Canal Outlets 321n6f

Canal Outlets Department of Civil Engg UET Lahore, Pakistan

Introduction • An outlet is a hydraulic structure conveying irrigation

water from a state owned distributary to privately owned water course.

• The outlets are large in number as compared to other

irrigation structures in an irrigation system and hence their design and type has maximum bearing on the equitable distribution of water.

• Therefore proper design of outlet is of utmost importance.

Canal Outlet

Tail cluster

Pakka nakka (Turnouts)

Essential Requirements of an outlet •

An outlet should be strong and be without movable parts to minimize tempering

•

Tempering by cultivators should be readily detectable

•

The outlet must carry its fair share of silt from parent channel

•

It should be able to work with small working heads

•

It should be simple so that construction is easy

•

The total cost of installation and maintenance should be minimum

Types of Outlets • Non-Modular Outlets – It is one in which the discharge is dependent upon the difference of head in water course and parent channel. – Hence, a variation in either affects the discharge.

• Semi-Modular (Flexible) – It is one in which the discharge depends upon the water level in distributary only and is independent of water level in water course. – This is achieved by producing hydraulic jump within the flume length.

• Modular (Rigid) Outlets – It is one in which the discharge is independent of the water level in water course and parent channel. – It can be fixed for any discharge value. This is achieved by creating a free vortex and destroying any extra head more than allowed for in the designed discharge.

Types of Outlets Semi-Modular

Modular

(Flexible)

(Rigid)

Non-Modular Pipe or Barrel Type, with drowned flow at d/s Scratchley outlet (a pipe type, with a cistern d/s)

Weir-Type

Orifice-Type (OSM)

Harvey Stoddard outlet

Kennedy Gauge Outlet

Crumps open flume outlet

Crumps adjustable proportion al module

Jamrao type open flume

(Crumps APM)

Adjustable orifice semi module

With moving parts Without moving parts

Gibbs module Khanna module Ghafoor rigid flume module

Ref: Mahbub SI, Gulhati ND (1964), Irrigation outlets, Paper No 264, Pakistan Engineering Congress, Lahore.

Pipe outlet

Ref:. http://www.idoub.com/doc/113998814/6/CANAL-OU

Harvey’s Outlet

Gibb’s Module

Characteristic of Outlets • Flexibility: It is defined as the ratio of rate of change of discharge in outlet to the rate of change of discharge in parent channel. F = (dq/q)/(dQ/Q) = (m/n)(D/H) (H/D) is the setting of an outlet

n is exponent of discharge eq (Q=K1Dn) for canal & m is exponent of discharge eq (q=K2Hm) for outlet. For Trap. channel with ½:1 side slope, n is 5/3, and for open flume outlet m is 2/3. Therfore F =0.9 D/H. If we set the crest of outlet at 0.9D depth below water level, the F will be 1 & the outlet will be proportional outlet.

• Flexibility is the capacity of an outlet to vary its discharge with the change in the discharge of the distributary.

• If F=1 • If F>1 • If F<1

Proportional Hyper-proportional Sub-proportional

Characteristic of Outlets • Sensitivity: It is the ratio of rate of change of discharge of an outlet to the rate of change in the level of distributary water surface, i.e. normal depth of channel S = (dq/q)/(dG/D)

• Here, S is the sensitivity and G is the gauge reading of a

gauge which is so set that G = 0 corresponds to the condition of no discharge through the outlet (i.e., Q0 = 0).

• Sensitivity can also be defined as the ratio of the rate of change of discharge of an outlet to the rate of change of depth of flow in the distributary channel.

S = nF n = 5/3 for wide trapezoidal channel with side slope ½:1

Characteristic of Outlets • Efficiency: It is equal to the ratio of the head recovered (or the residual head after the losses in the outlet) to the input head of the water flowing through the outlet.

• Minimum Modular Head: it is the minimum head required for the proper functioning of the outlet as per its design.

• Modular Limits: The extreme values of any parameter at which a module or a semi module ceases to be capable of acting as such.

• Modular Range:

The range of conditions between the modular limits within which a module or semi module works as designed.

Characteristic of Outlets • Coefficient of Discharge:

In order to use the outlet as a measuring device the coefficient of discharge should remain constant in the full modular range.

• Silt Drawing Capacity: It is vital that the outlets should draw their fair share of silt. This avoids silting or scouring and consequently remodeling of distributary.

• In a distributary system the absorportion losses are generally taken as 10-15% and therefore the silt conducting power of outlets should be around 110-115% as compared to 100% of distributary to enable them to draw their proportional share.

Characteristic of Outlets • 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.

• Immunity from Tempering:

There is tendency on the parts of cultivators to draw more than their lawful share of water by tampering with the outlets. Therefore the outlets must be tamper proof.

Selection of Type of Outlet • A rigid module (Modular) or a Flexible module (Semimodular) with a constant coefficient of discharge is the best selection if the discharge and the water levels are constant in the distributary and necessary working head is available.

• But the problem of choice becomes quite complex when both the discharge and levels are likely to change.

