Rollover, drowning, and discontinuous retreat: Distinct modes of barrier response to sea-level rise arising from a simple morphodynamic model

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 119 (2014): 779–801, doi:10.1002/2013JF002941.We construct a simple morphodynamic model to investigate the long-term dynamic evolution of a coastal barrier system experiencing sea-level rise. Using a simplified barrier geometry, the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux. With barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response, the modeled barrier system demonstrates four primary behaviors: height drowning, width drowning, constant landward retreat, and a periodic retreat. Height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea-level rise. On the other hand, width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash-driven landward migration. During periodic barrier retreat, the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens. This periodic retreat, which occurs even with a constant sea-level rise rate, arises when overwash rates and shoreface response rates are large and of similar magnitude. We explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to the maximum overwash flux and the shoreface response rate, barrier response can be particularly sensitive to the sea-level rise rate and back-barrier lagoon slope. Overall, our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea-level rise rate, sediment supply, or back-barrier geometry.This research has been supported by the National Science Foundation grant #CNH-0815875, the Strategic Environment Research and Development Program, and the Coastal Ocean Institute of the Woods Hole Oceanographic Institution.2014-10-0

Rollover, drowning, and discontinuous retreat: Distinct modes of barrier response to sea-level rise arising from a simple morphodynamic model

We construct a simple morphodynamic model to investigate the long-term dynamic evolution of a coastal barrier system experiencing sea-level rise. Using a simplified barrier geometry, the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux. With barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response, the modeled barrier system demonstrates four primary behaviors: height drowning, width drowning, constant landward retreat, and a periodic retreat. Height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea-level rise. On the other hand, width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash-driven landward migration. During periodic barrier retreat, the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens. This periodic retreat, which occurs evenwith a constant sea-level rise rate, arises when overwash rates and shoreface response rates are large and of similar magnitude. We explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to themaximum overwash flux and the shoreface response rate, barrier response can be particularly sensitive to the sea-level rise rate and back-barrier lagoon slope. Overall, our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea-level rise rate, sediment supply, or back-barrier geometry

A geomorphic enthalpy method: Description and application to the evolution of fluvial-deltas under sea-level cycles

Fluvial deltas are composites of two primary sedimentary environments: a depositional fluvial region and an offshore region. The fluvial region is defined by two geomorphic moving boundaries: an alluvial-bedrock transition (ABT), which separates the sediment prism from the non-erodible bedrock basement, and the shoreline (SH), where the delta meets the ocean. The trajectories of these boundaries in time and space define the evolution of the shape of the sedimentary prism, and are often used as stratigraphic indicators, particularly in seismic studies, of changes in relative sea level and the identification of stratigraphic sequences. In order to better understand the relative role of sea-level variations, sediment supply, and basin geometry on the evolution of the ABT and SH, we develop a forward stratigraphic model that accounts for curvature changes of the fluvial surface and treats the SH and ABT as moving boundaries (i.e., internal boundaries that are not known a priori and their location must be calculated as part of the solution to the overall problem). This forward model extends a numerical technique from heat transfer (i.e., enthalpy method), previously applied to the evolution of sedimentary basins, to account for sea-level variations, including eustatic sea-level cycles. In general, model results demonstrate the importance of the dynamics of the fluvial surface on the system response under a large range of input parameter values. Specifically, model results suggest that time lags in the ABT response during sea-level cycles can result in geologically long-lived river incision in the upper and mid portions of the fluvial surface during sea-level rise. These results suggest that the relationship between the coastal onlap configuration of strata and relative changes in sea level is complex, and therefore not necessarily a good indicator of contemporaneous sea-level changes

Exploring the role of organic matter accumulation on delta evolution

We explore the role of plant matter accumulation in the sediment column in determining the response of fluvial-deltas to base-level rise and simple subsidence profiles. Making the assumption that delta building processes operate to preserve the geometry of the delta plain, we model organic sedimentation in terms of the plant matter accumulation and accommodation (space made for sediment deposition) rates. A spatial integration of the organic sedimentation, added to the known river sediment input, leads to a model of delta evolution that estimates the fraction of organic sediments preserved in the delta. The model predicts that the maximum organic fraction occurs when the organic matter accumulation rate matches the accommodation rate, a result consistent with field observations. The model also recovers the upper limit for coal accumulation previously reported in the coal literature. Further, when the model is extended to account for differences in plant matter accumulation between fresh and saline environments (i.e., methanogenesis versus sulfate reduction) we show that an abrupt shift in the location of the fresh-salt boundary can amplify the speed of shoreline retreat

Morphodynamic Modeling of Fluvial Channel Fill and Avulsion Time Scales During early Holocene Transgression, as Substantiated by the Incised Valley Stratigraphy of the Trinity River, Texas

The Trinity River system provides a natural laboratory for linking fluvial morphodynamics to stratigraphy produced by sea-level rise, because the sediments occupying the Trinity incised valley are well constrained in terms of timing of deposition and facies distribution. Herein, the Trinity River is modeled for a range of base-level rise rates, avulsion thresholds, and water discharges to explore the effects of backwater-induced in-channel sedimentation on channel avulsion. The findings are compared to observed sediment facies to evaluate the capability of a morphodynamic model to reproduce sediment deposition patterns. Base-level rise produces mobile locations of in-channel sedimentation and deltaic channel avulsions. For scenarios characteristic of early Holocene sea-level rise, the Trinity fluvial-deltaic system progrades 13m/yr, followed by backstepping of 27m/yr. Avulsion is reached at the position of maximum sediment deposition (located 108 km upstream of the outlet) after 3,548 model years, based on sedimentation filling 30% of the channel. Under scenarios of greater base-level rise, avulsion is impeded because the channel fill threshold is never achieved. Accounting for partitioning of bed-material sediment between the channel and floodplain influences the timing and location of avulsion over millennial time scales: the time to avulsion is greatly increased. Sedimentation patterns within the valley, modeled and measured, indicate preference toward sandy bed material, and the rates of deposition are substantiated by previous measurements. Although the results here are specific to the Trinity River, the analysis provides a framework that is adaptable to other lowland fluvial-deltaic systems

