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Recurved barrier spits occur in a wide variety of environments, from active delta complexes to rocky coasts, where spits extend depositionally from a shore that is otherwise eroding. Although controls on spit orientation are often presented in the literature a posteriori (i.e. after the spit has been observed), there surprisingly remains no general model that predicts spit shape and orientation in terms of external variables, such as wave climate, sediment supply, and embayment depth. We study spit shape controls using the Coastline Evolution Model (CEM), a numerical model that evolves the plan-view coast based upon the processes of alongshore sediment transport and barrier overwash maintaining a minimum critical barrier width. Model results demonstrate that the directional distribution of approaching waves serves as a first-order control on spit shape, with waves from multiple directions playing a vital role in spit extension and reshaping. Surprisingly, we find that boundary effects, namely the rate of change of the updrift coast location, play a similarly important role in spit shape. The depth of the platform upon which a spit grows plays another important role, with deeper platforms tending to accommodate more sharply curved spits. Every day, spits act as a type of messenger in disguise, revealing wave forcing, sediment supply, and local geometry. +
Reduction of nitrogen (N), phosphorus (P), and suspended sediment (SS) load has been a principal focus of Chesapeake Bay Watershed management for decades. To evaluate the progress of management actions in the Bay's largest tributary, the Susquehanna River, we analyzed the long-term seasonal trends of flow-normalized N, P, and SS load over the last two to three decades, both above and below the Lower Susquehanna River Reservoir System. Our results indicate that annual and decadal-scale trends of nutrient and sediment load generally followed similar patterns in all four seasons, implying that changes in watershed function and land use had similar impacts on nutrient and sediment load at all times of the year. Above the reservoir system, the combined loads from the Marietta and Conestoga Stations indicate general trends of N, P, and SS reduction in the Susquehanna River Basin, which can most likely be attributed to a suite of management actions on point, agricultural, and stormwater sources. In contrast, upward trends of SS and particulate-associated P and N were generally observed below the Conowingo Reservoir since the mid-1990s. Our analyses suggest that (1) the reservoirs' capacity to trap these materials has been diminishing over the past two to three decades, and especially so for SS and P since the mid-1990s, and that (2) the Conowingo Reservoir has already neared its sediment storage capacity. These changes in reservoir performance will pose significant new kinds of challenges to attainment of total maximum daily load goals for the Susquehanna River Basin, and particularly if also accompanied by increases in storm frequency and intensity due to climate change. Accordingly, the reservoir issue may need to be factored into the proper establishment of regulatory load requirements and the development of watershed implementation plans. (Published in Science of the Total Environment (2013); available at http://dx.doi.org/10.1016/j.scitotenv.2013.02.012. For a pdf pre-print, please contact Qian Zhang at qzhang19@jhu.edu.)
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Reef islands are carbonate detrital landforms perched atop shallow reef flats of atolls and barrier reef systems. Often comprising the only subaerial, inhabitable land of many island chains and island nations, these low-lying, geomorphically active landforms face considerable hazards from climate change. Sea-level rise and wave climate change will affect sediment transport and shoreline dynamics, including the possibility for wholesale reorganization of the islands themselves. Here we apply a hierarchical modeling approach to quantify the potential responses of reef island systems to future changes. Using parameterizations of sediment transport pathways and feedbacks from previously presented XBeach modeling results, we investigate how sea-level rise, change in storminess, and different carbonate production rates can affect the profile evolution of reef islands, including feedbacks with the shallow reef flat that bounds the islands offshore (and lagoonward). Model results demonstrate that during rising sea levels, the reef flat can serve as a sediment trap, starving reef islands of detrital sediment that could otherwise fortify the shore against sea-level-rise-driven erosion. On the other hand, if reef flats are currently shallow (likely due to geologic inheritance or biologic cementation processes) such that sea-level rise does not result in sediment accumulation on the flat, reef island shorelines may be more resilient to rising seas. This simplified modeling approach, focusing on boundary dynamics and mass fluxes, including carbonate sediment production, provides a quantitative tool to predict the response of reef island environments to climate change. +
Relating scientific results generated from modeling, remote sensing, or instrumental measurements of Earth’s surface to topics of social relevance often poses challenges for theoretical scientists. Theoretical science is treated as divorced from social matters, as a matter of definition, unless specific problems are solved, then it shifts into an applied realm and is viewed as spatially constrained. A paradox results: if scientific results are “merely” theoretical, yet intended to be universal, then it is undetermined whether results would follow everywhere, but if they are collected from actual measures and observations it is undetermined whether the same conditions are everywhere and apply to all spatial and temporal scales. The problem is particularly acute for Earth’s surface which is both physical and has high value for all human beings and infrastructures and all life. It is also the object of theoretical models. So, it is critical for human life and its connections to all life that models aim to model the spatial and temporal reality of Earth’s surface including regional variabilities with their constraints. Social relevance is a value that can be viewed together with physical characterizations as part of understanding landscapes.
