Property:CSDMS meeting abstract
From CSDMS
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Rivers exhibit high temporal variability in their constituent concentrations, which can result in distinct hysteresis patterns of suspended sediment (SS), particulate organic carbon (POC), and dissolved organic carbon (DOC) during various high-flow events. In mountain rivers, this variability may be controlled by the mobilization of streambed sediment, which is enriched with organic carbon and other nutrients and can be evaluated by the entrainment of the armor layer, a group of coarser particles that act as a protective layer to finer sediment. In this study, we test if resulting hysteresis patterns could be controlled by the extent to which the armor layer was mobilized by analyzing POC, DOC and SS patterns during monsoon-driven flow events in two river reaches at the Valles Caldera National Preserve, New Mexico. We collected continuous water samples over numerous flood hydrographs using automated ISCO samplers while also monitoring flow depth, turbidity, and fluorescent dissolved organic matter (fDOM) using YSI EXO2 sondes. Turbidity and fDOM were included because they can be used as proxies for SS and DOC, respectively. To quantify armor mobilization, we deployed tracer particles of different sizes and mapped their location before and after each event. We found that constituent hysteresis changes between clockwise and counterclockwise for different flow events, each with their own streambed mobilization characteristics. Opposite hysteresis occurred for DOC and fDOM than for POC, SS, and turbidity. Additionally, there were poor correlations between DOC and SS, while POC and SS were found to be well correlated. Our results imply that DOC hysteresis might not be related to the release of fines from the streambed, whereas POC is. These findings suggest that the temporal variability of SS and POC can be partly understood by the mobilization and suspension of fine sediment not only seasonally, but also over individual high-flow events. We show how the mobilization of the armor layer can be an important contributor to this variability.
Rivers flowing across permafrost limit the residence time of floodplain soil organic carbon (SOC) by transferring riverbank material to the fluvial network. In this way, permafrost riverbank erosion limits production of greenhouse gases (GHG) in a deepening permafrost active layer. Concurrently, arctic riverbanks are laden with permafrost ice wedges and channels themselves are seasonally occupied by ice; these factors are known to affect fluvial potential to erode and remove material from floodplains. However, we do not know how ice impacts rates of riverbank migration through permafrost, especially in small arctic watersheds where observations are limited. We hypothesize that bank migration into permafrost is best described by melting of bank ice but is also described by slumping of the active layer, and fluvial carrying capacity. First, we develop a model of thermal riverbank erosion. Next, we detail plans to incorporate slumping and mechanical erosion into a holistic icy riverbank erosion model. We will characterize riverbanks along the Canning this field season and use or measurements to calibrate our model. Then, we will test which mechanisms are most significant in forming permafrost riverbanks, as well as describe their evolution under warming mean annual air temperatures. This work is also useful for predicting the future contribution of the arctic rivers to their basin-wide carbon budget. This work has immediate importance for people who traverse or depend on arctic landscapes, but especially those who live within them. Arctic landscape response to climate change is just as much a story about the loss of place and vanishing resources as it is about a dynamic earth system. +
Rivers in natural settings are frequently characterized by downstream variations in channel width. However, the effect of width variations on bed topography and sorting patterns remains poorly understood, especially under conditions of changing sediment and hydrologic regimes. In this study we use two-dimensional numerical modeling to systematically explore how the amplitude and wavelength of sinusoidal width variations affect the shape and location of bars, sorting patterns of surface sediment, and the movement of a sediment pulse. We perform simulations with sediment regimes consisting of constant sediment supply, no sediment supply, and a sediment pulse with no background sediment supply. We also perform steady and unsteady flow simulations to explore the combined effect of hydrograph shape and width variations. Preliminary results indicate that width variations force riffle-pool topography with riffles coincident with wider channel sections and pools at narrow sections. The amplitude of width variations is the dominate factor controlling riffle-pool relief. The wavelength of the width variations controls whether central or side bars develop in the wider channel sections. These numerical simulations are complimented with ongoing physical experiments in a laboratory flume and can potentially be used to guide stream restoration and river management practices under conditions of varying sediment and hydrologic regimes. +
