Property:CSDMS meeting abstract
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Alluvial rivers record the external drivers of change, such as climate, tectonics and anthropogenic disturbances, and they code their dynamics in their bankfull channel geometry and planform geometries and longitudinal profiles. The key to understanding the past and predicting the future alluvial rivers is learning to interpret the language of the grains of sediment and to decode the responses they record. 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 by linking sediment transport and river morphodynamics, including planform (width) adjustment as a function of excess shear stress (following Parker, 1978, and Dunne et al., 2018), thereby linearizing the sediment-transport response to changing river discharge. Through quantifying the changes in the river's bed elevation, cross section, and channel slope, this one-dimensional, physics-based model captures the internal dynamics of this inherently complicated system and ultimately provides the critical information about how the river is responding to both natural and anthropogenic disturbances. We now further aim to understand and model how sediment is transported and where it is deposited within sand-bed alluvial river networks, how different portions of the alluvial river network respond and how the behavior of those network components are impacted over both human and geological time scales. In order to do this, we use an updated network model approach (by Wickert A., GRLP v2.0.0-alpha). This latest release works as a core network engine, allows integration of different process modules and ultimately provides a new network-model platform where we can include SRLP model. Through the addition of SRLP module, we present examples of long-profiles of sand-bed alluvial river networks under a variety of base level and water- and sediment-supply boundary conditions and investigate the mainstem river and both upstream and downstream tributary responses over time. Finally, we compare the response of the model of linked sand-bed tributaries to the one of gravel-bed rivers to further discuss the effects of variations in grain- and reach-scale dynamics on the longitudinal evolution of these two classes.
Along Andean-type convergent margins, the preserved stratigraphic successions in retroarc foreland basins record complex interactions between oceanic plate subduction, overriding lithosphere deformation, and surface processes. Modeling their interactions and their impacts on basin stratigraphy helps to distinguish the geological footprint of the operating processes. We use a source-to-sink landscape evolution model, Badlands, to investigate the basin stratigraphic formation in response to changes in subduction morphology, hinterland orogenic uplift, overriding lithosphere strength, and surface erosional efficiency. Our modeling results reveal distinguishable responses of basin sedimentation to the imposed tectonic and surface forcings. Firstly, with sufficient sediment supply (i.e., the basin is filled with sediments), subduction at higher slab dip leads to development of shallower and narrower basins, with increasing volume of fluvial and shallow-water deposits accumulation. For mechanically thicker overriding plates, a deeper foreland basin tends to develop, though the basin width does not show consistent changes with increasing lithosphere strength. When sediment supply is further enhanced by either increasing orogenic uplift rate or surface erodibility, the basin sedimentation extends horizontally while the basin depth changes in an opposite way. Secondly, our basin subsidence analysis reveals strong impact of flexural rebound at the foredeep on modifying the basin morphology and strata dipping. We further found positive correlations between the flexural rebound and the progradation of fluvial deposits at the foredeep. Lastly, by normalizing the basin width to orogenic belt width and basin depth to maximum foreland flexure, we categorize the basins to be accommodation-dominant and supply-dominant, which helps to evaluate the impact of varying each contributing process on the basin development. Overall, our source-to-sink models reveal the complex interactions between surface and tectonic forcings, and highlight the huge potential of extracting their signals from the geological record.
