Property:CSDMS meeting abstract
From CSDMS
This is a property of type Text.
2
The lower Mississippi River drains a watershed of over 3.2 million square kilometers. The continental flux of water, sediment, and nutrients passes through the state of Louisiana in the last stretch of its journey to the Gulf of Mexico. A portion of the river detours, pronounced during high flow events, to the gulf through a series of natural and manmade diversions. Systemic understanding of the Mississippi River sediment and water resources partitioning among various outlets or diversions is crucial to the sustained function of the Northern Gulf of Mexico’s communities, habitats, and industries. This study discusses the development and application of a Delft3D FM 3-dimensional hydrodynamic, salinity, and temperature model of the Northern Gulf of Mexico. We used this model to analyze and quantify the tradeoffs among various management scenarios for freshwater allocation in the lower Mississippi River through existing and proposed infrastructure and natural openings. We also explored the possibility of varying the operational strategies of existing structures to investigate the changes in service and protection to communities in the receiving basins. To maximize the benefits of the Mississippi River’s water, sediment, and nutrients, this study emphasizes the continued analysis of management scenarios as an important step in the preservation and protection of the coast of the Gulf of Mexico while sustaining the support of relevant industries. We synthesized scoring metrics to facilitate communication of the efficacy of various management scenarios. The scoring metrics provide an evaluation framework covering physical, ecological and indirect socioeconomic criteria. This approach can be used for other complex natural systems to explore viable strategies and tradeoffs balancing ecosystem services with socioeconomic interests. +
The lower shoreface, a transitional subaqueous region extending from the seaward limit of the surf zone to beyond the closure depth, often serves as a sediment sink or source in sandy beach environments over annual to millennial time scales. Despite its important role in shoreline dynamics, however, the morphodynamics of the lower shoreface remain poorly understood. Previous work highlights discrepancies between equation-based theoretical equilibrium contours and bathymetric data, indicating that models may not accurately reproduce real shoreface cross-shore profiles. Here, we combine energetics-based suspended sediment transport formulae (Ortiz & Ashton 2016, JGR-ES) with wave climate and sedimentological data from Rockaway Peninsula, NY, to understand controls on shoreface morphology and differences between modeled and empirical equilibrium profiles. Analyzing a full wave climate time series from Wave Information Studies (WIS) spanning 40 years at one hour intervals reveals how different components of the wave climate affect suspended transport rates, particularly at varying depths. This results in a different steady-state, or equilibrium profile compared to one computed using single wave parameter inputs. The computed profile shape further changes when computations include reduction in sediment settling velocity due to offshore sediment fining, based on field observations. These profile are then compared to USGS bathymetric shoreface profile shapes at Rockaway and other locations. Our preliminary results appear to rectify the gap between modeled and empirical equilibrium profiles, moving towards a more thorough understanding the evolution of the lower shoreface. +
A
The majority of process studies on alluvial fans have focused on gravely fans. Many fan systems, however, are sourced from basins composed of fine-grained sediments. Deposition on such fans involves deposition from hyperconcentrated- or mud-flows. Many of such fans occur where there is sufficient vegetation to affect and, often, obscure depositional processes.
The modeling effort to be presented is motivated by the occurrence of fine-grained alluvial fans on Mars that feature a network of distributaries floored with coarser sediment and what we interpret to be fine-grained overbank deposits that comprise the bulk of the sediment. We have identified active fine grained fans in the arid Atacama desert deriving sediment from the higher Andes and lowland deposition dominated by muddy sheetflow sediment.
We are constructing a simulation model for deposition on such fans based on the fan-delta model of Sun et al. (2002). The model routes water and sediment through multiple distributaries that can branch, recombine, and avulse. Modeling flow and bedload sediment through the distributaries is relatively straightforward, but overbank deposition and avulsion processes are more problematic to characterize realistically (e.g. avoiding development of "holes" in fans or preventing evolution to a fixed distributary pattern). Our observation of overbank processes on the Atacama fans demonstrates the importance of sedimentation by long shallow sheetflow floods in addition to local levee aggradation. These processes are being implemented into our fan model. +
2
The modern Ohio River network is a Rubrik’s Cube for anyone interested in dynamic river reorganization. Throughout the Quaternary, the cyclic growth of North American ice sheets forced the Ohio drainage network to oscillate between a north-flowing (towards the Gulf of St. Laurence or Hudson Bay) and west / south-flowing (towards the Gulf of Mexico, i.e., the modern river) configuration. These cycles produced a network of overprinted paleo valleys that reflect multiple episodes of river reorganization (the so-called “Teays” paleo river network). The overprinted nature of these valleys makes it very difficult to assess the timing of specific stream capture events. In order to unravel this complex history of river reorganization, geomorphologists can begin by constraining the timing of individual stream capture events that do not overprint older episodes of drainage reversal.