Anthropogenic controls on overwash deposition: Evidence and consequences

Accelerated sea level rise and the potential for an increase in frequency of the most intense hurricanes due to climate change threaten the vitality and habitability of barrier islands by lowering their relative elevation and altering frequency of overwash. High-density development may further increase island vulnerability by restricting delivery of overwash to the subaerial island. We analyzed pre-Hurricane Sandy and post-Hurricane Sandy (2012) lidar surveys of the New Jersey coast to assess human influence on barrier overwash, comparing natural environments to two developed environments (commercial and residential) using shore-perpendicular topographic profiles. The volumes of overwash delivered to residential and commercial environments are reduced by 40% and 90%, respectively, of that delivered to natural environments. We use this analysis and an exploratory barrier island evolution model to assess long-term impacts of anthropogenic structures. Simulations suggest that natural barrier islands may persist under a range of likely future sea level rise scenarios (7-13mm/yr), whereas developed barrier islands will have a long-term tendency toward drowning

Quantifying the relationship between evaporation and precipitation variations and mangrove health: Insights from a Mangrove Islands in Belize

Along tropical coastal intertidal zones, mangroves provide numerous beneficial ecosystem services such as coastal defense, blue carbon storage, and fauna and flora habitat. Despite their socioeconomic and biological importance, mangroves are under threat due to a number of factors, including deforestation, pollution, and droughts. In particular, droughts result from a deficit in precipitation with respect to evaporation rates over prolonged time periods, which can lead to an increase in the concentration of soil stressors such as sulfide as sulfate and consequently a decrease in mangrove health. To quantify the relative importance of changes in evaporation and precipitation rates on mangrove response with respect to other factors, we analyze a low-lying mangrove island in Belize, where human development, freshwater inputs from land, and overland flow are negligible. First, we determined a normalized difference vegetation index based on Landsat 8 satellite data as a proxy for mangrove health from 2013-2022 and found that mangrove health generally improves during the wet season (June through November) and declines during the dry season (December through May). Second, we extracted precipitation and evaporation rates from publicly available satellite data and found that net evaporation rates (i.e., evaporation minus precipitation rates) are generally negatively correlated with our metric for mangrove health. That is, when the net evaporation rate increases increase, the percentage of mangrove area under stress increases and vice versa. Consistent with previous work, these results suggest that net evaporation rates are a good predictor of mangrove stress and potentially mangrove area loss

Chasing Boundaries and Cascade Effects in a Coupled Barrier-Marsh-Lagoon System

The long-term dynamic evolution of an idealized barrier-marsh-lagoon system experiencing sea-level rise is studied by coupling two existing numerical models. The barrier model accounts for the interaction between shoreface dynamics and overwash flux, which allows the occurrence of barrier drowning. The marsh-lagoon model includes both a backbarrier marsh and an interior marsh, and accounts for the modification of the wave regime associated with changes in lagoon width and depth. Overwash, the key process that connects the barrier shoreface with the marsh-lagoon ecosystems, is formulated to account for the role of the backbarrier marsh. Model results show that a number of factors that are not typically associated with the dynamics of coastal barriers can enhance the rate of overwash-driven landward migration by increasing backbarrier accommodation space. For instance, lagoon deepening could be triggered by marsh edge retreat and consequent export of fine sediment via tidal dispersion, as well as by an expansion of inland marshes and consequent increase in accommodation space to be filled in with sediment. A deeper lagoon results in a larger fraction of sediment overwash being subaqueous, which coupled with a slow shoreface response sending sediment onshore can trigger barrier drowning. We therefore conclude that the supply of fine sediments to the back-barrier and the dynamics of both the interior and backbarrier marsh can be essential for maintaining the barrier system under elevated rates of sea-level rise. Our results highlight the importance of considering barriers and their associated backbarriers as part of an integrated system in which sediment is exchanged

Analytical and Numerical Solution of a Generalized Stefan Problem Exhibiting Two Moving Boundaries With Application to Ocean Delta Formation

A model associated with the formation of sedimentary ocean deltas is presented. This model is a generalized one-dimensional Stefan problem bounded by two moving boundaries, the shoreline and the alluvial-bedrock transition. The sediment transport is a non-linear diffusive process; the diffusivity modeled as a power law of the fluvial slope. Dimensional analysis shows that the first order behavior of the moving boundaries is determined by the dimensionless parameter 0 ≤ Ra b ≤ 1-the ratio of the fluvial slope to bedrock slope at the alluvial-bedrock transition. A similarity form of the governing equations is derived and a solution that tracks the boundaries obtained via the use of a numerical ODE solver; in the cases where the exponent θ in the diffusivity model is zero (linear diffusion) or infinite, closed from solutions are found. For the full range of the diffusivity exponents, 0 ≤ θ → ∞, the similarity solution shows that when Ra b \u3c 0.4 there is no distinction in the predicted speeds of the moving boundaries. Further, within the range of physically meaningful values of the diffusivity exponent, i.e., 0 ≤ θ ∼ 2, reasonable agreement in predictions extents up to Ra b ∼ 0.7. In addition to the similarity solution a fixed grid enthalpy like solution is also proposed; predictions obtained with this solution closely match those obtained with the similarity solution