Canal Outlets
Canal Outlets
Canal Outlets
Outlet discharge = q
Full supply depth in parent channel
=D
Discharge of canal = Q
Hs = distance between canal FSL
and lowest point of the roof block
Width of throat = Bt
Depth of water above crest u/s = G
Flexibility = F
Min. Modular Head = Hm (Working head)
Definitions
• Criteria of performance for various outlets
Flexibility
The ratio of the rate of change of discharge of an outlet (dQ0/Q0) to the rate
of change of discharge in the distributary channel (dq/q) is termed
Flexibility. Thus, F =
Where F is the flexibility; q is the discharge through the outlet and Q is the
discharge of the distributary or canal. For the field channel q = KHm ; where
K is constant; m is outlet index; H is the head at outlet. K and m depend on
the type of outlet.
Now dq = mKHm-1dH ; = = m (i)
Since any change in the water depth results in an equal change in the head causing
flow, then dH = dD. Thus the expression for flexibility become F =
Thus setting is defined as the ratio of the depth of sill or the crest level of the
module below the full supply of the distributing channel to the full supply depth of
the distributing channel. Thus Setting =
Definitions
• Inthecase of proportionate outlet setting equal to the ratio of outlet index to
channel index i.e. setting =
For a channel of trapezoidal shape Q α D5/3 so n= 5/3; similarly for orifice
type outlet q α H1/2
Then setting = = = 0.3. Hence the pipe out let to be proportional, the outlet
is set at 0.3 times the depth below the water surface
• Hyper Proportional Outlet: A hyper proportional outlet is the one in which
the flexibility is greater than one i.e. the discharge in the outlet changes by a
larger percentage than the percentage in the discharge of the distributing
channel. Thus, for a hyper proportional outlet F > 1.
• 1 or if the setting is higher than required for proportionality then it will
be hyper proportional.
Definitions
•• Sub Proportional Outlet : A sub proportional outlet is the one in which the
flexibility is less than one i.e. the discharge in the outlet changes by a smaller
percentage than the percentage change in the discharge of the distributing
channel. Thus, for a sub proportional outlet F< 1.
• 1 or if the setting is lower than required for proportionality then it will
be sub proportional.
• Sensitivity: it is defined as the ratio of the rate of change of discharge of an
outlet to the rate of change in the level of the distributing surface, referred
to normal depth of the channel. Thus
• S = (i)
• where S is the sensitivity of the outlet;
• q = Discharge through the outlet ;
Definitions
• Dq = Change in the discharge of the outlet ;
G = Gauge reading, so set that G = 0 when q = 0 ;
D = depth of water in the distributing channel;
dG = dD
• S = where = (ii). Comparing eq(i) and (ii) S = nF. It means that the sensitivity of a
rigid module is zero.
• Efficiency: Efficiency is defined as the ratio of the head recovered to the head put in.
Less is the working head required for functioning of the outlet, more will be its
efficiency
• Drowning Ratio: It is the ratio between the depths of water level over crest on the
downstream and upstream of the module.
• Minimum Modular head: The minimum modular head or loss is the minimum loss of
head or the differences between the upstream and downstream water levels which is
essential to maintained to enable the module to pass its design discharge.
Definitions
• Modular Limits and Range: The modular limits of an outlet are the upper
and lower limits of any one or more factors beyond which an outlet is
incapable of acting as a module or semi module. Modular range is the
range between the modular limits. It is the range of various factors which
a module or semi module works as designed.
• Adjustability: The adjustment of module may range from complete
reconstruction to the provision of some mechanical arrangement
by which readjustment can be made at little cost. Readjustments are
required in view of the revision of areas under command and because
of change conditions in the distributary.
outlets are suitable for low head conditions. Examples of non modular
outlets are submerged pipe outlets, masonry sluice and orifice etc.
Non Modular Outlets
•• One
of the example of non modular outlet is pipe outlet. The diameter of pipe varies
from 10 to 30 cm and are laid on a Concrete foundation to prevent uneven
Settlement and leakage. They are generally Fixed horizontally at an angle to the
direction of flow. The head loss H through the outlet is given by
f friction factor (0.005 for clean iron pipe & 0.01 for slightly encrusted iron pipes
A is cross section area (m2); d is the pipe diameter (m) and L is the pipe length (m)
Non Modular Outlets
••The discharge through the outlet is given as where k = coefficient of discharge
= 6 for submerged case.
watercourse and by lowering the water level in it. This will increase the head
Minimum modular head is 0.22 H o, where Ho is the depth of water over the
center of the orifice.
The water discharges at atmospheric pressure from bell mouth orifice into
the truncated cone. The water is led further through cast iron expansion
pipe to a concrete pipe and from there to the water course.
Types of Canal Outlets
Discharge through this module can be found out as following:
𝑞=𝐶𝑎
√2 𝑔 𝐻 𝑜
a = Cross section area at the throat ;Ho is mentioned in the figure
The discharge formula for the Crump’s open flume outlet is given
as:
Q = C Bt H3/2
selection if the discharge and the water levels are constant in the distributary and
necessary working head is available. But the problem is become more complex when both the
proportional.
• For channels running with full supply for a certain period and remaining closed for some
other period i.e. rotational running it is desirable to have hyper proportional or high flexibility