Property:CSDMS meeting abstract
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The evolution of human-flood systems is shaped by complex interactions between hazards, policy decisions, individual risk perception, and the exposure of properties. This complexity is further stressed by the changing climate conditions, making it crucial to understand how these systems will evolve and which regions and populations will be most affected. In this regard, we calibrated socio-environmental models across US coastal communities with historical records of flooding hazards, National Flood Insurance Program (NFIP) economic losses, NFIP policy purchases, housing density, and housing values. Next, we forced future projections of sea level rise, storm surge, and rainfall intensity under Shared Socio-economic Pathways (SSP) SSP245 and SSP585 up to 2100 for each coastal communities, and forecasted the future flooding loss, NFIP active policies, housing density, and housing values. We found significant regional and demographic variations in human-flood dynamics. The Pacific coast, due to high rainfall and storm surge threshold has less exposure, but a more sensitive housing market and NFIP participation rate. In contrast, the Atlantic and Gulf coasts are more exposed to hazards but have a less sensitive housing market and NFIP participation. Relative to historical average, we forecast flood loss to increase by 130% (SSP585) and 25% (SSP245) with a modest policy coverage of 16% (SSP585) and 13% (SSP245). Furthermore, we predict socially vulnerable communities to experience disproportionately more economic loss with a slow policy uptake rate, leading to a growing insurance coverage gap under both climate scenarios. Finally, we tested the effect of heightening levees across the US coast on future flood risk, and found that levee investment can stabilize housing markets, but it won’t eliminate flooding risk entirely due to increased rainfall intensity. Understanding how human-flood systems co-evolve under climate risk helps to recognize population and property at risk and make robust mitigation strategies.
The extent to which chemical and mechanical erosion each contribute to the erosion of cave passages in limestone is an open question. In mixed cave riverbeds that are partially alluviated and partially exposed limestone bedrock, we sometimes see clearly scalloped bedrock. The uniquely soluble properties of limestone imply that these scallops that tessellate to comprise the scalloped bedrock are the result of chemical dissolution. However, because we see silt, sand, and gravel, and because when we visit the same reach of the cave river many times, we see those sediment deposits shift in size and location, we infer that there may also be physical abrasion from sediment impacts on the scalloped bedrock surface. In this paper, we compare the equations that describe dissolution of limestone with those that describe abrasion of bedrock to prove that dissolution and abrasion may be co-occurring processes. Using our numerical model, DKARST (Does karst abrasion result in scalloped tunnels?), in conjunction with previous data from dissolution studies, we quantified parameters that delineate four distinct erosional zones according to the likelihood of contribution to overall erosion from dissolution, abrasion, or both processes combined. We then generalized those erosional zones to a range of scalloped bedrock morphology characteristic wavelengths. Our investigation of the role of mechanical erosion to the scalloping of bedrock in caves provides insight into the settling velocities of particles in turbulent flow over rough beds, as well as the relative roles played by mechanical and chemical processes in broader scale landscape evolution, particularly in karst regions dominated by carbonate bedrock. +
The formation and evolution of channel networks is a critical control on coastal landscapes and fluvial stratigraphy. Analysis of drainage networks often divides them into two regions: a dendritic upstream catchment with behavior governed by erosional processes resulting from the interaction of climate and tectonics, and a transition to a distributary reach governed by depositional processes close to the coast. The landscape built by these larger coastal distributaries is typically dominated by low-relief floodplains and numerous smaller stream networks. Despite the importance of these networks in governing the routing of fluids and sediments that build these landscapes, network geometries and characteristics remain poorly studied and understood. The northern Gulf of Mexico coastal plain is a depositional landscape characterized by the channels and deposits of large fluvial systems that have been prograding into the Gulf of Mexico since the Mesozoic, and hosts smaller stream networks locally known as the Coastal River Basins. Using a compilation of lidar bare earth elevation datasets we systematically identify and map these tributary stream networks across the coastal plain. We calculate for each basin a series of stream metrics that include local relief, slope, and length/contributing area. Additionally, our high-resolution (2m) elevation data allows for detailed analysis of the stream heads and drainage divides between each identified basin. We find that the basin divides for these networks are older distributary channel belts built by the larger fluvial systems. This indicates that the organization and geometry of these coastal networks is initially set and controlled by depositional processes, but the resulting basin morphology is nearly identical to those of drainage networks in predominantly erosional settings. We explore how drainage networks can form in depositional settings as the consequence of sedimentary processes such as river avulsion and ridge formation, with important implications for understanding drivers of drainage network formation, the speed and scale of drainage reorganization in coastal settings, flow routing during floods, and fundamental controls on the creation and preservation of fluvial stratigraphy.
