[Acoustics] OSW Memo 2006.04 - Availability of the Loop Method report

[Kevin Oberg]

The memo announcing the availability of the Loop 
Method report by David Mueller and Chad Wagner is 
below.  The pdf version can also be downloaded 
from http://hydroacoustics.usgs.gov/policy/

Please note that the step-by-step procedure for 
applying the loop method (in the report appendix) 
has been slightly revised and is available at 
http://hydroacoustics.usgs.gov/policy/.  It is 
recommended that this revised appendix be printed 
out and placed in the subject report.  Also, a 
copy of the LC program for applying the loop 
method can be downloaded from: 
http://hydroacoustics.usgs.gov/equipment/software/


Kevin Oberg

============================

August 31, 2006


In Reply Refer To:
Mail Stop 415

OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 2006.04

SUBJECT:  Availability of the report “Application 
of the Loop Method for Correcting Acoustic 
Doppler Current Profiler Discharge Measurements 
Biased by Sediment Transport” by David S. Mueller 
and Chad R. Wagner (Scientific Investigations 
Report 2006-5079) and guidance on the application of the Loop Method

This memorandum (1) announces the availability of 
the report “Application of the Loop Method for 
Correcting Acoustic Doppler Current Profiler 
Discharge Measurements Biased by Sediment 
Transport” (SIR 2006-5079), and (2) provides 
guidance on the application of the loop method.

The loop method may be used to correct a 
discharge measurement made with an acoustic 
Doppler current profiler (ADCP) that is biased 
low by sediment transport, often referred to as a 
moving bed, and to make the required moving-bed 
test (OSW Technical Memorandum 2002.2).  When a 
moving bed is present, the use of a 
differentially corrected global positioning 
system (DGPS) for the navigation reference is the 
preferred method of making a discharge 
measurement with an ADCP.  However, there are 
situations where a DGPS is unavailable or cannot 
be used reliably.  In these situations, the loop 
method is a valuable alternative for correcting 
biases in discharge measurements caused by a moving bed.

The loop method must be applied properly or it 
may produce incorrect results.  Anyone planning 
to use the loop method should read and follow the 
subject report (SIR 2006-5079), which describes 
the procedures, limitations, and uncertainties 
associated with the loop method. Some key aspects 
of the loop method are summarized here.

1.  The ADCP compass must be properly calibrated 
using the manufacturer’s internal compass 
calibration routines.  For Rio Grande 
instruments, the compass should be evaluated 
after the calibration is completed.  It is 
recommended that the compass error during 
evaluation should be less than 1-degree.

2.  The navigation reference must be set to 
bottom track in the ADCP data-collection software (WinRiver or RiverSurveyor).

3.  Bottom track must be maintained throughout 
the loop.  Loss of bottom track during the loop 
will cause inaccuracies in the computed 
moving-bed velocity.  The amount of bottom track 
data that can be lost without significant impact 
on the method is difficult to specify.  The 
hydrographer must consider how much data are lost 
and if the lost data are accurately represented 
by adjacent data.  If it is determined that lost 
bottom track is adversely impacting the loop 
method, the loop method may not be appropriate 
for that location and flow condition and another 
method of determining the moving-bed bias or 
means of measuring the discharge may be needed.

4.  The loop should begin with the boat located 
at a fixed, nonferrous marker.  The boat is 
maneuvered to make the discharge measurement at a 
uniform speed back and forth across the channel, 
including while turning the boat at the far 
shore.  The boat must return to the exact same 
starting point marker.  Uniform speed is required 
to obtain a spatially uniform sampling of the 
moving-bed conditions throughout the cross 
section.  If boat speed varies, the moving-bed 
computation will be biased by the part of the 
cross section with the most data.

5.  The duration of the loop should be 3 minutes 
or greater, and the boat speed should not exceed 
1.5 times the mean downstream water velocity.

When appropriate, using the loop method as a 
moving-bed test has the advantage of measuring 
bed conditions throughout the part of the cross 
section that can be directly measured with the 
ADCP. The loop method, therefore, results in a 
more representative moving-bed test than a 
single-location stationary moving-bed 
test.  Either a loop test or stationary test can 
be used to satisfy the moving-bed test 
requirement described in OSW Technical Memorandum 2002.2.