Robust estimate of the critical shear stresses and flocculation is a prerequisite for analyzing the sediment dynamics of tidal flats. To determine the impact of clams (Meretrix meretrix Linnaeus) on critical shear stress and flocculation, in situ measurements were made on both a bare flat and a flat inhabited by abundant clams. on the Jiangsu Coast, China. Near-bed in situ floc size, grain size distribution of suspended particles in sea water, suspended sediment concentration (SSC), salinity, and currents were measured for three consecutive semidiurnal tidal cycles simultaneously at the two stations. Based on the observational data, mean floc size measured by LISST-100x was found to be more than three times the size of dispersed suspended sediment in water samples, suggesting remarkable flocculation processes were occurring. Correlation analysis indicated that the flocculation and break-up process in the study area appeared to be controlled by the variations in SSC and turbulent shear. Negative relationships were found between SSC, turbulent shear and floc size for both stations, but for a given SSC, floc size was usually larger at the clam aquaculture site. The comparison between the two sites shows that the near-bed in situ floc size in the aquaculture mudflat (mean 88 μm) was 70% larger than that in the bare tidal flat (mean 51 μm), suggesting significant biological modulation on the flocculation processes, as the hydrodynamics were very similar between the two sites. The content of extracellular polymeric substances was obviously higher in the sediment layer below the surface seabed at aquaculture site. Further, we find that the sediments on the flat inhabited by clams were more erodible and had a lower critical shear stress for erosion (0.13 N m-2) due to the physical and biological activities of clams. The results further show that the critical shear stress for deposition on the flat with M. meretrix (0.13 N m-2) was 30% greater than that in the bare flat (0.10 N m-2). Our results suggest that changes to the critical shear stress for erosion and deposition caused by the activity of clams can alter the sediment dynamics and geomorphologic processes of flats, as well as abundant filter feeders alter floc properties and enhance flocculation by excretion of exopolymer particles.
Salmonine fishes (salmon and trout) are resilient and have evolved to survive environmental perturbations, including flood, drought, and wildfire. The effects of these perturbations are translated through the landscape by rivers, where aquatic communities can be severely impacted. For instance, after wildfire, rivers can experience increased frequency and magnitude of flash floods, ash and nutrient loading, increased sediment flux from runoff and debris flows, destabilization and physical alteration of fluvial habitat, stream temperature impairment, and either loss or gain of refuge (e.g. deep pools, woody debris, riparian vegetation). Depending on the severity, any one of these effects could drive the extirpation of fish populations, and the response and survival of fish gets increasingly complex when faced with multiple environmental perturbations. Historically, the extirpation of fish populations would not have been as significant a risk to the extinction of entire species or subspecies of salmonids, as unrestricted migration allowed for recolonization by neighboring populations. However, increasing river disconnectivity, due to the introduction of physical barriers, has put native fish species at greater risk of extinction after natural catastrophes. In order to evaluate the viability and recovery of fish populations after catastrophe, we have developed a multi-site structured population viability analysis (PVA) model that is designed to incorporate factors that are unique to the spatial distribution of catastrophe and migration in fluvial networks. Specifically, our multi-site PVA provides the flexibility to vary both the duration and severity (i.e., multi-year catastrophe and habitat recovery) of vital rate adjustment (survival and growth). Our model also allows for a multi-mechanistic approach to vital rate adjustment after catastrophe – this is a particularly important advancement, as fluvial habitats located within the fire perimeter often experience distinctly different impacts than those outside of but downstream of fire. Both of these improvements are necessary as the negative impacts of wildfire on fish habitat and vital rates can last for years or even decades, and commonly used PVA modeling software only allows for impairment to last for one year. Additionally, previous models allow for a “one, all or radial spreading” approach to the spatial distribution of catastrophe, which works for disease but is inconsistent with the flow routing of catastrophe in stream networks. Finally, we have also developed a new metapopulation migration model that accounts for bidirectional river connectivity, a characteristic of migration unique to fluvial environments. Migration behavior in this model is driven by simple probabilities of life-stage structured dispersal and migration distances, measures of habitat suitability (including post-catastrophe adjustment), and site population densities. To demonstrate the utility of our multi-site PVA, we apply it to a case study of Bonneville Cutthroat Trout after the Twitchell Canyon Fire in the Fish Lake National Forest, Utah. The impact on and recovery of trout populations after wildfire was monitored across 14 sites of variable hydrologic, temperature and physical impairment (both within and outside of the fire perimeter). Using these observations along with maps of stream connectivity barriers, we model trout population viability and recovery after wildfire in this site. We also compare our results to model simulations using single year impairment, more similar to that of previous PVAs. Finally, we demonstrate the potential improvements on population recovery through simulations removing individual fish barriers throughout the network. This model presents a new framework for directly linking parameters of landscape change that may vary in both spatial and temporal distribution to the viability of fish populations after natural catastrophe. Plans for future model development include linking the PVA with models of fish bioenergetrics and landscape evolution, which can provide spatially variable predictions of changes in discharge, stream temperature and sediment fluxes after fire. Ultimately, we hope to develop and provide a new management tool for evaluating the overall vulnerability of aquatic organisms to wildfire in watersheds throughout the Intermountain West.
