Date of Award

5-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Testik, Firat Y

Committee Member

Khan , Abdul A

Committee Member

Hayter , Earl J

Abstract

An extensive laboratory experimental campaign was undertaken in order to elucidate the entrainment and dilution characteristics of constant-volume non-Newtonian fluid mud gravity currents. Three experimental series were conducted in a lock-exchange tank. The first experimental series quantified ambient entrainment using a light-opaqueness technique, the second experimental series measured vertical concentration profiles using a siphoning technique, and the third experimental series provided visual observations for the investigation of the deposited sediment layer.
In the first experimental series, a technique similar to the so-called light attenuation technique was developed to find the boundary of the current, allowing for the calculation of entrainment velocity. It was found that the entrainment velocity is dependent on different parameters in the different propagation phases. On the whole however, the entrainment characteristics of a gravity current are found to be governed by a competition between the entrainment-inhibiting density stratification effects and the entrainment-favoring effects of turbulence. An entrainment velocity parameterization for non-Newtonian fluid mud gravity currents is developed based upon the experimental observations.
In the second experimental series, four unique initial conditions were studied, with an array of siphoned samples being withdrawn throughout the head and body of the gravity current. From these samples, the functional form of the vertical concentration profiles inside the head of low-concentration gravity currents is proposed. The higher-concentration gravity currents revealed the presence of a lutocline in the current head and body. The presence of a lutocline has been observed for constant-release gravity currents, but to the authors' knowledge this is the first measurement of a lutocline in constant-volume currents. The siphoned samples permitted for the quantification of dilution due to ambient entrainment at different concentration contours in the gravity current. The analysis of the experimental observations shows that the initial reduced gravity is inversely proportional to the growth rate of the denser regions of the gravity current.
In the third experimental series, visual observations were employed to study abrupt transitions, which is a phenomenon where the bulk of the suspended sediment in a propagating gravity current drops out. It was found that the presence of a lutocline can trigger an abrupt transition, a conclusion that extends the currently accepted theory on the root of such behavior. A semi-empirical equation was proposed that accurately predicts the location of the abrupt transition. The results of this investigation are anticipated to be valuable for prediction of the behavior of fluid mud gravity currents in dredge disposal operations.

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