Property:CSDMS meeting abstract
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2
Barrier island response to sea level rise depends on their ability to transgress and move sediment to the back barrier, either through flood-tidal delta deposition, or via storm overwash. Our understanding of these processes over decadal to centennial time scales, however, is limited and poorly constrained.
We have developed a new barrier inlet environment (BRIE) model to better understand the interplay between tidal dynamics, overwash fluxes, and sea-level rise on barrier evolution. The BRIE model combines existing overwash and shoreface formulations with alongshore sediment transport, inlet stability, inlet migration and flood-tidal delta deposition. Within BRIE, inlets can open, close, migrate, merge with other inlets, and build flood-tidal delta deposits. The model accounts for feedbacks between overwash and inlets through their mutual dependence on barrier geometry.<br><br>Model results suggest that when flood-tidal delta deposition is sufficiently large, barriers require less storm overwash to transgress and aggrade during sea level rise. In particular in micro-tidal environments with asymmetric wave climates and high alongshore sediment transport, tidal inlets are effective in depositing flood-tidal deltas and constitute the majority of the transgressive sediment flux. Additionally, we show that artificial inlet stabilization (via jetty construction or maintenance dredging) can make barrier islands more vulnerable to sea level rise. +
Barrier islands and other coastal landforms are highly dynamic systems, changing in response a spectrum of disturbances from multi-decadal ‘press’ disturbances like sea-level rise (SLR) to often more intense episodic perturbations like storms. As a result, multiple stable ecomorphological states exist on barrier islands. In this study, we use a probabilistic Bayesian network approach to investigate the likelihood of shifts among alternative equilibrium states on Fire Island, New York under three scenarios of shoreline change driven by sea-level rise (SLR). Specifically, we highlight areas that are most likely (i) to become inundated, (ii) to shift from one non-inundated state (or landcover type) to another (e.g., a forest becomes beach), or (iii) to remain in the current landcover state. We explore the effects of these changes on the availability of coastal ecosystem types, piping plover habitat, and anthropogenic development. +
Bedload flux is notoriously challenging to measure and model with its dynamics, therefore, remains largely unknown in most fluvial systems worldwide. We present a global scale bedload flux model as part of the WBMsed modeling framework. The results show that the model can very well predict the distribution of water discharge and suspended sediment and well predict bedload. Bedload predictions’ sensitivity to river slope, particle size, discharge, river width, and suspended sediment were analyzed, showing that the model is most responsive to spatial dynamics in river discharge and slope. The relationship between bedload and total sediment flux is analyzed globally and in representative longitudinal river profiles (Amazon, Mississippi, and Lena Rivers). The results show that while, as expected, the proportion of bedload is decreasing from headwater to the coasts, there is considerable variability between basins and along river corridors. The topographic and hydrological longitudinal profiles of rivers are shown to be the key driver of bedload longitudinal trends with fluctuations in slope controlling its more local dynamics. Differences in bedload dynamics between major river basins are attributed to the level of anthropogenic modifications, flow regimes, and topographic characteristics. +
Bedrock lithology has been shown to strongly influence how rivers and landscapes respond to tectonic perturbations, yet the specific variables and mechanisms that set how lithology controls river erosion are poorly understood. Recent field and modeling work suggests that one important lithologic control on channel response may be the delivery of large, generally immobile boulders from hillslopes to channels. This raises the possibility that differences in boulder delivery rates between lithologies may cause substantial differences in how landscapes respond to tectonics. An intriguing recent study suggested that in the Mendocino Triple Junction (MTJ) region of northern California, bedrock lithology might control the frequency and size of boulders delivered to channels, and therefore govern channel steepness and river evolution (Bennett et al., 2016). We further test this hypothesis here.
The Central Belt of the Franciscan Complex, a mix of sheared graywacke and mudstone, contains large blocks of more resistant serpentinite, greenstone, and amphibolite that are delivered to channels by earthflows. The adjacent Coastal Belt generally lacks such boulders, and sediment delivery to channels is dominated by shallow landsliding. This geologic setting provides a unique opportunity to test whether boulder abundance exerts a first-order control on landscape form. We use a landscape-scale analysis of channel steepness and active width indices, local topographic relief, lithology, and mapped boulder occurrence to understand the differences between the catchments eroding the Central Belt and those eroding the Coastal Belt. We find that channels are steeper in the Central Belt than in the Coastal Belt, both across the whole MTJ region and when averaged over 10-50 km2 subcatchments. Channels are also generally narrower in the Central Belt. This result could reflect lithologic controls or spatial heterogeneity in erosion rates. To control for the latter, we construct clusters of neighboring subcatchments that are free of knickpoints to explore possible controls of lithologic makeup (percent of a subcatchment underlain by Central Belt rocks) on channel steepness independent of erosion rate variations. We find inconsistent relationships between lithologic makeup and channel steepness within a given cluster of catchments with similar baselevel history. Finally, we compared channel segments adjacent to hillslope failures with segments far from failures. Central Belt channels show greater absolute increases in steepness adjacent to hillslope failures, but relative increases in steepness are consistent between the Central Belt and Coastal Belt.
Our preliminary results suggest that Central Belt channels are steeper and narrower than Coastal Belt channels, but that the lithological influence on steepness is difficult to disentangle from the effects of spatially variable erosion rates. We are continuing to map in-channel boulder size distributions to assess the relative importance of intra- vs. inter-lithologic variability in setting boulder concentrations and landscape form.
Besides long-term monitoring in changes of thermal state of permafrost and active layer thickness, the knowledge of permafrost distribution at very fine scales (tens of meters) in discontinuous permafrost is still largely unknown in Qinghai-Tibet Plateau (QTP). A permafrost island was found by using geophysical investigations in the Heihe River Basin in northeastern QTP. Permafrost island was present at PT10 site beneath alpine steppe and coarse soil with a quality of gravel in surface soil (Fig. 1, Fig. 2). In contrast, permafrost is absent at SFGT site with density land cover area and relatively less gravel. The results showed that the ground surface temperature (5 cm) at PT10 site is lower in winter and higher in summer than the SFG site. The presence of permafrost is caused by soil conditions, especially by high thermal conductivity, based on field investigations. To address the controlling factors of permafrost presentences a 1D heat transfer model is used to compare the ground temperature difference between these two sites by only changing the soil conditions. +
Block-mantled hillslopes responding to river incision deliver large blocks of rock to channels. These blocks inhibit fluvial erosion by shielding the bed and reducing available bed shear stress. Block delivery by hillslopes in response to channel incision therefore feeds back on the boundary conditions felt by the hillslopes: larger numbers of blocks, or larger blocks, reduce the rate at which the hillslope boundary condition is lowering. This coupled set of feedbacks can lead to oscillatory behavior in both channels and hillslopes with periods of rapid channel incision interspersed with intervals of little to no incision. For a hillslope with a line supply of blocks (such as might originate from a resistant caprock overlying a less resistant layer), we expect that these feedbacks are strong only when the source of blocks is relatively close to the channel. Once the block source has retreated sufficiently far from the channel, blocks will weather away before reaching the channel and the oscillatory channel-hillslope feedbacks described above will cease. Our questions are 1) For how long after initial river incision through a caprock do oscillatory channel-hillslope feedbacks persist? and 2) How far must the block source retreat from the channel before such feedbacks become negligible?<br><br>We use the new BlockLab 2-D landscape evolution model to assess the spatial and temporal extent of oscillatory channel-hillslope feedbacks. We model a channel incising a lithological sequence consisting of a weak layer underlying a resistant caprock. Blocks from the caprock are delivered to the channel and inhibit river incision. We find that at early time, temporal variation in the erosion rate boundary condition felt by the hillslope is significant. As the resistant layer retreats further from the channel, variations in both the channel erosion rate and the resistant layer retreat rate decline. The rate of these reductions in variability with time is set by competition between 1) the ability of the hillslope to deliver multiple large blocks to the channel (a function of initial block size, block weathering rate, and the distance the blocks had to travel before arriving at the channel), and 2) the ability of the channel to overcome the erosion-inhibiting effects of blocks (set by fluvial discharge and the block erodibility coefficient). We find that after enough model time has passed, the resistant layer has retreated far enough from the channel that block effects on the channel are negligible and oscillatory channel-hillslope feedbacks no longer exist. This distance is primarily a function of initial block size and block weathering rate. Our results indicate that channel and hillslope evolution rates in block-mantled landscapes may be highly unsteady, depending on the strength of coupling between the channels and hillslopes.
