YSI SonTek RiverSurveyor, System Manual

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SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
79
Minimal Flow along the Edges
Flow along the riverbanks at the measurement site should ideally be near zero or small. In gen-
eral terms, good practice is to try to measure as much water in the cross-section as possible. This
will provide a lower systematic error for the discharge measurement by minimizing the ratio of
unmeasured flow to measured flow. However, the physical size of the RiverSurveyor ADP and
the minimum vessel clearance usually prevents measurements to the riverbanks.
To overcome these limitations and account for the total flow through the measurement site, Riv-
erSurveyor lets you specify a distance to the bank edge. The
RiverSurveyor program then uses an
equation (§
5.1.3
) to compute the flow along the edge assuming a constant triangular or vertical
streambed slope. Under normal circumstances, this procedure will provide a reasonable estimate
of the unmeasured flow along the river’s edge.
At times, however, it may be difficult to determine precisely the distance from the measurement
start or end-points and the edge of the river. As such, systematic errors associated with over or
under estimating riverbank flows will be significant when flow along the riverbank is high, but
can be easily reduced to insignificance by starting or ending in a section with little or no flow.