Outlined are considerations to help integrate values that express social relevance into the scope of how theoretical science is conducted and approached to address the scale of connectedness of Earth’s surface while expanding the human reach of Earth surface modeling. Inspiration has been taken from recent NSF sponsored initiatives, such as efforts to expand and unify through diversity and inclusion the critical zone (CZRN) and convergence research in Navigating the New Arctic, as well as the open science philosophy of CSDMS, and a recent Greenland Data workshop seeking to unify data management of Greenland. I contribute my synthesis to engage with others about how the application of computational approaches and landscape data management can be used to provide basic platforms for treating and comparing earth surface data at multiple temporal and spatial scales while having “convergence” of social spheres as an underlying consideration. Therefore, the infinite potential at any point on Earth’s surface is representable, relatable, and connectable numerically, and it recognizes and includes the realm of human beings as investigators and inhabitants of any landscape.
Computational thinking may be a source for thinking about expanding who are considered the inhabitants of landscapes and who studies the landscapes, i.e. including diverse identities in research while also unifying them through shared and recognized goals. Expanding the realm of theory in Earth surface processes to include model data about people, life, geographies, climates, processes and their change through time is a new science frontier. The idea that information has many dynamic layers and dimensions and that there are many ways to connect and relate them through time with computational approaches as a starting point may serve as a guide for integrating social value into theoretical research.
River delta morphology is shaped by complex interactions between sediment supply and discharge variability, influencing their resilience to environmental changes. This study employs Delft3D-Flow to investigate how different discharge scenarios—constant discharge, a unimodal flood hydrograph, and a monthly flood hydrograph—affect the long-term evolution of a river-dominated delta modeled after the Wax Lake Delta, Louisiana. Over a 50-year simulation period, results indicate that unsteady discharge promotes a more symmetric delta morphology with broader sediment deposition, while constant discharge leads to localized deposition near the river mouth, resulting in an elongated delta with fewer channels. These findings highlight the role of discharge variability in delta formation, with implications for coastal management and restoration strategies. +
River deltas are dynamic landforms that archive the complex interplay among sediment supply, water discharge, and sea-level change. Understanding how these factors shape delta morphology over centennial to millennial time scales remains a central challenge, particularly in the context of accelerating sea-level rise. To investigate fan delta evolution under a wide range of external forcings, we couple geometric and enthalpy-based numerical models with experimental data.
We analyze 14 experimental runs conducted in a tilting flume facility that produced isolated fan deltas over a sloped, non-erodible basement. Each run maintained a fixed water-to-sediment discharge ratio and implemented a sea-level scenario—either constant, rising, or falling. These experiments span both steady base-level conditions with varying sediment and water inputs, and sea-level rise scenarios. To extract morphodynamic data, we apply a computer vision algorithm to time-lapse imagery, enabling automated reconstruction of topset and foreset geometries.
