Property:CSDMS meeting abstract
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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. +
A
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? +
A
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.
A
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. +
As a rift evolves from its initiation until continental breakup it goes through a number of different phases that can be associated with distinct rifted-margin domains and major sedimentary basins. Seismic and geophysical data around the globe can give us glimpses into the progression through these domains, however, it is not well understood how the fault network evolves to produce them. Additionally, sedimentation and erosion are known factors that influence the longevity of an evolving fault and may affect the overall rift evolution. Previous work has qualitatively investigated the effect surface processes have on an evolving rift, however, there has not been a quantitative approach to analyze changes to the fault network through time.
To investigate the quantitative effect of surface processes on an evolving rift fault network, we utilized the two-way coupling between the geodynamics code ASPECT and the landscape evolution code FastScape to run 12 high-resolution 2D rift models. Using FastScape, we vary the erosional efficiency of the stream power law by changing the bedrock erodibility (Kf) from no surface processes to low (Kf= 10-6 m0.2/yr), medium (10-5 m0.2/yr), and high (10-4 m0.2/yr) efficiency. We then apply this to three different model setups that represent a wide, asymmetric, and symmetric rift. We analyze the models using the fault analysis toolbox (fatbox), which can track and correlate individual faults and their properties through time. Specifically, we utilize this toolbox to track the evolution of the number of faults and the cumulative fault system length and displacement through time and investigate how they change depending on the efficiency of surface processes and the rift type.
Through this analysis, we find that regardless of the rift type or the efficiency of surface processes the rift fault network evolves through up to five distinct phases: 1) distributed deformation and coalescence, 2) fault system growth, 3) fault system decline and basin-ward localization, 4) rift migration, and 5) continental breakup. While we find that surface processes do not exert a strong control on the phase progression or final rifted margin architecture, they do affect the temporal evolution of the fault network by increasing fault longevity. As faults live longer with greater surface processes, the fault network phases are prolonged and continental breakup is delayed. Additionally, greater surface process efficiency leads to fewer faults forming which causes a less complex fault network.
As climate change and environmental variability increase pressure on vulnerable communities, migration is one possible adaptation strategy. However, the decision to migrate is complex, and environmental factors are rarely the sole drivers of that decision. Rather, the decision to migrate is often influenced by a combination of economic, social, political, and environmental pressures. This is especially true in coastal communities in Bangladesh, where temporary migration has long been a method of livelihood diversification, and researchers are trying to understand how environmental factors influence existing migration flows. This work addresses a gap in current research by beginning to investigate how different “push” and “pull” drivers of migration might have distinctive variables that contribute to the ultimate decision to move or stay. In this study, random forest classification models are applied to a dataset consisting of household surveys from more than 1,200 households in southwestern Bangladesh to directly assess key variables that influence five types of migration in coastal communities: temporary migration within a village due to environmental stress, migration for education, migration for healthcare, migration for trade or commerce, and migration to visit relatives. This work demonstrates that these types of migration do have different drivers, which yields insights into the complex motivations that impact the decision to migrate. However, livelihood variables and individual aspirations were key for all investigated forms of migration. In the process, this work demonstrates that random forest models could be a powerful method for improving predictive accuracy of migration models to better inform migration policy and planning. +
As coastal regions become more developed, many communities are considering costly engineering solutions to address coastal change, including "soft" approaches, such as beach replenishments or dune constructions, and hard structures, such as seawalls, revetments, bulkheads, or groins. Given current rates of sea level rise and the associated shoreline losses that coastal communities face, however, it is unclear whether the benefits generated by these protection measures justify the costs. We are building a set of integrated geologic and economic models to better understand the coupled evolution of developed shorelines under alternative protection policies. The first model incorporates dune construction and sediment overwash relocation into a morphodynamic model for dune evolution. We use this model to assess the costs of constructing an optimal cross-sectional area for a long-term dune system, and we explore the “geo-economic” effects on ocean views that may be diminished by constructing a dune system of particular size seaward of protected properties. A second model simulates beach width dynamics for two adjacent communities, each with their own groin structure. We use the model to analyze both coordinated and uncoordinated strategies between the two communities, reflecting individual community decisions to protect or retreat. A third model incorporates beach nourishment practices into a morphodynamic model for barrier evolution that accounts for shoreface dynamics. Results show that the efficiency of beach nourishment can be affected by the dynamic state of the shoreface during each nourishment episode. In general, these models reinforce the need to refine numerical coastal management tools to incorporate bi-directional interactions between natural processes and human responses to shoreline change. +
