Initial Bathymetry Research Articles

Hindcasting and forecasting marsh ecomorphodynamics by integration of model with stratigraphic record

Marsh morphodynamics models are usually calibrated using sparse short-term measurements. Hence, their ability to predict long-term trajectories (centuries to millennia) is highly uncertain. Here I improved an existing model for marsh ecomorphodynamics and used it to hindcast the whole morphodynamic trajectory (2500 years) of an actual salt marsh in New England (USA). The model includes enough processes – among which tidal currents and waves, sand and mud transport, and vegetation population dynamics – to simulate a realistic marsh analogue that closely matches the modern topobathymetry and stratigraphy. Marsh trajectory is highly sensitive to initial bathymetry, mud supply from the nearshore, wave erosion processes, and vegetation dynamics. Each one of these factors is important yet difficult to calibrate on its own. Matching the long-term evolution with the stratigraphic record provides a global constraint for the processes and parameters in the model, increasing the confidence in future long-term predictions. This type of comparison complements those made at short times scales, and it should be used more routinely.

Sedimentary process of channel-feeding fan delta in the south of Albert Rift, Uganda: Insight from sensitivity analysis of forward stratigraphic modeling

Channel-feeding fan deltas without any conglomerates have recently been discovered on the steep slope in the Albert basin at the northern end of the west branch of the East African Rift. However, the new deposition system's mechanics and patterns are unknown. To reveal the sedimentary process of the distinctive fan delta, we integrate forward stratigraphic modeling (FSM) technology with interpretations of the core, well logs, and 3D seismic data. Firstly, a reference FSM model is constructed to describe the evolution of the fan delta, taking into account initial bathymetry, source supply, lake level, and fluvial discharge as independent variables. Then, the Orthogonal Experimental Design (OED) is employed based on the uncertainty analysis of each independent factor to generate FSM multi-realizations for formation thickness and Net-to-Gross. Sensitivity analysis of Multi-FSM realizations reveal that source supply is the most influential variable affecting formation thickness and texture. Combined with the tectonic evolution of Albert Basin, the formation dip during the deposition period is much smaller than present. Multiple incised channels were formed in the provenance area, which became the primary source system of the fan delta, facilitating the sediments' transportation over a certain distance to produce well-developed cross-bedding sandstone. The channel-feeding fan delta has a narrower plain and wider front, and the principal sand body is high-quality distributary channel. The discovery of the channel-feeding fan delta without any gravel formed on the steep slope of the rift basin enriches the theory of deposition in rift basins. In addition, the application of FSM technology helps to improve the quantification of sedimentary processes.

The Sensitivity of Tidal Channel Systems to Initial Bed Conditions, Vegetation, and Tidal Asymmetry

AbstractIn tidal environments, channel networks act as essential drainage pathways. Although the complex interactions between environmental factors have been studied extensively, the effects of the initial bathymetry on tidal network ontogeny are poorly understood. In this contribution, we used a numerical model to mimic a schematic tidal basin subjected to tidal forcing. The effects of the initial bathymetry and vegetation growth are analyzed by changing the features of randomly generated bed perturbation and the intertidal platform slope. Different perturbation densities mildly affect the growth of tidal networks, which, at equilibrium, share similar values in terms of channel length, tidal prism, and cross‐sectional area. The complexity and structure of channel networks are more sensitive to variations in the perturbation distribution. Increasing the initial bathymetry slope can shorten channels and reduce the tidal prism and drainage efficiency. Vegetation growth is found to invariably promote channel lengthening and narrowing, increasing the complexity and drainage efficiency of the system. An asymmetrical tidal forcing generally leads to longer channels and smaller unchanneled lengths. Under ebb‐dominant conditions, channels get deeper, and the increased channel length ensures a higher drainage efficiency. The insights of our study provide a deeper understanding of the environmental factors controlling the equilibrium morphology of tidal channel systems and their overall resilience. Further implications concern the restoration and management of coastal areas through the informed use of topographic changes and planting arrangements. Finally, accounting for the uncertainties associated with initial conditions is relevant when modeling other earth systems and comparing them with real systems.