Our first modeling approach uses a geometric framework that assumes conical fan delta shapes to estimate three-dimensional volumes and sediment partitioning between the subaerial topset and subaqueous foreset. The model quantifies key metrics (slope and opening angle), reveals a consistent relationship across scenarios between plan-view opening angle at the alluvial-bedrock transition and sediment/water discharge ratios: increasing sediment supply or decreasing water discharge produces narrower opening angles, and vice versa.
The second model is a moving-boundary numerical framework based on the enthalpy method, enabling more realistic geometries and dynamic responses. Simulations under continuous sea-level rise replicate key experimental observations: foreset starvation leads to abandonment of the submarine delta front, while the topset migrates landward and narrows. This retreating geometric adjustment is effectively captured by the model. Its simplicity enables efficient exploration of a wide parameter space, providing new insights into deltaic evolution and sedimentary prism development under changing environmental conditions.
The integrated modeling framework provides a quantitative foundation for linking external forcings to delta morphology and stratigraphy. Our long-term goal is to apply this approach to constrain past sea-level histories and sediment budgets from plan view geometries, with future applications to Arctic deltas and other climate-sensitive coastal systems.
River deltas ringing the Arctic Ocean coastline are unique landforms shaped by both highly seasonal cold region hydrology and permafrost features such as thermokarst lakes. These lakes trap, store, and modulate the timing and magnitudes of riverine freshwater, sediment, and nutrients. Future climate warming is expected to thaw permafrost, modifying lake coverage and therefore riverine flux delivery to the Arctic Ocean. How and where thermokarst lake coverage on deltas will change remains highly uncertain, in part due to the difficulty in separating perennially inundated thermokarst lakes which undergo thermal expansion in response to warming, from ephemeral wetlands resulting from interannual and seasonal hydrologic variability. We present a methodology that allows us to classify waterbodies as perennial lakes or ephemeral wetlands, by examining their presence in a 20 year record of Landsat imagery. By analyzing 12 deltas laying on a gradient of temperature and ice content, we find that perennial lakes and ephemeral wetlands have universal but distinct size distributions which result from different mechanisms forming the two waterbodies. We also find that colder deltas have larger lakes on average, mechanistically attributed to thicker and colder permafrost which supports larger lakes by preventing sub-lake unfrozen zones (i.e. taliks) from connecting to the sub-permafrost groundwater table. Lastly, we explore how differences in the spatial patterns of lakes across the deltas may relate to climate variability. These findings provide the basis for quantitative predictions for the trajectory of lake and wetland coverage on arctic deltas under projected warming. +
River morphodynamics can affect overbank flooding if changes to cross-sectional area or roughness reduce the flow conveyance capacity of the channel. Correspondingly, extreme floods can also cause drastic adjustments to river morphology on relatively short timescales. These co-occurring processes raise the question: how do flood dynamics and river morphology co-evolve? The difficulty of conducting rapid field measurements of river geometry during peak flows has limited existing research on channel adjustment and recovery during a flood hydrograph. Here, we leverage a one-month Delft3D simulation to investigate flood hydraulics and river morphodynamics in a November 2021 flood event in the Nooksack River, western Washington State (WA). This flood devastated river-adjacent WA communities along the Lower Nooksack. Flood waters additionally overtopped the levees near Everson, WA and traveled over 25 km north towards the Fraser River, causing extensive and costly damages across the U.S.-Canada border. To understand the feedbacks between river morphodynamics and floods, we analyze streamwise changes in flood hydraulics (flow velocity & shear stress) and morphodynamics (bed elevation change). Within the Everson overflow region, we find that spatial gradients in flow velocity shift between in-bank- and peak-flow conditions. Bed elevation changes are commonly located where hydrodynamic gradients intensify during peakflows. Importantly, at the Everson overflow location, bed deposition co-occurs with a drastic along-channel velocity decrease and overbank flow, suggesting that the hydraulics of overbank flooding can contribute to local morphodynamic adjustment. We additionally investigate how channel adjustments during the first November 2021 flood peak affect flooding during a secondary flood peak occurring two weeks later. These results are of particular interest to the Nooksack River floodplain managers who are eager for insights on the contribution of channel morphodynamics to flooding and who are actively investigating methods to alleviate overtopping including setback levees and dredging.
