Import flow measurements

Import flow measurements

HYMOS can treat two types of flow measuring methods:

• point velocity method
• moving boat method.

In both cases average velocities in a number of verticals in the cross-section are computed. From these velocities the discharge is computed by:

• mean-section method
• mid-section method.

The computational results are presented in tables and graphs. The condensed results can be stored in the database.

Mean and mid-section methods

The flow velocity is measured in verticals at distances w~i~ , i=1,m from a reference point on the river bank. The water depth at vertical i is denoted by d~i~ and the average flow velocity in that vertical by , see the underneath figure.

Figure 8.1: Definition sketch

Discharge computation

The discharge q~i~ in section i-1 to i is computed from:

• Mean-section method:
  
• Mid-section method:
  
  for i= 1,n+1, with:
  d~o~ = d~n+1~ = 0

The total discharge then follows from:

If the water levels at the beginning and at the end of the measurements are denoted by h~1~ and h~2~ respectively, then the representative water level h~Q~ is computed from:

• if |h~1~ - h~2~ | < 0.05 by: h~Q~ = ½ (h~1~ + h~2~ )

• if |h~1~ - h~2~ | ³0.05 by:
  To estimate the average velocity in a vertical in the cross-section from point measurements v~p~ at p * flow depth, following methods are available in hymos: (iso748 and iso/tr7178):

1. one-point method:

2. two-point method:

3. three-point method:

4. four-point method:

5. five-point method:

6. six-point method:

7. profile method, see Figure


Figure 8.2: profile
The last term in the equation assumes an extrapolation of the velocity profile to the bottom according to the power profile

Air- and wet-line correction

hymosallows you to include air- and wet-line corrections in case the depths are measured with a wire, see figure.

Figure 8.3: Air- and wet-line correction

The air-line correction cd is computed from:

cd = ab (sec (P) - 1)

where:

ab = air-line length
P = vertical angle in the sounding line

The vertical angle P is adjusted to the true vertical angle X in case the plane of the protactor makes a horizontal angle H with the direction of the flow according to:

X = arctan (tan(P)/cos(H))

The wet-line correction ge is expressed as a percentage aof the wet-line length de according to iso748-1979 (E) Annex C Table 2. So the true depth bf follows from:

bf = (ce - cd) (1 - a /100)

Moving boat method

In the moving boat method the combined boat- and river velocity is measured. To compute from this the actual river velocity either the angle between the current meter and the section line or the boat velocity is measured additionally.

Following methods are distinguished to compute the stream velocity in vertical i :

• v~i~ = v~v,i~ . sin a~i~
• v~i~ =

where:

v~i~ = point velocity in vertical i
v~v,i~ = combined boat and stream velocity
a~i~ = angle between current meter and section line
v~b,i~ = boat velocity at vertical i

The stream velocity is measured at a constant depth wd below the water surface. This velocity is transformed into the average velocity in the vertical by assuming a power law velocity profile:

with:
= average stream velocity in vertical i
c = power of power law velocity profile (5 £c £7)
d~i~ = flow depth at vertical i
wd = current meter depth

Width adjustment

Since the sum of the segment widths B _c may deviate from the actual river width B _m the computed segment widths are adjusted by the ratio B _m /B _c in the calculations.

Application

When the <Processing of Flow Measurements> function is selected the following form will be presented.

Station Codes

First you have to select a station code. This is the station, you want to process the flow measurement for. Select a station by clicking the checkbox in the stations list.

Current meters

Select a currentmeter by clicking the checkbox in the Current Meter list. Make sure the flow measurement is within the calibration period of your currentmeter.

Measurements

Select a flow measurement by double-clicking on the measurements textbox. The Windowsfile open dialogue will appear, from this dialogue you can select the flow measurement file.
The date of the selected flow measurement will be presented in the Measurement Date Textbox.

After you made your selections press <Execute>. The condensed current metering data and relevant geometric and hydraulic parameters of the river and cross-section as derived from the flow measurements will be stored in the database. This includes (see also Section Edit and Entry):

• date of measurement,
• measurement number,
• gauge zero,
• water level,
• discharge,
• gradient (dh/dt) in case of unsteady flow effects or fall (F) in case of backwater effects,
• width of cross-section at the water surface,
• wetted perimeter,
• cross-sectional area,
• average flow velocity, and
• hydraulic radius.

The results of the calculation are also presented in a report. By pressing <Report> from the functions tab you can analyse these results.