Salt and shale based minibasins are quasi-circular depression connected by submarine canyons of economic importance because they are prime locations of hydrocarbon reservoirs. The history of sedimentation of the minibasins is modulated by sea level changes, and it is strongly influenced by basin topography and continental shelf dynamics. Sedimentation in intraslope minibasins is generally described in terms of the “fill-and-spill” model in which the turbidity currents enter a minibasin and are reflected on the minibasin flanks. After the reflection, the turbidity currents pond and deposit the suspended sediment. As the minibasin fills, the current spill over the lowermost point of the minibasin flanks and reaches the next minibasin downslope, where the fill and spill process starts again. In this stage, deposition still occurs in the upslope minibasin with the formation of channel-levee complexes. In the last two decades field, laboratory and numerical studies focused on the description of (1) the large-scale stratigraphic architecture and evolution of the minibasins and (2) the behavior of the turbidity currents in the minibasin-canyon system. This notwithstanding, questions regarding the spatial distribution of the grain sizes in minibasin deposits, the role of the system geometry and of the flow characteristics of the turbidity current on the depositional pattern still need to be answered. The objective of the present study is to investigate with three-dimensional model simulations how the deposit characteristics change for increasing in slopes of the minibasin-canyon system. In particular, we are using a three-dimensional numerical model of turbidity currents that solves the Reynolds-averaged Navier–Stokes equations for dilute suspensions. Turbulence is modeled with a buoyancy-modified k–ε closure. The numerical model has a deforming bottom boundary to model the changes in elevation and grain size characteristics of the bed deposit associated with sediment erosion and deposition. Here we present the model validation against 1) 2D laboratory experiments of a horizontal minibasin in a constant width flume, and 2) 3D laboratory experiments on two linked minibasins. The model validation is performed comparing measured and simulated deposit geometries, vertical profiles of suspended sediment concentration and spatial distributions of sediment sizes in the deposit. In the near future, we will perform laboratory scale simulations by changing the slope of the experimental minibasin, i.e. the difference in elevation between the entrance and the exit points to study how the depositional pattern changes when the relative size of the ponded accommodation space, i.e. the space at a lower elevation than the spill point, and the perched accommodation space, i.e. the space under an ideal line connecting the spill point and the minibasin entrance.
Salt marsh provides critical estuarine habitat and shoreline protection, and is highly vulnerable to sea-level rise. Models of marsh accretion and resilience to sea-level rise rely on estimates of sediment supply, yet the factors governing sediment supply to marshes and its temporal variation are poorly understood. This presentation focuses on temporal variability in suspended-sediment concentration (SSC) and spatial gradients in SSC at the marsh edge, with two goals: 1) to identify processes important to sediment supply, and 2) to inform the choice of SSC values to use as input to marsh accretion models. We present data collected as part of an investigation of the influence of tides and wind waves on sediment supply to an estuarine salt marsh in China Camp State Park, adjacent to San Pablo Bay, in northern San Francisco Bay (tide range approximately 2 m). The long-term sediment accretion rate in the lower China Camp marsh is 3 mm/year. The marsh vegetation is predominately Salicornia pacifica, with Spartina foliosa occupying the lower elevations adjacent to the mudflat. The marsh is bordered by wide intertidal mudflats and extensive subtidal shallows. In the winter of 2014/2015 and the summer of 2016 we collected time series of SSC, tidal stage and currents, and wave heights and periods in the bay shallows, in a tidal creek, and (except for currents) on the marsh plain. On the mudflats, SSC depends strongly on wave energy, and also varies inversely with water depth, increasing toward the marsh edge and with decreasing tidal stage. Within the marsh, SSC is lower in the Salicornia-dominated marsh plain than at the marsh edge, as expected, but in the Spartina zone SSC is greater than at the marsh edge. This effect is greater in summer, when Spartina is significantly taller and denser, than in winter. SSC over the marsh was typically greater during flood than ebb tides in both seasons, indicating net deposition over the tidal cycle. However, median flood-tide SSC over the marsh, and the inferred deposition, were greater in summer than winter. We attribute the increased SSC and deposition in summer to greater sediment trapping in Spartina, followed by mobilization and transport of sediment onto the marsh during subsequent high tides.