One such event is likely present in Hocking Hills State Park in central Ohio, known for its hundreds of 30-50 m-tall waterfalls. These knickpoints were likely created when the upper reaches of the Salt Creek watershed were blocked by one of the ice sheets, forming a glacial lake that spilled over a drainage divide and rerouted the channel network from a west-flowing to a south-flowing configuration. The stream capture event would have also produced a local base level drop that created the knickpoints. This hypothesis implies that the knickpoints were all created at the same time; if true, we can constrain the timing of the capture event using catchment averaged erosion rates and knickpoint celerity models.
However, the hypothesis also implies that the waterfalls should be located at the same approximate χ value. This is not the case; rather, there is prominent, N-S trend in χ values. Without an explanation for this trend, any age constraints on the capture timing will be suspect.
We used Landlab-based landscape evolution models (LEMs) to explore several possible explanations for the trend in χ values. We found that following a single capture event, the trend can be explained by the specific combination of (a) the pre-capture channel topology; (b) the precise capture location; and (c) the spatial extent of different rock layers. We believe that this in an “Occam’s razer” scenario, because it allows the χ trend to be explained by a single, stream capture forcing. However, without the insights provided by our LEMs, we would have considered multiple forcings or stream capture events to be more likely. These simulations are a novel and interesting case study in how LEMs can be applied to understand unique complexities of specific field sites and also have important implications for using knickpoint celerity models to assess landscape evolution.
The morphodynamics of coast and estuarine environments are known to be sensitive to environmental change and sea-level rise. However, whilst these systems have received considerable individual research attention, how they interact and co-evolve is largely unknown. Through a novel coupling of numerical models, this research is designed to explore the complex behaviour of these systems in terms of fluid flows and sediment fluxes. This includes elucidating the relative influence of various controls on system behaviour and exploring the effects that variable sea levels and changing wave climates may have on their evolution over the mid to longer term.
This research is being carried out through the modification and coupling of the one-line Coastline Evolution Model (CEM) with the hydrodynamic LEM CAESAR-Lisflood (C-L). Progress to date includes a new version of the CEM that has been prepared for integration into C-L. This model incorporates a range of more complex sedimentary processes in quasi-2d and boasts a graphical user interface and visualisation.
The model is being applied and tested using the long-term evolution of the Holderness Coast, Humber Estuary and Spurn Point on the east coast of England (UK). Holderness is one of the fastest eroding coastlines in Europe and research suggests that the large volumes of material removed from its cliffs are responsible for the formation of the Spurn Point feature and for the Holocene infilling of the Humber Estuary. Over the next century it is predicted that climate change could lead to increased erosion along the coast and supply of material to the Humber Estuary and Spurn Point. How this manifests will be hugely influential to the future morphology of these systems and the flood and erosion risk posed to coastal communities. +
A
The morphodynamics of large anabranching sand-bed rivers is investigated using a numerical model of hydrodynamics, sediment transport, bank erosion and floodplain development, operating over periods of several hundred years. Model sensitivity to key parameters is examined, and simulated channel and natural river morphology are compared in terms of the statistical characteristics of channel width, depth and bar shape distributions, and mechanisms of unit bar, compound bar and island evolution. Model results provide insight into controls on the frequency of mobile sand bars and the stability of larger vegated islands. +
2
The morphology of the Earth’s surface is continuously evolving under multiple factors (tectonics, climatic, etc). As the interface between the lithosphere and the atmosphere, the critical zone provides the prime record of these changes and can be directly monitored. Understanding the physical processes that control temporal changes is important to quantify and predict them.
