Sediment mobility and memory effects distinguish transport and morphodynamics in fine sediment settings from sandy settings. This paper focuses on the morphodynamic modeling of fine sediment systems, and an adaptation of modeling procedures more commonly used in sand-dominated systems. An extensive dataset of short-term transport and morphological trends along with ancillary data is used to support the development, parameterization, and calibration of a morphodynamic model of the Lower Passaic River (USA). The model is subsequently extended to include a morphological acceleration procedure in order to enable computationally efficient morphodynamic simulations. The performance of the resulting model is first verified and subsequently validated using measured decadal-scale morphological change. Subsequently, the model is used to assess the current morphodynamic status and the long-term response of the river to forcings such as shipping, episodic storm events, and climate change, with results that are consistent with theoretical expectations.

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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. @en

Estuarine sediment dynamics are a consequence of various forcings (barotropic, estuarine circulation, and fluvial) that vary in space and time. Here, we present a study examining sediment dynamics in a narrow microtidal estuary, the Lower Passaic River in New Jersey, USA. The analysis incorporates measurements of suspended sediments, morphological change, sediment erodibility, and a numerical hydrodynamic model. The former two datasets are used to develop an understanding of sediment dynamics over the full range of hydrologic conditions, and the latter two datasets are used to interpret the behavior of the system. Subsequently, a conceptual picture is developed, one that classifies the morphological status of the system at any given time into three regimes dependent on river flow—regime I includes conditions when the system is importing sediments, regime II includes conditions when the system is exporting sediments by flushing a thin easily erodible surficial stratum termed the fluff layer, and regime III includes conditions when the system is exporting sediments by scouring more consolidated strata underneath the fluff layer. Regime III is relevant for the long-term morphodynamic equilibrium of the estuary by providing a mechanism that erodes and exports sediment accumulated under regime I conditions. Consequently, sediment dynamics depend not only on short time-scale processes such as the instantaneous river flow rate, but also on the time-history of river flow, and the long-term morphological progression of the system. These regimes represent a conceptualization of estuarine sediment transport dynamics and can be useful in the development of effective estuarine sediment management strategies.

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Numerical models of fine sediment transport depend on different approaches to parameterize the erosion properties of surficial sediment strata. These properties, namely the critical shear stress for erosion and the erosion rate coefficient, are crucial for reproducing the short-term and long-term sediment dynamics of the system. Methods to parameterize these properties involve either specialized laboratory measurements on sediment samples or optimization by model calibration. Based on observations of regular patterns in the variation of suspended sediment concentrations (SSC) over the tidal cycle in a small, narrow estuary, an alternate approach, referred to as the entrainment flux method, for quantifying the erosion properties of surficial bed strata is formulated and applied. The results of this method are shown to be analogous to the erosion data used to formulate the standard linear erosion formulation developed by various authors. The erosion properties inferred from the entrainment flux method are also compared to direct measurements of erodibility on sediment samples from the same site using the Gust microcosm apparatus. The favorable comparison of the two approaches suggests that the entrainment flux method can be used to infer and quantify the erodibility of surficial sediment strata in similar small and narrow estuaries. This method has certain advantages, chiefly its ease of implementation and the fact that it uses SSC time series which would typically be expected to be available for the study of or for model application at a given site. Guidelines for selecting the appropriate dataset for the application of the method are also presented.

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