River profiles are shaped by a combination of tectonic forcing, climatic history, and internal feedbacks. One example of internal dynamics is the self-formation of waterfalls in steep channels, which can cause erosion rates to both accelerate (‘fast waterfalls’) or decelerate (‘slow waterfalls) relative to waterfall-free reaches. We previously used a 1D stream power model with a waterfall rule to show that the self-formation of waterfalls above a threshold slope can alter river long profiles over km scales. In the 1D model, the formation of fast waterfalls results in a uniform-gradient zone maintained by a dynamic equilibrium, while slow waterfalls can cause autogenic knickpoints. However, it is not clear how these findings alter river profile form in a 2D setting in which hillslopes and channels are linked and adjacent basins can interact. Here, we ask the question: are long profile signatures from waterfall formation enhanced or erased by additional internal feedbacks, such as hillslope diffusion and planform channel adjustment? To address this knowledge gap, we implemented our waterfall model in a 2D setting, using Landlab and specifically the SPACE and hillslope diffusion components. Our results in 2D show agreement with the initial 1D implementation, and additional results show that self-formation of waterfalls can alter landscape-scale metrics, including drainage density and slope-area relationships. Additional exploration of natural variability, including stochastic rock strength show that increasing natural variability can expand the length of the affected profile. +
River-bed grain size distributions in fluvial systems set the initial condition for landscape change on the event to millennia scale. These distributions are used to infer characteristic flow conditions or estimate mass flux through a fluvial system, and this is often under the assumption that sediment grain-size distribution on the bed remains static over time. However, recent work has shown that grain size distributions can fluctuate over individual flow events, can be dependent on the sequence of successive flow events, and can change seasonally. This discrepancy can lead to order of magnitude differences in estimating sediment flux or characteristic hydrologic conditions. To constrain bed grain size evolution, we perform numerical simulations using a probabilistic, discrete model of a river bed in which computational cells represent grains randomly distributed across the surface of the bed. Patterns and timing of grain mobilization are determined according to distributions of entrainment thresholds for individual grains and flow rate dependent distributions of velocity fluctuations that vary over hydrographs. Entrained grains are replaced from a static distribution allowing the bed grain size distribution to evolve throughout the simulation depending on the imposed flow condition. Our preliminary model results from varying initial grain size distribution, individual event shape, and flow sequencing show a significant dependence on the range of grain sizes available for transport as well as the total duration of individual flow events, while seasonal variability has a moderate impact on bed grain size evolution. +
Rivers are key drivers of landscape evolution. Transient signals of base level fall propagate up rivers and cause increased erosion rates, which in turn increases channel steepness. In landscapes with horizontally layered rocks, erosion rates vary in both space and time as different layers are eroded and exposed at the surface, complicating how these landscapes respond to base level fall and influencing the topographic expression of different lithologies. Lithologic variations further influence river response by producing sediment, which can armor the channel bed and reduce bedrock erosion. Motivated by the lithologic variability found in the Guadalupe Mountains of Texas and New Mexico, we use the Stream Power with Alluvium Conservation and Entrainment (SPACE) model to test how sediment cover affects channel steepness and erosion rates in horizontally layered rocks. We simulate 1.2 million years of landscape evolution with an imposed uplift rate of 1.0 mm/yr in alternating layers of hard and soft rock, systematically varying the relative amount of alluvial cover. We compare the normalized channel steepness of the model output against the steepness values predicted by the stream power incision model, which is the most commonly used model for interpreting steepness variations in real landscapes. We find that in model runs with sediment cover, channel steepness is systematically higher than predicted by the stream power model in soft rock layers, and lower than predicted in hard rocks. Sediment cover also exerts a strong control on erosion rates. Sediment cover preferentially accumulates over soft rock layers, decreasing erosion rates in these layers while increasing erosion in the unarmored hard rock layers. As the degree of sediment cover is increased, the maximum erosion rate at any given point along the channel decreases. Increasing the sediment cover effectively decreases the erodibility contrast between hard and soft rocks, illustrating the importance of considering the role of sediment when interpreting channel profiles.