Sandwich Town Neck Beach, MA, USA is a 1300-m long barrier spit on the north shore of Cape Cod that has experienced chronic dune erosion during nor’easters. Repeated mapping with drones and photogrammetry shows that dune erosion varies spatially along the beach. We used the wave model SWAN to compare simulated waves with the observed morphological response along the barrier spit. Local refraction and dissipation of wave energy by the complex nearshore bathymetry produces alongshore variations in wave energy, with regions of diverging alongshore wave power corresponding to the regions of rapid erosion. These regions are also where lower beach profiles are steepest, waves break closest to shore, and wave runup is highest. These results suggest that, while erosion rates may be affected by the availability of sediment from up-drift sources, wave patterns play an important, and maybe dominant, role in determining alongshore variations in dune erosion at Sandwich. +
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Satellite and field observations find modern carbonate depositional systems to be self-organized, yet the processes generating such behavior are not fully understood. A 3-D forward model of carbonate reef growth rooted in cellular automata is developed to simulate the evolution of self-organized geometry through time. Carbonate landscapes are generated over spatial extents of several kilometers through time scales of millennia at meter-scale resolution. Classes in the model include carbonate factories (e.g., branching and massive coral communities, algal communities) and sinks (e.g., unconsolidated sand). Environmental factors include relative sea level and light intensity, and ecological controls are based on life history traits for the biological facies. Ecological processes within the model include mortality and colonization rates for biological classes, transition probabilities between facies, and rates of vertical accretion. The algorithm results in a self-organized landscape that emulates those observed in nature, such as rims and reticulate structures. Visualizations can be produced by accessing topographic and facies maps generated at each time step. This project’s goals are 1) to investigate which configurations of environmental parameters result in specific spatial motifs, 2) examine the effects of environmental perturbations on reef construction, and 3) understand the importance of biological and physical regimes on the generation of geomorphological features. +
Satellite remote sensing is a powerful tool for terrestrial hydrological studies. In particular studies of droughts and floods - hydrological extremes can be well accomplished using remote sensing. In particular, we will use data from the visible-infrared and microwave sensors on NASA platforms to studies the onset and propagation of droughts as well as spatial extent of flooding. In this talk we will present numerous examples of hydrological extreme events and the use of satellite remote sensing as a tool for mapping the spatial extent and the temporal persistence. The droughts of 1988 and 2012 in the United States Midwest, flooding in 1993 and 1998 are strong examples in United States. There have been numerous such events in Asia in India, Pakistan and China which have affected billions of people who depend on the land and agricultural productivity to a much greater degree than in United States. +
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Sea level rise and changes in storm climate will modify the intensity of cross-shore sediment transport processes in the coming decades and centuries. Chief among these processes is the opening of tidal inlets on barrier islands which can affect nearby erosion rates and shoreline positions on a decadal time scale. In addition, shoreline change related to sea level rise and overwash deposition could vary alongshore in ways that could be dynamically coupled to patterns of coastline sculpting from gradients in alongshore sediment flux.
