measured discharge from the correct velocities • Compute the correct total discharge NOTE: This approach is currently be validated for stationary moving-bed test.
Updates to SMBA • Corrected problem where SMBA would include a negative moving-bed velocity in computing the correction from multiple moving-bed tests. • Corrected multiple moving-bed test approach from a mean of the moving-bed tests to a true multiple linear regression of the tests.
Negative Moving-Bed Fix Old New Negative values were included here. Now they are set to zero.
Change to Linear Regression Previous Approach Regression Approach where “a” is a regression coefficient from regession of moving-bed velocities and near-bed velocities from the moving-bed tests.
Example Sample Data Old: a=0.092 New: a=0.093
SMBA Discharge Correction • Load and evaluate one or more moving-bed tests • This step uses the raw data file *r.000 or *.pd0 • If a moving bed exists click Adjust Q • Select all transects that comprise the measurement • This step uses ASCII output files that must be created from WinRiver or RiverSurveyor • Review the results and save to a file
WinRiver II Updates • WinRiver II 2.03 contained a change in the way depth screening was handled. • The USGS was unaware of the change and failed to identify the change during testing. • The affect of the change was identified by the USGS and users in China after the release. • The change only affected measurements using GPS as the reference where there was lost bottom tracking.
Evolution of Depth Computation • Traditionally, if any of the four beams processed a valid depth the simple average of the valid depths was reported and used as the depth for that ensemble. • In WinRiver II 1.0, an option was added to changed from a simple average to an inverse depth-weighted average. This option is enabled by default. • In version 2.03, TRDI added an additional level of screening to the depths and marked depths invalid if bottom tracking derived velocities were invalid.
Effect of Change (GPS Reference) Versions Prior to 2.03 Version 2.03
Rules in Version 2.04 • Moving-Boat measurements • Default: Box unchecked • Depths ARE NOT screened using bottom track velocity • Only affects discharge when using GPS as reference • User can choose to apply screening, if desired • Section by Section Measurements • Default: Box checked • Depths ARE screened using bottom track • Default maintains consistency with StreamPro section-by-section software • User can choose to remove screening, if desired
Summary • Use of the bottom track screening in section by section measurements may result in this method being unusable in some areas where bottom track cannot be maintained. Uncheck if this problem is observed. • We have not observed significant problems with erroneous depths where bottom track is not valid. • Checking or unchecking the box does not affect the computed discharge for moving-boat measurements unless GGA or VTG is used as the reference and invalid bottom track occurs during the measurement.
Hydroacoustics.usgs.gov • WinRiver 2.04 is available for download now. • LC and SMBA are available for download from the Hydroacoustics Forum. See instructions on the main hydroacoustics web site.
