YSI SonTek RiverSurveyor, Системное руководство

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SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
68
5.2. Velocity Profile Extrapolation
Previously, Section
5.1
( Discharge Calculations ) described the concept of measured and un-
measured areas. The RiverSurveyor program computes discharge in the unmeasured areas based
on the water current velocities in the measured areas. In the riverbank edges, the velocities are
based on the adjacent profiles. However, for the top and bottom unmeasured areas, the technique
for velocity calculations is different. This section explains how the water velocities in the top
and bottom areas are estimated.
5.2.1. Estimating Top/Bottom Water Velocities
Figure 5-6
shows a water velocity profile through the water column. The unmeasured parts of the
profile are marked – transducer depth, blanking distance, bottom estimate.
A technique known as Velocity Profile Extrapolation is generally the method used in the River-
Surveyor program to calculate the velocities above and below the measured area. A power law
velocity profile proposed by Chen (1991), and Simpson and Oltmann (1990), is used to calculate
these velocities:
b
z
z
u
u
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
• =
0 *
5 . 9
Equation 5-1. Velocity Profile Power Law
where u is the velocity at height z measured from the river bottom, u
*
is the bottom shear veloc-
ity, z
0
is the bottom roughness height, and b is a constant (equal to 1/6 according to Chen, 1991).
Equation 5-1
assumes the currents in the profile are traveling in approximately the same direc-
tion. In situations where this may not be the case (e.g., Stratified Flows – §
4.3.4
), one of the other
methods available in RiverSurveyor should be used.
5.2.2. RiverSurveyor Software and Velocity Profile Extrapolation
By default the RiverSurveyor program uses the 1/6
th
power law velocity profile (described
above) using the entire profile to calculate velocities in the top and bottom unmeasured areas.
If you need to change this method of velocity profile extrapolation in RiverSurveyor , select
Processing|Velocity Profile Extrapolation
. This will display the
Velocity Profile Extrapolation
dialog box (
Figure 5-7
).
Figure 5-6. Unmeasured areas of a water velocity profile