Jorge Lorenzo-Trueba

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 (7–13 mm/yr), whereas developed barrier islands will have a long-term tendency toward drowning.

Follets Island: A Case of Unprecedented Change and Transition from Rollover to Subaqueous Shoals

Follets Island, a transgressive island located on the upper Texas coast, is an ideal location to study barrier island transition from a rollover subaerial barrier to subaqueous shoals. This system also allows for an examination of coastal response to accelerated sea-level rise, storms, and sediment supply. The landward shoreline retreat rate during historical time is similar to the landward retreat rate of the bay shoreline, hence its current classification as a rollover barrier. However, the island has a limited and diminishing sand supply, which makes it even more vulnerable to erosion during storms and relative sea-level rise. Four core transects that extend from the upper shoreface to the back barrier bay are used to constrain the thickness of washover, barrier and upper shoreface deposits and to estimate sediment fluxes in the context of the overall sand budget for the island over centennial timescales. Stratigraphic architecture reveals two prominent transgressive surfaces. A lower flooding surface separates red fluvial-deltaic clay from overlying bay mud and an upper erosional surface separates back-barrier deposits from overlying shoreface and foreshore deposits.

Morphodynamics of Barrier Response to Sea-Level Rise

Barrier response to sea-level rise involves a dynamic interplay between the shoreface and the subaerial portion affected by overwashing. Focusing on feedbacks between these two, here we discuss a morphodynamic approach to modeling barrier transgression. In contrast with the steady transgression portrayed by morphokinematic models (which transport mass based on geometric considerations), a simple morphodynamic model predicts two modes of long-term barrier failure: width and height drowning. For barriers that survive sea-level rise, a most likely mode of barrier motion consists of punctuated and abrupt periodic transgression of the shelf, which can arise even from constant driving conditions. These intermittently migrational barriers spend most of their existence staying essentially in place, a stark contrast to the continuous behavior suggested by morphokinematic models of barrier retreat. Even small perturbations to a barrier system traversing the shelf in dynamic equilibrium can kick-start an oscillating retreat. Looking alongshore, shoreline interconnectivity can have a significant effect on shoreline behavior across both space and time. Overall, our morphodynamic modeling results motivate a need to investigate the internal dynamics of barrier systems to understand the full range of past and potential future response of barrier systems to sea-level rise.

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 (4.3 mm yr−1), the Trinity fluvial-deltaic system progrades 13 m yr−1, followed by backstepping of 27 m yr−1. 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.

CHASING BOUNDARIES AND CASCADE EFFECTS IN A COUPLED BARRIER — MARSHES — 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.

Impacts of Hurricane Storm Surge on Infrastructure Vulnerability for an Evolving Coastal Landscape

AbstractPredicting coastal infrastructure reliability during hurricane events is important for risk-based design and disaster planning, including delineating viable emergency response routes. Previ...

Natural and Human-Induced Variability in Barrier-Island Response to Sea Level Rise: Barrier Island Response to Sea Level Rise

Storm-driven sediment fluxes onto and behind barrier islands help coastal barrier systems keep pace with sea level rise (SLR). Understanding what controls cross-shore sediment flux magnitudes is critical for making accurate forecasts of barrier response to increased SLR rates. Here, using an existing morphodynamic model for barrier island evolution, observations are used to constrain model parameters and explore potential variability in future barrier behavior. Using modeled drowning outcomes as a proxy for vulnerability to SLR, 0%, 28%, and 100% of the barrier is vulnerable to SLR rates of 4, 7, and 10 mm/yr, respectively. When only overwash fluxes are increased in the model, drowning vulnerability increases for the same rates of SLR, suggesting that future increases in storminess may increase island vulnerability particularly where sediment resources are limited. Developed sites are more vulnerable to SLR, indicating that anthropogenic changes to overwash fluxes and estuary depths could profoundly affect future barrier response to SLR. Plain Language Summary Barrier islands, thin strings of islands offshore of mainland coasts, are the first line of defense for protecting estuaries and mainland population centers from storms. They are also important for tourism that drives many coastal economies. Sand movement to the top of and across barrier islands is how they keep pace with sea level rise (SLR), so restrictions to those processes may make barrier islands more vulnerable to SLR effects. In our study, we used observations from New Jersey, USA, as inputs to a model that forecasts barrier island changes in response to SLR. This is particularly important for New Jersey, which is expected to experience rates of relative SLR that are higher than average. We found that 28% of the barrier island was vulnerable to a moderate rate of SLR and 100% of the barrier island was vulnerable to a high rate of SLR. Furthermore, we found that barrier island vulnerability increased in heavily populated locations relative to less populated locations. This suggests that human changes to coastal systems likely impact the lifespan of barrier islands. If some barrier islands degrade faster than others, their ability to protect mainland coasts and sustain coastal communities and economies could be compromised.