On the Influence of Antecedent Morphology on Development of Equilibrium Bathymetry in Estuaries Past and Future

AbstractAlthough analytical and numerical models have been widely used to explore evolution and equilibrium morphology in tidal environments, less attention has been paid to examining the impact of initial bathymetry on the model outcomes. Here we use two‐dimensional idealized models with contrasting initial bathymetries to study how the interactions between antecedent morphology and tidal exchange processes determine the establishment of an estuarine equilibrium bathymetry, and how these interactions mediate the morphodynamic response to rising sea levels. In all model runs with sandy beds, inter‐tidal zones reach the equilibrium condition first and equilibrium profiles are similar for points close to mean seal level. However, key aspects like channel formation, residence time and energy dissipation do not evolve to the same state and are inherited from the initial bathymetry. This implies that responses to sea‐level rise (SLR) are different as well. Conversely, in cases with mud and sand input at the boundaries, equilibrium occurs more quickly and the equilibrium bathymetry and channel formation are dominated by the boundary mud concentration. General implications of the study are that predictions of coastal response to changes such as SLR depend on initial bathymetric conditions.

3D stratigraphic forward modeling of an ancient transgressive barrier system: A case study of accuracy and sensitivity

Accuracy of geological modeling and understanding of the controlling parameters in sedimentary systems evolution are challenges in both academy and industry. Uncertainty in the prediction of spatial and temporal facies distributions and difficulties in quantitative analysis of controlling parameters are current challenges. To increase geological model certainty and sedimentary process understanding, computational modeling has evolved exponentially during the last 50 years. In this study, 3D stratigraphic forward modeling technique is used to simulate an ancient transgressive barrier system of southern Brazil, deposited during the Lower Permian in the Paraná Basin, upper part of the Rio Bonito Formation. In the study area, the depositional systems are part of a large transgressive cycle that includes fluvial-estuarine facies at the base and barrier-lagoon deposits (associated with the development of salt marshes) at the top. Throughout the region, offshore strata cover the barrier-lagoon facies. The simulation successfully represented the retrograde evolution of the coastal barrier system, with longshore currents as the main transport mechanism of sediments. The evolution of the barrier system started with upper shoreface deposits overlying the lagoon system facies in the south. Later, lower shoreface sediments covered upper shoreface deposits. Then, shoreline migration continued landward. Finally, offshore transition deposits covered all previous deposits. Simulations and tests of controlling parameters allowed an adjustment to actual thickness and lithologies up to 83%. These simulations allowed for the analysis of model sensitivity, in particular to the influence of parameters that control sedimentary evolution. The main controlling parameter was sediment supply (volume and composition), followed by time step. Initial bathymetry and sediment velocity had less of an impact.

Can sediment supply variations create sequences? Insights from stratigraphic forward modelling

AbstractClassic sequence stratigraphy suggests depositional sequences can form due to changes in accommodation and due to changes in sediment supply. Accommodation‐dominated sequences are problematic to define rigorously, but are commonly interpreted from outcrop and subsurface data. In contrast, supply‐dominated sequences are much less commonly identified. We employ numerical stratigraphic forward modelling to compare stratal geometries forced by cyclic changes in relative sea level with stratal geometries forced by sediment discharge and water discharge changes. Our quantitative results suggest that both relative sea‐level oscillations and variations in sediment/water discharge ratio are able to form sequence‐bounding unconformities independently, confirming previous qualitative sequences definitions. In some of the experiments, the two types of sequence share several characteristics, such as an absence of coastal‐plain topset deposits and stratal offlap, something typically interpreted as the result of falling relative sea level. However, the stratal geometries differ when variations in amplitude and frequency of relative sea‐level change, sediment/water discharge ratio, transport diffusion coefficient and initial bathymetry are applied. We propose that the supply‐dominated sequences could be recognised in outcrop or in the subsurface if the observations of stratal offlap and the absence of coastal‐plain topset can be made without any strong evidence of relative sea‐level fall (e.g. descending shoreline trajectory). These quantitative results suggest that both supply‐dominated and accommodation‐dominated sequences are likely to occur in the ancient record, as a consequence of multiple, possibly complex, controls.

Study of 3D bathymetry modelling using LAPAN Surveillance Unmanned Aerial Vehicle 02 (LSU-02) photo data with stereo photogrammetry technique, Wawaran Beach, Pacitan, East Java, Indonesia

Coastal is an ecosystem that has unique object and phenomenon. The potential of the aerial photo data with very high spatial resolution covering coastal area is extensive. One of the aerial photo data can be used is LAPAN Surveillance UAV 02 (LSU-02) photo data which is acquired in 2016 with a spatial resolution reaching 10cm. This research aims to create an initial bathymetry model with stereo photogrammetry technique using LSU-02 data. In this research the bathymetry model was made by constructing 3D model with stereo photogrammetry technique that utilizes the dense point cloud created from overlapping of those photos. The result shows that the 3D bathymetry model can be built with stereo photogrammetry technique. It can be seen from the surface and bathymetry transect profile.