The formation of the branching channel network is controlled mainly by water discharge and the boundary shape of receiving basin. The understanding of channel morphology is important because it controls the sediment diversion in a river delta, and determines the sustainability of coastal zones. Numerical models of river deltas have improved remarkably over the past two decades. However, the long-term (millennial scale) simulation of real delta systems remains rare. Here, we attempt to reconstruct the Lafourche Delta channel network, active 1600-600 years before present, with a simple numerical model (Moving Boundary Model for Distributary Channel Networks MB_DCN). Runs with 10 basin boundary shapes and 6 river discharge rate scenarios using the Moving Boundary Model for Distributary Channel Networks (MB_DCN) show that each scenario produced distinguishing channel characteristics including a complex channel network, diverse progradation rates and channel numbers, and number of bifurcations. For the appropriate basin shapes, reasonable water discharges and common sediment transport parameters, MB_DCN produces a channel network that resembles the Lafourche Delta channel network morphology and progradation rates. Our preliminary results suggest that the basin boundary shape and water discharge are the most important control of the distributary channel network in terms of channel geometry and progradation rates. +
The frequency of high temperature events is increasing globally under the current climate change conditions. These extreme events have important consequences for society, affecting public health, the regional habitability and the global economy. We evaluate the changes in frequency and distribution of high temperature events over North America, using three different indices and a set of regional climate simulations from the Coordinated Regional Climate Downscaling Experiment (CORDEX). Our results show an increase in the number of high temperature days per summer, in addition to an increase in the frequency of heat wave events for the 21st century. The results reveal large variability among the regional climate models and boundary conditions from the driving models. The increase in the frequency of high temperature simulations examined over North America advocates for strategies to prevent potential effects on food availability, public health and the environment. +
The geologic history of major river canyons is strongly debated, as is the extent to which river canyons record climatic and tectonic signals. Fluvial and hillslope processes work in concert to control canyon evolution; rivers both set the boundary conditions for adjoining hillslopes and respond to delivery of hillslope-derived sediment. But what happens when canyon walls deliver boulders that are too large for a river to carry? River canyons commonly host large blocks of rock derived from resistant hillslope strata. Blocks have recently been shown to control the shapes of hillslopes and channels by inhibiting sediment transport and bedrock erosion. Here we present Blocklab, a 2-D model within the Landlab modeling toolkit that uses a hybrid discrete-continuum framework to track block transport throughout a river canyon landscape. This is the first process-based model for canyon evolution that incorporates the roles of blocks in both hillslope and channel processes. Our model reveals that two-way negative channel-hillslope feedbacks driven by block delivery to the river result in characteristic planview and cross-sectional river canyon forms. Internal negative feedbacks strongly reduce the rate at which erosional signals pass through landscapes, leading to persistent local unsteadiness even under steady tectonic and climatic forcing. Surprisingly, while the presence of blocks in the channel initially slows incision rates, the subsequent removal of blocks from the oversteepened channel substantially increases incision rates. This interplay between channel and hillslope dynamics results in highly variable long-term erosion rates. These autogenic channel-hillslope dynamics can mask external signals, such as changes in rock uplift rate, complicating the interpretation of landscape morphology and erosion histories. +
The growing complexity of landscape evolution models (LEMs) has broadened their use to answer a variety of questions, but approaches for statistically assessing model outputs remain underexplored. Here, we suggest enhancing the study of LEM outputs by utilizing Shannon Entropy, Moran's I, and Geary's C, which provide insights into dynamics and variations within and between simulations, both quantitatively and visually. Three experiments were used as case studies; a constant uplift (Experiment 1), a periodic alternating uplift (Experiment 2), and a spatially variable uplift (Experiment 3). Incorporating the proposed metrics as a comparison module in LEMs offers a methodical way to analyze variations in information content, gauge spatial consistency, and spot simulation divergence. Although our focus is on LEM outputs, similar techniques may be applied to any matrix-based data or digital elevation models (DEMs), allowing for thorough model evaluations and better decision-making in topographic analysis and landscape modeling investigations. This material was developed during the 2024 CSDMS Visiting Scholar Program, being supported by NSF under Grant Nos. EAR-2104102 and EAR-1917695. +