A
Breaking waves, especially plunging breakers, generate intense turbulence and is crucial in dissipating incident wave energy, suspending and transporting sediment in the surf zone. Therefore quantifying breaking-induced turbulence kinetic energy (TKE) is essential in understanding surf zone processes. Surf zone hydrodynamic data collected at the Large-scale Sediment Transport Facility (LSTF) at the U.S. Army Engineer Research and Development center were used here. One LSTF case, with irregular waves (3 s peak period), is examined here. This case resulted in dominantly plunging type of breaker. Waves and currents were measured simultaneously at 10 cross-shore locations and throughout the water column, with a sampling rate of 20 Hz. In order to separate orbital wave motion from turbulent motion, an adaptive moving average filter is developed, involving a 5-point moving average, with additional 3-point moving average at sections with more fluctuations. This adaptive moving average filter is able to maintain more wave energy as compared with the results from 7-point moving average, while resolve more turbulence energy as compared with the result from 5-point moving average.
The TKE was calculated based on the resolved turbulence. Large TKE was generated at the water surface associated with wave breaking and dissipated rapidly downward. The TKE decreased nearly one order of magnitude downward within 15 cm. The TKE reached a minimum value at approximately 50%-80% of the water depth, and increased towards the bottom due to the generation of bed-induced turbulence. The TKE flux during wave crest and tough indicate that, at the bottom and middle layers of the water column, the TKE is transported dominantly onshore, while for the top layer, it is transported mostly offshore. +
By using a fixed-mesh approach, morphodynamic models have some difficulty to predict realistic equilibrium hydraulic geometries with vertical banks. In order to properly account for bank erosion without resorting to a complicated moving mesh algorithm, an immersed boundary approach that handles lateral bank retreat through fix computational cells is needed.<br>
One of the main goals of the FESD Delta Dynamics Collaboration is developing a tested, high-resolution quantitative numerical model to predict the coupled morphologic and ecologic evolution of deltas from engineering to geologic time scales. This model should be able to describe the creation and destruction of deltas made of numerous channels, mouth bars, and other channel-edge features, therefore requiring an approach that is able to deal with the disruption, destruction, and creation of sub-aerial land. In principle, these sub-aerial land surfaces can be randomly distributed over the computational domain. <br>
We propose a new approach in Delft3D based on the volume of fluid algorithm, widely used in the literature for tracking moving interfaces between different fluids. We employ this method for implicitly tracking moving bank interfaces. This approach easily handles complicated geometries and can easily tackle the problem of merging or splitting of dry regions characterized by vertical vegetated banks. +
By using spatially-varying estimates of seabed bottom drag (z0) the performance of ocean current and tide numerical models may be improved. To an extent, the seabed database dbSEABED is able to supply these values from data on the seabed materials and features. But then adjustments for varying dynamic (wave, flow) conditions are also required. So the data and model must work closely together. We developed methods for calculating inputs of z0 for circulation models in this way. Preliminary outputs from this new globally capable facility are demonstrated for the NW European Shelf region (NWES). +
Cellular automata models have gained widespread popularity in fluvial geomorphology as a tool for testing hypotheses about the mechanisms that may be essential for the formation of landscape patterning. For instance, studies of braided rivers using cellular automata modeling suggested that erodible banks are an essential characteristic for formation of the braid-plain morphology. In wetlands with emergent vegetation and complicated flow patterns, distilling the relevant, nonlinear interactions to a relatively simple set of rules that can be used in cellular automata modeling poses challenges, but the advantage of doing so lies in the ability to perform sensitivity analyses or examine system evolution over millennia. Here I show how a hierarchical modeling strategy was used to develop a cellular automata simulation of the evolution of a regular, parallel-drainage patterned landscape in the Everglades. The Ridge and Slough Cellular Automata Landscape model (RASCAL) suggested that this landscape structure is stable only over a small range of water-surface slopes (the driving variable for flow)—a result that both explains the limited distribution of low-gradient parallel-drainage systems worldwide and would likely have not been detected had a non-hierarchical CAM been used. Additional sensitivity analyses with RASCAL show how interactions between flow, vegetation, and sediment transport can lead to a wide variety of other regular and amorphous landscape patterns, depending on the relative strength of physical and biological feedbacks. Comparisons between RASCAL and well-known CAM models of braided stream dynamics raise interesting questions about the level of complexity that need to be incorporated into models of transitional (low- to high-energy) environments such as wet meadows and small/intermittent streams. +
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Changes in landscape structure are known to affect species macroevolution largely by altering habitat connectivity. Species can disperse across a greater area when habitats expand. Habitat fragmentation reduces gene flow and increases rates of speciation. Conversely, a shrinking habitat increases the likelihood of species extinction.
We integrated macroevolution processes (dispersal, speciation, and extinction) into the landscape evolution modeling toolkit called Landlab. Here, we present a new Landlab component, BiotaEvolver that tracks and evolves the species introduced to a model grid. In one model, surface process components evolve the landscape and BiotaEvolver evolves the species in response to topographic change or other characteristics of the model set by the user. BiotaEvolver provides a base species and users can subclass this object to define properties and behaviors of species types.
We demonstrate BiotaEvolver using scenarios of drainage rearrangement and stream species. Stream captures and high macroevolution process rates occurred within a limited combination of parameters and conditions in hundreds of model runs. The number of species increased most rapidly after a response period following a perturbation. Species numbers declined then became stable after this period. +
Changes in upstream land-use have significantly transformed downstream coastal ecosystems around the globe. Restoration of coastal ecosystems often focuses on local-scale processes, thereby overlooking landscape-scale interactions that can ultimately determine restoration outcomes. Here we use an idealized bio-morphodynamic model, based on estuaries in New Zealand, to investigate the effects of both increased sediment inputs caused by upstream deforestation following European settlement and mangrove removal on estuarine morphology. Our results show that coastal mangrove removal initiatives, guided by knowledge on local-scale bio-morphodynamic feedbacks, cannot mitigate estuarine mud-infilling and restore antecedent sandy ecosystems. Unexpectedly, removal of mangroves enhances estuary-scale sediment trapping due to altered sedimentation patterns. Only reductions in upstream sediment supply can limit estuarine muddification. Our study demonstrates that bio-morphodynamic feedbacks can have contrasting effects at local and estuary scales. Consequently, human interventions like vegetation removal can lead to counterintuitive responses in estuarine landscape behavior that impede restoration efforts, highlighting that more holistic management approaches are needed. +
Changing sea level and ice volume since the Last Glacial Maximum (LGM, 26-19 ka) has been an intensively studied topic for decades, and yet we have still not been able to adequately close the water volume budget at the LGM. At the LGM, global sea level was depressed by approximately 125-135 m relative to the present level. Past researchers have attempted to account for the storage of this water as an estimated 52*106 km3 of land-based ice. However, relative sea level, ice sheet morphology, and isostacy studies at local and regional scales have been unable to reasonably place high enough ice volumes to meet this global total, accounting for only approximately 120 m of sea-level change. This discrepancy has resulted in the so-called ‘missing ice’ problem.
We propose that some portion of this ‘missing’ water was stored not as ice, but in lakes and groundwater. Thus far, no studies have attempted to determine the volume of water stored in lakes and groundwater at the LGM. Groundwater storage could potentially account for a large volume of water, reducing the missing water volume by a significant margin. Differing topography and recharge rates may have resulted in greater terrestrial water storage, which can help us to close the water budget. Indeed, many large proglacial and pluvial lakes are known to have existed and may indicate higher groundwater levels. Furthermore, assessing groundwater levels at 500 year intervals from the LGM to the present day can provide insights into changes in water storage and inputs to the ocean over time.
It is challenging to assess groundwater levels with precision since various factors, including evapotranspiration, topography, and sea level all play a role in controlling groundwater level at a particular location. However, a recent model (Reinfelder et al., 2013) was able to estimate modern groundwater levels on a global scale. By using this model in combination with modelled topography and climate data for the LGM and each 500 year time step, we are able to compare the volume of water stored in the ground from the LGM to the present day to test whether groundwater would be a viable reservoir for LGM water storage. The model provides depths to water table, thus allowing computation of changing storage volumes.
The model covers the entire globe at a resolution of 30 arc-seconds. The large datasets and iterative nature of the model require MSI’s computational power to perform the calculations. So far, preliminary results have shown that over a metre of additional sea-level equivalent water was stored in the ground at the LGM.
