YSI SonTek RiverSurveyor, Системное руководство

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SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
61
Most profile extrapolation routines assume the entire water column is heading in the same direc-
tion. In the case of bi-directional flow, this can create errors in the extrapolation of velocities in
unmeasured areas because both water current magnitude and direction are computed incorrectly.
This error will be so significant that the calculated direction will be incorrect by 180 degrees! By
default, RiverSurveyor will extrapolate current velocities using a power-fit curve applied over
the entire profile.
If you are using RiverSurveyor to calculate discharge, you must account for stratified flows. The
RiverSurveyor program contains a provision for these situations. There are a few ways that Riv-
erSurveyor can identify whether you are in an area with bi-directional flow:
•
When current directions are displayed in the
Contour Graph
(lower left window in River-
Surveyor ), an area of the cross-section will show directions that are about 180° from the rest
of the cross-section. This area will be typically near the horizontal center of the cross-
section, though its exact location will vary with river geometry and tidal elevation.
•
When
View|Profile Graph Variable
is set to
Profile Extrapolation
, the top and bottom portions
of the
Profile Window
will have different signs (e.g., the top of the line will be distinctly
positive and the bottom part of the line will be negative, or vice versa).
If you are performing a discharge measurement in areas of bi-directional current flow, correct
extrapolation of current velocities must be done in the
Velocity Profile Extrapolation
dialog box
(
Processing|Velocity Profile Extrapolation
; §
5.2.2
). For bi-directional flow conditions, select
In-
dependent Top and Bottom
. Select
Use range of cells
for both the
Top
and
Bottom
options. Enter
the number of cells ( n ) that best fits the flow conditions for the top water layer as:
Top
–
Use n
cells
. Enter the number of cells ( n ) that best fits the flow conditions for the bottom water as:
Bot-
tom
–
Use n cells
. Click
OK
to close the dialog and recalculate the velocities.
4.3.5. Wind
The effects of wind on water currents in rivers are generally low and localized to a thin surface
layer. However, in shallow rivers, the effect of wind on current velocities can be magnified. This
can be particularly enhanced in shallow, wide rivers where wind can influence current velocities
and where unmeasured areas make up a much greater percentage of the total cross-sectional area.
Typically, computer modeling of rivers and streams involves the use of two-dimensional (assum-
ing either lateral or vertical homogeneity) or three-dimensional computer models. SonTek’s
software is designed to provide input into both these types of models. Data files that can be input
into the model can be exported from either
RiverSurveyor or ViewADP programs. See §
4.2
(
Computer Modeling and Verification
) for more information.