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Hello everyone!
I have just noticed inconsistency in how screening (of near surface data) is applied to Sontek ADCP data, and how this can have a major impact on the development of Surface Alpha values for non-contact measurements.
My rather unattractive graphic shows:
- Top – Sontek M9 transect, screening set to 0.0m in RSQ or RSL
- Middle – merged image showing how Sontek interprets 0.16m of screening (the faint grey image) vs how QRI (can I abbreviate QRevINT to QRI…?!) does (the coloured image overlaid). It is very different.
- Bottom – screening set to 0.16m in QRevINT
Breaking the oldest rule in the book, and actually opening the (instruction) book, I find that in RSL, Sontek states screening is applied from water’s surface, which does appear to be the case, but in RSQ, it states that it is below the transducer (i.e. how QRI does it), but this does not actually appear to be the case.
In any case (ahem), this can all have big impacts on the determination of the Surface Alpha you want to apply to your surface velocimetry measurements.
Playing about with data from the shallow fast(ish) flowing river illustrated, my SA can be anything from 0.81 to 0.93, depending how much data I screen and if I go with the recommended Power/Power, or the rather more fashionable Constant/No Slip extrapolations.
In other words, that top extrapolation we always say needs to be checked, but sometimes only changes Total Q by say 0.5%, can make a rather significant 10% change to non-contact measurements.
Which all points to: we need to improve our ability to accurately determine the vertical velocity profile…..
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Interesting Nick! Good catch!
I thought QRI was a new piece of software that I had completely missed, I’ll just stick to Qrev-int for the time being.
If I remember correctly, there is development planned for Qrev-int to display SA, perhaps Dave can accomodate this then?
(Also, we are doing first tests with ADCP-mounted SVR in August if all goes according to plan!)
/Daniel
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SonTek does measure the screening distance from the water surface. This appears to be consistent with both RiverSurveyor and RSQ. QRev defines the excluded distance as the distance from the transducers. This definition made more sense to me as typically the excluded distance is used to remove data affected by flow disturbance, which is a function of the distance from the transducer and not from the water surface.
Dave
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