Performance Testing Of Semi Circular Contraction Critical Flow Flumes for Field Channels

Abstract

A laboratory experiment entitled Characteristics of Semi Circular Contraction Critical Flow Flumes for Low Discharges under Different Submergence Conditions was conducted in a hydraulic flume with motorized bed slope alteration facility at the Hydraulics laboratory, College of Agricultural Engineering, Bapatla, India to study the effect of percentage of contraction and submergence on flow characteristics like critical depth, location of critical depth, accuracy of measurement and total quantity of material used etc. The experiment consisted of three different contractions (20%, 40% and 60%) tested with three different discharges (10 ls -1 , 14 ls -1 and 18 ls -1 ) and four submergence levels (60%, 70%, 80% and 90%). Dimensional analyses were performed and end depth, throat width and discharge relationships were arrived. The discharges arrived conventionally based on critical depth and through equations derived through dimensional analysis were compared and presented. The deviation of discharge under free flow conditions depicted from Eq. a and Eq. b are within the range of ± 5% of actual discharge for all flumes. A single measurement of end depth in semi circular contraction critical flow flumes can be used for discharge computation in open channels, if the submergence conditions are below 80% in general. Semi circular contraction critical flow flumes can be used for discharge measurement in open channels with best accuracy of ± 5% equations developed. I. Introduction The rapidly increasing use of all available water resources and the increasing costs of on-farm irrigation development require economical use of irrigation water. Inequity in the distribution of water through the irrigation network cause lower irrigation efficiencies. To improve water management, it is strongly recommended that the irrigation water be accurately measured and regulated at all important points in an irrigation system. Cutthroat flume is one of the critical flow measuring devices for open channel flows. The flume has become popular compared to others mainly because of its simple geometry and horizontal bed, which make construction simple and reduce the cost. It is extensively used in irrigation systems in India. A circular flume is a cylinder installed axially in a prismatic channel can be used to measure discharge. However the circular flume, as presented by Hager (1985, 1986 and 1988) and Samani et al. (1991), has the disadvantage of trapping floating material, which affects reliability and function of structure. Replogle and Clemmens (1981) developed a portable flume to measure flow rates in standard concrete slip form irrigation canals. The flume resembled a broad crested weir with only two finished surfaces of long and flat weir crest and rounded approach ramp. Replogle and Clemmens (1981) reported that that critical flow flumes with contracted throat sections that are about twice as long as the maximum flow depth through the flumes produced stable and reliable calibrations accurate to ±3-5%. Samani and Megallanez (2000) developed a simple venturi flume for flow measurement in open channels. The flume was contracted using two half cylinders of PVC pipe which created a contraction. Three different half cylinders are used with 40%, 64% and 60% contractions. It has been shown that the modified cut throat can predict the measured flow rate with less than 5% error. A cost of comparison of the flume showed that it can be constructed with less than 60% cost of cutthroat flume due to lower material and labor requirements. Present research work is carried out to test the accuracy of measurement of water under different submerged conditions by preparing semi-circular contraction critical flow flumes.