Distribution and Measurement Structures for Canal Flows (Part - 2) - Civil Engineering (CE) PDF Download

 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 (Figure 6). 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. 

Figure 6. Open Flume 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. The open flumes can either be deep and narrow or shallow and wide in which case it fails to draw its fair share of silt. Generally, this type of outlet does not cause silting above the work, except when supplies are low for a considerable length of time.  The silt which gets accumulated gets washed away during high supplies. 

The open flume outlet is also cheaper than the Adjustable Proportional Module (APM), discussed below.  The discharge formula for the open flume outlet is given as: 

Q = C Bt H^3/2           (3)

Where Q (given in l/s) is related to the coefficient of discharge, C, as given in the table below; Bt is the width of the throat in cm; and H is the height of the full supply level of the supply channel above the crest level of the outlet in cm.  

The minimum head required to drive the outlet is about 20 percent of H. 

Adjustable Proportional Module (APM)     

There are various forms of these outlets but the earliest of them is the one introduced by E.S. Crump in 1992. In this type of outlet, a cast iron base, a cast iron roof block and check plates on either are side are used to adjust the flow and is set in a masonry structure (Figure 7). This outlet works as a semi-module since it does not depend upon the level of water in the watercourse. 

Figure 7. Plan and Section of Adjustable Proportional Module

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 cast. 

The roof blocks may also be built of reinforced concrete.  The face of the roof block is set 5 cm from the starting point of the parallel throat. It has a lamniscate curve at the bottom with a tilt of 1 in 7.5 in order to make the water converge instead of a horizontal base which would cause it to diverge. The cast iron roof block is 30cm thick.

As such, the APM is the best type of outlet if the required working head 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.   

The discharge formula for this type of weir is given as:  

Q = C B_t H ₁(H₂)^1/2           (4)

Where Q (given in l/s) is related to the coefficient of discharge, C, which is taken equal to around 0.0403; B_t is the width of the throat in cm; H₁  is the depth of head available, that is the difference between the supply channel full supply level and the outlet bed (crest) level;  and H₂  is the difference between the supply channel full supply level and the bottom level of the roof block (Figure 8) . 

Figure 8, Details of Block for Adjustable Proportional Module

The base plates and the roof block are manufactured in standard sizes, which with the required opening of the orifice are used to obtain the desired supply through the outlet. 

Tail clusters  

When the discharge of a secondary, tertiary or quaternary canal diminishes below 150 l/s, 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 (Figure 9).

Flow measurement in canals 

The available water resources per person are growing scarcer with every passing day. Although a region may not face a net reduction in water resources, the increasing population of the area would demand increased food production and consequently, agricultural outputs.  Such that an equitable distribution of water is ensured as far as possible with a command area, it is required to measure water at important points in the canal network. Measurements may also help in estimating and detecting losses in the canal. 

Further, at the form level, advanced knowledge of soil properties and soil moisture / plant relationships permits irrigation systems to be designed so that water can be applied in the right amount and at the right time in relation to the soil moisture status thereby obtaining maximum efficiency of water use and minimum damage to the land. This knowledge can be utilized most effectively only by reasonably accurate measurement of the water applied. 

The amount of water being delivered to a field of an irrigator should also be measured in order to make an assessment of water charges that may be levied on him. If the charge to the user of canal water is based on the rate flow, then rate-of-flow measurements and adequate records are necessary. Charges on the basis of volume of water delivered necessitate a volumetric measuring device.  Ideally, water flow should be measured at intakes from storage reservoirs, canal head works, at strategic points in canals and laterals and at delivery points to the water users.  The most important point for measurement is the form outlet which is the link between the management authority of the canal system and the user. 

The degree of need for a measuring device at the outlet varies according to the delivery system employed.  Delivery on demand usually relies up on the measurement of water as a basis for equitable distribution as well as for computing possible water charges.  Where water is distributed by rotation among farmers along a lateral (or distributary or minor canal) and the where the amount of water supplied to each farmer may be different, a measuring device at the turnout is required.  On the other hand, if farmers along a lateral receive water on the basis of area of land or crops irrigated measurement is not entirely necessary, but may still be desirable for other purposes, such as improvement of irrigation efficiency.  Similarly in all systems based on constant flow, measurement is not entirely necessary but may be advantageous. 

Where several farmers share the water of each outlet and the flow in the canal fluctuates considerably, each such outlet should be equipped with a measuring device, even if equitable distribution among outlets is practiced, so that each group of farmers will know the flow available at any one time from their respective outlet. 

Amongst the methods and devices used for measuring water in an irrigation canal network, the weir is the most practical and economical device for water measurement, provided there is sufficient head available. Measuring flumes are also used in irrigation networks and their advantage are smaller head losses,  reasonable accuracy over a large flow range, insensitivity to velocity of approach, and not affected much by sediment load. Propeller meters are used in many countries and are particularly suited to systems where no head loss can be permitted for water measurement and where water is sold on volumetric basis. For water measurement in small streams, particularly in field ditches and furrows and where head losses must be small, the deflection or vane meter has proved to be a useful device. Only the weir and the standing wave (hydraulic jump) type flume are discussed in this lesson as these are most commonly used. 

Weirs 

Weirs have been in use as discharge measuring devices in open channels since almost two centuries and are probably the most extensively used devices for measurement of the rate of flow of water in open channels. Weirs may be divided in to sharp and broad crested types. The broad crested weirs are commonly incorporated in irrigation structures but are not usually used to determine flow.  The types of sharp crested weirs commonly used for measuring irrigation water are the following: 

Sharp crested rectangular weir 

 A general view of this type of weir is shown in Figure 10. 

FIGURE 10. General view of a sharp created rectangular weir

Amongst the many formulae developed for computing the discharge of rectangular, sharp crested weirs with complete contraction, the most accepted formula is that by Francis and is given as: 

Q = 1.84 (L – 0.2H) H^3/2                (5)

Where Q is the discharge in m³/s; L is the length of the crest in meters; and H is the head in meters, that is, the vertical difference of the elevation of the weir crest and the elevation of the water surface in the weir pool.

Sharp crested trapezoidal (Cipolletti) weir 

 A general view of this type of weir is shown in Figure 11. 

The discharge formula for this type of weir was given by Cipoletti as: 

Q = 1.86 L H^3/2               (6) 

Where Q is the discharge in m³/s; L is the length of the crest in meters; and H is the head in meters. The discharge measurements using the above formula for the trapezoidal weir are not as accurate as those obtained from rectangular weirs using the Francis formula.

 Sharp sided 90⁰ V-notch weir

A general view of this type of weir is shown in Figure 12. 

FIGURE 12. General view of 90⁰ v-notch weir

Of the several well known formulae used to compute the discharge over 90⁰ V - notch weirs the formula recommended generally is the following: 

Q = 8/15 (2gCd)^1/2   H ^{5/2                     (7)}

Where Q is the discharge in m³ /s; g is the acceleration due to gravity (9.8m/s²); C_d is a coefficient of discharge; and, H is the head in meters.  The value of C_d varies according to the variation of H and can be read out from (Figure 13). 

FIGURE 13. Variation of c_d for 90⁰ V-Notch weir with H²/A

Each of these weirs has characteristics appropriate to particular operating and site conditions.  The 90⁰ V-notch weir gives the most accurate results when measuring small discharges and is particularly suitable for measuring fluctuating flows. Weirs require comparatively high heads, considerable maintenance of the weir or stilling pool and protection of the channel downstream of the crest.