YSI SonTek RiverSurveyor, Системное руководство

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SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
56
Obviously, a large number of points to measure translates into high operational costs in crew la-
bor. It also translates into high safety risks for the crew, especially during flood events. In addi-
tion, for some situations such as very wide rivers under tidal influence, the long time required to
do a cross-section makes it difficult to perform short-time scale monitoring of flow changes.
Typically, discharge measurements are done over several different stage/velocity conditions, and
the results are used to develop the discharge rating. This approach works well as long as: the lo-
cation of the continuous measurement (i.e., velocity and/or water level) is chosen correctly, the
continuous discharge values remain within the calibrated range, and the morphology of the river
does not change significantly (e.g., not much scouring or accumulation of sediments and debris).
Under these conditions this discharge rating method simply needs to be periodically re-calibrated
with instantaneous discharge measurements.
4.1.3. How does RiverSurveyor make instantaneous discharge measurements?
RiverSurveyor systems use a SonTek Acoustic Doppler Profiler (ADP) mounted on a vessel
(e.g.,
Standard and Mini RiverSurveyor systems) or a catamaran (e.g., RiverCat system) to col-
lect the data used to make an instantaneous measurement of river discharge. The ADP measures
the water current velocity simultaneously at multiple depth locations through the water column.
As the vessel (or catamaran) moves from one side of the river to the other, the ADP measures the
water velocities over
most of the cross-section. We say “most”, because there are two thin layers
of water (and in many cases, two areas along the river banks) that are unmeasured by the ADP:
•
The first unmeasured layer is located at the water surface (the distance from the water sur-
face to the top of the first cell).
•
The second unmeasured layer runs along the bottom from the last valid cell to the riverbed.
Note : In many cases, two other “unmeasured” areas exist – each riverbank. However,
estimates can generally be made for these areas in the RiverSurveyor program. The
grid in
Figure 4-6
shows the “typical” measurement areas encompassed by the grid.
By taking the vessel from one side of the river to the other, the ADP can quickly provide a virtu-
ally complete cross-section of the velocity field. This velocity field can then be combined with
the bathymetry of the section to measure instantaneous discharge. The concepts and methods in-
volved with discharge measurement are described in detail in
Section 5
–
Principles of Discharge
Measurement
.
By being able to measure velocities at many depth levels simultaneously, the ADP saves time by
eliminating the need to lower a single-point current meter to different depths. However, the ca-
pabilities of the SonTek ADP go much further. Besides measuring water velocities, the ADP is
capable of measuring:
•
The distance from the instrument to the bottom
•
The velocity of the instrument with respect to the bottom
This means you do not need to stop the boat, or maintain a constant course heading, while trav-
ersing the river. As the boat moves from one side of the river to the other, the ADP continuously
samples the water velocity field, the bottom depth, and the displacement of the boat (by integrat-
ing the boat velocity). As such, all the information needed to measure instantaneous discharge is
gathered by the ADP and used by the
RiverSurveyor program to compute the total discharge.
As mentioned, to determine discharge across a river using an ADP, you only need start on one of
the riverbanks, and “transect” the boat to the opposite side. During the transect, the boat does not