Along a quarter of the Beaufort Sea coast, back-barrier estuaries modulate the transport and transformation of nitrogen and carbon, impacting food webs and carbon budgets. These estuaries are adjacent to permafrost, a large carbon reservoir that contains ~1700 Gt of organic carbon that is thawing from rapid Arctic warming. Thawed dissolved organic matter and nutrients may be transported to the coastal ocean by groundwater and rivers, adding nutrients to the coast that may impact production and biogeochemical cycles. It is unclear what effect permafrost thaw will have on Arctic estuarine biogeochemistry, partly because present-day spatial and temporal variability of residence time and export in Arctic back-barrier estuaries is unknown and complicates efforts to predict future change. To investigate the residence time of water, as well as estuary-shelf fluxes, this study uses a numerical modeling approach. Specifically, a hydrodynamic model, the Regional Ocean Modeling System (ROMS), is being implemented for Arey, Kaktovik, and Jago Lagoons along the Beaufort Sea coast of northern Alaska. The model accounts for processes including local winds, rivers, and larger scale circulation. Analysis will focus on variations in circulation dynamics within the ice break-up and open water season of 2019. +
Along wave-influenced deltas, wave-driven longshore currents usually interact with the fluvial jet at the river mouth, creating a sharp gradient alongshore in sediment transport/deposition and hence a corresponding change in coastline morphology. When multiple channels intersect a delta coastline, morphological changes can take on a complex outlook driven by the multiplicity of the river channels and their ‘hydraulic groyne effect’, whereby the overall effect of the river jets is to limit the loss of sediment within the coastal littoral system and ensure shoreline stability. This study explores the dynamic relationships between waves and fluvial discharge along a coastline intersected by river mouths by employing a numerical model of an idealized delta coastline containing two river mouths. The modelling is undertaken using Delft3D, in order to simulate both sediment transport and wave propagation along with the accompanying changes in coastline morphology. Analysis focuses on the relative change in coastline morphology, updrift, and downdrift of the river channels, in relation to varying scenarios of the incident wave climate, fluvial input, and river channel geometry. Specifically, water discharge entering the basin is set temporally constant during a model run but is varied in the range of 500 - 2000 m3/s between runs. Further, 3 scenarios of fluvial sediment discharge corresponding to low, medium, and high sediment discharges, are incorporated into each fluvial discharge scenario. Finally, waves approach the coastline from an incident angle of <45o, generating longshore sediment transport proportional to its significant height and approach angle, which are varied between model runs in the ranges of 1.0 - 1.5 m and 15 - 42 degrees, respectively. The study is set to provide new insights into the morphodynamics of wave-influenced deltas resulting from the interaction of waves with fluvial discharges at interannual timescales. +
Although mangroves provide several beneficial ecosystem services, such as blue carbon storage, coastal protection, and nursery habitats, they rapidly decline due to human development and climate change. In particular, in areas in the Caribbean, such as Puerto Rico, climate change
will likely lead to an increase in evaporation over precipitation. Such an increase in drought-like conditions will drive porewater salinity to increase exceeding the threshold beyond which mangroves can survive. To improve our understanding of this interplay, we developed a numerical model using the Landlab Python library that describes the spatial distribution of vegetation and die-back in low-lying and undeveloped mangrove islands where freshwater inputs come solely from precipitation. We apply the model to a series of islands with elongated and asymmetric die-backs in La Parguera, a bay environment in southern Puerto Rico. Our model can explain the die-back shape and location for all islands as a function of the average net evaporation rate (i.e. evaporation – precipitation), the island's edge water salinity, and the mangrove soil dispersion coefficient, or the porewater exchange through tidal flushing. We gathered evaporation data from the Woods Hole Oceanographic Institute's OAFLUX project and precipitation data from the Tropical Rain Monitoring Mission, and quantified the soil dispersion as a function of the area of red mangroves, which was calculated via satellite imagery analysis. Additionally, we infered the outer edge salinity from the maximum canopy heights, gathered from Goddard's LiDAR, Hyperspectral, and Thermal Imager. In our model results, some islands presented a subtle bayward shift of the die-back. This can be explained by a higher island's edge water salinity on the landwards side, where bay depths are shallow and mixing with the rest of the bay is low. This spatial difference in salinity was consistent with the differences in canopy heights derived from LiDAR, and fell within the range of values reported in the literature. This portable modeling framework can be applied to other low lying mangrove carbonate islands with complex geometries.