Rivers exhibit high temporal variability in their constituent concentrations, which can result in distinct hysteresis patterns of suspended sediment (SS), particulate organic carbon (POC), and dissolved organic carbon (DOC) during various high-flow events. In mountain rivers, this variability may be controlled by the mobilization of streambed sediment, which is enriched with organic carbon and other nutrients and can be evaluated by the entrainment of the armor layer, a group of coarser particles that act as a protective layer to finer sediment. In this study, we test if resulting hysteresis patterns could be controlled by the extent to which the armor layer was mobilized by analyzing POC, DOC and SS patterns during monsoon-driven flow events in two river reaches at the Valles Caldera National Preserve, New Mexico. We collected continuous water samples over numerous flood hydrographs using automated ISCO samplers while also monitoring flow depth, turbidity, and fluorescent dissolved organic matter (fDOM) using YSI EXO2 sondes. Turbidity and fDOM were included because they can be used as proxies for SS and DOC, respectively. To quantify armor mobilization, we deployed tracer particles of different sizes and mapped their location before and after each event. We found that constituent hysteresis changes between clockwise and counterclockwise for different flow events, each with their own streambed mobilization characteristics. Opposite hysteresis occurred for DOC and fDOM than for POC, SS, and turbidity. Additionally, there were poor correlations between DOC and SS, while POC and SS were found to be well correlated. Our results imply that DOC hysteresis might not be related to the release of fines from the streambed, whereas POC is. These findings suggest that the temporal variability of SS and POC can be partly understood by the mobilization and suspension of fine sediment not only seasonally, but also over individual high-flow events. We show how the mobilization of the armor layer can be an important contributor to this variability.
Rivers flowing across permafrost limit the residence time of floodplain soil organic carbon (SOC) by transferring riverbank material to the fluvial network. In this way, permafrost riverbank erosion limits production of greenhouse gases (GHG) in a deepening permafrost active layer. Concurrently, arctic riverbanks are laden with permafrost ice wedges and channels themselves are seasonally occupied by ice; these factors are known to affect fluvial potential to erode and remove material from floodplains. However, we do not know how ice impacts rates of riverbank migration through permafrost, especially in small arctic watersheds where observations are limited. We hypothesize that bank migration into permafrost is best described by melting of bank ice but is also described by slumping of the active layer, and fluvial carrying capacity. First, we develop a model of thermal riverbank erosion. Next, we detail plans to incorporate slumping and mechanical erosion into a holistic icy riverbank erosion model. We will characterize riverbanks along the Canning this field season and use or measurements to calibrate our model. Then, we will test which mechanisms are most significant in forming permafrost riverbanks, as well as describe their evolution under warming mean annual air temperatures. This work is also useful for predicting the future contribution of the arctic rivers to their basin-wide carbon budget. This work has immediate importance for people who traverse or depend on arctic landscapes, but especially those who live within them. Arctic landscape response to climate change is just as much a story about the loss of place and vanishing resources as it is about a dynamic earth system. +
Rivers in natural settings are frequently characterized by downstream variations in channel width. However, the effect of width variations on bed topography and sorting patterns remains poorly understood, especially under conditions of changing sediment and hydrologic regimes. In this study we use two-dimensional numerical modeling to systematically explore how the amplitude and wavelength of sinusoidal width variations affect the shape and location of bars, sorting patterns of surface sediment, and the movement of a sediment pulse. We perform simulations with sediment regimes consisting of constant sediment supply, no sediment supply, and a sediment pulse with no background sediment supply. We also perform steady and unsteady flow simulations to explore the combined effect of hydrograph shape and width variations. Preliminary results indicate that width variations force riffle-pool topography with riffles coincident with wider channel sections and pools at narrow sections. The amplitude of width variations is the dominate factor controlling riffle-pool relief. The wavelength of the width variations controls whether central or side bars develop in the wider channel sections. These numerical simulations are complimented with ongoing physical experiments in a laboratory flume and can potentially be used to guide stream restoration and river management practices under conditions of varying sediment and hydrologic regimes. +
