Discharge measurement in moving bed conditions

When carrying out discharge measurements with an ADCP instrument using the moving boat method the usual way of determining the path along which the measurement was taken is "bottom track". This method uses the reflexion of acoustic pulses from the sole of the river to measure the velocity of the moving boat relative to the bed. This information is then used to calculate the path of the measurement.

If the streambed itself is in motion, however, using bottom track obviously introduces a velocity component upstream which in reality doesn't exist. Since this information enters the discharge calculation a moving bed condition leads to systematically lower discharges which aren't immediately obvious to the user.

Moving bed - Principle schematic

To detect a moving bed you need to incorporate extra checks into the measurement routine. A commonly used but time consuming practice is the loop test: Prior to each measurement a two-way crossing of the stream is performed returning the boat exactly to its starting position. If the software shows the finishing point of the measurement lying upstream of the starting point this indicates a moving bed which must be considered in post processing. While there are methods to correct for the bias of a moving bed (which can be as large as 10 - 15 % of the measured discharge) any type of numerical correction will introduce a degree of uncertainty that taints your results. So if in doubt these measurements should be discarded.

Loop test for detection of a moving bed

A moving bed often occurs during flood conditions. So in the very case where the data is particularly valuable as it lies on the sparsely populated part of the rating curve you risk having biased data.

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