The hazards faced by retreating barrier island systems to the increased rates of sea level rise predicted over the coming century and beyond lacks historic precedent. Consequently, exploration of the sedimentological record can provide key insights into how barrier systems might behave in the future. Continental shelves around the world preserve records of former barriers as relict deposits, providing a window into past behaviors. These relict barrier deposits are usually considered to originate from purely allogenic processes, or external environmental forcing, with barrier abandonment typically attributed to episodes of increased rate of sea level rise. However, using a cross-shore morphodynamic model, we show that the internal dynamics of migrating barriers can also result in autogenic deposition of relict sediments even under a constant rate of sea level rise. Subsequently, we propose that allogenic forcing from sea level rise and autogenic forcing from internal dynamics might interact to produce novel barrier retreat behaviors, with the potential to be recorded on the seabed by relict deposits. We model barriers through a range of scenarios with interacting autogenic and allogenic forcing, showing that the morphology of deposits might be used to infer the relative influence of external and internal processes. Intriguingly, our results demonstrate that the internal dynamics of barriers can both amplify and dampen losses of shoreface sediment to the seabed during increased rates of rise, in some cases with internal processes increasing the risk of barrier destruction. Future classification of relict deposits in the field could help explain if and when these allogenic/autogenic interactions have taken place, revealing long term hazards to modern barrier systems that have not previously been described. +
The high speed winds of a hurricane account for 95% of a hurricane’s storm surge. Thus, parametric wind models are vital components of numerical storm surge modeling. These parametric hurricane wind models are used as inputs for a storm surge computation to hindcast and forecast hurricane surge heights. These wind models are dependent on several input parameters including but not limited to the radius at which the maximum wind speed of the hurricane occurs and the speed of the maximum winds. The impact of these input parameters on the final surge computation is not well known. Our study is a sensitivity analysis of the effect of uncertainty in the input parameters on the uncertainty in the final computation of the storm surge model. This study will help us to understand the robustness of a parametric wind model, the parameters that must be precise in order to reduce model error, and can aid in model simplification. +
The impact of climate on tectonics has been the muse of tectonic geomorphologists for more than 30 years. However, few natural examples exist where connections between climate and tectonics are clear. Here, we present a study of the Sangre de Cristo Mountains (SCM), CO, a normal fault system at the northern tip of the Rio Grande Rift. The SCM represents an ideal natural setting to explore the impact of climate on spatial and temporal slip patterns along the range-bounding fault. Preserved glacial moraines and trimlines are used with the Glacier Reconstruction (GlaRe) toolbox to model glacial extents during the last glacial maximum (LGM). A simple line load model is used to explore the impact of glacial melting on clamping stress along the range front fault, and a flexural isostatic model is applied to estimate the footwall response to deglaciation. Results show that glacial melting reduces fault clamping stress, perhaps enabling accelerated fault slip in the post-glacial period. Flexural isostatic results suggest modest footwall uplift of ~4 m due to ice removal. We compare our results to fault displacement, measured from scarps preserved in Pleistocene and Holocene alluvial fans. The spatial pattern and magnitude of Holocene fault displacement are consistent with our flexural isostatic results. Furthermore, Holocene slip rates are at least a factor of three higher than Pleistocene slip rates. We infer that the flexural isostatic response to footwall deglaciation primarily controls the spatial and temporal fault slip patterns during the Holocene. Our results show that climate-modulated glacial ice loading and unloading can pace the spatial and temporal slip on a range-bounding normal fault system. +
The impact of supraglacial meltwater on the motion of the Greenland Ice Sheet is strongly correlated to spatial and temporal variability of meltwater input. Meltwater infiltrates the bed through moulins and can reduce effective pressure and, consequently, accelerate the ice. However, the subglacial conduit system evacuates the water and can adapt to accommodate different water inputs. The timing of water infiltration impacts the ability of the system to reach equilibrium state. With the progression of the equilibrium line higher up on the ice under warming climate, it is essential to predict how increased meltwater is going to affect ice motion. Understanding these processes will reduce uncertainty in global sea level rise predictions.
