Effect of particle shape on grain size, hydraulic, and transport characteristics of calcareous sand.

摘要

This study examines the grain size, fall velocity, initiation of motion, and transport rates of calcareous sand collected on Oahu, Hawaii. Through a series of laboratory experiments and data analyses, this study provides a comprehensive data set of calcareous sand characteristics and quantifies their dependence on particle shape.; Sand samples were selected from Oahu beaches. Sieve and settling techniques separate the samples into groups by sieve size and fall velocity, respectively. Individual grain properties such as shape factor, intermediate dimension, fall velocity, and nominal and equivalent diameters within those groups are presented. Evaluation of the grain size data by sieve and settling groups provides empirical relationships between the median sieve size of the samples and the corresponding nominal and equivalent diameters. Fall velocity and drag coefficient expressed respectively as functions of nominal diameter and Reynolds number show strong correlation over a wide range of shape factors.; These findings are used to interpret initiation of motion of nine calcareous sand samples. Unidirectional flume experiments provide the transport rate as a function of bed shear stress. Reference-based criteria are supplemented by visual observations to determine the critical shear stress. The results are compared with published data in terms of median sieve size and median nominal and equivalent diameters. The critical shear stresses of the irregular particles, in comparison with data for rounded particles, are higher in the hydraulically smooth regime and lower in the rough turbulent regime.; Finally, the transport of calcareous sand in unidirectional flow and its prediction through existing sediment transport models are examined. Seventy sets of sediment transport data are compared with direct predictions from five published models developed for siliceous particles. Corrections for grain size and hydraulic characteristics of calcareous sand developed in this study are applied and the results are compared with the direct calculations. The comparisons show that one model gives good results before calcareous sand corrections are considered and another responds well when the corrections are applied. This analysis provides guidelines to the application of existing sediment transport models to calcareous beaches and the gathered data lays a foundation for future model development.