To examine how cross-shore and alongshore patterns interact, we will couple the BarrieR island and Inlet Environment (BRIE) model to the Coastline Evolution Model (CEM). BRIE has also been coupled to Barrier3D, which resolves cross-shore processes at higher resolutions. CEM was previously used to hindcast the component of coastline change patterns arising from alongshore sediment flux gradients, without including the components related to inlets or overwash. Coupled model experiments using BRIE, Barrier3D and CEM exploring how contrasting sets of processes dynamically interact will inform upcoming work to improve long term hindcasting and forecasting of shoreline change on complex coastlines. +
Sea level rise presents an urgent threat to the occupants of river deltas. However, while low lying deltaic landscapes are at risk of significant drowning, the ability to harness a river’s sediment delivery system offers deltaic populations a mechanism to control the location and extent of land loss via land building sediment diversions. Despite their well-recognized importance there are few examples of diversions that have been intensively monitored throughout their development to the extent necessary to support engineering decisions.
In order to guide the operational design of two planned diversions in the Lower Mississippi River, we apply Delft3D to simulate diversion discharge through time as a function of the characteristics of the receiving basin. In both cases the conveyance channel connecting the river to the basin is prevented from eroding.
We find that diversions in basins that offer many outlets for flow are more likely to maintain their discharge over a ten-year time horizon. We also find that diversion performance is not significantly affected by substrate erodibilities in the range of those found in the Mississippi River Delta, but that artificially increased bed strength would lead to decreases in performance. Our work also sheds light on the spatial pattern of erosion near a diversion. We find that very little erosion into the substrate occurs away from the immediate vicinity of the outfall channel, but that the evolution of the proximal scour is a critical control on the sustainability of the diversion. Ecological considerations suggest that operating diversions at low flow might be useful, but this practice increases the risk of back flow from the receiving basin. +
Seagrass provides a wide range of economically and ecologically valuable ecosystem services, with shoreline erosion control often listed as a key service, but can also alter the sediment dynamics and waves within back-barrier bays. Here we incorporate seagrass dynamics into the existing barrier-marsh model GEOMBEST++ to examine the coupled interactions of the back-barrier bay with both adjacent (marsh) and non-adjacent (barrier island) subsystems. In our new integrated model, bay depth and distance from the marsh edge determine the location of suitable seagrass habitat, and the presence or absence, size, and shoot density of seagrass meadows alters the bathymetry of the bay and wave power reaching the marsh edge. While seagrass reduces marsh edge erosion rates and increases progradation rates in many of our model simulations, seagrass surprisingly increases marsh edge erosion rates when sediment export from the back-barrier basin is negligible. Adding seagrass to the bay subsystem leads to increased deposition in the bay, reduced sediment available to the marsh, and enhanced marsh edge erosion until the bay reaches a new, shallower equilibrium depth. In contrast, removing seagrass liberates previously-sequestered sediment that is then delivered to the marsh, leading to enhanced marsh progradation. Lastly, we find that seagrass reduces barrier island migration rates in the absence of back-barrier marsh by filling accommodation space in the bay. These model observations suggest that seagrass meadows operate as dynamic sources and sinks of sediment that can influence the evolution of coupled marsh and barrier island landforms in unanticipated ways. +
Sediment creep is ubiquitous and precedes failure (e.g. landslides) in most landscapes. Accurate modeling of sediment creep is therefore crucial for predicting both the long-term (>10 000 years) evolution of landscapes and the short-term (minute to centuries) evolution of landscapes and infrastructures. Current sediment creep transport laws used in landscape modeling are determined empirically over geological time scales and are diffusion-like (Roering et al,2001); yet the mechanics of sediment creep on all time scales remain poorly understood. As a result, creep models used in civil engineering, materials science, and geomorphology are largely disconnected in time scales, goals, and approaches. In particular, excess porous flow from rain infiltration is currently not a governing parameter of any creep model, while large rain events are known to trigger landscape failures.
Houssais et al. (2021) showed experimentally for the first time, that porous flow can be a leading cause of creep, and ultimately the failure (avalanching) of sediment piles, for flow strength (or pore pressure) far lower than classically admitted. Building on the results from Houssais et al., we propose a new equation for sediment creep consistent with the general formalism of the mechanical creep of disordered materials. In our equation, the creep sediment flux is a function of: topographic slope (similar to the equation from Roering et al.), porous flow intensity, grains and fluid properties, and, importantly, time.
