Calibration of Irrigation Headgates by Model Analysis

Introduction The purpose of this research project was to calibrate the slide gates used by the D.M.A.D. Company (Delta, Melville, Abraham and Deseret Irrigation Companies). These gates, which number more than 600, are located throughout the distribution system. Each gate is placed in a concrete box 4 feet wide, 3-1/2 feet deep and 4 feet long. The structure is used as a means of diverting the water and is also used as a measuring device. A similar structure, but with a different type of slide gate, was calibrated in 1914. At the time rating tables were prepared which listed the flow rate when the height of gate opening and the difference in water levels upstream and downstream from the gate were know. A number of measurements of flow through the present structures have been made in the field. These measurements have been utilized to relate the height of gate opening, b, and the difference in water levels upstream and downstream from the gate, ΔH, to the discharge rate, Q. Discharge rates were obtained by current meter measurements and were checked by making a systems analysis of the flow throughout the distribution system. The water users have recognized for some time many of the problems inherent in calibrating these structures in the field. One of the primary difficulties is caused by the scour holes which have been formed immediately downstream from most of the structures. The depth of flow downstream from the gate has had to be observed at the scour hole because the concrete structure is short and does not allow full recovery of the kinetic energy of the flow to take place within the structure. The term full recovery refers to the kinetic energy associated with the high velocities of the jet issuing from the gate opening being converted back to potential energy in the form of depth of flow. The point of full recovery is the point downstream from the gate at which the flow is essentially re-established and maximum depth occurs. The principle of full recovery is illustrated in Figure 1 with maximum depth of flow, and consequently full recovery occurring at yd. The measurement of water level at the scour hole is influenced by flow conditions in the structure and added dissipation of energy at the scour hole. To include the scour hole in the flow measurement system would present many problems and would require extensive calibration work because of the variability of the size of the scour holes and the effect of downstream conditions on the dissipation of energy at the scour hole. Although these problems could be overcome in the laboratory, the cost of such a study would be many times the cost of the research effort reported herein. In order to obtain a general calibration of the gate structure system, a rigid channel was placed downstream from the model gate structure. The rigid channel, in effect, was comparable to increasing the length of the concrete gate structure. This system allowed full recovery to occur in the channel. The slide gate has steel angels attached around its periphery, which act as guides during the operation of the gate. These angles are located on only one face of the gate. The gates have been placed in the structure with the angles facing either upstream or downstream in a more-or-less random fashion. The water users have observed that for the same change in water surface elevation upstream and downstream from the gate, and with the same height of gate opening, the discharge will be greater with the gate angels facing upstream. Consequently, it was necessary that was calibration of the gate structure system include both conditions