
Manual – Starflow Ultrasonic Doppler Instrument
Model 6526
Unidata Manual - 6526 Starflow User Manual Issue 17 01 2020.docx
Page 54
12.2.5
Effects of Surface Ripples
In shallow water the presence of ripples or small waves on the surface can cause complications.
The ripples may be caused by wind, or be a feature of the channel caused by obstructions in the
channel or by irregularities on the bank. Ripples will be moving and at a different velocity to the
water.
In shallow and clean water the Starflow can clearly see the surface. As the ripples move they
present a varying effect on the Doppler signal. At times the ripples will be positioned so that the
carrier reflects away from the Starflow. A minor signal is detected but many more signals are
detected from the water and a correct velocity is measured.
When the ripples move slightly one can become positioned such that it becomes an “acoustic
mirror” reflecting back a powerful signal and dominating the signals from the channel. The median
velocity will be biased toward the ripple velocity and will be incorrect.
In practice, a series of histograms in such a situation will move back and forth between the two
extreme conditions, logging an average that is not the correct velocity. At some sites ripples will be
a characteristic at certain levels. Positioning and orienting the Starflow so that it does not point
directly into a “mirror” ripple face is the best prevention.