• In order to design the flume properly, it is necessary to determine a minimum length of a prismatic channel before the flume itself (rectangular section and a transition section). Moreover, it is also necessary to determine a minimum distance of Parshall flume from a channel bend, confluence, shaft drop, hydraulic jump etc. These distances are dependent on a discharge, pipe diameter and/or channel dimension, a Parshall flume bottom level and a throat width. Considering a large number of variables, only a simplified relationship dependent on a throat width is used and the distances are only approximate values. More accurate values can be obtained upon request from Pars-Aqua pty.Ltd

...........................Stilling length /m/ .................... length from Parshall flume to bend, confluence, floodgate,
.................................................................................lower outlet, overflow, etc.

...........................Length of prismatic channel /m/ .... dlength of a rectangular channel section before the Parshall
...........................................................................................flume and a transition section

...........................Length of transition section /m/ .... length of section of widening/narrowing, change of cross-
....................................................................................sectional shape e.g. from rectangular to circular and vice versa

Following three tables show illustrative examples on approximate data for stilling lengths, length of prismatic channels and for lengths of transition sections. Optimum method of construction of this measuring structure can be based on a schematic given below.

Stilling length of conduit

Conduit length after the bend, confluence etc.e	12 x throat width
Conduit length behind the hydraulic jump, sluice gate, overflow structure, shaft drop, valve gate etc.	30 x throat width

Length of prismatic flume

Parameter/type of flume	P1	P2	P3	P4	P5	P6	P7	P8	P9
Length of prismatic flume (m)	0,05	0,1	0,5	1,6	2,4	3	4	6	8

The lenth can vari according incoming hydraulic conditions table above is only aproximately value. The design of manhole hs to be made individually ( provide Pars aqua - free ).

We can send you designs of manholes for all parshall flumes.

Length of transition section

- change of flow cross-sectional shape When a transition between different conduit shapes (for instance from circular to rectangular cross section) must be constructed in order to achieve a gradual widening we require that the angle between longitudinal channel axis and wall did not exceed 100. This will prevent the creation of wakes and disturbance from uniform velocity distribution field. In case of converging of the flume, this angle is higher and its values are shown in Table 3. Narrowed flume is then directly connected with the beginning of Parshall flume.

Parameter/type of flume		P1	P2	P3	P4	P5	P6	P7	P8	P9
angle of narrowing between Cephalo-caudal axis of flume and walls	maximal angle of converging	900	900	900	450	300	150	110	110	110
	recommended angle of converging	900	600	450	250	150	110	110	110	110

When a determination of stilling section is not correct, surface waves and non-uniform distribution of velocity field occur. It results in a decrease in accuracy because of a poor hydraulic design and false measurement of flow depth. Small surface waves cannot necessarily create a significant error in measuring and therefore their influence is necessary to verify by an independent method, e.g. using hydrometry. In case of proof of unacceptable errors in measurements, it is essential to stabilise the flow. For this purpose a variety of breakers or rectifiers can be used. If possible, rising the flume bottom can solve these problems. If inevitable, a new flume is installed.

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(c) 2003, PARS Aqua, s.r.o.