Low-lying Barrier Island Research Articles

Impacts of Hurricane Matthew on adjacent developed and undeveloped barrier islands in southeastern North Carolina

High-magnitude storms such as hurricanes can cause significant and potentially long-lasting morphological coastal change, particularly along low-lying barrier islands. This study investigated the impacts of Hurricane Matthew (2016) on neighboring undeveloped Masonboro Island reserve and engineered/nourished Wrightsville Beach barrier islands, located in southeast North Carolina. Using a combination of high-resolution pre- and post-storm RTK–GPS beach surveys, coupled with direct observations, storm surge and wave runup calculations and aerial imagery, a range of contrasting storm-induced coastal changes and impact regimes were identified across the two adjacent barriers. Storm impacts were especially pronounced across low-lying undeveloped central/southern Masonboro Island, which was dominated by significant overwash processes, leading to landward directed barrier crest migration. In contrast, only short-lived and minor collision with the dune base was observed at Wrightsville Beach, were storm impacts were dominated by a swash storm regime resulting in significant beach erosion. Field- and aerial based observations match well with modeled extreme water level estimates. This study offers a real-time example of how geomorphologically different neighboring islands respond to specific storm events, and how storm impact regime type and duration helps explain differences in barrier responses. Similar storm impacts are likely at other locations with comparable barrier island settings and differing coastal management approaches.

Physical modeling of progressive damage and failure of wood-frame coastal residential structures due to surge and wave forces

A physical model study was performed to examine the forces and response of 1:6 scale wood-frame coastal residential structures subjected to storm surge and waves. An on-grade and an elevated specimen were tested and exposed to regular waves with varying water depths and wave heights to simulate typical wave/surge conditions resulting from landfall hurricanes on low-lying barrier islands such as Hurricane Sandy that impacted the US East Coast in 2012 and Hurricane Ike that affected the US Gulf Coast in 2008. Results show that through careful design, wood-frame model specimens can be constructed to have similar strength and stiffness compared to full-scale structures. The two specimens were subjected to increasing surge and wave conditions, and the progressive damage was monitored using LiDAR. The on-grade specimen failed at a lower water level than the elevated specimen as expected. Both specimens showed failure modes consistent with the observed damage of residential structures in Ortley Beach, NJ, after Hurricane Sandy and on the Bolivar Peninsula, TX, after Hurricane Ike. The pressure distribution on the underside of the elevated structure and the resulting vertical forces on the on-grade structure were also examined in this study. The pressure distribution and the resulting vertical forces were significantly affected by water depth, wave height, and air gap. The results indicate that maximum vertical forces are positively correlated to wave height and water depth and negatively correlated to the air gap. The breaking type also affects vertical forces. Nonbreaking waves correlated to relatively small vertical forces, and waves breaking directly on the specimen correlated to peak vertical forces. Accelerometer data showed that the specimen decreased in stiffness during the tests due to progressive damage. An uplift pressure distribution equation was developed for an elevated residential structure as a function of wave height and air gap. The equation is generally conservative when compared to other data sets. The results and data presented in this study increase the current knowledge of the interaction between waves and residential structures, which may be useful to increase the resiliency of coastal communities.

Morphodynamic modeling of a low-lying barrier subject to hurricane forcing: The role of backbarrier wetlands

Along much of the world's coastline, coastal barriers serve as the first line of defense against oceanic and meteorological forces. Extreme storms cause large morphological changes on coastal barriers through high sediment transport rates, which may degrade their defensive capabilities. The understanding of morphological impacts is therefore important for coastal resiliency, but is often challenged by site-specific characteristics, such as land cover and sediment availability, and their poorly understood impacts on the governing physical processes. The Caminada Headlands, Louisiana, USA presents unique considerations for morphodynamic modeling with regard to its low-lying topography, variable land cover, nearshore muddy substrate and sand deficiency. This study investigates the effects of land cover and limited sediment supply on low-lying barrier island morphology under storm conditions by using physics-based numerical models. A high-resolution, local-scale sediment transport/morphodynamic model (XBeach) of the Caminada Headlands is verified for Hurricane Gustav's (2008) impact using pre- and post-storm LIDAR surveys. When accurate input data are used to create physics-based numerical models these tools are robust in hindcasting storm impacts and provide a wealth of information as to the governing processes, which is otherwise difficult to obtain observationally. The simulation results show that a short-duration overwash regime dominates the morphological change in this low-lying barrier and is influenced by backbarrier wetland deterioration. The morphological response to overwash is modulated by backbarrier land cover and topography, as reduced accommodation space limits landward transport during the subsequent inundation regime. An intact backbarrier marsh reduces landward washover sediment transport distances and promotes deposition at supratidal elevations. In light of these findings, simultaneous restoration/creation of backbarrier wetlands in conjunction with subaerial beach renourishment may be an effective form of increasing the resiliency of low-lying barriers subject to frequent overwashing.