• The following points may be noted; – For a temporary discharge variation a proportional semi module is desirable to distribute both excess or deficiency in the parent channel. – Seasonal variation in the slope require the use of outlets of low flexibility, i.e., sub-proportional.

Selection of Type of Outlet – For channels running with full supply for a certain period and remaining closed for certain other periods, i.e. rotational running, it is desirable to have hyper-proportional or high flexibility outlets in the head reaches. – The silt drawing capacity of outlet must be 110-115% assuming a 10-15% loss in parent channel. – In general rigid modules are desirable in the following circumstances • Direct outlets on a branch canal subject to variation in supply • In channels which sometimes carry extra discharge for specific reasons like leaching.

Open Flume Outlets • This is a smooth weir with a throat constricted sufficiently long to ensure that the controlling section remains with in the parallel throat for all discharges up to the maximum

• Since a hydraulic jump forms at the control section, the

water level of the watercourse does not affect the discharge through this type of outlet. Hence this is a semi-modular outlet.

• This type of structure is built in masonry, but the controlling

section is generally provided with cast iron or steel bed and check plates.

Open Flume Outlets

Open Flume Outlets • The discharge formula for the open flume outlet is given as:

Q = C Bt H3/2

• Where: Q is related to the coefficient of discharge, C, as

given in the table below; Bt is the width of the throat; and H is the height of the full supply level of the supply channel above the crest level of the outlet in ft. Bt C 0.2 ft – 0.29 ft, 0.3 ft – 0.39 ft Over 0.4 ft

2.90 2.95 3.00

Crump’s Open Flume Outlet Design • Data: • Outlet Discharge = q = 4 cfs • Full Supply Depth = D = 3.5 ft • Working Head = Hw = 1.0 ft • Discharge of Distributory = Q = 60 cfs • Design – 1. Canal Section – According to Lacey’s theory, design of distributory comes out to be Side Slope 1:0.5

D=5’ B=38’

Crump’s Open Flume Outlet Design • Setting: G= Setting of outlet

= 0.9 D

= 3.15 ft Head above crest of outlet = 3.15 ft

• Throat Width: • q= CdBtG3/2 • q= 2.9BtG3/2 • 4=2.9Bt(3.15)3/2

Assuming Cd = 2.9

• Bt= 0.2462 = 0.25 ft • Note: The value of Bt lies in between 0.2 to 0.29 ft. (min. is 0.2ft)

Crump’s Open Flume Outlet Design • Length of Crest: Length of Crest = 2.5 G = 2.5x3.15= 7.875=7.9 ft

• Radius of Transition: R = 2 G=6.30 ft

• Setting Back:

This distance by which the wall parallel to the distributary axis is to be set back bears the same ratio to the width of distributary as the discharge of the outlet to that of the distributary. Setback/width of distributary = q/Q Setting Back = 2.53 ft

Crump’s Open Flume Outlet Design • Transition in bed: The transition in bed is given by curve of radius= 2 G = 2x3.15=6.3 ft

• D/S Transition: The slope of d/s glacis (1:10 to 1:15) is not defined as it depend upon the bed level of the water course.

• Minimum Modular Head: MMH = 0.2G =0.2x3.15 = 0.63 ft < working head=1.0 ft (OK)

Crump’s Open Flume Outlet Design • Flexibility:

(m=3/2, n=5/3)

F=(m/n) (D/G) =(3/2/5/3)(1/0.9)

•

=1 Sensitivity: S = nF = 5/3 F = 5/3

• Efficiency: =100xHead recovered/Head put in =100x(Head put in-Working head)/Head put in =100 x (3.15-1)/3.15= 68.25%

Adjustable Proportional Module (APM) • In this type of outlet, a cast iron base, a cast iron • • • •

roof block and check plates on either side are used to adjust the flow and is set in a masonry structure This outlet works as a semi-module since it does not depend upon the level of water in the watercourse. The roof block is fixed to the check plates by bolts which can be removed and depth of the outlet adjusted after the masonry is dismantled. This type of outlet cannot be easily tampered with and at the same time be conveniently adjusted at a small cost. The APM is the best type of outlet if the required working head (MMH) is available and is the most economical in adjustment either by raising or lowering the roof block or crest. However, it is generally costlier than the other types of outlets and also requires more working head.

Schematic Diagram of Outlet Outlet discharge = q = Cd Y Bt (Hs)1/2

Roof Block

FSL

Full supply depth in parent channel = D Working head = Hw Discharge of canal = Q Width of throat = Bt Depth of water above crest u/s = G Flexibility = F Min. Modular Head = Hm Distance from tip of roof block to FSL = Hs Hs+Y = G, Hs=Y, G = 0.6 D for proportional

D

G

Hs V

Hm J Y

Tail Cluster

• When the discharge of a secondary, tertiary or quaternary canal

diminishes below 150 l/s (about 5 cusecs), it is desirable to construct structures to end the canal and distribute the water through two or more outlets, which is called a tail cluster. Each of these outlets is generally constructed as an open flume outlet

Thank you