Chemical erosion of regolith is of wide interest due to its role in Earth’s topographic evolution, the supply of nutrients to soils and streams, and the global carbon cycle. Theory and experiments suggest that chemical erosion rates (W) should be strongly controlled by physical erosion rates (E), which affect W by removing weathered regolith and regulating mineral supply rates to the regolith from its underlying parent material. A global compilation of field measurements reveals a wide range of relationships between W and E, with some sites exhibiting positive relationships between W and E, some exhibiting negative relationships, and others exhibiting a flat relationship within uncertainty. Here we apply a numerical model to explore the variety of W-E relationships that can be generated by transient perturbations in E in well-mixed regolith.
Our modeling results show that transient relationships between W and E during erosional perturbations can strongly deviate from steady-state relationships. These deviations ultimately result from the time lag in changes in W following imposed changes in E. As a consequence of the lag, a hysteresis develops in plots of W versus E during transients in E. This yields a positive relationship between W and E at some times during a transient perturbation, a flat relationship at other times, and a negative relationship at other times. The shape and duration of these transient hystereses can be modulated by climate and lithology, as the lag time increases linearly with a characteristic regolith production time and decreases with a characteristic mineral dissolution time, both of which are affected by climatic and lithologic factors. Our results show that even in the absence of variations in climate and lithology, however, a range of W-E relationships can be generated by a single perturbation in E. To the extent that these model results capture the behavior of chemical and physical erosion in natural landscapes, these results may aid interpretation of field measurements of W and E.
Climate change and reduced water availability in arid regions has important implications for how channels will change as they adjust to a new steady-state characterized by different riparian populations. While much study has been devoted to the effects riparian vegetation has on fluvial processes (Tal & Paola, 2010; Osterkamp & Hupp, 2010; Corenblit et al., 2009), the complexity of natural channels obscures exactly how these feedbacks modify long-term channel evolution, making prediction of the larger impacts of vegetation change on channel morphology difficult. In order to isolate the impact vegetation has on morphology, single channels that are variably vegetated along their length are desirable for study because flow conditions and long-term sediment flux change minimally between major tributaries (Bertoldi et al., 2011). Comparisons made in such dryland channels in Henry Mountains, Utah, USA, where groundwater springs juxtapose vegetated and un-vegetated reaches allow us to examine two hypotheses: first, that disruptions to normal fluvial processes caused by in-channel vegetation produce distinct morphological responses to floods at the scale of single flood events, and, second, that these responses accumulate on the timescale of multiple floods to produce channel morphologies in vegetated reaches that are fundamentally different from those in unvegetated reaches. Analysis of repeat airborne LiDAR data for these areas provides an opportunity to quantify morphological parameters and elevation differences, and to attempt to correlate these metrics with quantitative metrics of vegetation. Field observations from October, 2017 in this region agree with the results of LiDAR analyses and indicate that the presence of dense vegetation seems to produce more uniform cross-sectional shape with narrow, deeply incised channels supported by intense rooting on banks, and a longitudinal profile that is characterized by frequent vegetation-supported, non-bedrock knickpoints. Future work will involve modelling flood flows to determine the degree and areal extent of channel reworking during a flooding event and the influence of vegetation on shear stress for comparison with LiDAR differencing results.
Climate change has altered the frequency and intensity of hydrologic events like precipitation and flood, yielding vulnerability of communities dwelling in coastal and inland flood plains. Flood prediction and mitigation systems are necessary for improving public safety and community resilience all over the world at Country, continental and global scales. Numerical simulation of flood event has become a very useful and commonly used tool for studying and predicting flood events and susceptibility. One of the major challenges in hydraulic modeling is accurate description of river and floodplain geometries. The increased availability of high-resolution DEMs (e.g. LiDAR data) alleviates this challenge for floodplains but (with the exception of blue/green LiDAR surveys) not for river channels. Here we investigate the effect of river bathymetry data on numerical simulations of flood events. Two numerical models (GSSHA and Mike 21) were used for comparison in the results. Three channel geometry inputs were simulated for three river reaches of different sizes: DEM-captured elevation (water surface), hydraulic geometries (empirical estimation), and observed river bathymetry. +
Climate change has led to unprecedented precipitation events in the hyper-arid Atacama Desert of Northern Chile. On the coast of the El Salado watershed, legacy mine tailings infilled the watershed-ocean connection, while the river channel has been altered both by tailings and urbanization. Loss of life and destruction of infrastructure in a large flood event in 2015 resulted from the coupling of anthropogenic geomorphic changes with unusual climate events. We carry out unsteady two-dimensional simulations fully coupled with the sediment concentration to identify the influence of tailing deposits. The analysis incorporates high-resolution topography data from both pre- and post-flood, where the pre-flood scenario represents the presence of tailings, and the post-flood scenario reflects partial erosion of these deposits. Results highlight the important role of topographic alterations in enhancing the hazard to people and critical infrastructure.
Additionally, an upscaling methodology based on porosity is presented for an urban flood simulation in Santiago de Chile, adjacent to the Andean foothills. In this model, topographic information is included at the subgrid-level to optimize CPU time at the cost of some loss in the accuracy of the results. We analyze how accuracy is affected by gradually increasing grid resolution, specifically when estimating flood extent and associated hazards. Results suggest that the cell size can be increased up to the street width, capturing the main flow paths and hazards while significantly reducing the CPU time employed by classical models.
The integration of an upscaling scheme to model concentrated flows coupled with surface dynamics is particularly valuable for comprehensively assessing flood hazards, meeting real-time decision-making needs. +
Climatically controlled surface processes redistribute mass and modulate solid-Earth stress fields, potentially driving changes in tectonics. Examples of climatically-influenced tectonics exist in glaciated orogens, however this phenomenon has not been well documented in fluvial systems. Here we describe a previously undiscussed feedback between hillslope and fluvial processes that buffers climate-tectonic interactions, helping to explain the dearth of observations of climatically influenced tectonics in fluvial systems. Using remote sensing and field investigation, we quantify production, deposition, and transport of landslide sediments resulting from the 2009 Typhoon Morakot in Southern Taiwan, which delivered record-breaking rainfall triggering more than 22,000 landslides across 7800km^2. An annual landslide catalog facilitates use of area-volume scaling to estimate amount of landslide material distributed across a strong northward gradient in tectonic uplift in Southern Taiwan. Landslide volume and frequency exhibit similarly positive trends with distance from the southern tip of the orogen. Exploiting a wealth of publicly available imagery and elevation data, we map sediment aggradation throughout fifteen drainage basins and observe 10’s of meters of aggradation with the distribution tightly coupled to areas of greatest exhumation. Sediment transport modeling across the orogen suggests that areas of highest exhumation will be inundated with sediment over three orders of magnitude longer than less exhumed basins. Estimating the frequency of events like Typhoon Morakot, we expect the most active basins in the study area to have their channels buried by landslide sediment for up to 50% of any given time period, while less active basins will be able to incise nearly 100% of the same time period. This feedback suggests that as landscapes become more exhumed, the erosional buffering effects of extreme storms and earthquakes that cause widespread landslides are amplified, driving a negative feedback between climate driven surface processes and tectonics in fluvial systems.
Close to half a billion people live in deltaic regions worldwide, including in a number of mega-cities. River deltaic landforms act as central locations for agricultural production, livestock farming, and hydrocarbon extraction. The understanding of riverine sediment fluxes and associated delta morphology changes, aids in planning engineering works such as identification of flood-prone areas, installation of coastal defense structures, dam construction, and restoration activities of extensively altered areas.
The overarching goal of the study is to elucidate the interconnectivity between fluvial fluxes and associated landform changes in large global deltas. The following research questions are investigated: (1) Are changes in fluvial sediment flux to the delta directly linked to changes in delta morphology? (2) What are the magnitudes and trends of riverine sediment fluxes that can be expected throughout the 21st century? A multifaceted research approach combining (a) satellite remote sensing analysis of delta morphology changes (progradation/degradation), and (b) numerical modeling of riverine water and sediment fluxes, is used on selected large river deltas globally.