Hydrogeochemistry of near-surface groundwater on a developing barrier island (Spiekeroog, Germany): The role of inundation, season and vegetation

Low-lying barrier islands present ideal locations to investigate hydrogeochemical processes in sandy coastal aquifers, as they (i) host typical geographic features of the coast, including beaches, dunes, as well as salt marshes, (ii) are susceptible to the impacts of a global climate change, because of their low surface elevations, and (iii) are ubiquitous (>2,100) amongst the global coastlines. The aim of this study was to investigate the evolution of hydrogeochemical patterns in near-surface groundwater on developing barrier islands, using the Ostplate on Spiekeroog Island, Germany, as an example. The impact of inundation, season and vegetation on the hydrogeochemical processes were of particular interest. Six groundwater monitoring wells were installed for this purpose, covering main geographic features of the Ostplate, i.e., supratidal beaches, dunes and salt marshes. Groundwater samples were collected bimonthly from April 2016 to March 2017. Measured parameters included major ions, nutrients, dissolved organic carbon/matter (DOC/DOM), δ13C signatures of dissolved inorganic carbon, as well as δ34S signatures of dissolved sulfate and sulfide. We found major ion compositions to be a function of the annual flooding frequency, although cation concentrations were further impacted by calcite dissolution and cation exchange. The redox chemistry varied at the different sites and through seasons. Aerobic respiration was the dominant pathway for carbon oxidation at the dune locations, whereas organic matter mineralization in the salt marsh was also mediated by anaerobic processes. Regarding electron donors, the presence of labile DOM was linked to salinity, and terrestrial DOM signatures as well as gross inputs of organic carbon agreed with vegetation patterns. A physico-chemical interpretation of the pore water compositions revealed a potential for the authigenic formation of carbonate and phosphate minerals in the investigated sediments, triggered by the redox processes.

Effects of coupled consolidation and overwash processes on a low-lying headland system

Coastal barrier islands serve as the first line of defense against storm-induced hazards and coastal erosion. Due to sea-level rise and more frequently occurring storms, barrier island restoration and rehabilitation have become more important for coastal protection. The current-state-of-the practice for simulation of barrier island restoration design mostly overlooks consolidation as one of the governing processes. However, low-lying barrier islands, especially those built on loose deltaic or estuarine deposits, may further lose elevation due to consolidation and become more vulnerable to erosion. This study investigates the impacts of consolidation without overwash, overwash without consolidation, and coupled consolidation and overwash in Caminada-Moreau Headland in Louisiana, USA, which is built on loose deltaic deposits. Consolidation and overwash processes were simulated by using Settle3D and XBeach, respectively. The coupled consolidation and overwash simulations predicts twice the elevation loss predicted by the simulations that consider overwash or consolidation only indicating the critical role of consolidation in barrier island morphology. Based on the morphologic changes, feasible restoration strategies are developed and tested through numerical experiments. Our analyses indicate that the simultaneous restoration of the headland and back barrier marsh increases dune volume retention and project design criteria longevity when compared to the coupled simulation without a restored marsh.

