An experimental and numerical investigation of hyperpycnal flow
Date
2011
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Publisher
University of Delaware
Abstract
The understanding of river-borne sediment dispersal is important for several reasons. While predicting how rivers discharge sediment into the coastal environment quantitatively remains challenging, several studies have shown similar features in the interaction between river and ocean. Amid these similarities is the formation of freshwater plumes, positively and negatively buoyant, that extend from river mouths into the ocean. Research has been conducted to help understand how these plumes are created and determine their impact on sedimentation. A series of laboratory experiments were performed to study sediment-laden freshwater plumes within a saltwater flume. Both hyperpycnal and hypopycnal plumes were created by pumping freshwater with a given sediment concentration into the saltwater flume with a constant salinity. Fluorescent dye was added to the slurry to improve the visual distinction with the ambient saltwater. Observations were divided into three regimes: fully buoyant plumes, divergent plumes, and fully submerged plumes. These idealized plumes are used to investigate the effect of specific variables: initial sediment concentration of the slurry, the bottom slope used at the entrance of the tank, and the method by which the slurry is injected into the flume. Image analysis was performed using Matlab code to measure parameters in order to describe the plumes. Numerical models were used to further investigate the formation of these plumes, specifically the impact of particle settling velocity. It is found that the resultant plume structure cannot be simply described by the density ratio between the sediment-laden river plume and seawater. The injection method, bottom slope, and settling velocity are all critical factors affecting the plume form. In many cases, reverse buoyancy or divergent plumes are observed, consistent with several field observations based on point sensor measurements. The experiments designed are visual by design and will be used to disseminate results for educational purposes, aiding to expand this field of study. Results are applicable to other fields of study, waste water treatment for example, due to the general demonstration of density stratification and particle settling.