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

• Main Author: Smith, David A
• Other Authors: Cheung, Kwok Fai
• Published: University of Hawaii at Manoa 2009
• Online Access: http://hdl.handle.net/10125/6875

This study examines the grain size, fall velocity, initiation of motion, and sediment transport rates of calcareous sand collected on Oʻahu, Hawaiʻi. These characteristics are unique to calcareous sand owing to the irregular shape of the particles and are distinct from those of siliceous sand, which have been studied extensively with well-documented results. 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 the swash zones of Oʻahu 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 for 998 grains 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 sand samples and the corresponding nominal and equivalent diameters. The 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. Analysis of the data by flow regime shows that particle shape has stronger influence on the settling characteristics when unstable wakes develop behind the grains. These findings are used to interpret the initiation of motion of four natural and five sieved calcareous sand samples in unidirectional flow. Flume experiments provide the sediment transport rate as a function of bed shear stress up to bed-form development. Reference-based criteria are supplemented by visual observations to determine the critical shear stress. The results are compared with published data for rounded and irregular particles in terms of the median sieve size and median nominal and equivalent diameters over Reynolds number. 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. Flume experiments provide 70 sets of sediment transport data and the results are compared with direct predictions from five published sediment transport models developed for siliceous particles. Corrections for the 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 of the models 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. === xii, 99 leaves