Unifying multi-source data and modeling perspectives to investigate connectivity and material transport in coastal river deltas

Date

2022-11-29

Authors

Wright, Kyle Austin

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Abstract

River deltas are characteristically complex and dynamic landscapes that build from deposited sediment at the mouths of rivers. Deltaic systems tend to self-organize into an elaborate connected network of channels and islands which interact and exchange fluxes of matter and energy, and understanding this "hydrological connectivity" is of key interest to restoration planning—particularly along the Louisiana coastline, where a range of natural and anthropogenic threats have led to substantial land loss over the last century. Due to the range of spatial and temporal scales involved, characterizing connectivity requires a multi-scale and multi-perspective approach that integrates numerical modeling, remote sensing, and on-the-ground observations. In this dissertation, we leverage an unprecedented suite of observations in two coastal systems in Louisiana to advance our understanding of the causes, utility, and quantification of connectivity. In the first study, we utilized high-resolution remote sensing imagery to extract the channel and wetland network of the Wax Lake and Atchafalaya Delta system, and developed an image processing workflow to spatially refine an ANUGA hydrodynamic model mesh using this information. We find that our methodology can reduce model computational demand by ≈ 30% without any discernable loss in model accuracy when compared to water level measurements from in-situ gauges, airborne lidar, and radar interferometry. Our study is the first to use imagery for this purpose, and the flexible and open-source code can enable more objective and efficient model creation for other applications. In the second study, we use this hydrodynamic model in addition to the reduced-complexity Lagrangian model dorado to simulate patterns of material transport for a diverse range of materials in deltas. We quantify the magnitude of fluvial nourishment and connectivity by material, discharge conditions, and tidal conditions, and find that even small differences in local material properties lead to emergent system-scale differences in the partitioning of materials. Our results have important implications for understanding the eco-geomorphic evolution of deltas, and our modeling framework could have interdisciplinary implications for studying the transport of materials in other systems, including sediments, nutrients, wood, plastics, and biotic materials. In the third study, we adopt and refine geostatistical methods for quantifying connectivity in order explore the relationship between surface-water connectivity, suspended sediment, and land change in the Atchafalaya and Terrebonne basins. Using time-series of multi-spectral imagery, we characterize spatial patterns of structural connectivity across a range of scales in each basin, and find that length-scales of connectivity in the Terrebonne basin are nearly twice that of the Atchafalaya basin. Our results demonstrate an interesting relationship between connectivity, suspended sediment concentrations, and land change in each basin, and we discuss a novel dynamical framework that links the balance of sediment input and hydrodynamic forces to distinct "regimes" of land change in coastal systems. The results of these three studies provide a comprehensive analysis of the role of connectivity in coastal systems, and advance our understanding of how deltas evolve in response to fluvial inputs and external drivers. These results have critical implications for the successful management of restoration efforts.

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