Major outcomes of the study indicate that the synoptic capability of remote sensing provides a useful reconnaissance tool to infer on the rates at which the deltas change. An overview of global delta change is presented with a special focus on case studies with severe degradation and interesting flux estimates. The outcomes of the study yield a number of novel insights into fluvial fluxes of the 21st century and transform our analytical capabilities for studying delta morphology change and sediment flux dynamics in large rivers, globally. +
A
Close to half a billion people live on deltas, many of which are threatened by flooding. Delta flooding also imperils valuable ecological wetlands. In order to protect deltas, it is critical to understand the mechanisms of flooding and evaluate the roles of different forcing factors.
Delft3D, a widely used 3D hydrodynamic and sediment transport model, has been applied to the Wax Lake Delta in Louisiana in order to explore the impacts of wind, waves, and vegetation during extreme conditions. Using wind and pressure field inputs of Hurricane Rita in 2005, the simulation indicates that the deltaic hydrodynamics and morphologic changes are determined by the interactions of all three factors. Wind shows a large impact on water level and velocity, especially in the shallow water zone, where water level increases by ~2 m and water velocity increases by ~1 m/s. Waves, on the other hand, demonstrate almost no effect on water level and velocity, but significantly increase sediment transport due to increasing bed shear stress. Sediment deposition occurs primarily at the coast, when water floods higher elevated land and velocities start to decrease, leading to a significant drop in bed shear stress. Vegetation, a critical factor that influences deltaic hydrodynamics, is represented in the model by adding 2D roughness to the bed. The vegetated wetland and its surrounding area show a notably different pattern in erosion and deposition compared to the unvegetated simulations. The vegetated islands receive significant deposition, while adjacent channels become much more eroded because water is routed through channels when the surrounding vegetated islands are more difficult to erode.
To take into account the impact plant roots have on the soil (increase in soil strength and therefore an effectively reduction in erosion), a new root routine has been added to Delft3D. This routine mimics this process by increasing the soil critical shear stress required to reduce erosion. The modeled results indicate that more deposition appears on the vegetated root area, while more significant erosion simultaneously occurs at those sides of these islands that are facing the ocean. This illustrates that, while vegetation can protect land from erosion, it can also intensify erosion in the surrounding area. Therefore, the use of natural vegetation as a protection against coastal erosion processes requires more research.
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Coastal aquifers, vital freshwater sources for over a billion people globally, often face saltwater intrusion, especially in island freshwater lenses. Despite extensive studies on sea-level rise, storm surges, and over-pumping, the impact of droughts on coastal aquifers, particularly barrier island aquifers reliant solely on aerial recharge, remains understudied. Understanding recharge and salinization processes is crucial for sustainable water resource management amid potential climate change impacts. This study introduces a novel approach to assess a freshwater lens's response to drought conditions, incorporating in-situ observations, geophysical measurements, and numerical modeling. Examining a Northeastern U.S. barrier island's shallow unconfined aquifer during the 2020 drought, the research reveals a reduction in the freshwater lens volume during reduced recharge, emphasizing the vulnerability to droughts and the potential for recovery. Comprehensive studies in this area are essential for informed water resource management. +
Coastal areas globally face increasing threats from intensified weather events and rising sea levels, leading to challenges such as fluctuations in groundwater levels and salinity intrusions. This presents a significant concern for the Department of Defense (DoD), which manages over 1700 coastal sites worldwide, with several facing heightened vulnerability to these environmental changes. We aim to evaluate the susceptibility of DoD coastal sites to sea-level rise and saltwater intrusion, utilizing the Defense Regional Sea Level (DRSL) database that includes projections for five global sea-level rise scenarios and extreme water events. To achieve this, we have adopted a two-pronged strategy. First, we conduct an in-depth vulnerability analysis considering the current situation, sea-level trends, and topographic elevation. The vulnerability analysis aids in selecting sites for detailed further investigation. Subsequently, we formulate Reduced Order Models (ROMs), including Dynamic Mode Decomposition (DMD) and the Unified Fourier Neural Operator (U-FNO) for sites with a range of vulnerabilities. DMD and U-FNO are selected for their efficiency, enabling faster execution and thousands of runs to assess site vulnerability under future climate scenarios through the century's end. Trained on site-specific mechanistic models, both DMD and U-FNO accurately simulate current groundwater and salinity conditions, providing reliable forecasts of future impacts on DoD sites, utilizing data from the DRSL database and climate model projections. This approach clarifies the immediate risks and facilitates the transfer of essential knowledge throughout DoD's extensive network, fostering a deep understanding of global coastal vulnerabilities. Ultimately, this informs the development of targeted, effective mitigation strategies, safeguarding critical defense infrastructure against the impacts of climate change. +
Coastal communities facing erosion require beach maintenance for property protection and recreation. While some communities may have the means to pay for sand nourishment, others may benefit from their neighbor’s alongshore-transported sediments. If communities expect to free-ride off beach nourishment carried out by a neighbor, incentives favoring inaction may lead to narrower beaches overall. Recent work coupling human and natural systems found that coordination between neighboring communities is preferable economically to each community acting independently. Contrasting past work, we model two communities acting without knowledge of a jointly determined economically optimal nourishment program. Instead, nourishment behavior is triggered by a traditionally imposed minimum beach threshold and bounded by a predefined seaward edge. The goal is not to limit sand loss; rather, nourishment decisions are based on separate or joint benefit-cost assessments for two communities. We compare two management approaches: (1) sequential/decentralized decisions, where the updrift community chooses first and the downdrift community reacts second; and (2) simultaneous/coordinated decisions where both communities make a joint choice. We test how variable up/downdrift property values affect outcomes under these two approaches. Results suggest that communities do not always favor coordinating simultaneously. When both up- and downdrift communities have high property values, sequential/decentralized decisions are favored, leading to updrift over-nourishment to maintain beach width. This enhances alongshore sediment availability, thus providing higher marginal benefits for downdrift communities whom under-nourish. When the property values of the updrift community are low and the property values of the downdrift community are high, however, the outcome results in abandonment of property by the updrift community instead of coordinating with the downdrift community. Overall, we find that the distribution of property values across neighboring communities can be a driver for both strategy selection and the decision-making process.
Coastal ecosystems, infrastructure, and human health are vulnerable to extreme precipitation, flooding, and water-quality impacts. Integrating a hydrologic model (WRF-Hydro) into the Coupled Ocean Atmosphere Wave Sediment Transport modeling system (COAWST), which includes ocean (ROMS), atmosphere (WRF), surface-wave (SWAN, WAVEWATCHIII), sediment (CSTMS), and sea-ice components, offers the potential to investigate compound flooding and the dispersal of contaminants, sediments, and other material at the land-ocean boundary. Here, the new model coupling is described, along with an application to Hurricane Florence.
Extreme precipitation during Hurricane Florence, which made landfall in North Carolina in September, 2018, led to breaches of hog-waste lagoons, coal-ash pits, and wastewater facilities. In the weeks following the storm, historic freshwater discharge carrying pollutants, sediment, organic matter, and other debris was released to the coastal ocean, contributing to beach closures, algal blooms, hypoxic conditions, and other ecosystem impacts. The Cape Fear river basin, North Carolina’s largest watershed, is used as a case study. Progress in model coupling applied to this region includes (1) a two-way coupled ROMS and WRF-Hydro simulation in which fluxes between the ocean and hydrology models are computed from the pressure gradient at the ocean-land boundary, and (2) a one-way coupled simulation in which a WRF-Hydro simulation provides river point-source forcing in ROMS. The work as part of the one-way coupled simulation demonstrates how the pathways of land-sourced tracers can be tracked in the coastal ocean; a suite of different flood and wind scenarios are studied and used to map the arrival and departure times of threshold-exceeding contaminants that contribute to swimming advisories and other impacts. Next steps are described for continuing the ocean-hydrology model coupling efforts to improve forecasts of compound flooding and water quality impacts.
