Calculation scheme:
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Determination of range of flow
A range of flowrates may be determined in dependence on a type of sewer system. In case of a separate
sewer system, it is determined from directive values of production of water and maximum flowrate that
depends on a type of operation, technology, type of pumping device.
So called peak coefficients of non-uniformity are considered as well. In combined sewer systems a
minimum flowrate of sewage and maximum value of rain water are calculated using several methods.
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Demands on wastewater quality parameters
Concentration of suspended solids should not be too high and the wastewater must behave as Newtonian
liquid. The size of suspended solids should not be greater than the value given as 0,8 x w where w is
the throat width. It is quite important to prevent the flume throat from sediment problems. Chemical
content should not result in forming a foam on the water surface ? then the use of Parshall flume is
possible.
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Calculation of downstream conditions
After determining a range of flowrates (Qmin for sewage, Qmax for rain water) it is necessary to take
into consideration the worst possible influence of backwater from the recipient. This must be carried
out by the water-surface calculation using a well established hydraulic method, e.g. the step-by-step
method for steady non-uniform flow. Using the coefficient of flooding 0,6 or 0,7 (depending on the flume
size) we can calculate the water level Hh before the Parshall flume as Hh=Hd/06. Performing this for the
whole range of discharges, the optimum size of the flume can be selected.
Hydraulic Design of Parshall flume
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Flow regime
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Supercritical flow in outflow channel The flow regime is dependent on the Froude number. If
the flow after the Parshall flume is supercritical, i.e. the Froude number is greater than unity (Fr
> 1), these are recommended conditions. On the other hand, the flow before the flume must be
subcritical, because then a critical flow occurs when water flows over the flume. A hydraulic jump is
formed downstream of the supercritical flow part. The level of the floor in the throat and in a
diverging section is lowered, however, at the end of Parshall flume it again coincides with the
channel bottom. The structure is design to withstand a 5 years recurrence discharge without flooding.
In case of frequent backwater curve from the recipient, it is recommended to calculate water-surface
profile using a suitable hydraulic method, e.g. the step-by-step method. When a supercritical flow is
achieved within the whole range of discharges, it is possible to use the Parshal flume with shorter
length, see illustrative example P2 MsachtU.dwg.
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Subcritical flow in outflow channel If the flow after the Parshall flume is subcritical (Fr
< 1), i.e. the flume is operating in ?drowned? conditions, both upstream depth and depth at the
throat must be measured in order to determine the discharge. The water-surface profile must be
calculated behind the flume. The submergence ratio, which is the ratio of the downstream total head to
the upstream total head over the flume is determined. Another criterion is the maximum coefficient of
flooding which for Parshall flumes P1 up to P4 is equivalent to 0,6, for bigger flumes is equal to 0,7
(see the Chart for Parshall flumes). The length of outflow channel section is chosen within a range 5
- 10m. .
Note: It should be pointed out that neglecting the local loss in the transition sections that join channels
of different size can result in errors in hydraulic calculations.
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Flow conditions in the approach channel
- ait is necessary for the flow before the flume to be subcritical. The most simple and robust design is
achieved when the value of the Froude number is within a range of flowrates below 0,7 {the Froude number
is defined as in which hm is the hydraulic depth}. If the flow in the approach channel is supercritical,
it must be changed by the hydraulic design of the flume into subcritical flow. The hydraulic jump must be
for all discharges located sufficiently before the flume and hence a suitable length of the approach
channel must be selected. Moreover, a sufficiently high approach velocity must be achieved in order to
prevent sedimentation problems. In flumes P1 to P3 these speeds are usually low and sedimentation does
occur. The accuracy of measurement is not affected, if sediments do not reach the point, where the
critical depth occurs (this means close distance before the throat). At this point the velocities are high
enough and therefore the sedimentation is rare.
