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Add a flow measurement by filling in the text boxes and making your selection concerning the discharge computation method and units. Enter the number of verticals and per vertical the number of measuring points. After entering all values give a filename and press <Save> to save your data.
Types of 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):
- one-point method:
- two-point method:
- three-point method:
- four-point method:
- five-point method:
- six-point method:
- 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.
Layout of files
The Layout of the flow measurements file (.flw) is as follows
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