Although numerous approaches for deriving water depth from bands of remotely-sensed imagery in the visible spectrum exist, digital terrain models for remote tropical carbonate landscapes remain few in number. The paucity is due, in part, to the lack of in situ measurements of pertinent information needed to tune water depth derivation algorithms. In many cases, the collection of the needed ground-truth data is often prohibitively expensive or logistically infeasible. We present an approach for deriving water depth from multi-spectral satellite imagery without the need for direct measurement of water depth, bottom reflectance, or water column properties within the site of interest. The reliability of the approach is demonstrated for five satellite images, each at a different study site, with overall RMSE values ranging from 0.84 m to 1.56 m when using chlorophyll concentrations equal to 0.05 $\text{mg m}^{-3}$ and a generic seafloor spectrum generated from a spectral library of common benthic constituents. Sensitivity analyses show that the model is robust to selection of bottom reflectance inputs and errors in the atmospheric correction and sensitive to parameterization of chlorophyll concentration. +
An Extreme Value Analysis (EVA) model is realized for seafloor elevation changes in an area of shallow continental shelf in the North Sea. Extreme events have practical application in this area of abundant Unexploded Ordinance at the seabed and also wind energy projects. The events being examined are from the motion of seabed sediment in megaripples, sand waves, sand bars and sand sheets, but driven by normal and extreme swell- and wind-waves, tides and human activities. Changes of seabed elevation up to 8m in one year are observed, but rare.
The observational dataset for the study is a large, publicly available compilation of 3-decades of annual, hydrographic-standard bathymetric soundings in the German Bight, provided in gridded form at a spatial resolution of 50m. Counts of annual seabed elevation changes by elapsed time were compiled and related to the seabed features, such as tidal channels (which have previously been well studied).
The change statistics were compared to forms of the Generalized Extreme Value and Generalized Pareto distributions, per pixel and also by small morphodynamically uniform subareas. The Generalized Pareto distribution with coefficient c ≈ -6.0 to -6.5 appears to be the appropriate model, but adjusted according to water depths and locations on features.
The result suggests a method to statistically model seabed behavior including extreme events. +
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An Isopycnic Coordinate Ocean Model is used to represent the propagation of internal tides in the Bay of Biscay and their desintegration into solitons. To model important vertical variability of the thermocline, such as solitons, a non-hydrostatic model is necessary. In this study, we test the possibility of integrated non-hydrostatics terms under weakly nonlinear and nonhydrostatic approximation. Non-hydrostatic terms derived with this assumption, are directly added to the hydrostatic equations. We then address numerical problems : mesh size limitation responsible for numerical dispersion, numerical instabilities. After having investigated these problems analytically and tested the limitation, a stable method is proposed. Results for a 2D idealised configuration of the Bay of Biscay is described : the model is forced by the semi-diurnal tidal wave M2, two layers of different density are considered. The internal waves is desintegrated into solitons after few tidal periods. +
An accurate, three-dimensional Navier–Stokes based immersed boundary code called TURBINS has been developed, validated and tested, for the purpose of simulating density-driven gravity and turbidity currents propagating over complex topographies. The code is second order accurate in space and third order in time, uses MPI, and employs a domain decomposition approach for parallelism. It makes use of multigrid preconditioners and Krylov iterative solvers for the systems of linear equations obtained by the finite difference discretization of the governing equations. Various boundary conditions on the complex geometry are imposed via the direct forcing variant of the immersed boundary approach, utilizing a stable interpolation method. Bi- and trilinear interpolations are employed in such a way that the original discretization accuracy is retained with no additional restriction on the time step. Weak and strong scaling tests were performed for a uniform flow over array of spheres. We obtain very good scaling results as expected for multigrid solvers. We perform convergence tests via uniform flow over cylinder. Both skin friction and pressure coefficients show very good agreement with results reported by other authors.
Subsequently, a computational investigation was conducted of mono-, bi- and polydisperse lock-exchange turbidity currents interacting with complex bottom topography. Our simulation results are compared against laboratory experiments of other authors. Several features of the flow such as deposit profiles, front location, suspended mass and runout length are discussed. For a monodisperse lock-exchange current propagating over a flat surface, we investigate the influence of the boundary conditions at the streamwise and top boundaries, and we generally find good agreement with corresponding laboratory experiments. However, we note some differences with a second set of experimental data for polydisperse turbidity currents over flat surfaces. A comparison with experimental data for bidisperse currents with varying mass fractions of coarse and fine particles yields good agreement for all cases except those where the current consists almost exclusively of fine particles. For polydisperse currents over a two-dimensional bottom topography, significant discrepancies are observed. Potential reasons are discussed, including erosion and bedload transport.
