The friction factor of an open channel flow is generally affected by the characteristics of the fluid, flow, geometry of the channel and the characteristics of the channel boundary. Large research was carried out for the study of friction factor using one or more parameters like Froude number, relative flow depth, hydraulic mean depth, slope, flow Reynolds number, size of the particles, etc. However, in each study only limited numbers of the parameters were considered. In alluvial rivers depending upon the flow conditions, any of the above parameters may vary and predominate to change the friction factor. So, here an attempt was made to compute the friction factor following different methods and approaches based on the field data of Purna river and study the variation of conveyance with friction factor. The mathematical model to predict the friction factor for the Purna river at Mahuwa Gauging station was developed by using the average value of the friction factor obtained by various methods, approaches and the multiple regression analysis. The multiple regression analysis was carried out by using the nonlinear curve fitter of the Origin 7.5 software.

Mahuwa is one of the gauging stations on river Purna. In this paper, the past 15-year data collected from this gauging station was used to compute resistance to flow in terms of friction factor. The friction factor was computed for monsoon season. There are number of approaches used to compute friction factor but in this paper three approaches were used to compute friction factor. The field data of 15 years was analyzed and computer programing in Ms-Excel and Origin software was used to carry out analysis of the data. The relationship between friction factor and conveyance coefficient was established. The graphs were plotted for the above parameters using Origin software and on the obtained results, statistical analysis was carried out. The conveyance coefficient prediction may help in assessing resistance to the flow in alluvial channel. The entire analysis is based on average diameter of particles.

The prediction of flow resistance in alluvial channels is needed for two major purposes:

1. The estimation of stage discharge relationship; and
2. The estimation of sediment transport from the hydraulic characteristics of the channel by means of transport formula.

Moreover, knowledge of the resistance characteristics of alluvial streams is of great value when dealing with the location of bridges, training works, flood control works, navigation and channel improvement, backwater computation due to confluences and barrages, mathematical and physical modeling of flow, prediction of aggradations and degradation due to the presence of hydraulic structures and so on.

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