Regulating Structures: Structure
Regulating Structures: Structure
Regulating Structures: Structure
Structure
13.1 Introduction
Objectives
13.1 INTRODUCTION
The discharge released from a barrage into an irrigation canal has to be regulated
according to the demand and the available supply. The regulation of supplies as available
from the parent canal, to the different distributary canals has to be made as per the
rotation that is set out for the equitable distribution of water to the cultivators. For
effecting control, for this purpose, on the imgation system some structures are required to
he constructed at the barrage and also on the offtaking distributary canals.
Objectives
By the end of this unit you should be able to know about the basic composition, and
fundamental design parameters of the following hydraulic structures :
canal head regulators,
distributary head regulators, and
I cross regulators.
r'
SHEET PILE-, /
ds CONC.FLOOR $ 1 1 %
PIER ,
c CANAL
TOEW J
LOOSE APRON
t After fixing the crest level, waterway, number of spans and thickness of piers, the head
regulator is designed on the same principles as a weir. The canal is generally kept closed
when the highest flood passes down the river. This situation would lead to the worst static
condition and the floor thickness must be such as to withstand the uplift pressure under
this condition. The exit gradient for this situation should also be within safe limits. The
floor IS designed so as to counter the uplift pressure by its self weight, and also to
develop the necessary bending strength. For economic reasons, it may be necessary to
extend the piers upto the end of the floor to provide the required support to the upward
bending slab: thus, reducing the thickness of the slab.
In the trough portion where the jump would form, it is seen that the worst uplift situation
occurs when some water is passing into the canal. Hence, the safety of this part of the
floor should be checked for varying discharges inclusive of the nlaxirnum. Extending the
concrete floor upstream of the undersluices upto the end of the head regulator also
reduces the uplift pressures on the downstream floor of the regulator.
Access across the canal head regulator is provided by a bridge and for operating the gates
a working platform is incorporated in this arrangement.
Figure 13.4 :Section at B-B through Distributary Head Regulator (See Figure 13.3)
The distributary head regulator can be used as a metering structure for which suitable
head on crest versus discharge relationship can be determined for use by the field staff.
This relationship will help in monitoring and regulating the discharge in the distributary.
figure 135 :Section at A-A through a Cross Regulator (See Figure 13.3)
It is general practice to provide a cross regulator in the main canal downstream of the
offtaking canal. This permits the water level to be raised in the main canal, by means of
gates provided in the structure for which suitable grooves are provided (Figure 13.5).
when the supplies are low, to such a level that the offtaking canal can draw the desired
discharge. A cross regulator is necessary on all irrigation systems, because the
distributaries and field channels are required to be supplied with water according to a set
roster even when the parent canal is carrying low supplies.
C d Structures Economic and other special considerations dictate the provision of a bridge or fall along
with the cross regulator.
SAQ 2
(a) How is a distributary head regulator different froni a carla1 l~cadregulator'?
Explain in detail.
(b) What causes heavy silt to be drawn into a clislribulary cailal?
(c) How can you minirnise the entry of silt into offtaking canals? Give details
about these measures. Collect some actual field examples.
SAQ 3
What is a cross regulator and what are its functions'? Give one ficlil exainple
along with a sketch.
Q= c ~ B ~ P . . . (13.2)
where, Cd = coefficient of discharge, Regulating Stwctures
SAQ 4
(a) What are Ule design criteria for distributary head regulators? Give
.justifications.
(b) Describe Ult' design criteria for cross regulators giving the reasons for each
criterion.
1 L = length of crest,
4 HL = upstream water surface level - downstream water surface level, and
"I hd = downstream water surface level - crest level.
The wings have grooves for inserting and removing stoplogs. The road is taken across the
parent canal and the offtake by means of a bridge. The floor is designed on the basis of
Bligh's theory for the extreme condition which occurs when the offtaking canal is closed
while the parent canal is running full.
T
EL
_HALF PLAN
PITCHED
+--
DISTRIBUTARY
-
L SECTION
Figure 13.6 :Simple Distributary Head Regulator
C d Structures
13.7 VENTURI HEAD REGULATOR
The venturi head regulator consists of a flumed throat with suitable wings to attain the
normal bed width of the offtake downstream (Figure 13.7). Theoretically, the maximum
discharge passing through an open venturi flume occurs when critical tlow is obtained;
this flow is given by the expression :
DISTRIBUTARY
ROAD
PLAN
SECTION A - A
Example 13.1
Design a venturi head (regulator) given that:
(i) Discharge in parent canal = 12 cumec,
Bed width = 15 m,
Water depth = 1.5 m, and
Bed level = 100.00 m
li) Discharge in offtaking canal = I cumec,
Bed width = 3 m, and
Water depth = 0.6 m
Solutlon
FSL of parent canal = 100.00 + 1.5 = 101.5 m
Allowing for a drop in head equal to 0.2 m, and thus, keep the FSL of the
offtaking distributary at an RL of 101.5 - 0.2 = 101.3 m.
Bed level of offtaking canal = 101.3 - 0.6 = 100.7 m.
The width of the throat should at least not be less than the le, ;t of the following
values :
(a) (113)rd bed width of offtake, i.e., 1 m.
(b) As determined by
w= Q41.223
Canal Structures The following values of d corresponding to various values of Ware obtained as
shown below :
Any one of the above combination may be adopted. Say, the third one is adopted,
that is a throat width of 1.4 m and a head over the sill as 0.707 m. The sill level
will be 100.793 - 100.7 = 0.093 m above the bed of the offtake.
Set back of throat = 1.4 W + 0.6 = 1.4 x 1.4 + 0.6 = 2.56 m from FSL line of the
parent canal.
Length of the throat = 2.5 d = 2.5 x 0.707 = 1.768 in.
The floor should be designed by Bligh's theory with FSL acting in the main
canal, and with the offtake closed.
SAQ 5
What is n venturi head'? Can you think of its nnin atlvanlape over (%her
regulators?
SAQ 6
Design a venturl head (rcgulalor) for Lhe followliig c o n d ~ t ~ o: n ~
Parent canal, Q = 10 cumec, bed width = 12 nl and water deptli = 1.2 111,
bed level = 100 nl.
Offtaking canal, L) = 0.8 c h e c , bed width = 3 m, and water depth = 0.5 m.
13.8 SUMMARY
Canal head regulators situated at the barrage or weir site regulate the water entering the
main canal. The cross regulator assists in raising the water level in the parent channel to
feed the distributary canal when the supplies are low in the parent canal. The distributary
head regulator controls the discharge entering the offtaking canal from the parent canal.
Besides controlling the discharge these regulators also control the sediment entering the
main canal and the distributary canal.