YSI SonTek RiverSurveyor, System Manual

Page: 39 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
27
amples of these rivers in the United States include sections of the Mississippi River and the Mis-
souri River. Both rivers have well documented evidence of moving bed.
Generally, it is good practice to understand the bottom characteristics of the survey site before
collecting data with an ADP, as some soft bottoms with large amounts of plant growth can imi-
tate a moving bed. River bottoms with strong acoustic reflections (e.g., rocky bottoms) can typi-
cally be tracked at ranges that are 20-30% greater than the water-current profiling range.
The following items are most common methods for detecting a moving bottom. Refer to §
3.3.6
for a description of bottom-track terminology.
•
Hold the vessel at a stationary position. This can be done by anchoring the vessel or by
keeping a constant position between two known landmarks. Maintain this position for 5-10
minutes and record your data to a file in RiverSurveyor. The
Distance Made Good (DMG)
value should be close to zero. However, in moving bed situations, DMG will continually in-
crease, and the vessel track will display continual vessel movement upstream. An approxi-
mate estimate of the speed of the moving bed can be derived from the DMG/time interval.
•
Set a marker (buoy) at a location on one of the riverbanks and begin a RiverSurveyor meas-
urement from this point. Travel as you would do normally across to the opposite riverbank.
After you have reached the opposite bank, continue recording data and travel back (at the
same speed) to the marker located at the starting point of your measurement. If the
DMG is
significantly greater than zero, and the ship track screen shows the vessel upstream of the
original starting point, it is possible that the riverbed is moving. In this case, it is best to rely
on GPS positional inputs to make your discharge measurements.
•
A GPS receiver can be used to compare DMG from the GPS with the ADP’s DMG value.
Hold the vessel at a stationary position. This can be done by anchoring the vessel or by
keeping a constant position between two known landmarks. Maintain this position for 5-10
minutes and record your data to a file in
RiverSurveyor . GPS data should also be recorded in
RiverSurveyor during this time. Check the results of the GC-BC and D
BT
/D
GPS
values in the
GPS data display. If GC-BC is more than a few degrees, it is possible that the river has a
moving bottom at that location. If the value is consistently off by several degrees, it is also
possible that the
magnetic declination has not been set correctly within the RiverSurveyor
program.
The results of your moving bed tests (above) should be recorded and saved with your transect or
survey data. This information can be useful when analyzing data at a later time and when verify-
ing that a check measurement was made to confirm the presence (or lack of) a moving riverbed
during a discharge measurement set. The file names of the moving bed tests can be stored in the
Discharge Measurement Summary (§
7.8
).