Browsing by Subject "Delta morphology"
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Item Ecogeomorphology of fluviodeltaic systems : effects of vegetation on delta morphodynamics(2016-05) Piliouras, Anastasia Meri; Kim, Wonsuck; Mohrig, David; Johnson, Joel; Passalacqua, Paola; Jerolmack, Douglas; Twilley, RobertVegetation exerts a strong control on landscape dynamics that can greatly influence landscape morphology and sediment transport. This dissertation aims to increase our understanding of vegetated delta morphodynamics using physical experiments and fieldwork. The first study presents the results of the first set of delta experiments using alfalfa for vegetation. Plants amplified the depositional nature of experimental deltas such that channels were blocked by vegetation. Discharge fluctuations between floods and base flow increased channel relief and limited vegetation colonization to maintain the channel network. The second study seeks to understand the morphological differences between deltas with relatively high and low discharges and how channel-plant feedbacks change with discharge. High discharge deltas had statistically different channel networks and delta shapes compared to low discharge deltas, and the input discharge controlled how and where plants established on the delta, creating different vegetation patterns and channel-plant interactions. Regardless of discharge, plants enhanced channel bifurcation and smoothed shorelines. The third study examines the timescales controlling vegetated delta channel network development by comparing experiments with varying seeding densities and delta sizes. As deltas grow, their channel volume increases such that sufficiently large deltas can maintain an underfilled channel network with channel memory across several flood cycles. Plants initially enhance channel bifurcation, but if the number of plant patches on the delta does not continue to increase with delta size (i.e. patches merge), then vegetation can block channels and accelerate channel filling. There is therefore a competition between timescales of delta growth and patchiness development that determines if and when a channel network can be maintained. The last study utilizes seasonal observations from the Wax Lake Delta, Louisiana to understand the hydrodynamics of a single delta island. Flow in and around the island is most strongly controlled by seasonal changes in river discharge. However, low river discharge or proximity to the bay can increase the influence of tides such that flow through secondary channels connecting the islands and main channel network can be reversed. I hypothesize that plants increase water storage in the island lakes during rising tide to create favorable conditions for secondary channel flow reversal.Item Experimental investigation of ice-covered deltas : the effects of ice cover on delta morphology(2016-05) Lim, Ye Jin; Kim, Wonsuck; Levy, Joseph S; Mohrig, David; Johnson, JoelDeltas are dynamic systems that can provide important information on past climate conditions (Helland-Hansen and Martinsen, 1996; Hill et al., 2001; Kim et al., 2006; Bianchi and Allison, 2009). Arctic deltas have the potential to preserve significant information about climate change in one of the most temperature-sensitive regions of the Earth (Walker 1998; Walker and Hudson, 2003; Bianchi and Allison, 2009; Stocker, 2014). Here we present experimental results assessing the effects of ice cover on delta morphodynamics and depositional processes to identify signatures of ice cover presence during deposition. Ice cover drives spatially varying sediment transport on the subaqueous delta clinoform through sub-ice channels, which then leads to the development of (1) multiple extended delta lobes built by elongated, subaqueous sediment wedges and (2) highly variable bathymetry with increasing topographic roughness up to a water depth above which bottom-fast ice cover exists. These seascape features are unique to ice-covered deltas, and can therefore serve as diagnostic markers of past climate conditions and indicators of climate change captured on vulnerable Arctic coasts.Item Mixed-energy shallow-marine systems with emphasis on tidal influence(2016-12) Rossi, Valentina Marzia; Steel, R. J.; Kim, Wonsuck; Olariu, Cornel; Fisher, William; Longhitano, SergioThis research investigates mixed-energy shallow marine depositional systems (i.e., subject to the influence of river, wave and tidal currents), with particular emphasis on the role of tidal currents in controlling the final stratigraphic product. In modern coastal areas and in the rock record, many sedimentary systems bear the signature of changing and overlapping coastal processes. Understanding the evolution of the mixed tidal systems and their stratigraphic expression is fundamental both to the science of dynamic stratigraphy and for a proper exploitation of the stored natural resources. The research was carried out using four datasets: an outcrop dataset of measured sedimentological sections from the Jurassic Lajas Formation, Argentina; a dataset of previously published literature of process variability and sedimentary structures; a dataset of numerical simulations produced with Delft3D software; and an outcrop dataset of measured sedimentological sections from the Pleistocene Siderno Strait, Italy. The data here presented highlight the great degree of process variability in the rock record, and the importance of tidal currents in controlling deltaic morphology and stratigraphic architecture. The strata of the Lajas Formation show a clear process partitioning in different reaches of the deltaic system (proximal vs. distal and regressive vs. transgressive). In particular, tidal currents strongly reworked the delta front at times, creating sand-rich and amalgamated sandbodies. This project demonstrates that disentangling the signals of river, wave and tidal currents in the stratigraphy leads to a better interpretation of ancient mixed-energy systems. The study of the literature database shows that some sedimentary structures can be considered reliable indicators of a particular process (river, waves or tides), whereas other structures cannot be tied with confidence to any particular process. A process probability value is calculated for each sedimentary structure, and thus quantifies process variability and its uncertainty. This work encourages a new, more detailed and more quantified field methodology for facies sedimentology. The numerical modeling of tide-influenced deltas using Delft3D shows how different degrees of tidal influence in river-dominated deltas affect delta morphology and stratigraphy, when tidal currents are flowing perpendicularly to the shoreline. Increasing tidal influence induces deeper and more stable distributary channels that act as efficient conduits for sediment transport basinward. The delta-front geometry is also affected by tidal current reworking, evolving into a compound clinoform geometry. The research on the Siderno Strait, in contrast, highlights tidal influence on deltaic stratigraphic evolution when tidal currents flow parallel to the coastline. River-dominated deltas entering the tide-dominated strait tend to show a deflection of the delta-front sands in a direction parallel to the dominant tidal current. The delta-front sands became reworked by tidal currents into large dune fields within the strait.Item Physical modeling of a prograding delta on a mobile substrate : dynamic interactions between progradation and deformation(2016-08) Jung, Eunsil; Kim, Wonsuck; Mohrig, David; Olariu, CornelThe subsurface architecture of a prograding delta on a mobile substrate (e.g., salt) is a product of the complex interplay between deposition and subsidence. Previous studies focused mainly on structural deformation of a salt layer in response to tectonic forcing, leaving the dynamic feedback between sedimentation and subsidence unexplored. We present results from physical experiments of delta progradation on a mobile substrate. Five carefully designed experiments were performed to understand the effects of delta progradation rate on the shape and dimension of salt deformation and associated delta deposition. All of the runs had constant sediment and water discharges, but the water depth and mobile substrate thickness varied from 1 cm to 3 cm and from 2 cm to 4 cm, respectively. The results showed that increasingly deeper water depths slowed the shoreline progradation rate, while increasingly thinner salt thickness accelerated delta progradation. The experimental results also provided a wide range of shoreline advance and subsidence rates that show changes in the shape and dimension of the salt deformation structure. Runs with fast shoreline progradation showed isolated salt domes developed internally on the delta plain and a rough platform pattern along the shoreline due to lobes built by channel flow between upwelled salt structures. However, runs with slow shoreline progradation developed long connected salt ridges around the toe of the delta, limiting sediment transport beyond the ridges. This overall pattern in salt structures is time dependent. As a delta surface grows larger and the shoreline progradational rate autogenically decreases with time, chances to develop isolated salt domes decrease but more connected long salt ridges occur. Physical modeling of a delta on a mobile substrate is important in predicting the mechanism for large-scale salt basin stratigraphy under a high sediment supply that interacts with the substrate.