Coastal erosion and wetland loss are affecting Louisiana to such an extent that the loss of land between 1932 and 2016 was close to 5,000 km2. To mitigate this decline, coastal protection and restoration projects are being planned and implemented by the State of Louisiana, United States. The Louisiana Coastal Master Plan (CMP) is an adaptive management approach that provides a suite of projects that are predicted to build or maintain land and protect coastal communities. Restoring the coast with this 50-year large-scale restoration and risk reduction plan has the potential to change the biomass and distribution of economically and ecologically important fisheries species in this region. However, not restoring the coast may have negative impacts on these species due to the loss of habitat. This research uses an ecosystem model to evaluate the effects of plan implementation versus a future without action (FWOA) on the biomass and distribution of fisheries species in the estuaries over 50 years of model simulations. By simulating effects using a spatially-explicit ecosystem model, not only can the changes in biomass in response to plan implementation be evaluated, but also the distribution of species in response to the planned restoration and risk reduction projects. Simulations are performed under two relative sea level rise (SLR) scenarios to understand the effects of climate change on project performance and subsequent fisheries species biomass and distribution. Simulation output of eight economically
important fisheries species shows that the plan mostly results in increases in species biomass, but that the outcomes are species-specific and basin-specific. The SLR scenario highly affects the amount of wetland habitat maintained after 50 years (with higher levels of wetland loss under increased SLR) and, subsequently, the biomass of species depending on that habitat. Species distribution results can be used to identify expected changes for specific species on a regional basis. By making this type of information available to resource managers, precautionary measures of ecosystem management and adaptation can be implemented.
Coastal flooding is an increasingly prominent hazard in the northeast United States, causing both property damage and disruption of daily life. Tide gauge records provide historical water level data and are used to estimate current return periods of storm tides (tide level plus storm surge) from both hurricanes and nor’easters. We calculate the interannual joint probability exceedance curves for select tide gauges in the Philadelphia, New Jersey, and New York City megaregion using the quasi‐nonstationary skew surge joint probability method (qn‐SSJPM) from Baranes et al. (2020). Analysis of the probability of storm tides for hurricane versus nor’easter seasons will be discussed, including geographic variations of the storm tide exceedance curves. Results from this study can be compared to storm climatology and used by social scientists and city planners to assess risk associated with the flood hazard in the area. By understanding the ways that probability of storm tide in summer and winter may change in the future, communities can better plan and prepare for future hazards. +
Coastal foredunes are dynamic ecogeomorphic landforms that provide increased resilience for both natural habitats and developed communities. Despite their dynamic nature, dunes can be stabilized with vegetation and are therefore an adaptable nature-based solution that can be utilized for flood risk management. However, coastal habitats are rapidly changing and require modeling support to understand the effectiveness of vegetated dunes under changing environmental conditions. Most existing dune morphology models incorporate vegetation implicitly, using percent cover or plant height to affect sediment accretion and erosion, rather than explicitly simulating ecological processes such as mortality and dispersal. A coupled modeling approach that integrates process-based dune and vegetation models is necessary to better understand plant-sediment-water interactions and manage coastal dune systems. Through this work, we demonstrate the coupling of AeoLiS, a process-based aeolian sediment transport model with GenVeg, a generalized vegetation model under development in Landlab and parameterized with growth, functional morphology, and sand accretion of native and non-native plant species from a common garden experiment in Nehalem Bay State Park, Oregon. This work highlights how vegetation morphology affects dune building and resilience to better inform dune management and restoration actions. +
Coastal landscapes are dynamic, subject to drowning by sea level rise, erosion driven by alongshore transport, and inundation by large storm events. Coastlines are also highly developed. Along the U.S. coasts, communities continuously develop and implement beach management strategies to protect coastal infrastructure and maintain recreational value. From sediment source to sink, littoral cells often span many coastal communities. Even as physical processes grade along these littoral cells, separate communities along this coast possibly enact different management strategies. By expanding upon an existing alongshore-coupled dynamic model of coastal profile and barrier evolution, we analyze the feedbacks between alongshore and cross-shore processes as well as human response to local shoreline change across multiple communities within the same littoral cell. Incorporating the possibility of intercommunity cooperation allows us to valuate variable coastal resilience strategies for communities within a littoral cell, particularly the benefit of coordinated versus uncoordinated activities. Both sediment transport processes and a cost-benefit analysis for each community determine optimal beach management strategies. Model results provide insights useful for understanding coastal processes and planning, allowing for more robust coastal management decisions, which depend upon future rates of sea-level rise. +
Coastal-plain depositional systems such as fluvial deltas are archives of past external (allogenic) forcing, such as sea-level variations, and their evolution can be described by two geomorphic boundaries: the alluvial-basement transition or upstream boundary, and the shoreline or downstream boundary. Patterns of landward/seaward migration of the shoreline (i.e., transgression/regression) and the alluvial basement transition (i.e., coastal onlap/offlap) in the rock record are often used for reconstruction of past sea-level changes. Theories for stratigraphic interpretation, however, need to be adapted to deal with internal (autogenic) processes that could play a significant role, but are to date largely unexplored. In particular, in-situ organic matter accumulation via plant growth has generally received little attention despite accounting for a significant volume fraction in most fluvio-deltaic plains and likely affect their response to sea level variations. To fill this knowledge gap, we develop a geometric model for the long-profile evolution of a fluvio-deltaic environment that accounts for sea-level cycles and organic sediment dynamics. The model assumes that sedimentological processes (i.e., inorganic and organic sedimentation) operate to preserve a linear geometry for both the delta plain or topset, and the subaqueous offshore region or forest. Changes in topset length can occur via shoreline transgression/regression, or coastal onlap/offlap, and the magnitude and timing of these changes can be directly related to the amplitude, phase and frequency of the sea-level variations. The model predicts that the maximum organic fraction occurs when the organic matter accumulation rate matches the accommodation rate, an observation consistent with field observations from coal geology. Further, we find that organic matter accumulation during the topset aggradation and organic matter erosion and decay during topset degradation generally results in substantial increase in the coastal onlap/offlap amplitude, which can result in an overestimation of the sea-level variations. These results are consistent with the discrepancy in sea-level amplitude reconstructions between sequence stratigraphic models and geochemical models over the Cretaceous.
Coasts are among the most intensely used environments on the planet, but they also present dynamic and unique hazards including flooding and erosion. Over the next century, these risk are likely to intensify across many coastal localities due to changes in environmental conditions, including sea level rise and changing wave climate patterns as induced by climate change. Managing these hazards and protecting vulnerable areas is challenging and requires an understanding of the behavior of coastal systems and longer-term prediction of their future evolution in the face of a changing climate.
Many existing one-dimensional coastal evolution models can effectively simulate the evolution of coastal environments. However, due to their 1D nature, they are unable to model the additional and combined effects of a variable water level and sea level rise. Hence, a new model, the Coastline Evolution Model 2D (CEM2D), has been built that is capable of simulating these processes.
CEM2D has been built from the 1D parent model – the Coastline Evolution Model (CEM) - that was originally developed by Ashton et al. (2001), Ashton and Murray (2006) and Valvo et al. (2006). CEM2D has been developed accordingly to the underlying assumption and mathematical framework of CEM, but applied over a two-dimensional grid. At the core of this framework is the calculation of longshore sediment transport rates using the CERC formula and Linear Wave Theory. Wave shadowing calculations are also used to ensure that sediment transport is negligible in shadowed areas. The distribution of material across the shoreface is controlled by a steepest descent formula that routes sediment from higher to lower elevations across the domain according to defined thresholds, whilst maintaining the average slope angle.
CEM2D provides a step forward in the field of coastal numerical modelling. It fills a gap between one-dimensional models of shoreline change that provide insights into the fundamental processes that control coastal morphodynamics and more complex and computationally expensive two- and three-dimensional models that are capable of simulating more complex processes and feedbacks. Key applications of CEM2D include improving our understanding of the meso-scale morphodynamic behaviour of coastal systems, their sensitivities to changing environmental conditions and the influence that climate change may have on their evolution over centennial to decadal timescales.
Conservation biologist, modeler, blogger, nature photographer, animal friend, swing dancer, Ecopathologist… All these describe Adrian Dahood, who tragically lost her life along with 33 others in a diving boat fire off the coast of California. She will be remembered fondly, and her legacy as a scientist and policy expert will remain alive within the scientific community. Please check out her photos, blogs, postcards and scientific papers, and I hope she can bring a smile to your face as well. +
Coupled process-based numerical models have the potential to greatly enhance our understanding of the drivers of coastal change by allowing for detailed simulations of the processes involved within each model core of the coupling. However, producing accurate hindcasts and forecasts with these coupled frameworks can be challenging due to a wide array of parameters that interact nonlinearly across and within the individual model cores and the potentially substantial computational cost that limits both the number and duration of simulations that can be reasonably performed. Additionally, many model parameters (e.g., wave asymmetry and skewness or sediment transport coefficients) that are critical for model calibration are unitless coefficients in the model formulations and thus cannot be readily measured in the field. Here, we use Windsurf, a coupled beach-dune modeling system that includes Aeolis, the Coastal Dune Model, and XBeach, paired with two surrogate neural network models, to produce a pair of hindcasts and forecasts to replicate observed modes of dune and beach morphological change on a developed barrier island on the US Atlantic coast (Bogue Banks, North Carolina). The first neural network aids in the calibration process by allowing for the prediction of Windsurf’s error surface over thousands of potential parameterizations to rapidly identify a potential best calibration before actually running the model. Windsurf is then run within a genetic algorithm to further hone the collection model parameter settings. Once Windsurf is finished running, we use the output to train a second neural network which contains a Long Short-Term Memory (LSTM) layer to produce five-year forecasts of dune crest height and dune toe elevation. We test our results by comparing them to observed data collected in the field between 2016-2020 using Real-Time Kinematic Global Positioning System (RTK-GPS) and find our forecasts (from the hindcasts) produce reasonably accurate predictions of dune morphology change at interannual scale.