Temporal variability of meltwater input is difficult to measure on the ice sheet due to the difficulties in instrumenting constantly melting stream beds. Therefore, glacier dynamic models rely on surface mass balance models to simulate the discharge. Those models usually neglect spatial properties of the drainage basin and are not able to reproduce the peak meltwater discharge in supraglacial streams. Lags between peak melt and peak discharge vary from one stream to another, and factors influencing the delay between peak melt and peak discharge have not been thoroughly explored. For this reason, we propose to build a distributed and physically based model using Landlab to reproduce flow routing on the Greenland Ice Sheet. This model will produce discharge values on a grid using three grid layers that calculate: 1) meltwater production, 2) flow direction, and 3) water displacement velocity. Model inputs will be weather, elevation, and snow coverage data. This model will enable us to explore and extract the main parameters influencing lags and predict the spatial pattern of infiltration lags at an ice sheet scale. +
The impacts of climate change on extent of permafrost degradation in the Himalayas are not well understood due to lack of historical ground-based observations. The area of permafrost exceeds that of glaciers in almost all Hindu Kush Himalayan (HKH) countries. However, very little is known about permafrost in the region as only a few local measurements have been conducted which is not sufficient to produce the fundamental level of knowledge of the spatial existence of permafrost. We intend to simulate permafrost conditions in Western Himalayas in India using Hyperspectral and Microwave remote sensing methods and computational models for the quantitative assessment of the current state of permafrost and the predictions of the extent and impacts of future changes. We also aim to identify the strength and limitations of remotely sensed data sets when they are applied together with data from other sources for permafrost modelling. We look forward to modelling ground temperatures using remote sensing data and reanalysis products as input data on a regional scale and support our analysis with measured in situ data of ground temperatures. Overall, we approach to model the current state and predictable future changes in the state of permafrost in Western Himalayas and also couple our results with similar research outcomes in atmospheric sciences, glaciology, and hydrology in the region. +
The increasing demand for sediments as source material for beach nourishment projects highlights the need to understand inner-shelf transport dynamics. At cape-related shoals, from where sedimentary materials are customarily extracted, the variability in particulate transport and related bedform evolution are not well understood.
To analyze bed elevation variability at a shoal adjacent to Cape Canaveral, Florida, an acoustic Doppler current profiler (ADCP) was deployed in spring 2014 at the outer swale of Shoal E, ~20 km south east of the cape tip at a depth of ~13 m. ADCP-derived velocity profiles and suspended particle concentrations were used to quantify instantaneous temporal changes in bed elevation (dζ/dt) using a simplified version of the Exner equation. Using mass conservation, temporal (deposition and entrainment) and spatial gradients in suspended sediment concentrations were calculated, although neither bed-load fluxes nor spatial gradients in velocities were considered.