On the upscaling of process-based models in deltaic applications

Process-based numerical models are increasingly used to study the evolution of marine and terrestrial depositional environments. Whilst a detailed description of small-scale processes provides an accurate representation of reality, application on geological timescales is restrained by the associated increase in computational time. In order to reduce the computational time, a number of acceleration methods are combined and evaluated for a schematic supply-driven delta (static base level) and an accommodation-driven delta (variable base level). The performance of the combined acceleration methods is evaluated by comparing the morphological indicators such as distributary channel networking and delta volumes derived from the model predictions for various levels of acceleration. The results of the accelerated models are compared to the outcomes from a series of simulations to capture autogenic variability. Autogenic variability is quantified by re-running identical models on an initial bathymetry with 1cm added noise. The overall results show that the variability of the accelerated models fall within the autogenic variability range, suggesting that the application of acceleration methods does not significantly affect the simulated delta evolution. The Time-scale compression method (the acceleration method introduced in this paper) results in an increased computational efficiency of 75% without adversely affecting the simulated delta evolution compared to a base case. The combination of the Time-scale compression method with the existing acceleration methods has the potential to extend the application range of process-based models towards geologic timescales.

The connectivity of reservoir sand bodies in the Liaoxi sag, Bohai Bay basin: Insights from three-dimensional stratigraphic forward modeling

The distribution of sand bodies is complex in small half-graben rift basins influenced by differential tectonic subsidence. The characterization of reservoir sand bodies remains half quantitative because the depositional processes of the sand bodies are poorly understood and quantitatively investigated. Based on wells and seismic data, a three-dimensional stratigraphic forward modeling is conducted to quantitatively characterize the depositional evolution of reservoir stratigraphy and to analyze the connectivity of sand bodies as well as its controlling elements in Liaoxi Sag, Bohai Bay Basin. We integrate Controlling factors for sequence development including initial bathymetry of basement, subsidence, lake level change, sediment supply, and sediment transport Dionisos program to establish a 3D reservoir-sequence model for the middle part of the third member of Eocene Shahejie Formation. The model matches the wells data of the oilfield, such as lithologies, gamma ray, and resistivity, making it credible to further predict sandstone distribution. The sedimentary process of reservoir sand bodies appear four stages. In stage 1, two prototypes of delta-front sand bodies developed in the northwest. In stage 2, both the western delta-front sand bodies and eastern slope-fan sand bodies were widely developed. In stage 3, the delta-front sand bodies developed in large scale, whereas the slope fans were smaller. In stage 4, only a slope-fan sand body developed in the southeast, whereas lacustrine shale dominated in the rift basin.During the depositional evolution process, the western sand bodies were not connected to the eastern sand bodies. This disconnection is a function of source supply and sustained slope sinking caused by basin differential subsidence. We tested the interplay of tectonic subsidence and sediment supply through the stratigraphic modeling. Under the conditions of bilateral source supply, the limited number of sand sediments from western delta front could not be transported to the eastern structure high. Nevertheless, under assumptions of one source supply and strong water flow, the plentiful sand sediments could be carried onto the eastern structurally elevated position. Therefore basin differential subsidence and bilateral supply of source are the two controlling factors for the sand distribution in small half grabens. In addition, we found an interesting phenomenon that sand ratio in rifted basin center increases with subsidence decreasing. However, it will be a constant (upper limit) when subsidence declines to a threshold value. The threshold of “subsidence slope” (vertical subsidence divided by length of slope) varied with climate and sedimentary environment. In warm climate and delta environment, the threshold value is 2.3°.