Robust estimate of the critical shear stresses and flocculation is a prerequisite for analyzing the sediment dynamics of tidal flats. To determine the impact of clams (Meretrix meretrix Linnaeus) on critical shear stress and flocculation, in situ measurements were made on both a bare flat and a flat inhabited by abundant clams. on the Jiangsu Coast, China. Near-bed in situ floc size, grain size distribution of suspended particles in sea water, suspended sediment concentration (SSC), salinity, and currents were measured for three consecutive semidiurnal tidal cycles simultaneously at the two stations. Based on the observational data, mean floc size measured by LISST-100x was found to be more than three times the size of dispersed suspended sediment in water samples, suggesting remarkable flocculation processes were occurring. Correlation analysis indicated that the flocculation and break-up process in the study area appeared to be controlled by the variations in SSC and turbulent shear. Negative relationships were found between SSC, turbulent shear and floc size for both stations, but for a given SSC, floc size was usually larger at the clam aquaculture site. The comparison between the two sites shows that the near-bed in situ floc size in the aquaculture mudflat (mean 88 μm) was 70% larger than that in the bare tidal flat (mean 51 μm), suggesting significant biological modulation on the flocculation processes, as the hydrodynamics were very similar between the two sites. The content of extracellular polymeric substances was obviously higher in the sediment layer below the surface seabed at aquaculture site. Further, we find that the sediments on the flat inhabited by clams were more erodible and had a lower critical shear stress for erosion (0.13 N m-2) due to the physical and biological activities of clams. The results further show that the critical shear stress for deposition on the flat with M. meretrix (0.13 N m-2) was 30% greater than that in the bare flat (0.10 N m-2). Our results suggest that changes to the critical shear stress for erosion and deposition caused by the activity of clams can alter the sediment dynamics and geomorphologic processes of flats, as well as abundant filter feeders alter floc properties and enhance flocculation by excretion of exopolymer particles.
Salmonine fishes (salmon and trout) are resilient and have evolved to survive environmental perturbations, including flood, drought, and wildfire. The effects of these perturbations are translated through the landscape by rivers, where aquatic communities can be severely impacted. For instance, after wildfire, rivers can experience increased frequency and magnitude of flash floods, ash and nutrient loading, increased sediment flux from runoff and debris flows, destabilization and physical alteration of fluvial habitat, stream temperature impairment, and either loss or gain of refuge (e.g. deep pools, woody debris, riparian vegetation). Depending on the severity, any one of these effects could drive the extirpation of fish populations, and the response and survival of fish gets increasingly complex when faced with multiple environmental perturbations. Historically, the extirpation of fish populations would not have been as significant a risk to the extinction of entire species or subspecies of salmonids, as unrestricted migration allowed for recolonization by neighboring populations. However, increasing river disconnectivity, due to the introduction of physical barriers, has put native fish species at greater risk of extinction after natural catastrophes. In order to evaluate the viability and recovery of fish populations after catastrophe, we have developed a multi-site structured population viability analysis (PVA) model that is designed to incorporate factors that are unique to the spatial distribution of catastrophe and migration in fluvial networks. Specifically, our multi-site PVA provides the flexibility to vary both the duration and severity (i.e., multi-year catastrophe and habitat recovery) of vital rate adjustment (survival and growth). Our model also allows for a multi-mechanistic approach to vital rate adjustment after catastrophe – this is a particularly important advancement, as fluvial habitats located within the fire perimeter often experience distinctly different impacts than those outside of but downstream of fire. Both of these improvements are necessary as the negative impacts of wildfire on fish habitat and vital rates can last for years or even decades, and commonly used PVA modeling software only allows for impairment to last for one year. Additionally, previous models