Calculated values for instantaneous dζ/dt ranged from erosion at ~1e-3 m/s to accretion at 0.5e-3 m/s. Most of the variability was found at subtidal (<1 cycle/day) and tidal (~2 cycles/day) periodicities. Bed changes were small (<0.005 m/s) when tidal motions were important, e.g. from May 6 to 16, whereas subtidal motions at periods of 1 and 8 days dominated erosion/accretion events between May 16 and 31. Values suggest a bed erosion of 3.1e-3 m during ~30 days of the experiment, which was 2 orders of magnitude less, and had a contrary tendency to the average accretion of ~150e-3 m in 37 days measured between July 28 and September 3 at the edge of Southeast Shoal, i.e. ~5 km to the northwest.
In addition to the fact that measurements were not performed simultaneously at the same location, the discrepancy in dζ/dt could be attributed to the underestimation of bed changes due to the exclusion of bed-load fluxes. Despite several uncertainties, these findings provide preliminary evidence regarding the role of seasonal and storm-driven subtidal flows in particulate transport at cape-associated shoals. Our methodology can be used to inform numerical models of sediment transport and morphological evolution along inner continental shelves.
The influence of hydrodynamics on delta morphology is well-understood: fluvial, tidal and wave processes sculpt deltas into characteristic shapes that serve as geomorphic signatures of the underlying dynamics. This work examines how complex interactions between major rivers and tides influence the dendritic, island-dense morphology of the Ganges-Brahmaputra-Meghna Delta (GBMD). In the uppermost delta plain, fluvial processes dominate. Moving downstream, tides begin to interact with fluvial dynamics in a “mixed” process zone. Near the terminus of the GBMD in the Bay of Bengal, tidal processes take over, particularly in the western, abandoned lobes of the delta. This work focuses on how sediment transport and floodplain deposition patterns and rates change along that process transition. Using geomorphic metrics such as island area, aspect and channel sinuosity within new machine learning techniques, we resolved areas of the delta that display similar process signatures. Ideal island cases were selected from several zones across the fluvial-tidal transition. Using Delft3D, we modeled the patterns of geomorphic change that result from multiple flood ranges (low, medium and extreme) and sediment cases (-50%, average +50% suspended sediment concentration of 4 cohesive grain classes). Preliminary results indicate floods can deposit 2 – 3 cm of sediment per year across the delta, albeit in distinct patterns due to local differences in hydrodynamic processes. By using this highly-resolved nested model approach, deposition rates can be upscaled to estimate the amount of sediment being reworked within the individual process zones. The results obtained can then be used to illustrate how the different hydrodynamic zones contribute to the large-scale evolution of the delta, and explore how the system will respond to predicted global sea level rise over the next century. +
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The interaction of the subsiding, subtropical limb of the Hadley circulation and the easterly North Pacific trade winds establishes a persistent thermal inversion about halfway up the eastern flank of the Big Island of Hawaii. This restricts convective rainfall to the lower elevations, resulting in stream channels that cross an order-of-magnitude rainfall gradient, active ephemerally above the inversion and perennially below it. Above the inversion–capped cloud layer, precipitation is on the order of 400 mm/yr, and the landscape features thin, weakly-developed soils, gentle hillslopes, and ephemeral, shallowly incised bedrock streams and grassland gullies. Below the inversion, where rainfall is >3000 mm/yr, the perennial streams run through 50- to 100-m-deep gulches, with steep forested walls covered by thick tropical soils that are prone to landsliding. Meter- to 50-meter waterfalls are common downstream of the inversion layer, and incision of the deep gulches may proceed by upstream migration of these knickpoints from the coast. The positions of these knickpoints likely reflect the history of lava flows in these catchments, base level changes due to landsliding at the coast, and the statistics of water and sediment discharge above and below the trade inversion and through time.
This landscape has evolved entirely in the last 0.3 Ma, and thus under conditions of glacial-interglacial climate oscillations. During glacial periods, the inversion’s average elevation was likely depressed, although the magnitude of this depression is not well-constrained. An ice cap that was present on Mauna Kea altered the hydrology of the upper slopes of the mountain, providing a continuous source of meltwater to channels that, in the modern setting, are active only during winter storms and rare hurricane strikes. The frequency and intensity of such storms during glaciations are also not well-known.