Assessing Storm Impact on a French Coastal Dune System Using Morphodynamic Modeling

Muller, H.; van Rooijen, A.; Idier, D.; Pedreros, R., and Rohmer, J., 2017. Assessing storm impact on a French coastal dune system using morphodynamic modeling.Natural coastal dune systems provide protection against flooding from the ocean for many coastal communities around the world. An in-depth knowledge of their erosion mechanisms under oceanic stress can help to identify potential weak spots along the coastline and eventually prevent damage to the hinterland. The objective of this study was to assess the effects of hydro-morphodynamic phenomena on the coastal dune morphology during storm events and to evaluate their level of protection. This paper focuses on the physical processes that cause erosion and breaching of sandy dune systems, which are typically a function of the coastal morphology in combination with the hydraulic loading. An XBeach model was set up for the coast of Les Boucholeurs (France), which suffered from severe erosion, dune breaching (at six locations), and inundation of the hinterland during storm Xynthia in February 2010. The model was run for the full storm period, and it accurately reproduced the reported dune breach locations and dimensions with limited calibration. The model results were further utilized to study the temporal and spatial breaching dynamics and to understand the effect of the prestorm morphology on dune breach development. The results show that the dune behavior during the storm was strongly related to the submersion height of the dune crest and anticorrelated with its prestorm geometry (i.e. dune height and width). The governing parameters for the poststorm dune state are, in terms of alongshore-averaged erosion and in order of relative contribution: Submersion height over the dune crest, prestorm dune width, and prestorm dune crest height. Finally, additional simulations showed the limited effects of infragravity waves and the initial bathymetry used in the simulations.

Modelling the role of self-weight consolidation on the morphodynamics of accretional mudflats

We develop a consolidation module and merge it into a morphodynamic model to assess the role of consolidation on estuarine morphodynamics. We test the model using two settings: point models without hydrodynamic forcing to validate against two benchmark experimental datasets; and a profile model to simulate a mudflat restoration. The modelled self-weight consolidation influences the simulations by gradually reducing the bed level and decreasing the bed erodibility (i.e., increasing the critical bed shear stress). Both effects modify sediment transport processes on mudflats, leading to long-term morphodynamic effects. Depending on the initial bathymetry, the hydrodynamic forcing and the soil properties, the simulated morphological change of the restored mudflat may differ considerably with and without considering consolidation. The consolidation model developed can be utilised to assess the medium to long term effects related to estuarine development (e.g., wetland restoration) and aims to be a publicly available tool.

Sedimentary evolution of overlapped sand bodies in terrestrial faulted lacustrine basin: Insights from 3D stratigraphic forward modeling

Terrestrial faulted lacustrine basins usually have large variabilities in structural subsidence and multiple river sources. The facies in such basins are thus characterised by frequently changed facies and complex relationship of overlapped sand bodies (e.g. Liaoxi Depression of Liaodong Bay, Bohai Sea, China). Therefore, it's a great challenge to quantitatively describe evolution of sand bodies and their overlap relationship. In this study a three-dimensional stratigraphic forward model is used to solve this problem, which is corrected according to well log data. After the parameter optimization was verified against borehole strata thickness and lithology, this model had a thickness error within 8%, and the lithology distribution agreed with the well data as a whole.Four evolution phases of the Shahejie 3 Formation were recognised through the modeling. In the first phase, the lake level rose, and the eastern fan delta sand developed. In the second phase, the fan delta sand scaled up, and the braided river delta front sand developed at a small scale. In the third phase, the lake level shifted from falling to rising, the fan delta sand further expanded the scale, and the braided river delta developed at a large scale. Sands on both sides overlapped at a large scale. In the fourth phase, the lake level rose, the lake basin expanded, and lake shale was the main lithology in the study area. From the sedimentary evolution of Es3 formation, sand overlapped in the third stage. In 39.75 Ma, two parts of the sand body began to overlap and reached largest scale in 39.6 Ma, which was approximately 10 km2. Both then retrograded to a separation in 39 Ma when the overlap ended.Coupling stratigraphic forward simulation with uncertainty quantification allowed us to estimate the influence of each stratigraphic parameter on sand overlap. Fluvial discharge appears to be the most influential parameter while the initial bathymetry and lake level variations exert a minor to moderate impact.

Concurrent correction method for modeling morphological response to dredging an offshore sandpit

In modeling the morphological impact of an offshore dredging pit using a process-based model, the hydrodynamic conditions, often derived from regional scale models and sometimes incomplete measurements, may not be in equilibrium with the initial bathymetry profile before dredging. This lack of equilibrium causes a fast profile adjustment toward an equilibrium, which may bias the model results representing the direct effect of the pit.In this study, a method is described which uses the parallel computing technique and evaluates only the morphological changes due to a single factor, i.e., the dredging of a pit. The hydrodynamics and morphodynamics are assumed to be in equilibrium before initial dredging of the pit. The morphological evolution is computed based on the change of sediment flux due to the presence of the pit. The model can avoid the computational demanding spin-up period necessary to reach equilibrium conditions. In particular the concurrent correction method is efficient for scenario testing with a large amount of morphological settings and hydrodynamic conditions.