allow for a “one, all or radial spreading” approach to the spatial distribution of catastrophe, which works for disease but is inconsistent with the flow routing of catastrophe in stream networks. Finally, we have also developed a new metapopulation migration model that accounts for bidirectional river connectivity, a characteristic of migration unique to fluvial environments. Migration behavior in this model is driven by simple probabilities of life-stage structured dispersal and migration distances, measures of habitat suitability (including post-catastrophe adjustment), and site population densities. To demonstrate the utility of our multi-site PVA, we apply it to a case study of Bonneville Cutthroat Trout after the Twitchell Canyon Fire in the Fish Lake National Forest, Utah. The impact on and recovery of trout populations after wildfire was monitored across 14 sites of variable hydrologic, temperature and physical impairment (both within and outside of the fire perimeter). Using these observations along with maps of stream connectivity barriers, we model trout population viability and recovery after wildfire in this site. We also compare our results to model simulations using single year impairment, more similar to that of previous PVAs. Finally, we demonstrate the potential improvements on population recovery through simulations removing individual fish barriers throughout the network. This model presents a new framework for directly linking parameters of landscape change that may vary in both spatial and temporal distribution to the viability of fish populations after natural catastrophe. Plans for future model development include linking the PVA with models of fish bioenergetrics and landscape evolution, which can provide spatially variable predictions of changes in discharge, stream temperature and sediment fluxes after fire. Ultimately, we hope to develop and provide a new management tool for evaluating the overall vulnerability of aquatic organisms to wildfire in watersheds throughout the Intermountain West.
Salt and shale based minibasins are quasi-circular depression connected by submarine canyons of economic importance because they are prime locations of hydrocarbon reservoirs. The history of sedimentation of the minibasins is modulated by sea level changes, and it is strongly influenced by basin topography and continental shelf dynamics. Sedimentation in intraslope minibasins is generally described in terms of the “fill-and-spill” model in which the turbidity currents enter a minibasin and are reflected on the minibasin flanks. After the reflection, the turbidity currents pond and deposit the suspended sediment. As the minibasin fills, the current spill over the lowermost point of the minibasin flanks and reaches the next minibasin downslope, where the fill and spill process starts again. In this stage, deposition still occurs in the upslope minibasin with the formation of channel-levee complexes. In the last two decades field, laboratory and numerical studies focused on the description of (1) the large-scale stratigraphic architecture and evolution of the minibasins and (2) the behavior of the turbidity currents in the minibasin-canyon system. This notwithstanding, questions regarding the spatial distribution of the grain sizes in minibasin deposits, the role of the system geometry and of the flow characteristics of the turbidity current on the depositional pattern still need to be answered. The objective of the present study is to investigate with three-dimensional model simulations how the deposit characteristics change for increasing in slopes of the minibasin-canyon system. In particular, we are using a three-dimensional numerical model of turbidity currents that solves the Reynolds-averaged Navier–Stokes equations for dilute suspensions. Turbulence is modeled with a buoyancy-modified k–ε closure. The numerical model has a deforming bottom boundary to model the changes in elevation and grain size characteristics of the bed deposit associated with sediment erosion and deposition. Here we present the model validation against 1) 2D laboratory experiments of a horizontal minibasin in a constant width flume, and 2) 3D laboratory experiments on two linked minibasins. The model validation is performed comparing measured and simulated deposit geometries, vertical profiles of suspended sediment concentration and spatial distributions of sediment sizes in the deposit. In the near future, we will perform laboratory scale simulations by changing the slope of the experimental minibasin, i.e. the difference in elevation between the entrance and the exit points to study how the depositional pattern changes when the relative size of the ponded accommodation space, i.e. the space at a lower elevation than the spill point, and the perched accommodation space, i.e. the space under an ideal line connecting the spill point and the minibasin entrance.
