Many factors influence the efficient use of water in irrigated agriculture. Water may be lost through seepage and evaporations from irrigation channels in irrigation distribution systems. Irregular field topography and poor irrigation delivery systems can cause variable application of water and excess runoff and deep percolation in some parts of the field while not providing enough water to other parts of the field. The frequency, flowrate, and duration of water delivery to the farm is also a factor in efficient on-farm water use.
For farmers to effectively use irrigation water, they must be able to request and receive water when their crop needs it. Meeting the crop's water needs requires a flexible delivery irrigation system (Merriam, 1977). Though this system potentially increases efficiency of water use by providing it to the farmers on demand, it also has the disadvantage of varying flowrates and water surface levels in the supply canal. Fluctuations in the surface level of the canal cause changes in the canal turnout discharges to the fields (Clemmens, 1984). Changes in flowrate to the fields during an irrigation "set" can lead to over-watering or under- watering in different parts of the field.
A combination of a weir and an orifice turnout can be used in the canal to help alleviate the problem of varied discharges through the turnout. An orifice turnout in the side of a canal will give a discharge that is a function of the difference between the water level in the main canal and the water level in the lateral. If the water surface level in the lateral is fairly constant, a rise in the canal water level causes increased discharge, and a drop in the canal water level will reduce the flow through the orifice. A weir may be placed in the main canal downstream of where the outlet is located in the side of the canal. The weir in the main channel can provide for a more constant water level above the orifice turnout over a range of flows in the canal. Often the width of the canal is not sufficient for a straight weir to be used and still keep the water level within the desired tolerances.
Long crested weirs can be used in the canal to provide added weir crest length for any given width of canal. The long crested weir can maintain the water surface elevation within small tolerances as the flowrate in the canal fluctuates. Therefore, a long crested weir and orifice turnout combination can be used to provide a fairly constant turnout discharge in spite of fluctuations of flowrate in the canal. one disadvantage of using long crested weirs compared to some other standard types of weirs is that other weirs can be used to measure the flowrate in the canal accurately. These standard types of weirs have been calibrated for use as flow measurement structures, and accurate discharge equations are available for the standard weirs. However, because no accurate discharge coefficients have been determined for the long cree ed weir, its usefulness as an accurate measuring device is still in question.
The objective of this research is to develop an equation which can accurately predict the flowrate over a long crested weir. The procedures to accomplish this objective are as follows: (1) use scale models of long crested weirs with sharp crests in an open-channel in the University of Wyoming Civil Engineering Hydraulics Laboratory to determine the discharge characteristics of the weirs; (2) develop dimensionless Pi terms which are in terms of the weir geometry; (3) vary the weir geometry and flowrates to obtain varying values for the Pi terms; (4) use the Pi term values in a regression analysis to develop an empirical equation for the long crested weir discharge coefficient.
Hydraulic modelling will allow results from the model tests to be analyzed to determine discharge coefficients which will be applicable to full-size long crested weirs. With known discharge coefficients, long crested weirs constructed in the future may be used not only to maintain more constant canal water levels for orifice turnouts but also to measure the flowrate in the canal.
It is anticipated that long crested weirs used for flow measurement in the canal would maintain fairly constant water levels above the canal orifice turnouts and therefore provide the farmers with constant flowrates and allow for more effective on-farm water use. In addition, the weir should also enable the operators of the canal to better control the water in the canal. By knowing the flow into and out of the canals in the distribution system, the operators may be able to minimize water wasted due to excess flows at the end of the distribution system and also avoid the problem of not providing enough water to the farms at the end of the system.
Using the flow measuring weirs in unlined canals should give canal operators values of flow into and out of various reaches of the canal between outlets and may give indications of areas where seepage losses are unacceptably high. Knowing where the high loss areas of the canal are would give the canal management an opportunity to concentrate corrective measures where they would likely be most effective in improving canal performance. These anticipated benefits of long crested weirs should be valuable to the canal management as well as help increase the effectiveness of the delivery system in providing the proper flows so that farmers can make better use of the water provided to them.
This report contains four other chapters. Chapter Two contains a review of pertinent literature associated with weirs. Chapter Three describes the laboratory apparatus and the experimental procedures. Chapter Four presents the analysis of data obtained from the experiments and contains a discussion of the results of the data analysis. Chapter Five contains the summary and conclusions of the research.
93-13 Table of Contents
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