Date of Award

2016

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Enrica Viparelli

Abstract

Relatively common to the submarine setting are depositional sequences that begin with a lower erosional boundary, followed in ascending order by a either a graded or an inherently massive basal unit, a relatively coarse parallel laminated unit, a cross laminated unit, a relatively fine parallel laminated unit, and a capping layer of massive mud. These sequences are present in turbidity current, coastal storm and tsunami deposits, and their study is the key to understanding the physical processes governing sediment transport, erosion and deposition in submarine settings, to the reconstruction of global paleoclimate and paleoflow, to the exploration of hydrocarbon reservoirs and to the prediction of natural hazards associated with earthquake and landslide induced tsunamis. A common feature of these deposits is a basal erosional layer underlying a sandy massive unit, i.e. a unit lacking internal structure. Previous studies have shown that these sequences were deposited from waning energy flows, and the mechanism for the emplacement of the basal massive unit is thought to be associated with rapid deposition of suspended sediment. Here we present the results of laboratory experiments specifically designed to test the hypothesis that these massive units can also be emplaced by very intense bedload transport conditions corresponding to what is commonly called sheet flow in the engineering literature. The experiments clearly show that in bedload dominated systems 1) the transition from upper regime plane bed with standard bedload transport to upper regime plane bed with bedload transport in sheet flow mode is gradual

and occurs with the formation of downstream migrating antidunes and thus can be representative of waning energy flows, and 2) in the presence of bedload transport in sheet flow mode, the internal fabric of the emplaced deposits is massive. Noting that in systems transporting medium to coarse sand bedload transport in sheet flow mode occurs for less intense flows than suspended transport, the results of this work demonstrate that the use of current suspension-based models may result in an overestimation of the flow velocities and bed shear stresses of extreme events. Finally, the analysis of time series of bed elevations reveals that the gradual transition from upper regime plane bed with standard bedload transport to upper regime plane bed with bedload transport in sheet flow mode can be modeled in terms of normal probability functions of bed elevations with mean equal to the mean bed elevation and standard deviation that varies with the flow properties. The definition of probability density functions of bed elevations as a function of the flow conditions represents the first and necessary step towards the implementation of a continuous morphodynamic modeling framework for non-uniform bed material that is able to numerically reproduce the internal fabric of the emplaced deposit.

Rights

© 2016, Ricardo Rafael Hernández Moreira

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