The subject report describes an analysis of the 
uncertainty associated with the loop method and 
concludes that the uncertainty is 0.02 ft/s.  A 
measured moving-bed velocity of less than 0.04 
ft/s (2-sigma) may be caused by method 
uncertainty rather than sediment transport.  When 
using the loop method, a measured mean moving-bed 
velocity of at least 0.04 ft/s indicates the 
presence of a moving-bed.  If the measured moving 
bed velocity exceeds 0.04 ft/s, the ratio of the 
mean moving-bed velocity and mean water velocity 
should be computed by dividing the mean 
moving-bed velocity by the mean water 
velocity.  If this ratio is greater than 0.01, 
the apparent bed movement will cause at least a 
1-percent negative bias in the computed discharge 
and a method that accounts for or corrects for a 
moving bed should be used. NOTE: The guidance 
provided in this paragraph supersedes the 
guidance described in Step 4, Processing for 
Moving-Bed Test, in the Appendix­Step-by-Step 
Procedures for Using the Loop Method of SIR 2006-5079.

EXAMPLE 1: The mean water velocity is 2 ft/s and 
the loop method measured a moving bed of 0.03 
ft/s.  It can be assumed that there is no moving 
bed condition because the moving bed velocity is less than 0.04 ft/s.

EXAMPLE 2: The mean water velocity is 2 ft/s and 
the loop method measured a moving bed of 0.05 
ft/s.  The ratio of the mean moving-bed velocity 
and mean water velocity should be computed by 
dividing 0.05 ft/s by 2 ft/s to yield 0.025, 
which is greater than 0.01 and indicates that a 
method that accounts for or corrects for the moving bed should be used.

EXAMPLE 3: The mean water velocity is 6 ft/s and 
the loop method measured a moving bed of 0.05 
ft/s.  The ratio of the mean moving-bed velocity 
and mean water velocity should be computed by 
dividing 0.05 ft/s by 6 ft/s to yield 0.0083, 
which is less than 0.01 and thus no significant 
bias in discharge will be caused by a moving bed.

The mean moving-bed velocity measured by the loop 
method can also be used to correct the measured 
discharge for the bias caused by the moving 
bed.  The loop method correction only adjusts the 
discharge; the measured velocities are still 
biased low.  To use the loop method to correct 
the measured discharge, the discharge should be 
measured in accordance with OSW Technical 
Memorandum 2002.02 and the moving-bed velocity 
measured using the loop method as described in 
the subject report (SIR 2006-5079).  The subject 
report presents two methods, mean and 
distributed, for applying this correction to the 
discharge measurement.  The mean-correction 
method, which can be easily computed by hand, 
uses the mean moving-bed velocity and the 
cross-sectional area perpendicular to the flow to 
compute the discharge under calculated due to the 
moving bed.  This discharge is simply added to 
the measured discharge to obtain the final 
corrected discharge.  The second correction 
method distributes the mean moving-bed velocity 
across the cross section based on the near-bed 
water velocity in each ensemble or profile. This 
distributed correction should provide a more 
accurate correction for those cross sections that 
have significant spatial variations in depth and 
moving-bed velocity. Computations needed to apply 
the distributed correction method are best 
accomplished by a computer program such as the program Loop Correction (LC).

Loop Correction is a computer program written to 
determine if a moving bed is present from the 
loop method data and to correct the measured 
discharge using the distributed correction 
method.  The computer program reads ASCII files 
in the standard ASCII output format from Teledyne 
RD Instruments WinRiver.  SonTek/YSI 
RiverSurveyor can also output files consistent 
with this format.  The LC program first requests 
the user load the loop data file.  The mean 
moving-bed velocity is computed and the direction 
checked to verify that it is in the upstream 
direction.  The criteria for determining if the 
data reflect a moving bed are applied and 
reported to the user.  If a moving-bed is 
detected, the user can select the files 
(transects) for the discharge measurement.  The 
program will read the files and compute the 
corrected discharge.  The results of the entire 
process are displayed for the user and can be 
saved to a file and printed for inclusion in the measurement archive.

A copy of subject report, SIR 2006-5079, can be 
downloaded from: http://pubs.usgs.gov/sir/2006/5079/

A revised Appendix for the subject report (SIR 
2006-5079) of the step-by-step procedures for 
using the loop method that are consistent with 
this memorandum can be downloaded 
from:  http://hydroacoustics.usgs.gov/policy/. 
It is recommended that this revised Appendix be 
printed out and placed in the subject report.

A copy of the LC program can be downloaded from: 
http://hydroacoustics.usgs.gov/equipment/software/

If you have any questions or comments about the 
policies and guidance in this memo, please 
contact David S. Mueller (dmueller at usgs.gov), 
Kevin Oberg (kaoberg at usgs.gov), or the OSW 
Hydroacoustics Work Group (hawg at simon.er.usgs.gov)


                                                                                     Stephen 
F. Blanchard (signed)
                                                                                     Chief, 
Office of Surface Water



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Kevin Oberg
USGS - Office of Surface Water
Voice:  217.344.0037 extension 3004
Cell:   217.840.9739
http://hydroacoustics.usgs.gov/