In this context, we aim to constrain the effect of physical rock weathering on erosion rates and their variation over seasonal cycles. We focus our studies on marly badland catchments in the southeast of France. The Draix-Bléone Critical Zone Observatory allowed data collection and experiments over the last 35 years and represents an ideal environment for this project (Mathys et al, 2005). The marly badland of Draix are subject strong weathering and erosion processes, caused by a variety of physical processes, resulting in the formation of a spatially and temporally variable regolith layer. Significant production of regolith is observed during the winter and rapid washing of slopes during the spring and early summer (Bechet et al., 2016). Based on regolith characteristics from the field we will build a 1D model of the dynamic of the regolith. Characterizing the seasonal variability and climatic dependence of regolith production is a prerequisite to predict yearly variations in sediment flux and its evolution under changing climate conditions.
We sampled the upper part of the regolith in the Draix catchment, in four targeted places, to obtain grain size distributions and water contents. Characteristics of the detrital cover that affect the rate of weathering. High-resolution photogrammetry records will enable comparing surface changes (roughness, thickness, grain size) over the seasons. Furthermore, we cleaned a 1m² surface on a ridge of regolith to monitor weathering processes and estimate regolith production during each season. We aim to repeat this exercise at the end of each season; the resulting difference in thickness removed should represent the new regolith formed
Two years of field campaigns are scheduled. We will use our field observations on the temporal variation of regolith characteristics to inform a 1D model of regolith dynamics. In parallel, the second goal of the project will be to spatialize and implement the latter description into a landscape evolution model based on Landlab (Hobleyet al., 2017) to simulate the effects of regolith dynamics on catchment-scale erosion. The development of this new module will be helpful to follow critical-zone evolution in different soil cover contexts.
References:
Bechet, J., J. Duc, A. Loye, M. Jaboyedoff, N. Mathys, J.-P. Malet, S. Klotz, C. Le Bouteiller, B. Rudaz, and J. Travelletti (2016), Detection of seasonal cycles of erosion processes in a black marl gully from a time series of high-resolution digital elevation models (DEMs), Earth Surf. Dynam., 4, 781–798, doi: 10.5194/esurf-4-781-2016.
Hobley, D. E. J., J. M. Adams, S. S. Nudurupati, E. W. H. Hutton, N. M. Gasparini, E. Istanbulluoglu, and G. E. Tucker (2017), Creative computing with Landlab: an open-source toolkit for building, coupling, and exploring two-dimensional numerical models of Earth-surface dynamics, Earth Surf. Dynam., 5, 21–46, doi: 10.5194/esurf-5-21-2017.
Mathys, N., S. Klotz, M. Esteves, L. Descroix, and J. M. Lapetite (2005), Runoff and erosion in the Black Marls of the French Alps: Observations and measurements at the plot scale, Catena, 63, 261–281, doi: 10.1016/j.catena.2005.06.010.
A
The motion of sediment in water is caused by fluid pressure gradient forces, primarily drag, on sediment grains. Turbulence-resolving experiments show significant temporal and spatial variability of fluid and sediment motion and particle forces at all stages of sediment transport. The signature of turbulence structures and their modification by sediment is apparent from incipient motion to vigorous suspension.<br>
This presentation introduces a numerical model that combines large eddy simulation (LES) of turbulence and the distinct element method (DEM) of granular motion. The LES and DEM models are fully coupled in momentum. Information from the LES is used to specify forces on the DEM particles, and those particle forces are given in an equal and opposite direction in the filtered and discretized Navier-Stokes equations at each grid cell in the finite volume LES.