We present here the first results of landscape dynamics from the implementation of our new sediment creep function in landlab, for the case of idealized berms (or coastal natural dams), before they breach. The long-term goal of this effort is to compare the model to our topographic and hydrogeologic observations of berms (pre-)breaching on the coast of Monterey County, CA, that occur each winter, as large rain episodes hit the land. This specific case is a good way to test our model validity over time scales from 1 minute to 1 month. In our presentation, we will show preliminary results of the berms creep (pre-breaching) dynamics, using over-simplified equations for the groundwater flow. In the future, we intend to develop a Landlab component of our new creep function, which could be used with Groundwaterdupuitpercolator, a landlab component recently developed to model groundwater flow while modeling landscape dynamics (Litwin et al., 2020, 2022).
In the end, once this model is validated, it will allow us to model sediment creep at all time and rate scales, and better predict chances of, and monitor, sedimentary failures, such as breaching and landslides. Our new model for sediment creep fundamentally addresses our needs for better understanding and forecasting landscape response to changing climate patterns.
Houssais, M., C. Maldarelli, and J. F. Morris, “Athermal sediment creep triggered by porous flow,” Physical Review Fluids, vol. 6, no. 1, p. L012301, 2021.
Litwin, D. G., G. E. Tucker, K. R. Barnhart, and C. J. Harman, “Groundwaterdupuitpercolator: A landlab component for groundwater flow,” Journal of Open Source Software, vol. 5, no. 46, p. 1935, 2020.
Litwin, D. G., G. E. Tucker, K. R. Barnhart, and C. J. Harman, “Groundwater affects the geomorphic and hydrologic properties of coevolved landscapes,” Journal of Geophysical Research: Earth Surface, vol. 127, no. 1, p. e2021JF006239, 2022.
Roering, J. J., J. W. Kirchner, L. S. Sklar, and W. E. Dietrich, “Hillslope evolution by nonlinear creep and landsliding: An experimental study,” Geology, vol.
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Sediment delivery to low-lying coastal zones must keep pace with, if not exceed, the rate of sea level rise in order to maintain a positive surface elevation. Deltaic lowlands are vulnerable to both sea-level rise and changes in river discharge, but whether the floodplains and coastal areas will ultimately drown depends on a balance of aggradation, eustatic sea level rise and subsidence. The Ganges-Brahmaputra (G-B) Delta is an example of a densely populated coastal system that could be flooded by rapid sea level rise within the next century. Annual monsoonal river flooding and cyclonic storm surges are the principal mechanisms by which sediment is distributed across the G-B floodplain and coastal plain. Stratigraphic reconstructions show that sedimentation in the upper floodplain was more than doubled under the Early Holocene enhanced monsoonal regime, suggesting that the delta may withstand an increase in monsoonal intensity, flooding, and tropical cyclones that are currently predicted in ensemble Community Climate System Model scenarios.
In an effort to improve predictions of climatic forcing on aggradation rates in the G-B floodplain and lower delta, direct sedimentation measurements are paired with a series of model components coupled within the CSDMS Modeling Tool (CMT). A sediment flux model, a floodplain sedimentation model and a tidal-plain sedimentation model will be linked to explore the response of the G-B river system to a future sea-level rise and changes in river discharge. Model algorithms will be validated by sedimentation data collected in 2008 and 2012 from the tidal delta (The Sundarbans National Reserve mangrove forest) and the highly cultivated fluvial-dominated delta plain. Field data will also be compared to model outputs by constraining the spatial patterns of sedimentation across the delta front. In this talk, we present initial sedimentation results and discuss controls on heterogeneous patterns of deposition in the tidal versus fluvial dominated parts of the delta. Early results from individual model components will also be discussed in an attempt to integrate current understanding of the G-B System into a numerical modeling framework.
