YSI SonTek RiverSurveyor, System Manual

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SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
80
6.5. How to Make Good Measurements from a Moving Vessel
The RiverSurveyor ADP is usually mounted to a boat or tethered platform ( RiverCat ) to measure
discharge in a river. Two basic measurement techniques apply:
•
Motoring the boat across the river from one bank to the other. Examples are shown in
Figure
4-1
,
Figure 4-2
, and
Figure 4-3
.
•
Pulling the boat across the stream with a small line (usually called a tagline) fixed to each
bank. The RiverCat is usually tethered with the line running from the catamaran to a person
located along a fixed river crossing (such as a highway bridge or foot crossing). Another
popular RiverCat tow method is to have one person on each riverbank and pull the RiverCat
from one side to the other. Examples are shown in
Figure 4-4
and
Figure 4-5
.
In either of these situations basic operation guidelines are very similar and are noted in the fol-
lowing paragraphs.
The RiverSurveyor measures discharge by collecting depth and velocity data in the river as the
vessel transects from one riverbank to the other. This basic method is described in Section
5.1
–
Discharge Calculations
. The following items detail the steps that should be taken during a River-
Surveyor discharge measurement.
•
The vessel should be stationary before beginning or ending a measurement. This will enable
you to easily determine an accurate distance to the starting and ending riverbanks using ei-
ther a measuring tape or a laser rangefinder. After the boat is stationary at the start, data col-
lection can begin.
•
Data collection will end after the vessel is stationary on the opposite riverbank. Good prac-
tice is to collect two or three profiles while stationary at both the start and the end of the
measurement section. This is because the profiles collected along the edges of the measure-
ment section are generally used to estimate edge discharges (§
5.1.3
).
•
Good practice is that the vessel velocity should be no greater than the average water veloc-
ity. As a rule, the slower a measurement can be made, the more data are collected and the
better the data quality. This rule holds true in steady-flow rivers, but should be carefully
evaluated when measuring in conditions where: (1) flow rates are changing rapidly, (2)
there is a time limit on the duration of the measurement, or (3) water velocities are well be-
low the safe operating speed of the vessel. In these cases, slightly higher vessel speeds will
still provide reliable results, but vessel speed should be minimized whenever possible.
•
Vessel speed and direction should remain steady during the measurement. Rapid accelera-
tion or deceleration, or sharp changes in direction, will tend to bias the bottom-track data
and should be kept to a minimum.