An evaluation of shields parameter and surface structure development in well-sorted gravel beds
The sediment entrainment process in gravel-bed channels has been studied for decades. A key study (Shields, 1936) that served as the foundation for subsequent work indicated that the initial motion of well-sorted sediment begins at a Shields parameter equal to 0.06. Additionally, there has not been consistency in the method used by workers to define entrainment. The first aim of this study was to examine which methods give closer results to what Shields found, and which are more practical and/or precise. Four series of flume experiments using two very well sorted gravel mixtures used by Shields were executed to analyze four methods used to establish entrainment and quantify the extent to which surface structure formed. Results show that, although all values of the Shields parameter lie below the original value of 0.06, the extrapolation method gives results closest to what Shields derived from his flume experiments. The second aim for the study was to analyze the degree to which surface structure influences entrainment in well-sorted gravel. The results indicate that the very well sorted mixtures did not exhibit surface structure commonly found when streambeds consist of gravel-sized sediment. Based on the findings, it is apparent that there is a heavy influence on grain characteristics for entrainment and structure development, and that there perhaps should not be a universally allocated value corresponding to critical shear stress.