YSI SonTek RiverSurveyor, System Manual

Page: 84 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
72
6.1. Selecting the Right Measurement Platform
The following topics are covered in this section:
•
What are some guidelines for selecting a measurement platform?
•
What type of vessel (moving platform) should be used for measurements?
6.1.1. What are some guidelines for selecting a measurement platform?
Of course, selecting the right equipment for the job will always produce the best results. This is
particularly true when making discharge measurements with a RiverSurveyor system. Second in
importance only to the actual vessel operation is using the right vessel or
measurement platform .
The measurement platform will have a significant effect on the reliability and quality of the data.
While this manual cannot list all possible options, our purpose here is to simply explain general
requirements for a measurement platform and some suggestions.
Let us start by relating to you a common phrase heard from experienced RiverSurveyor users –
“slow and steady”. This refers to the desired method of operating a platform during a RiverSur-
veyor measurement.
In all RiverSurveyor applications, the measurement platform will have to be transected across
the river to collect discharge. During the transect, wind, currents, and waves can make operating
the platform a very difficult process if the platform is not stable enough for the conditions. In ad-
dition, these conditions can quickly become a safety issue. The following are two details that
should be considered for any moving platform used to collect data with RiverSurveyor.
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Stability:
- The moving platform should be able to cross the river with a minimum amount of
pitch/roll/heave.
- The more stable the platform, the less motion the transducers will experience when col-
lecting data.
- Good stability will help to maintain a high degree of measurement accuracy by minimiz-
ing the variability caused by the transducers averaging different volumes of water as they
move through the water.
•
Speed:
- The moving platform should be able to maintain a relatively low and steady speed when
collecting data.
- Platform speed is a significant factor in the quality of a measurement. First, the slower
the platform speed, the more data that is collected. As a result, a lower measurement
standard deviation will be achieved. Second, the slower the platform speed, the better the
resolution of the velocity profile and river bottom.
- To maintain low speeds from a motorized vessel, it is important that the minimum speed
can be achieved without having to constantly engage/disengage the throttle. Constantly
engaging/disengaging the throttle has a tendency to cause the platform to jerk, which
causes unnecessary motion of the ADP.
- Another method to slow the speed of a transect is to orient the platform almost directly
opposing the flow direction (commonly called crabbing ) and drift slightly downstream
ultimately crossing the river diagonally. This will reduce the relative speed between the
ADP and the platform, and allow you to maintain steady control as well.