Transport and Morphodynamics in a Fine Sediment Estuary: From Conceptual Understanding to Numerical Modeling

This dissertation presents a study of fine sediment transport and morphodynamics in estuarine settings using data from the Lower Passaic River (LPR), located in New Jersey, USA. Originally a relatively shallow system, it has been dredged and deepened for navigation purposes from the late-1800s onwards, along with other modifications such as wetland reclamation, shoreline armoring, construction of bridges, etc. The last such dredging occurred several decades ago, and although the subsequent long-term morphological trend has been one of infilling, morphological trends over the short term (inter-annual durations) are more variable, with some years experiencing erosion and others experiencing infilling. Therefore, this dissertation seeks to understand the processes driving the long- and short-term morphological trends and the processes controlling the long-term morphodynamic equilibrium of the estuary. The dissertation approaches this problem by first assessing the small-scale (spatial and temporal) transport processes responsible for morphological evolution over the short term. Subsequently, it assesses the large-scale system dynamics from a morphodynamic perspective and the processes driving the variations thereof. Finally, the information gained from the small- and large-scale assessments is used to support the development and application of a morphodynamic model. Sediment transport, and consequently morphodynamics, in starved-bed or erosion-limited fine sediment systems is a non-equilibrium process related to the availability of mobile sediment. This defines one time-scale of transport in such systems, that of the tidal period. During such conditions, transport is associated with the dynamics of a thin layer (2-4 mm thick in the LPR) of easily-erodible surficial sediments termed the fluff layer. Based on variations in suspended sediment concentrations that follow the oscillatory tidal currents, an analytical method referred to as the entrainment flux method for quantifying fluff layer erodibility (specifically, the critical shear stress for erosion and the erosion rate coefficient) was formulated and applied. The results of the entrainment flux method are analogous to the erosion data used to formulate the well-known standard linear erosion formulation; the inferred erosion properties are also comparable to direct measurements of erodibility on sediment samples using a Gust Microcosm. The favorable comparison with the direct measurements suggests that the entrainment flux method can be used to quantify the erodibility of the fluff layer in such systems. Further to the various time-scales of transport in fine sediment systems, another time-scale is that spanning episodic scouring events. In the LPR, such scouring conditions are primarily associated with high river-flow events occurring every few years. During such conditions, depending on river flow-rate, erosion can extend beyond the fluff layer and up to tens of centimeters in the bed; consequently, sediment dynamics during such conditions is dependent on the fluvial forcing. However, during non-event conditions, sediment dynamics are controlled by barotropic and baroclinic circulation. In order to understand and quantify the dynamic impact of the various forcings on transport, an extensive dataset consisting of suspended sediment fluxes, inter-annual morphological change, sediment erodibility, and a numerical hydrodynamic model was analyzed. The former two datasets were used to develop an understanding of sediment dynamics over the full range of hydrologic conditions, and the latter two datasets were used to interpret the system behavior. Subsequently, a conceptual picture was developed, one that classifies the instantaneous morphological status of the system into three regimes dependent on river flow --- under Regime I the system imports sediments, under Regime II the system exports sediments by flushing the fluff layer, and under Regime III the system exports sediments by scouring the less-erodible strata underneath the fluff layer. Regime III is relevant for the long-term morphodynamic equilibrium of the estuary by providing a mechanism that scours and exports sediment accumulated under Regime I conditions. Limited information from the literature suggests that such a conceptualization of sediment dynamics may be common to estuaries characterized by starved-bed transport. These regimes also imply that transport in such systems also depends on the time-history of river flow and the long-term morphological progression of the system, i.e., the system develops a memory (represented by the availability of mobile sediment) that influences subsequent morphological response. The conceptual and quantitative information on transport and sediment dynamics in the LPR was used as the basis for the development of a process-based morphodynamic model. Key processes of relevance in fine sediment settings were formulated and parameterized in the model. Specifically, these include sediment mobility considerations that lead to erosion-limited transport, either due to armoring effects or decreasing sediment erodibility with depth in the bed. The model framework also includes morphological upscaling using the Morfac approach, with specific formulations and considerations relevant for morphodynamics in fine sediment settings. Model performance was assessed against various metrics including suspended sediment concentrations and fluxes, and short- and long-term morphological change. Although the model does not capture measured morphological response at local scales over the short term, it predicts the large-scale spatial and temporal (river flow-dependent) short- and long-term morphological trends of the system. The model was subsequently applied to assess the long-term morphodynamic evolution of the estuary in response to changes in various forcings, with results that are conceptually and theoretically explainable. The results support the application of the morphodynamic model using Morfac for studying the long-term morphodynamic evolution of such fine sediment systems. The overall conceptual findings, and the analytical and numerical methods developed in this dissertation are generally applicable to fine sediment systems characterized by starved-bed conditions. For instance, features such the presence of a fluff layer and its relatively high erodibility, and transport dynamics modulated by river flow have been observed in other systems as well. Similarly, concepts of sediment mobility and erosion-limited transport are also well known in the literature. This dissertation seeks to add to the body of knowledge for such systems by formulating a new method for quantifying the erodibility of the fluff layer, by presenting a conceptualization of sediment dynamics over the full range of hydrological conditions, by presenting a morphodynamic model framework that accounts for sediment mobility and erosion-limited transport, and by extending the applicability of the Morfac approach to fine sediment settings.