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. . The problem of predicting flow resistance coefficient with sufficient accuracy. is of great interest to hydraulic engineers. Realistic estimate of hydraulic resistance is important to the proper design and analysis of irrigation and drainage networks. Resistance (roughness) coefficient is a compound function of the geometric properties of the channel, the fluid properties, and the varied flow conditions prevalent in the open channel system. The purpose of this study is to demonstrate the response of the applied method of irregular geometric properties computation, the used numerical scheme, and the distance interval adopted in the numerical model on the calibrated value of resistance coefficient. The rectangular grid scheme of the method of characteristics is applied with different methods of irregular geometric properties computation and different value of distance interval. While, the Leap-Frog finite difference scheme is used with only one method of geometric properties computation and one value of distance interval. Measured field data of Tanta Navigation canal are used to feed and calibrate the numerical models. Results of numerical experiments illustrate that inaccurate channel geometry representation in the numerical models leads to incorrect value for the calibrated value of roughness coefficient. Findings also demonstrate that the applied numerical scheme as well as the schematization to describe the channel geomeby by a series of discrete representation along the reach being modeled has significant effect on the calibrated value of the roughness coefficient. Manuscript received from Dr. ~ous ta fa A. El Enany Accepted on : 7 19 I 2002 Engineering Research Journal Vol 25, No 4,2002 Minuiiya University, Faculty Of Engineering, Shebien El-Kom ,Egypt, ISSN 1110-1180 The main issue of the problem of flow in alluvial channels centers on the estimation of their hydraulic resistance. Knowledge of this factor is essential for accurate calculation of the sediment transport rate, stage-discharge relationships, channel stability, waterways design, and prediction of aggradation and degradation due to the presence of hydraulic structures. The resistance (roughness) coefficient is influenced by size, shape, and arrangement of granular material forming the wetted perimeter, geometry of the bed patterns (ripples, dunes, and antidunes) formed by the flow, rate of sediment transport, irregularity of channel cross sections, variation of shape and size of channel cross sections, channel alignment (meandering), vegetation characteristics and obstructions [14]. Each of these components contributes to the total resistance coefficient. One of the characteristics of irregular channels is the geometric properties variation of cross sections, Area (A), top width (B), and wetted perimeter (P), along the channel reach. The non-uniform properties of channel cross sections (channel irregularity) affect the conveyance and storage characteristics of the channel reach. Channel irregularities control the variation of flow from section to section and, at particular section, the variation of flow with stage. Theses irregularities are directly responsible for modifications of flow characteristics along the channel reach. The measured cross sections of irregular channel must be chosen'to ensure excellent feeding of the geometrical characteristics into the numerical model. They must be specified at locations along the channel reach where significant cross sections changes occur, i.e. the spacing of the measured cross sections, AX, depends on the degree of variation of cross section characteristics. Often this is a matter of economic balancing the value of additional accuracy versus the additional costs of data acquisitions. The user of a computer program must decide the distance interval, AX, to be used in the computation and whether some of cross sections should be deleted (i.e., using larger distance interval) or additional ones introduced (say be interpolation). It is generally true that using smaller value of distance interval (step) in the numerical solution tends to give better accuracy, but requires more computation time [lQ and 201. There are different methods to simulate the measured geometric properties of cross sections in the numerical model. The numerical model user must consider how best to embody the physical characteristics and properties of the prototype system to the model. The objective of this research is to study the effect of the used method for treatment of the measured geometric properties, the used distance interval, and the applied numerical scheme on the calibrated value of roughness coefficient. Numerical experiments are implemented using the rectangular grid scheme of the method of characteristics and the Leap-Frog scheme. Different methods of treatment of irregular geometric properties and different values of distance interval are applied. Measured field data of Tanta Navigationcanal-are used to carry out the numerical experiments. , ,

EFFECT OF GEOMETRY COMPUTATIONS AND NUMERICAL SCHEME ON THE RESISTANCE COEFFICIENT FOR UNSTEADY FLOW IN IRREGULAR CHANNELS