To quantify these effects, we would like to use climate models to inform landscape evolution models. A key difficulty in coupling these types of models is the separation of time and spatial scales involved. Global climate models typically run on grids of 1 degree or more, at temporal resolution of seconds and run lengths of years to decades. Landscape evolution models (LEMs) reside at the other end of both dimensions, with typical spatial resolutions of meters to km and temporal resolutions of years or decades. The entire duration of a climate model run may be shorter than the timestep of a typical LEM.
We report initial results from our efforts to bridge the relevant scales by downscaling large-scale climate model output for last-glacial and modern times with NCAR’s regional-scale Weather Research and Forecasting (WRF) model. The predicted precipitation fields are input to a hydrologic model to generate realistic discharge statistics useful for landscape modeling. This modeling chain may be validated for the modern climate using atmospheric observations, including the modern distribution of inversion height, and USGS stream gauge data. For glacial periods, the ability of the weather model to correctly predict snowlines on Mauna Kea provides a first-order point of calibration.
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The interplay between economic development and climate change exerts countervailing effects on human wellbeing, particularly concerning temperature-related mortality. While economic growth may enhance adaptive capacity and healthcare access, climate change intensifies extreme heat events, posing significant health risks. Carleton et al. (2022) and Barrage (2024) have quantified the effects of climate change and income growth on temperature-related mortality, providing insights into future health trajectories. To discern when climate change impacts on mortality become distinguishable from natural variability, this study introduces the concept of Time of Emergence (ToE). By identifying the ToE, this research – one of the first to apply ToE analysis to climate impacts rather than climate hazards – assesses the moment when climate-induced mortality becomes detectable beyond natural fluctuations, offering insights into the timing and magnitude of extreme-temperature impacts. The findings unveil that in warm regions, climate change may impede or nullify the reductions in temperature-related mortality typically driven by development, underscoring the urgency for targeted adaptation measures and policy interventions to mitigate health risks associated with climate change. +
The kinematics of sediment grains on a riverbed govern bedload transport, influencing water quality, aquatic ecosystems, river morphology, and landscape evolution. Previous studies suggest that hop distance distributions exhibit Gaussian-like behavior for large hops and exponential-like behavior for a mixture of short and long hops. Assuming flow hydraulics as the primary driver of these variations in hops, we numerically investigate the effects of flow conditions on the ensemble distributions of hop distances, particle velocities, travel times, and resting durations. This was done through high-fidelity simulations using coupled fluid dynamics/discrete element method (CFDEM), which integrates LIGGGHTS with OpenFOAM. For monodisperse grains (0.5 mm), hop distances follow a Weibull distribution, while velocity distributions are exponential, with scale factors increasing with flow strength. The relationship between streamwise hop distances and travel times follows Lx ∼ Tpα, with α ranging from 1.50 to 1.76. Travel times exhibit an exponential distribution, whereas resting times follow a Weibull distribution, both with scale factors increasing under higher flow conditions. For bidisperse grains (0.5 mm and 1 mm), the velocity distributions of each grain size independently follow an exponential distribution, while the combined velocity distribution conforms to a hypoexponential distribution - a weighted sum of exponential distribution of both sizes. This result suggests that, despite mechanical interactions between grains, the velocity distributions of each grain size remain statistically independent. These findings provide insight into the probabilistic nature of bedload transport and the role of flow dynamics in shaping sediment motion.
Keywords: Velocity distribution, hop distance, travel time, resting time +
The last 22 ka, since the Last Glacial Maximum (LGM), is known for significant millennial scale changes in global climate (Barker and Knorr, 2021). Sedimentary deposits in lacustrine and marine basins bear archives of corresponding changes in sediment accumulation. Yet given the scale that the global climate exerts on geomorphological processes on Earth’s surface, generalizations of the relationship between the climate and the erosion remain inconclusive. Whether the possible generalizations could even be applied to all regions has also remained unclear.