Parameterization of turbulent sediment transport processes is the basis of any well founded model of morphodynamics in fluvial and marine environments. Current parameterizations rely on a mixture of theory and empirical evidence. LES-DEM simulations can be performed in conditions that are difficult to reproduce in the laboratory and that stretch the limits of theory. It is hard to build an apparatus that can produce sediment transport under field-scale cnoidal waves, on sloping beds, with currents of arbitrary direction, and a range grain size distributions. Further, even in simple unidirectional flows only rough empirical relations exist for the critically important suspended sediment rate of entrainment.<br>
Validation of the LES-DEM approach is essential before development of transport relations for large-scale morphodynamic models. A series of LES-DEM simulations of unidirectional flow over flat beds of medium sand, ranging from no transport, to bedload, to vigorous suspension are presented. Simulations of flat sand beds under oscillatory waves and unidirectional flow downstream of a backward-facing step are compared to laboratory measurements. Simulations over ripples and through vegetation are also presented.<br>
Examples of some of the simulations can be previewed at the links below.<br>
START_WIDGET"'-287efb8f8ebb33d7END_WIDGET<br>
START_WIDGET"'-97536af3d75166ecEND_WIDGET<br>
START_WIDGET"'-e3e977584442e979END_WIDGET<br>
2
The movement of sea ice is influenced by a number of factors, from winds to ocean currents. As climate change continues to occur rapidly, understanding sea ice drift in the Arctic is a key parameter to understanding the effects of rising temperatures in the region. Recent literature has shown that the Arctic and the Antarctic are most affected by global warming, which raises questions regarding climate justice, as most of the carbon emissions causing anthropogenic climate change are produced in other regions. To analyze this impact, we employ artificial intelligence to predict sea ice drift velocity based on external features. Machine learning is the process of computers gaining insights by seeing and correlating large quantities of data. Using external parameters, including wind speed, and drift velocity ground truth as the inputs of the model, we train multiple different architectures and compare the results. Particularly, we experiment with a convolutional neural network (CNN), a random forest (RF), and a support vector machine (SVM). We also experiment with various model specifications. This research leads to a greater understanding of the Arctic’s response to climate change. +
The natural elevation of the vast, flat landscape of the lower Ganges-Brahmaputra-Meghna (GBM) remains remarkably stable despite persistent relative sea level rise (rSLR). This stability stems from the tight coupling of the land and tides through a robust negative feedback induced by periodic flooding with sediment-rich water. As water levels increase, the inundation depth and duration also increase resulting in more sediment deposition. This has a stabilizing effect and largely negates the initial increase in water level such that the elevation surface appears unchanged. We refer to this stable elevation as the equilibrium elevation.
Here, we investigate the strength of the inundation feedback and the resulting equilibrium elevation. We identify three main controls on this feedback - (1) annual rate of rSLR, (2) mean tidal range (TR), and (3) mean suspended sediment concentration (SSC). We explore the realistic parameter space of each using a simple, zero-dimensional mass balance model. Specifically, we ask (1) what equilibrium elevations are feasible, (2) how these equilibrium elevations compare to tides (e.g., relative to mean sea level (MSL) or mean high water (MHW)), and (3) how equilibrium elevation impacts the duration (hydroperiod) and intensity (depth) of a typical inundation cycle. Results show an incredibly robust feedback for most conditions with the notable exception of low SSCs (< 0.1 g/L). This low, yet realistic value of SSC represents a tipping point at which the equilibrium elevation drops precipitously. At higher rates of rSLR (> 8mm/yr) and lower TR (< 2 m) the equilibrium elevation results in complete drowning of the platform. +
A
The overall size of the Chesapeake Bay “dead zone” is quantified by the Bay’s hypoxic volume (HV), i.e., the volume of water with dissolved oxygen (DO) less than 2 mg/L. In order to improve estimates of HV, DO was subsampled from the output of three dimensional model hindcasts at times/locations matching the set of 2004-2005 stations monitored by the Chesapeake Bay Program. The resulting station profiles were then input into an interpolation program to produce Bay-wide estimates of HV in a manner consistent with non-synoptic, cruise-based estimates. Interpolations of the same stations sampled synoptically as well as multiple other combinations of station profiles were examined in order to quantify uncertainties associated with interpolating HV from observed profiles. The potential uncertainty in summer HV estimates resulting from profiles being collected over two weeks rather than synoptically, averaged ~5 km^3. This is larger than that due to sampling at discrete stations and interpolating/extrapolating to the entire Bay (2.4 km^3 ). As a result, sampling fewer, selected stations over a shorter time period is likely to reduce uncertainties associated with interpolating HV from observed profiles. A function was also derived, that, when applied to a subset of 13 stations, significantly improved estimates of HV. Finally, multiple metrics for quantifying Bay wide hypoxia were examined, and cumulative hypoxic volume was determined to be particularly useful, as a result of its insensitivity to temporal errors and climate change. A final product of this analysis is a nearly three-decade time series of improved estimates of HV for Chesapeake Bay. (Submitted March 2013 to Journal of Geophysical Research. For a pdf pre-print contact Carl Friedrichs at cfried@vims.edu .) +