Coupling models from different domains (e.g., ecology, hydrology, geology, etc.) is usually difficult because of the heterogeneity in operating system requirements, programming languages, variable names, units and tempo-spatial properties. Among multiple solutions to address the issue of integrating heterogeneous models, a loosely-coupled, serviced-oriented approach is gradually gaining momentum. By leveraging the World Wide Web, the service-oriented approach lowers the interoperability barrier of coupling models due to its innate capability of allowing the independence of programming languages and operating system requirements. While the service-oriented paradigm has been applied to integrate models wrapped with some standard interfaces, this paper considers the Basic Model Interface (BMI) as the model interface. Compared with most modeling interfaces, BMI is able to (1) enrich the semantic information of variable names by mapping the models’ internal variables with a set of standard names, and (2) be easily adopted in other modeling frameworks due to its framework-agnostic property. We developed a set of JSON-based endpoints to expose the BMI-enabled models as web services, through storing variable values in the network common data form file during the communication between web services to reduce network latency. Then, a smart modeling framework, the Experimental Modeling Environment for Linking and Interoperability (EMELI), was enhanced into a web application (i.e., EMELI-Web) to integrate the BMI-enabled web service models in a user-friendly web platform. The whole orchestration was then implemented in coupling TopoFlow components, a set of spatially distributed hydrologic models, as a case study. We demonstrate that BMI helps connect web service models by reducing the heterogeneity of variable names, and EMELI-Web makes it convenient to couple BMI-enabled web service models. +
Crop models are used to simulate crop development, yield and irrigation requirements, but their performance can be influenced by environmental and management conditions such as climate and irrigation strategies. Hence, performing a sensitivity analysis on these models is crucial to identifying influential parameters which informs model calibration. Here, we performed a global sensitivity analysis (Morris Screening method) on crop yield and irrigation on 34 crop parameters using the AquaCrop-OSPy model. This analysis is done for corn in Sheridan, KS under different water treatments (irrigated and rainfed) for varying meteorological scenarios represented by past years annual precipitation (normal-2021, wet-2019 and dry-2002). Thresholds of 0.3t/ha and 20mm are used for yield and irrigation respectively to identify influential parameters. Overall, parameter importance varies for yield and irrigation: parameters related to biomass and yield, root and canopy development, and irrigation strategy are the most influential for yield while those related to irrigation strategy, and root and canopy development are the most influential for irrigation. In general, yield was responsive to fewer parameters in rainfed conditions and simulations with drier meteorological conditions. The normal and wet scenarios have similar influential parameters with varying order of influence for yield under irrigated conditions. However, under rainfed conditions, the normal scenario only has two influential parameters (minimum effective rooting depth and the excess of potential fruits, a parameter related to biomass and yield), while 8 parameters related to biomass and yield production, water stress, and root development are influential during the wet scenario. Yield under irrigated conditions during the wetter years (receiving normal and high precipitation) tends to be impacted by water and temperature stress parameters. The influential parameters will further be analyzed using the Sobol method to calculate each parameter's influence on the output’s variance and interaction with other parameters, and ultimately used to guide model calibration.
Debris flows pose a hazard to infrastructure and human life. However, predicting debris flows remains a challenge due to uncertainty in initiation mechanisms, and the difficultly in appropriately parameterizing the resistance equations that describe flow velocities. Additionally, one of the limitations to progress in modeling debris-flow timing is the lack of empirical data from natural watersheds that can be used for parameter estimation and validation of predictions. Most quantitative measurements of debris flows are conducted in flumes, or unique watersheds where debris flows are known to occur annually, both of which suggest particularly remarkable conditions that may not reflect the majority of conditions where debris flows are manifested. This research addresses those challenges by using measured debris-flow timing in nine watersheds that were burned by a wildfire in 2009 to calibrate and test debris flow model parameterizations. Debris-flow timing was captured using pressure transducers attached to the channel bed. We used a kinematic wave rainfall-runoff model that we developed in python using the landlab environment to model flow timing. We separated the nine study watersheds into two categories: calibration and testing. For the calibration watersheds, model parameters were estimated based on prior research and then changed iteratively using a storm with known rainfall to minimize an objective function of the observed and modeled flow timing. Following hundreds of model realizations, we arrived at a set of best-fit parameters for saturated hydraulic conductivity (Ks) and the Manning’s roughness parameter (n). We found that a single value of Ks could be used in each of the model watersheds because, following wildfires, this parameter is typically reduced to very low values with a relatively small variance. In contrast n varied systematically as a function of upstream contributing drainage area, and thus values of n could be estimated for uncalibrated basins. When Ks and n were applied to test basins without any calibration we found that a reasonable result in estimated debris-flow timing was attained. These results suggest that given the appropriate scaling estimates it may be possible to estimate debris-flow timing within minutes and to capture multiple debris-flow surges separated by several hours.
Debris flows pose a significant threat to downstream communities in mountainous regions across the world, and there is a continued need for methods to delineate hazard zones associated with debris-flow inundation. Here we present ProDF, a reduced-complexity debris-flow inundation model for rapid hazard assessment. We calibrated and tested ProDF against observed debris-flow inundation at eight study sites across the western United States. While the debris flows at these sites varied in initiation mechanism, volume, and flow characteristics, results show that ProDF is capable of accurately reproducing observed inundation extent across different geographic settings. ProDF reproduced observed inundation while maintaining computational efficiency, suggesting the model may be especially applicable in rapid hazard assessment scenarios. +
Debris flows, sediment laden gravity driven fluvial processes, are a common issue in Southern California. They often occur during peak streamflow, making precipitation an important predictor for debris flow activity. However, the low temporal sampling of precipitation data used to calculate streamflow is often insufficient to forecast peak flows accurately. Here, we evaluate the effect of precipitation data resolution on discharge using 30-minute IMERG-early data averaged over different time intervals to model streamflow. We apply the results to a dimensionless discharge threshold model to predict debris flow locations. The streamflow values were calculated with the Distributed Hydrology Soil Vegetation Model and the debris flow model was programmed to be compatible with the Basic Model Interface (BMI). BMI was selected for this project because it standardizes model coupling, which enabled a hydrologic driven landslide model to run efficiently. The landslide model follows Tang et al. (2019) to produce dimensionless discharge and debris flow threshold values for stream segments. This can be used to predict where we would likely see a debris flow based on the given streamflow data. We ran these models with precipitation data of different temporal resolutions and evaluated their effect on dimensionless discharge. The model was able to capture a portion of debris flows using higher temporal resolution precipitation data. Of the 138 stream segments evaluated, 122 were predicted to have a dimensionless discharge value above the calculated thresholds when using 30-minute data, which largely matched observations from aerial imagery. In contrast, lower temporal resolution data did not capture these results. Initial debris flow predictions using high resolution precipitation data coincide in stream segments that experienced landslides. We conclude that high resolution precipitation data is critically important for predicting debris flow events. +
Decision making is a cultural process fundamental to slowing environmental destruction in all its guises. Although crucial to understanding environmental decision making, working toward a viable interdisciplinary model that could be used across problems and sites is not without obstacles. In order for coupled models to capture realistic lag times and interactions between social choices and the environment, algorithms of decision making must incorporate the influence of spatial-temporal local differences. This is especially true for coupled human-earth system models or agent-based models designed to inform policy. Here we provide a case study from the Paraná Delta of Argentina where a neighborhood assembly fights against pollution in the delta caused by an engineering failure. We combine components of a decision making framework with concepts from cultural and geographic theory, and then filter the combination through ethnographic description and interpretation to track how local culture influences decisions, and hence, lag times between actions and outcomes. Although fundamental to human decision making processes, sociocultural dynamics are often left out of formal behavioral modules coupled to environmental models. Through this experiment, we expand the capacity of such a framework for carrying cultural meaning and social interaction. +
Degradation of ice-rich permafrost is caused by rapid Arctic warming. Likely this degradation already has altered the water balance by increasing runoff and flooding. But here we ask, how do the hydrological changes in river systems, in turn, affect the permafrost conditions? How does river flooding affects permafrost thermal state in floodplains and deltas? What if the timing of river flooding changes with Arctic warming?