As one of the three major Asian marginal seas in the western Pacific, the SCS occupies less than 1% total ocean area while accommodating 15% atoll (25434.6 km2) in the globe (GSA, 2009), which mainly distribute in the Xisha, Zhongsha and Nansha Islands. Atolls in the SCS are generally ellipse-shaped with a longer axis extending in the NE-SW direction and a wider southwest reef platform compared to the northeast. One possible explanation ascribed such features to the monsoon circulation (northeast and southwest monsoons blow alternatively in winter and summer) over the SCS (Zeng, 1984). Waves and currents influence the atoll development by (1) sediment suspension and transportation that can influence the transparency of the water, thus the symbiotic algae and the coral growth, (2) supply of dissolved oxygen and nutrient and (3) removal of metabolic wastes under normal weathers, while storm waves can cause large-scaled breakage, transportation and reconfiguration of reefs (e.g. Chappell, 1980; Storlazzi et al., 2005). Yet, little data was available regarding the hydrodynamic conditions of the forereef of the SCS atolls. Here, we conducted in situ tripod mooring observations (ADCP, ADV & CTD) for at least one tide cycle in 15-18 m water depth at the southeast forereef of three typical atolls – Xiaonanxun (NX), Anda (AD) and Kugui (KG) Reef – in the SCS, respectively, and collected coral sediment samples at different zonation of atolls in September 2017. During the observation periods, tide elevations varied by ca.1 m in all the three sites, with the highest 1.16 m in AD and lowest 0.96 m in KG. Mean flow velocity turns out to be as weak as about 0.1 m/s, with the weakest ~0.05 m/s in KG. Wave influence appears to be strongest in NX, with the significant wave height of ~1 m, in contrast to the 0.6 m and 0.4 m in AD and KG, respectively. The hydrodynamic observations under normal weathers should be able to transport the fine reef debris alone, with limited sediment transport rates of 0.61, 0.01 and 0.64 m3/m per tidal period in the observations in NX, AD and KG, respectively. Coarse coral rubbles and gravels might be only transported during extreme weathers. More observations and modeling work are needed, e.g. simulations of waves’ influence on atoll sedimentary systems’ development with XBeach.
A
As part of the Mediterranean Landscape Dynamics (MedLand) project to create a modeling laboratory for human-landscape interaction, we have developed a suite of landscape evolution tools in the GRASS GIS environment. The core of this tool set is a Python script to estimate sediment transport for hillslopes, gullies/rills, and small channels, and simulate resulting terrain change for high-resolution 3D digital landscapes. Because it takes advantage of raster-optimized routines in GRASS, it is very fast on normal desktop systems, making it ideal for simulating long-term landscape change resulting from human activity, climate change, or other drivers. We provide examples of how this landscape evolution model is being used in the MedLand project. +
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Assessing the tsunami hazard in regions with infrequent or no instrumental or historical records of tsunamis is a challenge for emergency managers. In the absence of these records, coastal geologists rely on evidence of past tsunami inundation from buried sedimentary deposits to identify the presence of a tsunami hazard and to determine the recurrence of past events. One persistent challenge in assessing tsunami hazard from sandy coastal deposits is inferring the relative magnitude of past tsunamis from characteristics of the deposits. Recent reanalysis of field data from the 2011 Tohoku-oki earthquake and tsunami show that the volume of onshore sandy tsunami deposits is highly correlated with offshore tsunami magnitude, seafloor deformation, and fault slip. To further explore these relationships, we employ a Delft3D-FLOW hydrodynamic and sediment transport model to simulate onshore tsunami deposit volume from offshore slip of the 2011 Tohoku-oki earthquake and tsunami. We use the Satake et al. (2013) tsunami source model to derive the hydrodynamic boundary conditions for the sediment transport simulations. The Delft3D-FLOW model uses van Rijn (2007) sediment transport formulations and coefficients and a two-dimensional, vertically layered grid to model sediment transport with the effect of suspended-sediment induced density stratification on the vertical turbulent mixing. We model how variation in offshore slip affects tsunami deposit volume for a wide range of sediment sources, offshore and onshore slopes, and boundary roughness conditions. Model results show a strong correlation between onshore tsunami deposit volume and adjacent offshore co-seismic slip if ample sediment is available in the model to be eroded and transported. These results are consistent with data from the 2011 Tohoku tsunami at sites with sufficiently wide beaches and without shoreline armoring. We continue to test the model to evaluate sensitivity to parameters that may not be well known for paleo-tsunamis such as width of fault rupture, paleo-topography, and changes in sea level. Ultimately, this approach may be able to reconstruct past tsunami magnitudes and improve assessment of tsunami hazard.
* Satake, K., Fujii, Y., Harada, T., & Namegaya, Y. (2013). Time and space distribution of coseismic slip of the 2011 Tohoku earthquake as inferred from tsunami waveform data. Bulletin of the seismological society of America, 103(2B), 1473-1492.