Finally, we investigate the influence of a three-dimensional Gaussian bump on the deposit pattern of a bidisperse current. The suspension includes two particle sizes with a settling velocity ratio of 10. As the current travels over the bottom topography, we record instantaneous deposit profiles and wall shear stress contours. As the current impinges on the obstacle, it becomes strongly three-dimensional (see Fig. 1). Comparison of the final deposit profiles near the Gaussian bump against the case of a flat surface shows a smaller influence of the topography on the fine particles than on the coarse ones. Due to lateral deflection, deposition generally decreases near the bump, while increasing away from it. Some distance downstream of the obstacle, the deposit profiles lose their memory of the bump and become nearly uniform again. Instantaneous wall shear stress profiles are employed in order to estimate the critical conditions at which bedload transport and/or particle resuspension can occur in various regions.
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An enduring obstacle to reliable modeling of the short and long term evolution of the stream channel-hillslope ensemble has been the difficulty of estimating stresses generated by stream hydrodynamics. To capture the influence of complex 3D flows on bedrock channel evolution, we derive the contribution of hydrodynamic stresses to the stress state of surrounding bedrock through a Smoothed Particle Hydrodynamics (SPH) approximation of the Navier-Stokes (N-S) equations. The GPU-accelerated SPH solution locally integrates the N-S equations by discretizing the flow into millions of particles which communicate local motions to neighbor particles using a smoothing kernel. Coupling the flow solutions to the stress-strain formulation of the Failure Earth Response Model (FERM) provides three-dimensional erosion as a function of the strength-stress ratio of each point in the computational domain. This novel approach allows the resulting geomorphic response to be quantified for bedrock channels with bends, knickpoints, plunge pools, and other geometric and hydrodynamic complexities. Strength parameters used in FERM (tensile strength, cohesion, and friction angle) are readily constrained with field observations. Fluvial stresses calculated with SPH are added to the other components of the total stress state, such as slope-generated and tectonically-generated stresses. From the coupling of SPH and FERM we gain 3D physics-based erosion and a dynamic link between complex flows and hillslope dynamics in a finite element framework. Initial results indicate that the inertial forces generated by a simple 45° bend in a bedrock channel exceed the shear forces by a factor of two or more. Capturing these inertial forces and their 3D erosive potential provides a more complete understanding of the stream channel-hillslope ensemble. +
An enduring obstacle to reliable modeling of the short and long-term evolution of the stream channel-hillslope ensemble has been the difficulty of estimating stresses generated by stream hydrodynamics. To capture the influence of complex three-dimensional (3D) flows on bedrock channel evolution, we derive the contribution of hydrodynamic stresses to the stress state of the underlying bedrock through a Smoothed Particle Hydrodynamics (SPH) approximation of the Navier-Stokes equations as calculated by the DualSPHysics code (Crespo et al., 2015). Coupling the SPH flow solutions to the stress-strain formulation of the Failure Earth REsponse Model (FERM) (Koons et al., 2013) provides three-dimensional erosion as a function of the strength-stress ratio of each point in the computational domain. From the coupling of SPH and FERM we gain a 3D physics-based erosion scheme and a two-way link between complex flows and hillslope dynamics in a finite element framework. +
Analysis of topography can reveal signals resulting from both past and currently active tectonic regimes. In central Aotearoa New Zealand today, the Marlborough faults transfer plate boundary motion from the Hikurangi subduction zone to the highly oblique Alpine fault. The rocks of the Marlborough region have hosted active structures since the mid-Cretaceous when they sat at the edge of the Gondwana margin. Here we use tectonic geomorphology in conjunction with geological observations to unravel the long-term tectonic history of this plate boundary transition zone with emphasis on variations along and across strike, with depth and in time. To understand the active deformation occurring under the present tectonic regime, as manifested by recent complex faulting during the 2016 Mw 7.8 Kaikōura earthquake, we focus on understanding the 3D structure of the region as well as the development of, and control by, inherited structures. Cretaceous restoration of eastern Marlborough suggests that the major faults formed during extension of Te Riu-a-Māui Zealandia preceding breakaway from Gondwana. Overall, given the uncertainties of the reconstruction, the Cretaceous structural similarity of paleo-Marlborough with wider South Zealandia seems a remarkably clear and consistent match. How much of the distinctive landscape of Marlborough is due to the constraints of the current plate boundary versus the influence of tectonic inheritance? +
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Analyzing patterns of shoreline change between repeated LIDAR surveys reveals disparate styles of behavior on different temporal and spatial scales (Lazarus and Murray, GRL 2007; Lazarus, Ashton, Murray, Tebbens, and Burroughs, in review). We use wavelet analysis to investigate the mean variance (or spectral power) of cross-shore shoreline change, as well as the alongshore locations exhibiting high variance, across a range of scales. The time spans between surveys range from one to 12 years. On scales of a kilometer and less, the variance of shoreline change does not increase with the length of time between surveys. On these spatial scales, significant changes in shoreline location tend to occur in localized zones, and these zones shift from one time period to another rather than accumulating. Incidentally, the variance across these scales also exhibits a power-law behavior, even though different processes are known to dominate shoreline change on different scales within the range from 10-103 m. However, on scales larger than a kilometer, a peak in the variance appears, and both the magnitude of the variance and the alongshore scale of maximum variance increases over time; on these scales of a few to ten kilometers, shoreline changes do accumulate.
