The design of a proportional outlet (F1) is thus governed by the
condition
H/Dm/n. (9.28)
The ratio H/Dis called the setting ratio of the outlet.
For an orifice or pipe-type proportional outlet (m1/2) from a
trapezoidal channel (n5/3), the setting ratio H/Dis 0.30, whereas for an
open flume (Crump type, m3/2) proportional outlet it is 0.90.
(a) Classification of outlets
- Non-modular type.This is an outlet in which the discharge depends
upon the difference in level (H) between the water levels in
the distributing (supply) channel and the water course (field
channel). A submerged pipe outlet (Fig. 9.25), or an orifice, is a
common example of this type. Referring to the pipe outlet layout in
Fig. 9.25, any conventional pipe flow solution will give the outlet dis-
charge, e.g. an iterative procedure combining Darcy–Weisbach and
Colebrook–White equations.
- Semi-modular or flexible type.The discharge through this type of
outlet (semimodule) is only affected by the change in the water level
of the distributing canal. A pipe outlet discharging freely, an open
flume outlet and an adjustable orifice outlet are the common types of
flexible outlet. As the discharge through this type of outlet is
independent of the water level in the field channel, interference by
farmers is minimal. The pipe outlet is usually constructed with a
flexibilityF1, so that the outlet discharge changes by a smaller per-
centage than the change in the distributing channel discharge. An
open flume type outlet is shown in Fig. 9.26. The entrance is so
designed that the flume takes a fair share of the silt. The discharge
through this type of outlet (e.g. Crump) is given by
QCbH3/2(m^3 s^1 ) (9.29)
wherebis the throat width (in m), His the head over the crest (in
m), and Cequals 1.71 (theoretically 0.544g1/2; actual range 1.60–1.66).
An adjustable orifice semimodule (AOSM) type outlet, consisting of
an orifice followed by a gradually expanding downstream flume, is
shown in Fig. 9.27. This type of outlet is commonly used in the
Indian subcontinent, and is considered to be one of the best forms of
outlet.
- Modular type.This type of outlet (rigid or invariant module) delivers
a constant discharge within set limits, irrespective of the water level
fluctuations in the distributing channel and/or field channel. Gibb’s
402 DIVERSION WORKS
