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5.8.6
Flow calculation rectangular weir
Function
The block calculates the amount of water (volume flow) flowing over a rectangular overflow
weir depending on the measured flow over height according to the formula of Poleni (see
also DIN 19557).
Q
μ
b
g
h
Figure 5-8
The formula applies only to the complete flow without influence of bottom water on the flow
amount (backwater).
The formula contains no reduction factor that takes into account influent speed, stream in
form and direction or constructions features. Note that such influences can result in
significant deviations between the calculated result and the actual flow.
The flow coefficient μ is the correction value for the deviations between the actual and
calculated flow, For rectangular weirs the following values are specified depending on the
shape of the weir crest:
Table 5- 2
Crest shape
Wide, sharp-edged
Wide, rounded edges
Completely rounded wide flow
Tilted back weir flap with rounded edges of the
weir body
Sharp-edged with ventilation of the stream
Rounded, vertical upstream slope, inclined
downstream slope (standard profile)
Roof-shaped, rounded crest
* Schröder, Euler, Schneider, Knauf: Grundlagen des Wasserbaues, Hydrologie, Hydraulik und Was-
serrecht; Werner-Verlag Düsseldorf (1994)
RTU3030C
Operating Instructions, 09/2017, C79000-G8976-C382-04
Flow amount [m
/s]
3
Flow coefficient (without dimension without including g)
Width of the inflow [m]
Gravitational acceleration [m/s
Flow over height (water level in the head water - weir height) [m]
Formula from Poleni for flow calculation of rectangular weirs
Flow coefficient μ for horizontal rectangular weirs *
]
2
Illustration
Programming
5.8 Counter and summation blocks
0.49 .. 0.51
0.50 .. 0.55
0.65 .. 0.73
0.62 .. 0.64
μ
0.75
0.79
237
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