Dear colleagues,
I am trying to measure low inflow velocities (I expect less than 0.015 m/s) in a stream using ADCP. Comparing to another approach, I found that both approaches are in a good agreement, however, the main point is that both approaches gave opposite velocity values (i.e., positive and negative).
As a result, I wonder to what extent we can trust the velocities obtained by ADCP in a quasi static stream?
Thanks
Both StreamPro and Workhorse ADCP have what is called the built-in "high precision mode", i.e. "pulse to pulse coherent" mode. The high precision mode is designed for and great for low flows, but it has limitation in profiling range. Moreover, in your low flow condition, you may consider using the stationary ADCP method instead of the moving-boat method. In fact, the velocity less than 0.015 m/s is basically the limit to any velocity measuring instruments. What is the another approach you used to compare with ADCPs?
Thanks Dr. Huang,
I am using another tomographic approach similar to the AVMs developed by Hiroshima University, called the Fluvial Acoustic Tomography System (FAT).
I have measured many streams but not with ADCP or FAT. With the current meters I have used, it is up to the operator to recognize direction of flow such as downstream or upstream in eddy. When getting negative value where it should be positive may have something to do with how the equipment catalogues flow direction, and as suggested by Dr Huang, the moving boat method might affect.
Other options in estimating flow under such low velocity is to find a more confined cross section which would increase velocity or measure the time it takes filling a known volume as the flow discharges from a culvert. If an experimental catchment, installing a v notch weir or flume might help quantify the flows using the rating curve for the type structure.
To clarify,
Basically, I am trying to quantify the flow speed based time of travel approach. In other words, computing the velocity based on the time needed for an acoustic pulse to transmitted from a transmitter and received by the other receiver. Thus, the sound speed is the: distance / time of travel.
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