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Sediment dynamics on Arctic shelves can impact coastal geomorphology, habitat suitability, and biogeochemical cycling, and are expected to be sensitive to changes in sea ice extent. Variability in coastal erosion, for example, has been related to variations in waves due to changes in sea ice extent, as well as water temperature. Yet, it remains unclear how changes in sea ice extent will impact hydrodynamic and sediment transport conditions on the continental shelf. To analyze this, we are using a coupled hydrodynamic - sediment transport numerical model, the Regional Ocean Modeling System (ROMS) - Community Sediment Transport Modeling System (CSTMS). The model is implemented for the Alaskan Beaufort Sea shelf and currently accounts for winds, sea ice, offshore currents, rivers, waves, and multiple sediment classes. Ongoing work includes finalizing model inputs. The model is being run for the 2019 open water season when sea ice retreats 100 - 300 km offshore. Analysis will focus on spatial and temporal variations in current velocities, waves, bed shear stresses, and sediment fluxes. Preliminary results show that the time-averaged depth-averaged currents, and likely sediment fluxes, are directed eastward along the shelf. Additionally, the largest bed shear stresses occur near the coast and on the shelf-slope break. Future work includes additional analyses, as well as sensitivity tests to better understand how a lengthening open water season and changing weather conditions may influence shelf sediment dynamics. +
Sediment dynamics on Arctic shelves can impact coastal geomorphology, habitat suitability, and biogeochemical cycling, and are sensitive to changes in sea ice extent. Variability in coastal erosion, for example, has been related to variations in waves due to changes in sea ice extent, as well as water temperature. Yet, it remains unclear how changes in sea ice extent will impact hydrodynamic and sediment transport conditions on the continental shelf, motivating this study. To analyze this, we are using a coupled hydrodynamic-sediment transport numerical model, the Regional Ocean Modeling System (ROMS) - Community Sediment Transport Modeling System (CSTMS). The model is implemented for the Alaskan Beaufort Sea shelf and currently accounts for winds. Ongoing work includes accounting for waves, sea ice, and setting up open boundary conditions. In order to analyze variations in hydrodynamics and sediment transport, the model will be run for two open water seasons representing time periods where sea ice retreats 100-300 km offshore. Analysis will focus on spatial variations in current velocities, waves, and bed shear stresses, as well as how model estimates vary between the two time periods. Future work involves accounting for sediment transport in the model and performing sensitivity analyses to better understand how a lengthening open water season may influence the shelf sediment dynamics. +
Sediment transport is a universal phenomenon responsible for the self-organization of bedforms and dunes seen on the surfaces of many planetary bodies. The smallest of these patterns are wind, or impact ripples. Encoded in the sizes and propagation speeds of impact ripples is direct information about the local transport and environmental conditions: sediment fluxes, wind speeds, grain size, etc. However, to get at this information we must understand the processes that govern ripples dynamics. Because of the complexity of sediment transport, our current understanding of ripples is almost purely empirical, and the parameter space of the system has barely been explored.
To aid at the process of understanding impact ripple dynamics in arbitrary environments we turn to a discrete element model (DEM) of sediment transport. Simulated ripples sizes from the DEM quantitatively agree with wind-tunnel and field data and therefore the DEM can be used as an experimental tool to explore the state space of the system. Preliminary experiments suggest that ripple wavelengths scale with the average hoplength of eroded grains, but only above a threshold. Below this threshold wavelengths stagnate and ripples begin to propagate upwind. These “antiripples” have not previously been predicted or observed. Yet simulations suggest that they are persistent for many planetary conditions such as those on Venus and even Earth (for large enough grain sizes). We present additional findings for a range of environmental conditions found in our solar system and beyond, and thus map out a more complete space of possible states for ripple formation in the Universe. +
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Sedimentary delta formation varies over a wide range of time and space scales. Reduced-complexity models offer a worthwhile means of retaining key dynamics and phenomena in delta morphodynamics through employing approximate but physically reliable descriptions of governing transport equations. To that end, we developed a cellular rule-based model, using a “directed” random-walk to determine the flow field, coupled with empirically based sediment transport schemes, following an Exner equation combining bedload and suspended load. Preliminary results provide physically reasonable 3-dimensional topographical features, as well as dynamic processes like channel avulsions and bifurcations. Stratigraphy is also recorded. The flexibility of the modeling framework makes each building block to be updated separately, which will allow for the ready extension to include additional phenomena such as waves and tides. +
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Sequence is a modular 2D (i.e., profile) sequence stratigraphic model that is written in Python and implemented within the Landlab framework. Sequence represents time-averaged fluvial and marine sediment transport via differential equations. The modular code includes components to deal with sea level changes, sediment compaction, local or flexural isostasy, and tectonic subsidence and uplift. Development of the code was spurred by observations of repetitive stratigraphic sequences in western Turkey that are distorted by tectonics. +
