Models of alongshore sediment transport during quiescent conditions, storm-driven barrier island morphology, and poststorm dune recovery are integrated to assess decadal barrier island evolution under scenarios of increased sea levels and variability in storminess (intensity and frequency). Model results indicate barrier island response regimes of keeping pace, narrowing, flattening, deflation (narrowing and flattening), and aggradation. Under lower storminess scenarios, more areas of the island experienced narrowing due to collision. Under higher storminess scenarios, more areas experienced flattening due to overwash and inundation. Both increased sea levels and increased storminess resulted in breaching when the majority of the island was not keeping pace and deflation was the dominant regime due to increased overtopping. Under the highest storminess scenario, the island was unable to recover elevation after storms and drowned in just 10 years. Plain Language Summary Barrier islands protect mainland coastal communities during storms. In the future, the effects of storms and sea level rise (SLR) threaten barrier islands with increased inundation and loss of land. Barrier islands can keep pace with SLR by moving sand across the island during storm events to maintain height and width. However, if storms are too intense or sea levels are too high, the island may drown. This study uses computational models to assess the future response of a barrier island to higher sea levels and changes in frequency and intensity of storms (storminess). We found that the barrier island exhibits five behaviors in response to storms and SLR: keeping pace by maintaining height and width, losing width but maintaining height, losing height but maintaining width, losing height and width, and gaining height and width. These behaviors shifted based on the amount of SLR and storminess. Both increased SLR and increased storminess resulted in less of the island keeping pace and more of the island losing height and width; in some cases, this caused channels to be cut through the island. Under the most frequent and intense storm scenarios, the island lost significant amounts of land and was unable to recover.
Histogram showing the distribution of number of tropical cyclones (for 10-year period) across the realizations for each storminess bin, as well as the total distribution across all 1,000 realizations .
Barrier islands are dynamic environments that experience gradual change from waves, tides, and currents, and rapid change from extreme storms. These islands are expected to change drastically over the coming century due to accelerated sea-level rise and changes in frequency and intensity of storm events. The dynamic nature of barrier islands coupled with the importance of these environments make it critical for natural resource managers to understand how habitats on barrier islands are changing or may change over time to determine when and where management actions may be needed. In this study, we applied a habitat change assessment framework, which included exploring areal coverage and distribution changes and change component analysis. Change component analysis, which breaks differences into net gain/loss and allocation difference (i.e., habitat oscillation), has not previously been used to study barrier island habitat evolution. Here, we demonstrate the approach using habitat predictions from a geomorphic modeling effort on Dauphin Island, Alabama (USA). We explored differences of habitat predictions for potential island configurations with and without a beach and dune restoration action under future conditions related to sea level and storminess. We found a potential linkage between landward migration of barrier islands and exchange, an output of change component analysis. The hypothesis may be tested to explore whether this linkage applies over space and time and whether the approach is applicable to monitoring landward migration of coastal wetlands. Collectively, our results highlight the utility of change component analysis for monitoring and quantifying barrier island habitat change and migration.
The U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency are changing their perception of dredged material, from a byproduct of the dredging process to a valuable resource. The negative perception of navigation dredged material is codified under the 1972 Clean Water Act Section 502, which specifically defines "dredge spoils" as a pollutant, along with solid waste, sewage, and garbage. However, navigation dredged material is typically a mixture of sand, silt, clay, and possibly gravel. These sediments resources are critical to controlling flood risks and providing environmental benefits. This document provides details regarding the use of dredged material to support NNBF through strategic placement. Strategic placement is the process of placing sediment at one location with the expectation that hydrodynamic and possibly aerodynamic forces will transport specified classes of that sediment to desired locations. Strategic placement is a beneficial use option that may have less negative impact on the final receptor sites and often can be performed at a reduced cost when compared to direct placement (such as beach nourishment). Cost controls are critical to developing sustainable dredged sediment management plans that address the Federal Standard, which guides the disposal and placement of dredged material.
Coastal ecosystem management typically relies on subjective interpretation of scientific understanding, with limited methods for explicitly incorporating process knowledge into decisions that must meet multiple, potentially competing stakeholder objectives. Conversely, the scientific community lacks methods for identifying which advancements in system understanding would have the highest value to decision-makers. A case in point is barrier island restoration, where decision-makers lack tools to objectively use system understanding to determine how to optimally use limited contingency funds when project construction in this dynamic environment does not proceed as expected. In this study, collaborative structured decision-making (SDM) was evaluated as an approach to incorporate process understanding into mid-construction decisions and to identify priority gaps in knowledge from a management perspective. The focus was a barrier island restoration project at Ship Island, Mississippi, where sand will be used to close an extensive breach that currently divides the island. SDM was used to estimate damage that may occur during construction, and guide repair decisions within the confines of limited availability of sand and funding to minimize adverse impacts to project objectives. Sand was identified as more limiting than funds, and unrepaired major breaching would negatively impact objectives. Repairing minor damage immediately was determined to be generally more cost effective (depending on the longshore extent) than risking more damage to a weakened project. Key gaps in process-understanding relative to project management were identified as the relationship of island width to breach formation; the amounts of sand lost during breaching, lowering, or narrowing of the berm; the potential for minor breaches to self-heal versus developing into a major breach; and the relationship between upstream nourishment and resiliency of the berm to storms. This application is a prototype for using structured decision-making in support of engineering projects in dynamic environments where mid-construction decisions may arise; highlights uncertainty about barrier island physical processes that limit the ability to make robust decisions; and demonstrates the potential for direct incorporation of process-based models in a formal adaptive management decision framework.
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