We interpret these observations as follows: On scales of a kilometer and less, each wave event creates an alongshore-heterogeneous pattern of shoreline change, with the alongshore locations of accentuated shoreline change depending on the characteristics of the waves (height, period, deep-water approach angle) and how those waves interact with heterogeneities on the seafloor—bathymetric features on the inner continental shelf are associated with shoreline change on the kilometer scale (List REFSXXX), and those in the surf zone and swash zones produce changes with alongshore scales on the order of one hundred meters and ten meters, respectively . Repeating such shoreline changes over many wave events superimposes essentially independent patterns of change, with effectively no memory of previous changes. The cumulative changes on scales of a few to ten kilometers, in contrast, suggest a diffusion of plan-view coastline shape; the relationship between the length scales of the variance peak over different time scales are consistent with diffusion, given estimates of effective diffusivity for this coastline (REF ANDREW, JORDAN). Apparently, on large alongshore length scales, the residual alongshore sediment flux that emerges from the many disparate wave events and associated complicated smaller scale patterns of sediment transport can be treated as related to shoreline orientation (the gradient in shoreline location)—the way that a long-term, large-scale, gradient-related flux of soil creep on hillslopes emerges from the complicated smaller-scale patterns of tree throw, gopher burrows, etc..
Answers to scientific questions often involve coupled systems that lie within separate fields of study. An example of this is flexural isostasy and surface mass transport. Erosion, deposition, and moving ice masses change loads on the Earth surface, which induce a flexural isostatic response. These isostatic deflections in turn change topography, which is a large control on surface processes. We couple a landscape evolution model (CHILD) and a flexural isostasy model (Flexure) within the CSDMS framework to understand interactions between these processes. We highlight a few scenarios in which this feedback is crucial for understanding what happens on the surface of the Earth: foredeeps around mountain belts, rivers at the margins of large ice sheets, and the "old age" of decaying mountain ranges. We also show how the response changes from simple analytical solutions for flexural isostasy to numerical solutions that allow us to explore spatial variability in lithospheric strength. This work places the spotlight on the kinds of advances that can be made when members of the broader Earth surface process community design their models to be coupleable, share them, and connect them under the unified framework developed by CSDMS. We encourage Earth surface scientists to unleash their creativity in constructing, sharing, and coupling their models to better learn how these building blocks make up the wonderfully complicated Earth surface system. +
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Anthropogenic activities associated with climate change and urbanization in coastal deltas (i.e.
groundwater extraction, coastal engineering and urban loading) have resulted in freshwater
degradation through the upwelling of saline paleowater. Factors controlling the preservation of
paleowater, and the initiation of exfiltration and subsequent upwelling of saline water are not yet
well understood. This research uses coupled morphodynamic-hydrogeologic modeling to
evaluate the groundwater response to geomorphic change. Delft3D is used to model the
formation of coastal deltas throughout the Holocene and create generic three-dimensional
distributions of sediment deposits characteristic of fluvial, wave, and tidal dominated deltas. The
generated sediment deposits are used to create three-dimensional effective grain-size maps by
convoluting the spatial distribution of each grain-size. This accounts for the combined effect of
multiple grain-sizes while preserving basin-scale heterogeneity commonly seen in highly
heterogeneous depositional environments. The effective grain size maps are used as the
geologic input for density-dependent groundwater flow and solute transport modeling. Results
are expected to show that the degree of aquifer heterogeneity correlates to the balance of fluvial
and marine morphological forces shaping sediment deposition. Spatial variability in basin-scale
aquifer heterogeneity is anticipated to control the exfiltration and upwelling patterns of saline
paleowater in deltaic environments. The modeling approach taken in this research is novel and
allows for the investigation of evolving groundwater systems with changes in landscape. Results
of this study will allow for the assessment of delta vulnerability to freshwater degradation from
upwelling saline paleowater, based on morphological classification. In the future, this research
may be used to help determine which deltas are most at risk for salinization and where science
and engineering efforts can be most beneficial to society.