The present study uses the Sedflux stratigraphic model to simulate the Late Pleistocene evolution of the Eastern Beaufort Continental Shelf, Canadian Arctic. During this period, the proximity and the dynamics of the Laurentide Ice Sheet created a complex glacial environment. Modeling such environments thus presents challenges. Modules and input parameters have to be able to simulate major fluctuations in sea-level and sediment supply, an ever evolving source of sediments, a large outwash plain, sudden outburst floods, permafrost aggradation, glacial isostasy, etc. In addition, detailed understanding of glacially-influenced environments in general and the glacial history of the local region specifically make it difficult to estimate parameters such as sediment supply. This poster thus presents the challenges and the potential solutions in using SEFLUX to simulate the stratigraphy of a glaciated shelf such as the Beaufort Shelf. +
2
The propagation of environmental signals through the sediment routing system and their subsequent preservation or removal from the rock record is a central theme in current stratigraphic research. The identification of cyclicity and order in stratigraphic sequences with regard to vertical facies successions, thicknesses, and grain size trends is often used as indicator of preservation of non-random, extra-basinal signals (i.e. climate, tectonics, and base level). However, it is less clear to what extent the processes that alter these signals post-deposition (re-working, scour, and erosion) enhance or diminish cyclicity and order within preserved sediments. Furthermore, stratigraphic trends are often identified in subjective, qualitative terms and may be based more on a priori perception of order derived from depositional systems models than statistically robust trends inherent in the sediment archive. Here, we use a statistical metric to objectively evaluate order vs. disorder in the stratigraphic record in an attempt to identify the likelihood of a disordered (random) response to orderly (non-random) depositional processes. We utilize a quantitative geochemical and sedimentological dataset from the Ganges-Brahmaputra-Meghna delta (GMBD) to identify distinct fluvial sediment packages (defined as meter to 10s of meters thick sand packages similar in scale and character to modern bar forms) and statistical trends in their vertical successions across the delta. We begin by considering that the boundaries of these fining-upwards packages are defined by >50% increases in grain size from one sample to the next in a vertical succession (although other thresholds are evaluated as well). A runs metric “r” is then calculated by identifying streaks of increasing or decreasing sediment package thicknesses and volume weighted mean grain size. This metric is then compared to the output of a Monte Carlo simulation of 5000 synthetic boreholes created by random shuffles of the observed borehole data to determine the likelihood of a similar succession of sediment body thicknesses and grain size trends being generated by chance. Preliminary results indicate that the vast majority of observed thickness successions in the GBMD are statistically “disordered”, with regional variability correlated to discrete geomorphic provinces within the delta. Of note, sediment thickness trends from the main braidbelt exhibit the lowest probability of being generated by random chance, followed by the lower delta plain, and lastly by Sylhet basin, a semi-enclosed sub-basin in northeast Bangladesh that has experienced episodic occupation by the mainstem Brahmaputra River throughout the Holocene. Similar results (with some notable exceptions) are found within grain size runs analyses, with Sylhet basin exhibiting the least amount of order with regard to vertical changes in grain size. Previous studies have identified Sylhet basin as a site of rapid mass extraction, suggesting a possible inverse relationship between stratigraphic order and rates of sediment extraction in fluvial systems. These results lay the groundwork for future studies in the utility of simple statistical measures in identifying random vs. ordered successions of sediment packages as indicators of process-response relationships preserved in the stratigraphic record.
The recent incursion of Data Analytics and Big Data has inspired many fields to venture in. Although a late comer, as compared to financial and bioinformatic areas, geosciences have fast picked up momentum in past two years. We will summarize here quantitative efforts, which require computational means beyond a laptop, in machine learning, deep learning and visualization. The examples will be drawn from (1) delineation of three-dimensional sub-surface three -dimensional fault structure illuminated by tens of thousands of hypocenter from earthquake aftershocks in central Italy using unsupervised machine learning (2) Recurrent Neural Networks (RNN) for delineating earthquake Patterns Based on Complete Seismic Catalog created by large-scale finite element Modelling (3) A highly efficient computational interactive Virtual Reality (VR) Visualization Framework and workflow for Geophysical exploration (4) forecasting the intensity trend of the Earth's natural electromagnetic pulse field signal prior to large earthquakes using chaos theory and radial basis functions (RBF) as deep neural network. +
A
The response of the wave-dominated coasts to sea-level rise is dominated not by inundation, but rather by the dynamic response of sediment transport processes to perturbations of the sea level. In a regime of sea level change, the predominant response of the wave-dominated shoreface depends upon the time-dependent response of the shoreface itself to changes in sea level as well as the potential changes to the shoreline. Sediment transport processes on the shoreface remain poorly understood, complicating predictions of equilibrium shoreface shapes and even net sediment transport directions. However, presuming an equilibrium geometry, energetics-based, time-averaged relationships for cross-shore sediment transport provide a framework to understand the characteristic rates and types of shoreface response to perturbations to either the sea level or the shoreline boundary. In the case of a sea-level rise, we find that the dominant perturbation for a barrier system is not the sea-level rise itself, but rather the movement of the shoreline by overwash. The characteristic response time of the shoreface itself increases significantly at depth, suggesting that the lower shoreface response to a sea level change can be significantly delayed. To study the interactions between the characteristic timescales of shoreface evolution and barrier overwash, we apply a numerical model of barrier profile evolution that couples shoreface evolution with barrier overwash. This integrated model provides a tool to understand the response of barrier systems to changes in sea level over the late Holocene to the modern. The model also investigates the potential behavior of barrier systems as they (and their human occupants) respond to predicted increased rates of sea-level rise over the coming centuries. +
