Flow Measuring Structures

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Flow Measuring Structures Flow measurement structures are required in irrigation canals in order to facilitate the distribution of water through out the system and to keep account for seepage losses,

1 Flow Measuring Structures Flow measurement structures are required in irrigation canals in order to facilitate the distribution of water through out the system and to keep account for seepage losses, etc. However, in the smaller channels the flow measurement structures or devices are closely associated with local water management practices of an irrigation command. Several individuals have carried out investigation on flow measurement structures and have developed discharge rating relationship for them, but it must be noted that both national and international organisations are responsibility of Technical Committed TC 113 "Measurement of liquid flow in open channels. A list of standards relating to flow measurement structures is given in Table 1. ISO Standards ISO 1438/1 ISO 4360 ISO 4361 ISO 4359 ISO 6417 ISO 3846 ISO 3847 ISO4374 ISO 4377 ISO 748 ISO 1070 ISO 1100 ISO 2425 Thin plate weirs and flumes Triangular profile weirs. Round nosed weirs Standing wave flumes for different throat section viz, Rectangular, U-shape, Trapezoidal. Compound gauging structures. Rectangular broad -crested weirs. By the brink depth method. Round nose horizontal crest wiers. Flat V weirs. Liquid flow measurement in open channel by velocity area method. Liquid flow measurement in open channels by the slope area method. Established and operation of a gauging station and determination of the stage discharge relations. Measurement of flow in tidal channels * International Organisation for Standardization * On the suggestion of India, in 1954 the technical committee ISO/TC 30 of the ISO took upon the task of standardization of flow measurements in open channels set up a separate subcommittee.

2 Labyrinth Weir as High-Capacity Field Outlet Irrigation engineers have been forced to adopt new solutions to the engineering problems in order to ensure that irrigation projects are viable, or to deal with specially adverse conditions. Updating of existing spate irrigation systems which have been operating along traditional lines with reasonable success for substantial periods of time required a novel approach to build the structures involved. The important feature of these spate irrigation systems is that they are fed from non perennial rivers (normally dry but occurring flash flood flows when storms occur in the upper catchments). Diversion from these steep rivers was traditionally made by temporary earth banks which are often washed away during the flood period. Figure shows a plan view of a section of canal which includes a high- capacity field outlet having a cross - regulator immediately down stream. Closure of the cross - regulator cause backwater in the canal, and the water levels and the extent of the backwater is determined by the head required over the weir crest to discharge the necessary flow of water. A short length of weir crest would result in high backwater levels and a long length of crest would result in a relatively small increase of water levels due to backwater. The importance of the increase of water levels and of the extent of backwater requires raising of the canal banks, involving substantial investment. The labyrinth weir is one such solution. This should be cheaper than a straight weir having the same length as the developed length of a labyrinth weir. In Figure a simple two - cycle labyrinth weir has been shown near outlet. The configuration of labyrinth weir is determined by experiment. With some configurations, there is a possibility of, the nappes meeting from two of the sloping sides of the labyrinths forming a jet which may cause scour in the downstream. A Labyrinth weir is characterized by a broken axis in plan, the total length thus being compressed in concertina (Small musical instrument resembling an accordion but having button like keys) form into the space available on site. The purpose of the Labyrinth weir is to increase the discharge per unit width for a given operation head. Another advantage of this weir can be raised for the same maximum elevation of water level, thereby gaining substantial storage capacity.

3 5.5 m Note: This can develop submeged turbulent rollers at very low discharges Simple weir

4 1.2 m 5.5 m Note: Distributes the flow over a greater surface area Simple weir with splash plate Steps 1 and 2 = 1.83 m Steps 3 = 1.52 m Weir with cascade 1 2 3

5 Note: When a simple weir is used energy dissipation would not be very effective. Depending on the Tail water level, a hydraulic jump forms. In order to dissipate the energy possible alternatives are shown above. A simple weir with splash plate will help in distributing the flow over a great surface and the baffle blocks will assist in break up of submerged jump. The turbulence level is not reduced by this combination. Further cascade of splash plate will estimate the submerged jump. Reference: Don Richarad and others, Low Head Dam Safety with Hydraulic Models, Proceedings National Hydraulic Engineering Conference, ASCE, 1987, pp m Note: Longer length, lower head, reduced over action Typical Labyrinth weir

6 UPSTREAM SURFACE PROFILE DOWNSTREAM SURFACE PROFILE SECTION - 'XX' UPSTREAM CHANNEL DOWNSTREAM CHANNEL X PLAN LABYRINTH WEIR

7 Dry rubble pitching 1:1 Cross-regulator Warped masonry wall Stop logs or gates : Curved masonry wall Curved masonry wall :1 High-capacity field outlet upstream of a cross regulator. Plan view of V high capacity field outlet. Design outflow 5 m 3/s 1:3 Scale 1:100 With the low ratios of head to crest length, the effectiveness of the labyrinth weir configuration can be measured by a weir equation, such as Q= C g Lh 3/2 d in which, C d is the coefficient of discharge, L the developed length of the weir crest and h the head over the crest. High values of C d indicate an efficient structure, where as the head h inevitable reduces as the length L increases.

Open Channel Flow. Ch 10 Young, Handouts

Open Channel Flow. Ch 10 Young, Handouts Open Channel Flow Ch 10 Young, Handouts Introduction Many Civil & Environmental engineering flows have a free surface open to the atmosphere Rivers, streams and reservoirs Flow in partially filled pipes