Kyle Straub, PhD , Tulane University
The proposed research for this project had two components. The first component involves augmenting a previously developed levee growth model by Straub and Mohrig to track the evolving composition of levees bounding straight channels in aggradational settings and to compare model results against reduced scale laboratory experiments. The second component of the project focuses on improving our ability to predict the composition of inner and outer levees bounding compound submarine channels. Levees of compound channels form important repositories for thin-bedded fine-grained sands. Our investigation will focus on collecting experimental data which characterizes the growth and composition of inner and outer levees bounding a compound channel. This data will then be used to identify processes important to levee construction in compound channels. During the first year of this project we have made significant advances in the first component of the project, outlined below.
Motivated by observations of levee stratigraphy in seismic data and levee morphodynamics observed in physical experiments we have developed a new levee growth model. This model, developed in the last year, couples a simple advection settling scheme for suspended sediment with a vertical suspended sediment profile for partially-channelized turbidity currents. Use of an advection-settling scheme is supported by small advection length scales for settling sediment in overbanking flows compared to most levee widths. The channelized (parent) suspended sediment concentration profile in our model is defined for multiple grain sizes using a two-layer method. Suspended sediment concentration below the height of the velocity maximum is defined by the Rouse equation, while the concentration above this height is defined by a near-Gaussian relationship. In our model only the current fraction situated above the elevation of the levee crest is used in the advection settling calculation. As a result early levee growth is associated with coarse and relatively stratified overbanking flow compared to later periods of growth when channel relief is greater. We use this model to link channelized flow properties to both levee morphology and texture. The evolution of levee morphology, specifically levee taper, is shown to depend on the Rouse number of the parent flow with high Rouse number flows corresponding to steep levees. Initial exploration of levee texture has focused on the distribution of particle diameters in the parent flows. We find that a flow composed of a narrow distribution of particle diameters results in a coarser levee than a flow with the same median grain size but broader particle size distribution.
In conjunction with the development of our new levee growth model, time was spent in the last year constructing an experimental facility capable of testing our model. This facility, termed the Tulane Deepwater Basin went operational in June of this year. The basin is 6 m long, 4 m wide, and 2.2 m deep, and is outfitted with an XYZ data collection carriage which houses instruments capable of characterizing the flow field, sediment transport field and the topography of the evolving sediment surface. This grant provided the funds for the first set of experiments in this new basin, including purchase of sediment and instrumentation to characterize the flow field. The construction and operation of experiments in this basin is the centerpiece of PI Straub’s research campaign at present. The first experiment in this basin has lasted several months, but is currently near completion. In this experiment we have run a sequence of turbidity currents down a straight channel and characterized the evolving flow field, sediment transport field, and evolving topography of this system which is associated with actively aggrading levees. Data quality resulting from this experiment has been very high and is currently being compared to predictions from our numerical model.
Funds associated with this project have also been used to support summer salary for a graduate student, Christopher Esposito, who is working on this project as part of his PhD dissertation. This grant has also supported travel to the 2011 AAPG conference held in Houston, TX.
Goals for the coming year include 1) comparing results from our first experiment to our numerical model and 2) performing a second set of experiments that will document the growth of inner and outer levees in a compound channel.