2
The role of climate change on landscapes is one of the most difficult remaining challenges in geomorphology. It is thought that climate primarily modifies landscapes through sediment production and transport in rivers. However, collecting the data needed to resolve the relationship between climate and sediment transport has remained elusive. This issue stems from a lack of a methodology that can work in a wide variety of river environments. Furthermore, this problem is made pressing by a need to understand the coming effects of human-induced climate change.
To address this problem, I developed a model to capture sediment transport using luminescence, a property of matter normally used to date sediment deposition. Luminescence is generated via exposure to background ionizing radiation and is removed by exposure to sunlight. This behaviour is sensitive to sediment transport and could potentially be used to infer sediment transport parameters. I derive the model by performing a simultaneous conservation of sediment mass and absorbed radiative energy expressed as luminescence. The derivation results in two differential equations that predict the luminescence at any point in a river channel network. The model includes two key sediment transport parameters, the sediment transport velocity and the storage-center exchange rate. From these parameters, other key sediment transport variables such as the characteristic transport length-scale and the sediment virtual velocity can be calculated. These parameters can be constrained by determining the model’s luminescence parameters through field measurement and lab experiments.
I test my model against luminescence measurements made in rivers where these sediment transport parameters are well known. I find that the model can reproduce the observed patterns of luminescence in channel sediment and the parameters from the best-fit model runs reproduce the known sediment transport parameters within uncertainty. The success of the model, and the advent of new technology to measure luminescence using portable devices, suggests that it may now be feasible to collect critical sediment transport data cheaply and rapidly. This method can now be used to test outstanding hypotheses of the influence of climate on sediment transport.
The sand-bed river long-profile evolution model (SRLP) is a complete mechanistic model, describing both transient and steady-state long-profile evolution of a transport-limited sand-bed river with self-forming channel width adjustments by linking sediment transport and river morphodynamics. SRLP allows for planform adjustments as a function of excess shear stress, which is directly related to the critical stress of the bank material (following Parker, 1978, and Dunne and Jerolmack, 2018), thereby linearizing the sediment-transport response to changing river discharge. This one-dimensional, physics-based model captures the internal dynamics of this inherently complicated system, quantifies the changes in the river's bed elevation, cross section, and channel slope, and ultimately provides the critical information about how the river is responding to both natural and anthropogenic disturbances. Our work so far has explored the variability in topographic responses of both steady-state and transient sand-bed channel long-profiles to the changes in sediment and water supply and base level by utilizing SRLP. We also further build on this understanding of how sand-riverbed mobility and the physical factors controlling it couples to the channel form is a key mechanistic link for predicting river response to those external perturbations. Therefore, we now consider how the equilibrium geometry of sand-bed rivers varies as a function of physical controls such as grain size, bed roughness, and bank strength, all of which modify the effective stress available for sediment transport. Then we evaluate the path to steady‐state long-profiles using numerical morphodynamic experiments by SRLP. +
The sediment bed and the water column are tightly coupled in shallow water systems including large portions of the continental shelf. For instance, the continental shelf seafloor receives ~48% of the global flux of organic carbon to the seabed and the shelf benthic flux serves as a key source of nutrients for sustaining marine life. Observational studies of sediment-water exchange require concurrent measurements in both compartments; however, these are difficult to obtain and rarely available. Numerical modelling provides a valuable alternative approach to observational studies, however many previous modeling efforts used simple sediment-water parameterizations that did not capture the nonlinearities of benthic-pelagic coupling. Here, we present a coupled benthic-pelagic model that includes realistic representations of biogeochemical reactions in both compartments, and the fluxes at the interface. The model is built on the modeling algorithms for sediment-water exchange in ROMS and expanded to include carbonate chemistry and anerobic reactions in the seabed. The updated model is tested for three sites where benthic flux and porewater concentration measurements are available in the northern Gulf of Mexico summer hypoxic zone. Model-data comparison demonstrates the robustness of the calibrated model in reproducing the porewater concentration-depth profiles of O2, DIC, TA, NO3 and NH4, as well as the benthic fluxes of the former three. Further sensitivity experiments reveal that labile material input, bio-diffusion intensity and anerobic mineralization pathways are the three major factors regulating the benthic fluxes and porewater concentrations of O2, DIC and TA. To conclude, our model results provide important insights into the variation of sediment-water exchange under different environmental conditions. This model has the potential to be used as a research and management tool to quantify the role of shelf sediment in driving bottom water hypoxia and acidification over continental shelves.