A comparative study of physical and numerical modeling of tidal network ontogeny

We investigate the initiation and long‐term evolution of tidal networks by comparing controlled laboratory experiments and their associated scaling laws with outputs from a numerical model. We conducted numerical experiments at both the experimental laboratory scale (ELS) and natural estuary scale (NES) and compared these simulations with experimental data and field observations. Sensitivity tests show that initial bathymetry, frictional parametrization, sediment transport, and bed slope terms play an important role in determining the morphodynamic evolution and the final landscape. Consistent with experimental observations, the morphodynamic feedbacks between flow, sediment transport, and bathymetry gradually lead the system to a less dynamic state, finally reaching a stable network configuration. In both the ELS and NES simulations, the initially planar lagoon with large intertidal areas is subject to erosion, indicating ebb‐dominance. Based on quantitative analyses of the ELS and the NES simulations (e.g., geometric characteristics and relationship between modified tidal prism and cross‐sectional area), we conclude that numerical simulations are consistent with laboratory experiments and show that both type of models provide a realistic, albeit simplified, representation of natural systems. The combination of laboratory and numerical experiments also allowed us to explore the possibility of reaching a long‐term morphodynamic equilibrium. Both the physical and numerical models approach a dynamic equilibrium characterized by negligible gradients in sediment fluxes. The equilibrium configuration appears to be consistent with traditional relationships linking tidal prism and cross‐sectional area of the inlet. Finally, this contribution highlights the significance of complementary research between experimental and numerical modeling in investigating long‐term morphodynamics of tidal networks.

Numerical modeling of the impact of sea level rise on tidal basin morphodynamics

AbstractThe morphod ynamic adaptation of estuaries to sea level fluctuations has been subject of geological studies based on sediment core analysis and qualitative modeling efforts. Limited attention has been paid to understanding bathymetric evolution based on a detailed process level. The current study aims to explore governing morphodynamic processes and timescales by application of a 2D, process‐based modeling approach. The starting point of the analysis is an 80 km long and 2.5 km wide basin. Starting from a sandy flat bed, stable channel‐shoal patterns emerge within a century under semidiurnal tidal forcing. We impose a gradual rise in sea level (up to 0.67 m per century) and compare the results with a run excluding sea level rise (SLR). Model results show that SLR drowns the basin so that intertidal area disappears. This process generates tidal asymmetry reflected by an emerging M4 tidal constituent. The basin shifts from exporting to importing sediment reflected by shoal patterns migrating in the landward direction. The landward sediment transport remains too limited to compensate for the loss in intertidal area and to restore equilibrium within a millennial time scale. Further sensitivity tests on initial bathymetry, tidal amplitude forcing, and rate of SLR show that shallow basins with limited tidal forcing are most vulnerable to SLR.

Modelling the effects of tidal range and initial bathymetry on the morphological evolution of tidal embayments

Tidal embayments are characterized by a wide variety of landscape features, often including either complex tidal channel networks or extensive flood-tidal deltas. The origin of these features and the influence of hydrodynamic drivers and initial geological setting on their long-term characteristics are essentially unexplored. A model was applied to simulate the long-term morphological evolution of tidal embayments, with the purpose of providing insight into the environmental conditions that lead to the differences in tidal embayment morphology. Numerical simulations indicated that the interaction between hydrodynamics, sediment transport, and the evolving topography gives rise to the formation of channel networks. The tidal range and the depth of the initially unchannelized tidal basin controlled the way in which the morphology evolved and determined the timescale over which channels and intertidal areas developed. Channel network formation occurred more rapidly when the tidal range increased and/or when the initial basin depth decreased. Tidal basins with a large initial depth showed the development of a flood-tidal delta and for these deep basins channel incision could remain absent over long timescales. Both tidal range and initial bathymetry affected final basin hypsometry and channel network characteristics, including the channel density and the fraction of the basin occupied by the channels. All the simulated morphologies, with different combinations of the tidal range and depth of the basin, evolved towards a state of less morphodynamic activity for which the relative intertidal area was proportional to the ratio of tidal amplitude to basin depth.

