Manual – Starflow QSD Model 6527B and 6537A
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 08 05 2023.docx
Page 102
In a channel with laminar flows, the shape of the peak remains symmetrical. If other factors
disturb the velocity profiles within the field of view of the Starflow QSD, the histogram may
become distorted. This may introduce a bias to the computed velocity.
If the shape of the histogram is the same from cycle to cycle, then it represents the Starflow
QSD view of the channel and not some transient aberration. Hydrographers must decide if
this represents a valid velocity measurement or if the site is unsuitable.
17.7.
The Nature and Location of the “Reflectors” in the Water
The Starflow QSD measures echoed Doppler shifts from anything acoustically reflective in
the signal beam. It cannot discriminate where the echo is coming from, and the bigger,
closer reflectors will generate a stronger signal than smaller or more distant reflectors.
Reflectors can include:
All types of particles in suspension.
Alluvial saltation loads such as sand, pebbles and rocks moving at or near the bed.
Leaves, sticks, and organic material.
Surface waves from wind or velocity.
Bubbles and entrapped air pockets.
During the time it takes to traverse the signal beam, a particularly good reflector can be
measured 100 or more times. The velocity of this reflector may bias or dominate one
velocity measurement. This will tend to be averaged out during the logging period if it only
happens occasionally.
If it happens consistently, it is a site characteristic. The relevance of this to the mean
channel velocity will need to be understood and compensated for in processing.