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Any code that attempts to simulate large scale geophysical flows and their effect on topography needs a way to couple local flow properties to a rate of sediment erosion or deposition. However, the mechanisms responsible for a particle’s entrainment into a flow are poorly understood. Early erosional models setup a force balance between the fluctuating hydrodynamic forces acting on a particle and the adhesive forces holding a particle to the substrate. Later researchers eschewed this force balance in favor of an energy balance. They claim that a particle is constantly receiving energy from turbulent fluctuations in the flow near the surface, and that a particle will become entrained when it receives a critical amount of energy.
Despite all the work that has gone into deriving an erosion model based on theory, the most popular, and most accurate erosion model used in geophysical codes is the Garcia-Parker model, which is a simple fit to several sets of experimental data. But because their model is empirical, it’s impossible to know under what circumstances the model can and cannot be reasonably applied. A theoretical model would be much more desirable for precisely this reason.
Our goal is to better understand the mechanisms of particle entrainment through the use of direct numerical simulation. We are using a code developed at Lawrence Livermore National Laboratory, which solves the incompressible Navier-Stokes equations and uses a Lagrange multiplier method to enforce the correct boundary condition on the surface of the particles within the computational domain. With this method, we are able to accurately simulate the motion of thousands, or even tens of thousands of particles in an external flow in two or three dimensions. With this code, we can study in detail the coupling between local flow structures and the forces on a particle, which will hopefully lead to a better, theoretically based model for erosion. +
Arctic coasts have been impacted by rapid environmental change over the last 30 years. Warming air and water temperatures and the increased duration of the open water season, correlate with increases in the rate of already rapid erosion of ice-rich bluffs along the Beaufort Sea coast. To investigate longer-term changes in near-shore wave dynamics and storm surge set up as a result of sea-ice retreat, we coupled two simple modules.
Following Dean and Dalrymple (1991), we model wind-driven setup as a function of wind speed and direction, azimuth relative to the shore-normal, fetch and bathymetry. The wave module calculates the wave field for fetch-limited waves in shallow water based on the Shore Protection Manual (1984). For a given wind speed, dynamic water depth and fetch, we predict the significant wave height and wave period. Both modules require fetch as a controlling parameter. Sea-ice influenced coasts, are unique in that fetch is spatially variable due to the geometry of the shoreline and temporally variable as the location of the sea ice edge moves through the sea ice free season. We determine the distance to the sea ice edge using daily Nimbus 7-SMMR/SSM/I and DMSP SSMI Passive Microwave Sea Ice Concentration data. The sea ice edge is defined at a threshold sea ice concentration of 15%. We find a good match between the model predictions and our observed records of meteorological conditions and nearshore water level and waves along the Beaufort Coast in the summers of 2009 and 2010.
Over the period 1979-2012, fetch has increased significantly. In our study area near Drew Point, Alaska, the open water season itself lengthened from ~45 days to ~90 days. In the 1980’s and early 1990’s wave dynamics were fetch-limited during a significant period of the open water season. More recently, the distance from the coast to the sea ice edge shifts extremely rapidly (often 100’s of km over 1-2 weeks); fetch therefore only minimally influences wave dynamics as offshore distance exceeds the 140 km threshold over most of the open water season. Wave heights and surge set-up events on average have not changed in magnitude significantly, but storm surge set up events have increased in frequency.