A NOVEL CELLULAR AUTOMATA APPROACH TO ESTUARINE MORPHODYNAMIC MODELLING

The need for reliable modelling techniques for the prediction of morphological change in coastal settings has become increasingly important to coastal planners and policy makers in recent years due to the effects of accelerated sea level rise and a shift in approach from coastal defence to coastal management. In this research a new cellular automata based model is developed in order to bridge the gap between current bottom-up, process based models and top-down behaviour oriented models of estuarine morphodynamic evolution, and make predictions of morphological change over medium time-scales (one year to several decades). The key processes of tidal flows, waves, sediment transport and salt marsh ecology are represented in simplified form in order to capture the complex interactions and feedbacks that occur between them and which ultimately determine how the morphology will develop in response to environmental change. The initial bathymetry of the estuary is represented by a cluster of rectangular cells in a CA domain. Tidal flows are estimated using a new routing model, which shows good agreement with a conventional 2D hydrodynamic model but is much more computationally efficient.

Development of crescentic bars for a periodically perturbed initial bathymetry

[1] The development of crescentic bed patterns starting from an ‘undisturbed’ beach (i.e. one only perturbed so as to simulate background noise and thereby initiate morphological development) is compared with that starting from a beach where monochromatic (and in some cases bichromatic) bed patterns pre-exist, using a fully non-linear model (Morfo55). A wide range of different lengthscales and amplitudes is investigated. The findings suggest that the pre-existence of crescentic bed-forms can influence the subsequent morphological development of this beach significantly, but that this development is related to the undisturbed beach development. Whether a pre-existing bed-form remains under certain forcing conditions depends on the position of the pre-existing lengthscale along the undisturbed linear growth rate curve: If the pre-existing mode shows significant linear growth in the undisturbed scenario, this pre-existing lengthscale remains. Otherwise the pre-existing mode will be replaced by a bed-form with a lengthscale closer to the undisturbed fastest growing mode. A pre-existing lengthscale that is significantly longer than the undisturbed fastest growing mode will be replaced by a higher harmonic of the original lengthscale; for shorter pre-existing lengthscales, this will be the linear fastest growing mode. An increased amplitude of the pre-existing bed-form accelerates the development toward a new stable situation. For small initial amplitudes, the initial response of the system to pre-existing bed-forms could theoretically be described using linearized equations alone. The migration rates of both the pre-existing and newly arising modes correspond to the migration rate of these modes in the undisturbed scenario.

A numerical model to simulate the formation and subsequent evolution of tidal channel networks

We present a numerical model that simulates morphological change as a result of the interactions between hydrodynamics, sediment transport and bed elevation change. Numerical simulations indicate that these morphodynamic interactions can lead to the initiation of tidal channels and potentially give rise to large-scale channel networks. We perform a sensitivity analysis to show how model outcome is sensitive to the numerical scheme adopted, hydrodynamic and morphological time-steps, and initial bathymetry. Furthermore, the formation of tidal channels and intertidal areas affects both the large-scale flow patterns, as well as the asymmetry between flood-and ebb-tidal currents.

Seasonal-scale nearshore morphological evolution: Field observations and numerical modeling

A coupled waves–currents-bathymetric evolution model (DELFT-3D) is compared with field measurements to test hypotheses regarding the processes responsible for alongshore varying nearshore morphological changes at seasonal time scales. A 2001 field experiment, along the beaches adjacent to Grays Harbor, Washington, USA, captured the transition between the high-energy erosive conditions of winter and the low-energy beach-building conditions typical of summer. The experiment documented shoreline progradation on the order of 10–20 m and on average approximately 70 m of onshore sandbar migration during a four-month period. Significant alongshore variability was observed in the morphological response of the sandbar over a 4 km reach of coast with sandbar movement ranging from 20 m of offshore migration to over 175 m of onshore bar migration, the largest seasonal-scale onshore migration event observed in a natural setting. Both observations and model results suggest that, in the case investigated here, alongshore variations in initial bathymetry are primarily responsible for the observed alongshore variable morphological changes. Alongshore varying incident hydrodynamic forcing, occasionally significant in this region due to a tidal inlet and associated ebb-tidal delta, was relatively minor during the study period and appears to play an insignificant role in the observed alongshore variability in sandbar behavior at kilometer-scale. The role of fully three-dimensional cell circulation patterns in explaining the observed morphological variability also appears to be minor, at least in the case investigated here.