ESTIMATION OF HYDRAULIC ROUGHNESS PARAMETERS FOR BOULDER STREAMS

Abstract

Resistance characteristics of boulder reach streams differ significantly from those of low gradient / sand bed rivers. Hence estimation of flow resistance in these rivers cannot be achieved through techniques empirically developed for lower gradient rivers. To address the problem. the knowledge about the hydraulic geometric parameters of the boulder reach stream viz. Width (W), depth (d),hydraulic Radius (R), Particle size ( D50 & D84), water slope (So),velocity (V), discharge(Q) and Froude number (Fr) are required for analyzing a variety of problems related to flow measurement in a river.Quantify flow resistance becomes challenging because of the complex interaction of the different forms of flow resistance in steep mountain streams, usually represented by the Manning's ruggosity coefficient 'n'. The broad goals of this work are to increase understanding of basic physical processes and effects of 1-lydraulic Roughness parameters in the measurement of flow for boulder streams. The study also seeks to elucidate how hydraulic roughness parameters of boulder reach streams differ from low-gradient streams in terms of hydraulic and their interaction to the geometry of streams, size and gradation of bed material etc. In addition, all of these investigations are intended to contribute analysis of hydraulic parameters, discharge! flow estimation in boulder reach streams. Some of these cases have been critically analyzed in this study and investigations have been carried out to try to get answer to some questions. It is realized that the available equations for the prediction of Manning's ruggosity coefficient 'n' for boulder reach streams have limited applicability because of small data bases used in their development and / or non consideration of an entire range of flow conditions and effect. Their accuracy, in general, is very low with errors amounting 25-35 %or even more. A large volume of published datasets (212), representing a wide range of hydraulic and physical conditions has been compiled via literature survey of similar previous studies in the field of study and an attempt has been made to propose/develop relationships through linear empirical correlations using Multiple linear regression analysis in SPSS 16.0 software. Resulting as well as existing equations for the estimation of Manning's ruggosity coefficient 'n' are analyzed and inferences are drawn from their performance. The methods used to analyze the estimation of Manning's ruggosity coefficient 'n' for boulder reach stream are subsequently recommended for practical engineering applications. Suggestions are also made for future study