YSI SonTek RiverSurveyor, System Manual

Page: 38 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
26
and GPS are two totally independent methods of measuring position and distance traveled –
one will not correct the other.
•
Bottom-track is a relative position system (e.g., my current position is 5 m North of my po-
sition 10 seconds ago). GPS is an absolute position system (e.g., my current position is X
2
Latitude and Y
2
Longitude, and my last position was X
1
Latitude and Y
1
Longitude). There-
fore, as a significant distance is traversed (typically more than 1000 m), the uncertainty as-
sociated with positional information from bottom-track increases. The uncertainty from
GPS will remain the same. So when traversing shorter distances (0.1 to 1000 m), it is pref-
erable to use bottom-track because of the lower uncertainty; for longer distances (e.g., over
1000 m), GPS can minimize the positional uncertainty. For distances between these limits,
either source should be adequate, and a combination of the two can be helpful.
•
Bottom-track does not require any additional equipment or serial ports to receive data input.
3.3.5. What are the limitations of bottom-track?
While there are several benefits to using bottom-track, it does have a few limitations.
•
Depth Limit
•
Vessel Navigation
•
Moving Bed
Depth Limit
A basic limitation of bottom-track is that it can operate in waters that have a depth no greater
than approximately 1.3 times the maximum profiling range of the ADP.
Table 3-1
lists the maxi-
mum profiling range and the maximum bottom-track depth for each of the different RiverSur-
veyor system frequencies.
Vessel Navigation
When operating from a moving platform such as a boat, erratic course changes during the aver-
aging interval may cause inaccurate vessel velocity vectors. Generally, the best discharge and
velocity measurements are made when vessel course and speed remain constant over the entire
measurement. It is good practice to maintain the vessel at a speed no greater than the water cur-
rent speed, though environmental/safety conditions may preclude this recommendation. More in-
formation on vessel navigation can be found in §
6.5
–
How to Make Good Measurements from a
Moving Vessel
.
Moving Bed
Moving bed is a phenomenon that occurs when a high concentration layer of suspended sediment
or other fine material is pushed downstream along the riverbed by the flow of water. If a moving
bed is present, the measured discharge value will always be lower than the true discharge value.
This is because the bottom-track speed will determine the vessel speed relative to the moving
bed. As such, the speed of the moving bed is unaccounted for in the discharge measurement.
This phenomenon also becomes an issue when the layer of riverbed material is significantly
thick such that either isolating this layer from the bottom, or profiling through it, becomes uncer-
tain. In addition, it becomes difficult for the ADP to measure the true water depth because there
is no clear point where the water column ends and the bottom begins. Moving bed situations are
common during large flood events or in rivers associated with high sediment load transport. Ex-