The following points highlight the two main devices used for measuring the flow of water in channels. The devices are: 1. Weirs 2. Flumes.
Device # 1. 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.
They types of sharp crested weirs commonly used for measuring irrigation water are the following:
1. Sharp Crested Rectangular Weir:
A general view of this type of weir is shown in Fig. 4.29.
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) H3/2
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.
2. Sharp Crested Trapezoidal (Cipolletti) Weir:
A general view of this type of weir is shown in Fig. 4.30.
Fig. 4.30: General View of a Cipolletti Weir
The discharge formula for this type of weir was given by Cipoletti as:
Q = 1.86 LH3/2
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 Fracis formula.
3. Sharp Sided 90° V-Notch Weir:
A general view of this type of weir is shown in Fig. 4.31.
Of the known formulae used to compute the discharge over 90° V-notch weir the formula recommended generally is the following:
Q = 8/15(2gCd)12H5/2
Where Q is the discharge in m3/s; g is the acceleration due to gravity (9.8m/s2) Cd is a coefficient of discharge; and H is the head in meters. The value of Cd varies according to the variation of H and can be read out from (Fig. 4.32).
Each of these weirs has characteristics appropriate particular operating and site conditions. The 90° V-notch weir gives the 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.
Device # 2. Flumes:
Flumes are flow measuring devices that works on the principle of forming a critical depth in the channel by either utilizing a drop or by constructing the channel.
These two forms of flumes for flow measurement are:
1. Flume with a Vertical Drop:
This type of structures can be utilized to negotiate a fall in the canal bed level. One of these, the standing wave flume fall developed at the Central Water and Power Research Station (CWPRS), Pune, has been standardized and documented in Bureau of Indian Standard code IS: 6062-1971. “Method of measurement of flow of water in open channels using standing wave flume-fall” and shown in (Fig. 4.33). Because of the inherent free flow conditions, the measurement of flow requires only one gauge observation on the upstream side.
The discharge equation for this structure is given by the following equation:
Q = 2/3 (2g)1/2 CBH1.5
Where Q is the discharge in m3/s; g is the acceleration due to gravity C is the coefficient of discharge (= 0.97 for 0.05 < Q < 0.3 m3/s and = 0.98 for 0.31 < Q < 1.5 m3/s); B is the width of the flumed section, also called the throat and H is the total head, that is, the depth of water above crest plus the velocity head.
The other type of flume type of fall is the one called the Central Design Office (CDO) Punjab type fall, which is simple and robust in construction. Upto lm drop, a glacis is used on the downstream side (Fig. 4.34) and if the drop exceeds lm, the crest ends in a drop wall (Fig. 4.35). The structure is often combined with a bridge, an intake, an intake of a third degree canal or both.
2. Flume with a Constricted Section:
This type of structures for measuring water discharge creates a free flow condition followed by a hydraulic jump by providing a very small width at some point with in the flume. These are also further divided into two types- Long and Short-throated flumes. In the former, the construction is sufficiently long to produce flow lines parallel to the flume crest for which analytical expression for discharge may be obtained.
In the short-throated flumes, the curvature of water surface is large and the flow in the throat is not parallel to the crest of the flume, Hence, due to the non-hydrostatic pressure distribution, there is not analytically derived expression for discharge but has to be calibrated from actual measurements. However, these flumes require small lengths and are economical than long throated flumes. One of the commonly used short- throated flumes is the Parshall flume (Figure. 4.36).
The flume consists of a short parallel throat preceded by a uniformly converging section and followed by a uniformly expanding section. The floor is horizontal in the converging section, slopes downwards in the throat, and is inclined upwards in the expanding section. The control section, at which the depth is critical, occurs near the downstream end of the construction. There are standard dimensions of Parshall flumes which are available commercially and may be had from the reference “Design of Small Canal Structures” of USSR (1978).
One of the advantages of this type of flume is that it operates with a small head loss, which permits its use in relatively shallow channels with flat grades. For a given discharge, the loss in head through a Parshall flume is only’ about one fourth that required by a weir under similar free flow conditions. The flume is relatively insensitive to velocity of approach. It also enables good measurements with no submergence (that is free flow with a hydraulic jump downstream) or with submergence (that is the jump is drowned by the downstream water level) as shown in (Fig. 4.39).
The velocity of flow within the flume is also sufficient to eliminate any sediment deposition within the structure during operation. A disadvantage of the flume is that standard dimensions must be followed within close tolerance in order to obtain reasonable accuracy of measurement. Further, the flumes cannot be used close to an outlet or regulating devices. Parshall flumes can be constructed in a wide range of sizes to measure discharges from about 0.001m3/s to 100m3/s.