Erosion rates are a first-order response to climate of a region. The variability of erosion rates through time are needed for dating of buried surfaces, quantifying soil carbon budgets, and assessing landscape stability. Until now, a truly global analysis of comparing interregional erosion rates has not been available. Recent work in Madoff and Putkonen (2022) addresses this by generating global maps of regional erosion rates since the LGM. These results are supported by corresponding published sediment accumulation rates in sink areas corresponding to given watershed. Results show the spatial extent of higher erosion rates and larger ranges of variability through time in the Arctic and subarctic in contrast to the tropics and mid-latitudes. These results also indicate that the regional variability decreases the further from the past ice sheets a given location is. Finally, a clear take home message from these results is that the regional erosion rates vary both through time and space for the past 22 ka.
* Barker, S., Knorr, G., 2021. Millennial scale feedbacks determine the shape and rapidity of glacial termination. Nature Communications. 12.
* Madoff, R.D., Putkonen, J., 2022. Global variations in regional degradation rates since the Last Glacial Maximum mapped through time and space. Quaternary Research. 1–13. https://doi.org/10.1017/qua.2022.4 +
The long-term (3000 years) morphodynamics of backbarrier tidal basins is studied using a shallow-water hydrodynamic and wind-wave model (Deltf3D-FLOW-WAVE), modified to include fully-coupled marsh organogenic accretion, biostabilization, drag increase, and wave-induced marsh edge erosion. The latter process is implemented with a novel probabilistic algorithm. In simulations run with only sand, a flood tidal delta forms adjacent to the inlet, but marshes do not establish. In simulations run with only mud, instead, marshes establish at the basin margins and prograde seaward. If enough mud is supplied to the basin from the shelf, marsh progradation counteracts edge erosion. Marsh progradation does not completely fill the basin, but leaves open a few km-wide channels, large enough for waves to resuspend sediment. Starting from a basin (almost) filled with marshes, a drop in the external mud supply or an increase in the rate of relative sea level rise cause the basin to empty out by marsh edge erosion, while the marsh platform, aided by reworking of the sediment released by marsh retreat and mudflat deepening, keeps pace even with fast rates (10 mm/yr) of relative sea level rise. Even if the marsh does not drown, the marsh retreats faster if the rate of sea level rise increases, because more sediment is sequestered to fill the newly created accommodation space and is thus not available for marsh progradation. This study suggests that prediction of marsh erosion requires a basin-scale sediment budget, and that edge erosion, not platform drowning, is likely to dominate marsh loss. +
The low-lying tidal reaches of the Ganges-Brahmaputra delta relies on a system of polders (embanked landscapes) to prevent against tidal inundation and storm surge. These polders have increased the total habitable and arable land allowing the region to sustain a population of ~20 million people. An unintended consequence of poldering has been the reduction of water and sediment exchange between the polders and the tidal network, which has resulted in significant elevation offsets of 1-1.5 m relative to that of the natural landscape. Tidal River Management (TRM) and other engineering practices have been proposed in order to alleviate the offset. Previous work suggests if implemented properly with sufficient suspended sediment concentrations (SSC), TRM can be effective on timescales of 5-20 years. However, communities must also agree on how and when to implement TRM. Here, we expand previous numerical simulations of sediment accumulation through field-based constraints of grain size, compaction, and sea level rise. We then model human decision-making for implementation of TRM practices.
Our sediment model employs a basic mass balance of sediment accumulation as a function of tidal height, SSC, settling velocity, and dry bulk density. Tidal height is determined from pressure sensors and superimposed sea level rise rate, as defined by the representative concentration pathways of the IPCC. SSC varies within a tidal cycle (0-3 g/L) and seasonally (0.15-0.77 g/L). Multiple grain sizes (14-27 µm) are used as proxies for settling velocity by Stokes’ Law. Dry bulk density (900-1500 kg/m3) is determined from sediment samples at depths of 50-100 cm. The human dimension is introduced through an agent-based model for community decision-making regarding TRM. +