The shape of gravel-bed rivers controls aquatic habitat, fluvial hazards, landscape evolution, and river response to human impacts. Gravel-bed river shape, on average, obeys the expectations of an equilibrium model in which the width-averaged bankfull shear stress is slightly greater than the critical shear stress required to move the median bed grain size. However, it is important to understand not just average equilibrium form but also responses of river shape to changes in formative conditions, both to elucidate process dynamics and to enable applied prediction. Many natural and human-driven changes to channels, and especially river restoration interventions, involve altering the flow resistance of gravel-bed rivers by changing macro-roughness. Despite rich bodies of literature on 1) macro-roughness effects on flow and sediment transport and 2) controls on gravel-bed river shape, understanding of macro-roughness effects on reach scale channel form evolution remains limited.
We ask how macro-roughness affects gravel-bed river geometry, and how rivers respond to changes in macro-roughness as might occur during river restoration. We develop a simple numerical model for gravel-bed river form in the presence of macro-roughness and use it to investigate trajectories and timescales of channel response to macro-roughnesss changes. Model sensitivity analysis reveals that greater macro-roughness drives channel widening, bed aggradation, and steepening, as the channel remains adjusted to maintain constant shear stresses on the bed and banks regardless of roughness. The presence of a floodplain reduces widening but increases steepening at high roughness values, because high roughness drives flow overbank and increases the width/depth ratio relative to a confined channel. Comparison against topobathymetric data and UAV imagery from the North Fork Snoqualmie River, WA shows that modeled trends in channel adjustment to macro-roughness are generally consistent with data from the field site, but that the field site exhibits significant variability due to processes and feedbacks not captured by our model. We expect our model to be useful for establishing a priori expectations for the effects of process-based restoration projects on river form.
The shoreline is a boundary where survey methods change dramatically, where the time dimension is extremely important, where sediment fluxes are very large, where flotsam is trapped, and where numerical/physical singularities occur as the water depth goes to zero. The shoreline boundary oscillates; and as sea-levels rise what is now shore will become sea. Models of likely response of shorelines require detailed data on the sediment/beach/soil substrates.
We investigated how to obtain the best supporting data using the Louisiana area as example. We investigated to what extent the marine and terrestrial data were already in harmony, and what challenges remain in trying to make one seamless dataset. Of course, technologies like LIDAR carry out highly detailed imaging that achieves this to an extent. But we are focused on direct samplings of the ground-truthing type on which physical properties, fabrics, chemical compositions, grain types, genesis, can be directly determined.
Difficulties: Terrestrial surveys have a different data topology, more focused on soil polygons and boreholes; offshore mappings focus on point-samplings, for instance grabs and cores. Soil descriptions focus on layer-profile identities such as “Mollisol”; offshore datasets focus on bulk textures and compositions. Strong semantic differences exist. Terrestrial areas are greatly modified by agriculture and construction.
Positives: We discovered several information-integration pathways for merging the data from the two realms. Exhaustive searching uncovered data on the onshore soils and riverbed sediments to match the marine data (e.g. dbSEABED). Named geographical locations are linkable with coordinates through gazetteers. Computational methods exist to merge polygon and point data sets. In semantics, glossaries provide some information to link onshore and offshore descriptions. Seamless mappings are demonstrated, useful in support of cross-shore morphodynamic models. +