Salt marsh provides critical estuarine habitat and shoreline protection, and is highly vulnerable to sea-level rise. Models of marsh accretion and resilience to sea-level rise rely on estimates of sediment supply, yet the factors governing sediment supply to marshes and its temporal variation are poorly understood. This presentation focuses on temporal variability in suspended-sediment concentration (SSC) and spatial gradients in SSC at the marsh edge, with two goals: 1) to identify processes important to sediment supply, and 2) to inform the choice of SSC values to use as input to marsh accretion models. We present data collected as part of an investigation of the influence of tides and wind waves on sediment supply to an estuarine salt marsh in China Camp State Park, adjacent to San Pablo Bay, in northern San Francisco Bay (tide range approximately 2 m). The long-term sediment accretion rate in the lower China Camp marsh is 3 mm/year. The marsh vegetation is predominately Salicornia pacifica, with Spartina foliosa occupying the lower elevations adjacent to the mudflat. The marsh is bordered by wide intertidal mudflats and extensive subtidal shallows. In the winter of 2014/2015 and the summer of 2016 we collected time series of SSC, tidal stage and currents, and wave heights and periods in the bay shallows, in a tidal creek, and (except for currents) on the marsh plain. On the mudflats, SSC depends strongly on wave energy, and also varies inversely with water depth, increasing toward the marsh edge and with decreasing tidal stage. Within the marsh, SSC is lower in the Salicornia-dominated marsh plain than at the marsh edge, as expected, but in the Spartina zone SSC is greater than at the marsh edge. This effect is greater in summer, when Spartina is significantly taller and denser, than in winter. SSC over the marsh was typically greater during flood than ebb tides in both seasons, indicating net deposition over the tidal cycle. However, median flood-tide SSC over the marsh, and the inferred deposition, were greater in summer than winter. We attribute the increased SSC and deposition in summer to greater sediment trapping in Spartina, followed by mobilization and transport of sediment onto the marsh during subsequent high tides.
Sandwich Town Neck Beach, MA, USA is a 1300-m long barrier spit on the north shore of Cape Cod that has experienced chronic dune erosion during nor’easters. Repeated mapping with drones and photogrammetry shows that dune erosion varies spatially along the beach. We used the wave model SWAN to compare simulated waves with the observed morphological response along the barrier spit. Local refraction and dissipation of wave energy by the complex nearshore bathymetry produces alongshore variations in wave energy, with regions of diverging alongshore wave power corresponding to the regions of rapid erosion. These regions are also where lower beach profiles are steepest, waves break closest to shore, and wave runup is highest. These results suggest that, while erosion rates may be affected by the availability of sediment from up-drift sources, wave patterns play an important, and maybe dominant, role in determining alongshore variations in dune erosion at Sandwich. +
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Satellite and field observations find modern carbonate depositional systems to be self-organized, yet the processes generating such behavior are not fully understood. A 3-D forward model of carbonate reef growth rooted in cellular automata is developed to simulate the evolution of self-organized geometry through time. Carbonate landscapes are generated over spatial extents of several kilometers through time scales of millennia at meter-scale resolution. Classes in the model include carbonate factories (e.g., branching and massive coral communities, algal communities) and sinks (e.g., unconsolidated sand). Environmental factors include relative sea level and light intensity, and ecological controls are based on life history traits for the biological facies. Ecological processes within the model include mortality and colonization rates for biological classes, transition probabilities between facies, and rates of vertical accretion. The algorithm results in a self-organized landscape that emulates those observed in nature, such as rims and reticulate structures. Visualizations can be produced by accessing topographic and facies maps generated at each time step. This project’s goals are 1) to investigate which configurations of environmental parameters result in specific spatial motifs, 2) examine the effects of environmental perturbations on reef construction, and 3) understand the importance of biological and physical regimes on the generation of geomorphological features. +