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Arctic hydrological processes impose an important feedback on permafrost thermal conditions. Changes in permafrost hydrology could accelerate its thawing, resulting in a positive effect on permafrost carbon decomposition rates. Therefore, it is important to understand how geomorphic and other landscape processes control permafrost distribution and its properties such as soil saturation, ice content, active layer thickness (ALT) and temperature. The Advanced Terrestrial Simulator (ATS) is a collection of hydro-thermal processes designed to work within a flexibly configured modeling framework. ATS includes the soil physics needed to capture permafrost dynamics, including ice, gas, and liquid water content, multi-layered soil physics, and flow of unfrozen water in the presence of phase change. In this study, we directly address one of the tasks of the NGEE-Arctic project by modeling the effect of climate and environmental drivers on ALT and permafrost thickness and its distribution along the subarctic hillslope. Model runs demonstrate the likely role of vegetation-snow-permafrost-hydrology interactions by exploring snow depth and organic layer influence on horizontal and vertical patterns of permafrost. Understanding changes in hydrologic flow paths and soil moisture is important to predict evolution of ecosystem and biogeochemical processes that control climate feedbacks. In addition, hillslope flowpaths, vegetation, soil organic matter distribution, variation in soil depth and mineralogy are important components of the subgrid spatial extent of permafrost. This study explores the ways to improve the quality of the permafrost predictions at the subgrid scale and contribute to the better modeling of the permafrost related processes at the pan-Arctic scale. +
Arctic rivers play a crucial role in transporting sediment and nutrients from permafrost landscapes to the Arctic Ocean, influencing both landscape evolution and biogeochemical cycles. These river systems are undergoing significant transformations due to decreasing snow, intensified summer precipitation, altering vegetation, and permafrost thaw. Over a seasonal cycle the thermal state of Arctic rivers changes as their beds and banks thaw. Long-term observations indicate a rise in Arctic river discharge. However, our understanding of the complex mechanisms governing sediment transport in these rivers remains limited.
To address this gap, we focus on the Canning River, a gravel-bed river situated in continuous permafrost in Alaska. Previous studies on small nearby rivers during the 1970s suggested that sediment transport is hindered during the ice break-up flood because the channel bed remains frozen while cold river water starts running, slowing the sediment bed from thawing. This would imply a decoupling of sediment transport from water discharge, at least seasonally, in Arctic rivers. To investigate this hypothesis, we conducted fieldwork during the summers of 2022 and the spring of 2023, representing periods of high river discharge with differing thermal states. Our data collection included measurements of discharge, temperature, suspended sediment fluxes, grain size distributions, seismic signals, ground temperature, and river ice thickness, which we compared to a historical 5-year river discharge record.
We model how the river freezes to its bed over extended stretches during winter, and how it forms aufeis up to 2 meters thick despite limited water flow. Observed water temperature around the ice break-up period hovers around 0°C, potentially requiring several days to thaw the matrix sands and prohibiting pebble movement, according to our thermal model. Conversely, by mid-summer, water temperature rises to approximately 12°C. Although mid-summer river discharge peaks are lower, suspended sediment increases substantially during intense rainfall events, indicating a strong coupling with river discharge. These initial findings suggest that annual sediment transport might amplify with warming conditions, as the river water may no longer freeze to the channel bed and as summer flows intensify.
As a foundation of many ecological systems, vegetation is often a central component of ecological models used for forecasting and management. Many models are narrowly constrained by the system, species, and/or processes of interest and lack the ability to simulate specific management actions. This specificity limits their applicability to new, nonstationary, or actively-managed systems. The objective of this work is to create a Landlab component that combines an individual-based model design with grid-based model components to describe vegetation dynamics within and between grid cells. GenVeg is process-based, incorporating polymorphic plant-scale processes such as photosynthesis, dispersal, and seasonal allocation of biomass resources. Plant taxonomic principles are used to adapt the model methods based on the species (or representative species) of interest. Feedbacks between plants, plant communities, and the local physical environment utilize existing Landlab components and grid geometry to represent vegetation dynamics across the landscape. GenVeg is designed to be applied at a scale on the order of 10s to 1000s of meters over years to decades, which are scales relevant to ecosystem management and engineering planning. While the component is still under development, we will demonstrate its use within a dune environment utilizing coastal water levels and soil moisture to drive vegetation distribution across an idealized foredune system. +
