YSI SonTek RiverSurveyor, System Manual

Page: 89 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
77
6.4. Selecting a Measurement Cross-Section
The following topics are covered in this section:
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Why should I be careful when selecting a cross-section location?
•
What factors do I need to consider when selecting a cross-section?
6.4.1. Why should I be careful when selecting a cross-section location?
Selecting a suitable measurement location is quite subjective at times and involves considera-
tions such as access, width, bottom-depth, and deployment possibilities. The RiverSurveyor can
operate in a wide variety of river conditions. However, repeatability and accuracy requires that
basic assumptions be reasonably met concerning the measurement; for example, homogenous
flow (low turbulence), proper clearance, representative beam depths, etc. All these assumptions
can contribute a slight amount of variance when totaled together and may exceed typical criteria
for reliable measurements if used in an unsuitable measurement location.
A properly selected cross-section will easily yield measurements that are repeatable to within a
few percent. In contrast, a poorly selected cross-section may yield measurements that are not
easily repeatable and typically exceed five percent when compared with one another.
6.4.2. What factors do I need to consider when selecting a cross-section?
A measurement location should be in an area where:
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The river cross-section (streambed) and flow-lines are reasonably uniform (low turbulence)
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Flow along the riverbanks is small (relative to its mid-section)
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Depth changes are relatively gradual
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Measurement depths are within the specified operating range of the RiverSurveyor ADP.
While it may be impossible to select a cross-section that fulfills all of the above criteria, it is best
to select a section that best represents the flow through the area of interest. Also bear in mind
that when measuring the total discharge through a section, the section may be traversed several
times. Multiple measurements are made across the cross-sectional area for the following reasons:
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To confirm repeatability of measured results.
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To evaluate temporal variation in flow patterns.
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To average out any spatial variations.
A few examples of good measurement locations are:
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Uniform stream depths and velocities. This probably represents the best measurement con-
ditions possible. Rarely will this condition exist, however, in some situations the conditions
will be very close.
•
Narrow sections with gradual banks. In general, flow will tend to be slightly higher and
steadier in a narrower section as opposed to a wider section.
•
Low (or zero) flow along the riverbanks. In general, this will enable you to measure more
water out in the river and estimate less water along the edges, which is always preferred.