RiverSurveyor How-to: The Loop Method
In this short video illustration, the Loop Method procedure you see here follows the procedure documented by prominent and reputable water monitoring agencies such as the USGS. SonTek recommends that this step-by-step procedure is reviewed prior to attempting a Loop Method measurement using the RiverSurveyor Live software.
For moving-boat discharge measurements, the RiverSurveyor M9/S5 measures boat speed using either bottom-tracking or an integrated DGPS system. Bottom-tracking requires that the riverbed be stationary (motionless) in order to properly compute boat-speed. During certain flow conditions, sediment will be transported along the riverbed (i.e. a moving-bed) and will produce a bias in the bottom-tracking results which will ultimately result in a biased discharge measure-ment. DGPS is a good alternative to bottom-tracking because it is not affected by the moving-bed. In some cases DGPS will not be available or reliable at your measurement site. As an alternative, when used properly the Loop Method can be used to correct the bottom-tracking bias produced by the moving bed.
The Loop Method is used to correct discharges that are biased by sediment transport (moving-bed) when bottom-tracking is used as a reference to measure boat speed. The basic premise is based on a “back and forth crossing closure error” where the apparent upstream movement of the ship-track is used along with the measurement time to determine the mean velocity of a moving bed. In this case, the discharge will be biased LOW by a certain percentage based on the velocity magnitude of the moving-bed.