We develop a first-order heat budget approach to simulate evolving river flood water temperature over the seasonal inundation period. Solar radiation, air temperature and wind control the different components of heat exchange between the atmosphere and the river water surface. An additional term specifically calculates the exchange of heat between the river water and the channel bed and subsurface. Then, this river and flood water temperature is coupled to the Control Volume Permafrost Model (CVPM), which models detailed thermal state of shallow permafrost. We apply the combined model to the Kuparuk river floodplain and delta, a medium-sized river system on the North Slope of Alaska. Results indicate that permafrost underlaying the floodplain warms during inundation, and the active layer thickness (ALT) can increase for more several meters with sustained standing water. Permafrost underlying the floodplain farthest laterally from the main channel is only warmed by the short-lived spring snowmelt flood. We find that earlier arrival of the spring freshet and associated earlier inundation onset, as well as the increase of river discharge can significantly increase subsurface permafrost temperature, and lead to the deepening of the active layer. The sedimentary characteristics of the deposits in the floodplain are an important controls on the response of permafrost thermal state to inundation. River corridors, especially in the continuous zone of permafrost in the Arctic, are increasingly vulnerable to future changes in timing and magnitude of freshwater flooding as a result of earlier spring snowmelt and river breakup, and increasing river discharge.
Delivery of large blocks of rock from steepened hillslopes to incising river channels inhibits river incision and strongly influences the river longitudinal profile. We use a model of bedrock channel reach evolution to explore the implications of hillslope block delivery for erosion rate-slope scaling. We show that incorporating hillslope block delivery results in steeper channels at most erosion rates, but that blocks are ineffective at steepening channels with very high erosion rates because their residence time in the channel is too short. Our results indicate that the complex processes of block delivery, transport, degradation, and erosion inhibition may be parameterized in the simple shear stress/stream power framework with simple erosion-rate-dependent threshold rules. Finally, we investigate the effects of blocks on channel evolution for different scenarios of hydrologic variability, and compare and contrast our results with those of more common stochastic-threshold channel incision models. We show that hillslope-derived blocks have a different signature in erosion rate-slope space than the effects of constant erosion thresholds, and propose characteristic scaling that could be observed in the field to provide evidence for the influence of hillslope-channel coupling on landscape form. +
Delta environments, on which over half a billion people live worldwide, are sustained by sediment delivery. Factors such as subsidence and sea level rise cause deltas to sink relative to sea level if adequate sediment is not delivered to and retained on their surfaces, resulting in flooding, land degradation and loss, which endangers anthropogenic activities and populations. The future of fluvial sediment fluxes, a key mechanism for sediment delivery to deltas, is uncertain due to complex environmental changes which are predicted to occur during the coming decades. Fluvial sediment fluxes under environmental changes were investigated to assess the global sustainability of delta environments under potential future scenarios up to 2100. Climate change, reservoir construction, and population and GDP (as proxies for other anthropogenic influences) change datasets were used to drive the catchment numerical model WBMsed, which was used to investigate the effects of these environmental changes on fluvial sediment delivery. This method produced fluvial sediment fluxes under 12 scenarios of climate and socioeconomic change which are used to assess the future sustainability of 47 deltas, although the approach can be applied to deltas, rivers, and coastal systems worldwide. The results suggest that fluvial sediment delivery to most deltas will decrease throughout the 21st century, primarily due to anthropogenic activities. These deltas will likely become unsustainable environments, if they are not already, unless catchment management plans are drastically altered. +
Delta integrity is a function of adequate fluvial sediment supply since the form at the shoreline is the result interaction between fluvial and basinal processes. Globally, sediment supply to river deltas has been on the decline. Specifically, present sediment supply to the Niger Delta is less than what is required for a sustained growth. Anthropogenic intervention in the lower Niger Basin and within the delta is the main control of the decrease in sediment supply. Changes in shore form is a main consequence of shifting volume of sediment supply in the Niger Delta region. This study attempts a morphodynamic analysis of shoreline changes along the Niger Delta using recent high resolution remote sensing techniques within the Google Earth Engine Platform. Attempt will also be made to characterise the spatial or temporal variability in shoreline dynamics along the Niger Delta with a view to establish the drivers of change. The study will also attempt to model the future evolution of the Niger Delta given present forcing scenarios. The research is within the overall framework of ensuring a sustainable development within the Niger Delta coastal zone in order to preserve its huge economic and ecological potentials for future generation. +
A
Delta morphology is traditionally explained by differences in fluvial energy and wave and tidal energy. However, deltas influenced by similar ratios of river to marine energy can display strikingly different morphologies. Other variables, such as grain size of the sediment load delivered to the delta, influence delta morphology, but these models are largely qualitative leaving many questions unanswered. To better understand how grain size modifies deltaic processes and morphologies we conducted 33 numerical modeling experiments using the morphodynamic physics-based model Delft3D and quantified the effects produced by different grain sizes. In these 33 runs we change the median (0.01 – 1 mm), standard deviation (0.1 – 3 φ), and skewness (-0.7 – 0.7) of the incoming grain-size distribution. The model setup includes a river carrying constant discharge entering a standing body of water devoid of tides, waves, and sea-level change. The results show that delta morphology undergoes a transition as median grain size and standard deviation increase while changing skewness has little effect. At low median grain size and standard deviation, deltas have elongate planform morphologies with sinuous shorelines characterized by shallow topset gradients ranging from 1 x 10<sup>-4</sup> to 3 x 10<sup>-4</sup>, and 1 - 8 stable active channels. At high median grain size and standard deviation, deltas transition to semi-circular planform morphologies with smooth shorelines characterized by steeper topset gradients ranging from 1 x 10<sup>-3</sup> to 2 x 10<sup>-3</sup>, and 14 - 16 mobile channels. The change in delta morphology can be morphodynamically linked to changes in grain size. As grain size increases delta morphology transitions from elongate to semi-circular because the average topset gradient increases. For a given set of flow conditions, larger grain sizes require a steeper topset gradient to mobilize and transport. The average topset gradient reaches a dynamic equilibrium through time. This requires that, per unit length of seaward progradation, deltas with steeper gradients have higher vertical sedimentation rates. Higher sedimentation rates, in turn, perch the channel above the surrounding floodplain (so-called ‘super-elevation’) resulting in unstable channels that frequently avulse and create periods of overbank flow. That overbank flow is more erosive because the steeper gradient causes higher shear stresses on the floodplain, which creates more channels. More channels reduce the average water and sediment discharge at a given channel mouth, which creates time scales for mouth bar formation in coarse-grained deltas that are longer than the avulsion time scale. This effectively suppresses the process of bifurcation around river mouth bars in coarse-grained deltas, which in turn creates semi-circular morphologies with smooth shorelines as channels avulse across the topset. On the other hand, finest-grained (i.e. mud) deltas have low topset gradients and fewer channels. The high water and sediment discharge per channel, coupled with the slow settling velocity of mud, advects the sediment far from channel mouths, which in turn creates mouth bar growth and avulsion time scales that are longer than the delta life. This creates an elongate delta as stable channels prograde basinward. Deltas with intermediate grain sizes have nearly equal avulsion and bifurcation time scales, creating roughly semi-circular shapes but with significant shoreline roughness where mouth bars form.
