An Investigation on Gravel-Bed Roughness Characterization

The accurate and quantitative description of gravel bed roughness is essential in hydraulic studies. Traditional approach for characterization of random bed roughness relies on grain size distribution curve. However، in recent years، as an alternative approach، gravel bed is considered as a random field of bed elevations which varies point by point in longitudinal and transversal directions. In fact، in new approach، a more comprehensive description of the gravel bed is provided by the statistical analysis (e. g. correlations، spectra، structure functions) of bed elevations. In present laboratory study، the results of employing these two approaches to an artificial gravel bed are reported. Existing methods for estimation of grain size distribution curve (sieve analysis، image analysis and caliper measurements) are reviewed and their results are compared. The comparison shows that grain size measurement using caliper is the most suitable method. Moreover، comparison between characteristics of artificial gravel bed in present study and natural water-worked gravel bed in rivers، obtained from statistical analysis، shows that despite the simplicity of the employed method for creating our laboratory gravel bed، most of their characteristics are different. This observation highlights the importance of statistical characterization of roughness. Therefore، in laboratory study of gravel bed، it is recommended to precisely determine roughness characteristics. This will quantify the difference of natural river beds and artificial rough bed in experimental flumes.