Computer-based analysis of hydraulic design variables for uniformly sloping microirrigation system laterals

Abstract

Adequate analysis of lateral hydraulics is a very important concern for the design and evaluation of microirrigation systems. One of the main tasks of the lateral hydraulic calculation is to determine the pipe geometric characteristics (pipe size and length) with the required operating inlet pressure head, and the total friction head losses along the lateral line, assuming that the total flow rate at the inlet, characteristics of the emitter, and the acceptable level of uniformity are known in advance. This paper aims to present an efficient computer program in Visual Basic 6.0, named "Multi-flowCAD," which is based on a stepwise computation algorithm for determining the hydraulic design variables (pipe size, pipe length, and operating inlet pressure head) and the hydraulic flow characteristics (emitter outflow-pressure head distribution, lateral discharge, and total friction losses) along the energy-grade line. The stepwise algorithm takes into account the velocity head change and variation of the Reynolds number, which affects the selection of the proper friction coefficient formula to be applied along the different reaches of the pipeline, whereas some additional minor local losses resulting from emitter connections are neglected. An illustrative example of determining the operating inlet pressure head by a trial-and-error-procedure to achieve the design criteria (initial and boundary conditions) is presented. For any desired uniformity level, the procedure also provides an opportunity to evaluate the influence of different uniform line slopes on the pipe geometric characteristics (pipe size and length) and on the corresponding hydraulic variables (required operating inlet pressure and downstream-end pressure heads and total friction head losses). On the basis of the computer outputs, variations of the operating inlet pressure head, total friction head losses, and uniformity coefficients depend on the examined pipe lengths ranging between 25 and 250 m and the internal diameters ranging between 10 and 21mm in the horizontal pipe case, and were depicted graphically in dimensionless form for practical purposes. The present software technique was implemented successfully with the highest degree of accuracy because only the basic equations of the hydraulics of steady-state pipe flow condition were sensitively used in each pipe section between successive outlets. Examinations of various design configurations clearly showed that the performance of the Multi-flowCAD is satisfactory, and its results are in close agreement with those obtained by previous research. © 2014 American Society of Civil Engineers.