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Delta shoreline structure has long been hypothesized to encode information on the relative influence of fluvial, wave, and tidal processes on delta formation and evolution. However analyses and comparisons of deltaic shorelines have typically been qualitative or utilized relatively coarse quantitative metrics. We ask whether robust quantification of shoreline structure would enable mapping of deltas to a physically-based space in which the relative influence of the different processes could be compared, as has recently been done using a sediment flux budget approach. To explore this question, we analyze Landsat-derived shorelines from more than 50 deltas across the globe. Since the shorelines exhibit variability on scales ranging from tens of meters to tens of kilometers, we propose a multiscale characterization of shoreline structure by mapping the shorelines to a univariate series, through a macro-scale convexity-informed framework, and using localized multi-resolution analysis via wavelets to quantify shoreline variability across a range of spatial scales within and across deltas. Specifically, we focus on the relative energy contributed by meso-scale features (river mouths) and small-scale (less than 1 km scale features). We find that distinct classes of deltas naturally emerge in that metric space, which we attribute to the different processes driving the sources and sinks of sediment in these systems. The analysis suggests the potential towards a quantitative, process-based classification of delta morphology via multi-scale analysis of shoreline structure. +
Deltaic, estuarine, and barrier coasts are experiencing unprecedentedly fast rates of morphological changes, which constitute a threat to people, infrastructures, and economies. Predicting these changes in the future could help to develop cost-efficient mitigation and adaptation plans. Here I present recent progresses in simulating large scale and long term coastal evolution using a new morphodynamic-oriented model. Through opportune simplifications the model simulates tides, surges (hurricanes), wind waves, swells, sand/mud/organic sediment, stratigraphy, and vegetation in a numerically-efficient way. The model reproduces the self-organization of barrier islands and the formation of marshes in the backbarrier/estuarine region. The model emphasizes how mud supply is a major driver for the long-term retreat of marshes. The model also simulates how riverine inputs into backbarrier basins – for example through man-made river diversions – can reduce both marsh edge erosion and barrier island retreat. +
Deltas are home to approximately 7% of global population and play a crucial role in regional food security owing to the favorable conditions for agriculture. As a result, these areas are often heavily irrigated as humans strive to use the local water resource to maximise production. This study aims to incorporate irrigation practices into the LISFLOOD-FP hydrodynamic model to determine the impact of irrigation on the flood dynamics of the Mekong Delta, one of the most intensively irrigated deltas. Irrigation data is based on global databases of irrigation area, crop type and crop calendars, supplemented with local information allowing for this approach to be used across irrigated areas around the world. This study therefore builds upon the localized estimates of flood storage capacity of paddy fields through the region and generates a new estimate across a wider area that is subsequently used to assess the impact on the hydrodynamics and flood inundation pattern. It is envisaged this approach can be used for future analysis of the impact of the changing irrigation practices of the Mekong Delta. +
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Deltas are the important interface between continents and oceans, providing home to over half a billion people. The unique environment supports a wide variety of diverse ecosystems and is highly susceptible to a broad spectrum of interacting forces. Therefore it is critical to understand its current and future changes, especially against the background of climate change and human impact, something that could be explored by studying its historical evolution process. Delta evolution is mainly governed by: a) sediment load supply from its contributing river, and 2) ocean dynamics (e.g. waves, tides). Fluvial sediment supply to a delta fluctuates over time either e.g. due to shifts in climate or, on shorter time scales, due to human interference (e.g. deforestation which could increase sediment supply or the emplacement of dams and reservoirs that reduces the sediment supply). How does this affect the morphology of a delta? Waves interact on deltas by dispersing fluvial sediment, reshaping its shoreline, how will it be illustrated in delta’s shape and morphology?
To study this, we explored hypothetical delta evolution scenarios given the following boundary conditions: a medium size upstream drainage basin (~80,000km2) with, as base case, a typical Mediterranean climate. The analysis is done through coupling two numerical models, HydroTrend and CEM. HydroTrend, a climate-driven hydrological transport model, is applied to replicate freshwater and sediment flux to the delta, and subsequently a coastline evolution model (CEM) is applied to simulate the according changes in the delta’s coastline morphology. A component-modeling tool (CMT) developed by CSDMS, is used to couple the models for this study. Several scenarios are considered that take into account: 1) stepwise increasing fluvial sediment supply, to the delta and 2) the release time of these stepwise sediment increases by changing the storm intensity for periods of time.
Preliminary model experiments will be presented demonstrating: 1) the capability of the CMT to couple models that represent different process domains and were developed and designed independently (i.e. without the intentions of such coupling), 2) the impact of changes in fluvial sediment on deltas.
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Deltas are threatened not only by climate and environmental changes (sea level rise, soil salinization, water shortages and erosion), but also by socioeconomic factors (high population density, intensive land use). These processes threaten people’s livelihoods and wellbeing, and as a result, there is a growing concern that significant environmental change induced migration might occur from deltaic areas. Migration, however, is already happening for economic, education and other reasons (e.g. livelihood change, marriage, planned relocation, etc.). Migration has multiple, interlinked drivers and depending on the perspective, can be considered as a positive or negative phenomenon.
The DECCMA project (Deltas, Vulnerability & Climate Change: Migration & Adaptation) studies migration as part of a suite of adaptation options available to the coastal populations in the Ganges delta in Bangladesh, the Mahanadi delta in India and the Volta delta in Ghana. It aims to develop a holistic framework of analysis that assesses the impact of climate and environmental change, economics and governance on the migration patterns of these areas. The project will test plausible future scenarios and evaluate them by considering a range of perspectives.
The dynamic Bayesian Network integrated model of the DECCMA project formally brings together the project elements in fully coupled, quantitative assessment framework. The presentation introduces the overall integration concept and describes the household decision-making component in detail. This component is based on a detailed household survey from delta migrant sending and receiving areas. We describe the model structure, and contrast the model setup and sensitivities across the three study areas. In doing so we illustrate some key causal relationships between changes in the environment, livelihoods and migration decision. The outputs of the integrative modelling is used to objectively evaluate the simulated environmental, social and economic changes for decision makers including the benefits and disadvantages of migration as an adaptation option.
Deltas exhibit spatially and temporally variable subsidence due, in part, to faulting that lowers the land surface over time, thereby converting subaerial land to open water. In light of expected billion-dollar investments globally to redirect sediment via channel diversions and thus restore delta land, it is crucial to understand whether discrete faulting-induced subsidence events drive distributary channel networks to reorganize. Here, we take inspiration from examples from two deltas of faulting with documented surface expression and with distinct flux-to-shoreline symmetries: the symmetric-flux Selenga River delta (Russia) and the asymmetric-flux Mississippi River delta (Louisiana, USA). Using simulations with the DeltaRCM numerical model resembling these deltaic landscapes, we examine distributary network reorganization to faulting-induced subsidence over a range of surface area and slip displacement. Our findings indicate that in a symmetric-flux delta system, the duration of fault surface expression is strongly and non-linearly related to displacement, because slip above a threshold length-scale drives wholesale channel network reorganization, whereas smaller displacement does not. In contrast, displacement is only weakly related to network reorganization in the asymmetric-flux simulations. In this environment, faults located in areas of the delta not maintaining a surface-water connection to the main channel at the time of the subsidence event do not instigate network reorganization. Moreover, for the range of surface area and slip displacement we examined, areas of faulting also do not significantly influence the distributary network at later times. Nevertheless, all faulting events in simulated deltas, with both symmetric and asymmetric flux, create accommodation space and so inhibit the construction of subaerial land to some degree. +
Densely populated coastal deltas worldwide face cascading flood and salinization hazards associated with sea-level rise, storm surges, dwindling sediment supplies, and land subsidence. One of the greatest hurdles to hazard prediction stems from quantifying the land-subsidence component, which exhibits significant spatial and temporal variations across any given delta. Here, we present a delta-subsidence model capable of quantifying these variations. The model is built upon fundamental principles of effective stress, conservation of mass, and Darcy flow; as well as constitutive relations for porosity and edaphic factors (e.g. roots, burrows). For an input sediment column and deposition rate, we quantify the depth-profile of vertical land motion over time, allowing for direct comparison with field observations spanning various depths, timescales, and methods (e.g., GPS stations; Rod-surface-elevation tables; C14 and OSL ages). Preliminary results demonstrate the model can accurately resolve decadal-scale subsidence patterns on the Ganges-Brahmaputra delta, including subsidence hotspots associated with fine-grained lithologies, buried Pleistocene paleovalleys, and river embankments constructed in the 1950’s. This predictive subsidence model can improve assessments of coastal flood hazards on the Ganges-Brahmaputra and other deltas worldwide; and help inform ongoing billion-dollar restoration efforts facing crucial decisions as to where and when coastal barriers, sediment diversions, and settlement relocations will be implemented in the coming century. +