Field measurements and hydrodynamic modelling to evaluate the importance of factors controlling overwash

Overwash hydrodynamic datasets are mixed in quality and scope, being difficult to obtain due to fieldwork experimental limitations. Nevertheless, these measurements are crucial to develop reliable models to predict overwash. Aiming to overcome such limitations, this work presents accurate fieldwork data on overwash hydrodynamics, further exploring it to model overwash on a low-lying barrier island. Fieldwork was undertaken on Barreta Island (Portugal) in December 2013, during neap tides and under energetic conditions, with significant wave height reaching 2.6 m. During approximately 4 h, more than 120 shallow overwash events were measured with a video-camera, a pressure transducer and a current-meter. This high-frequency fieldwork dataset includes runup, overwash number, depth and velocity. Fieldwork data along with information from literature were used to implement XBeach model in non-hydrostatic mode (wave-resolving). The baseline model was tested for six verification cases; and the model was able to predict overwash in five. Based in performance metrics and the verification cases, it was considered that the Barreta baseline overwash model is a reliable tool for the prediction of overwash hydrodynamics. The baseline model was then forced to simulate overwash under different hydrodynamic conditions (waves and lagoon water level) and morpho-sedimentary settings (nearshore topography and beach grain-size), within the characteristic range of values for the study area. According to the results, the order of importance of factors controlling overwash predictability in the study area are: 1st) wave height (more than wave period) can promote overwash 3–4 times more intense than the one recorded during fieldwork; 2nd) nearshore bathymetry, particularly shallow submerged bars, can promote an average decrease of about 30% in overwash; 3rd) grain-size, finer sediment produced an 11% increase in overwash due to reduced infiltration; and 4th) lagoon water level, only negligible differences were evidenced by changes in the lagoon level. This implies that for model predictions to be reliable, accurate wave forecasts are necessary and topo-bathymetric configuration needs to be monitored frequently.

Legal Issues When Managing Public Roads Affected by Sea Level Rise: Georgia

The low-lying barrier islands and beach communities of Georgia are increasingly vulnerable to sea level rise. Coastal areas near Tybee Island, for example, have already experienced approximately ten inches of sea-level rise since 1935, and this trend is expected to accelerate in the future.1 As low-lying areas like Tybee Island become more frequently inundated, damage to critical infrastructure such as roads will result. Such regular damage will make maintaining roads more costly. To avoid expensive maintenance, decision-makers may face hard questions about whether to continue maintaining public roads or whether it is more cost-effective to abandon them. Sea level rise and coastal flooding will not follow jurisdictional boundaries, and their effects will likely touch all levels of government simultaneously. For Georgia’s coastal communities, adapting to sea level rise will be a group effort that will need to happen at a local level. The state, counties, and municipalities have a legal duty to maintain existing public roads. In cases where a poorly maintained road results in harm to life or property, the government entity could face a lawsuit for failing to maintain the road or negligently performing its responsibilities. Depending on the circumstances, a court may hold that the entity must pay damages or that the entity must continue to maintain the road adequately, or both. This paper first addresses the threshold question of jurisdictional control of roads for government entities faced with questions about maintaining or abandoning roads damaged by increased coastal flooding. It then explains the multi-step analysis that governments must take to evaluate their duties to maintain roads in order to avoid liability. The paper discusses in detail how counties and municipalities can discontinue their duties to maintain roads through the process of abandonment. Lastly, this paper discusses how abandonment can lead to takings liability. This paper is not intended to provide legal advice. Rather, it is a policy paper designed to highlight potential legal questions that are likely to arise in the context of community adaptation to sea level rise. Communities facing such questions should consult with their legal counsel when considering adaptation actions.

POST-STORM DUNE RECOVERY IN CAPE LOOKOUT NATIONAL SEASHORE, NC

Coastal dunes are often the first and primary form of defense against destructive surge and waves that accompany extreme storm events. Beach grasses are known to affect dune height, width, and stability, contributing to the dune’s ability to protect the hinterland from wave and flooding hazards (Hacker et al. 2012). However, the interaction and feedbacks between dune development and properties of beach grasses (e.g., species, density) is not fully understood. In particular, our knowledge of the ecomorphodynamic processes controlling the recovery of coastal dunes following storms and the long-term ability of dunes to adapt to changes in climate remains inadequate. The objective of this interdisciplinary research is to characterize the temporal and spatial variability of coastal foredune recovery following major storm events and the subsequent impact of this recovery on future vulnerability. The study region consists of three low-lying barrier islands within the Cape Lookout National Seashore (CALO) along the central coast of North Carolina. The 90 km stretch of coast exhibits spatial variability in dominant dune grass species, grass cover density, coast orientation, beach slope, and wave energy. Using physical and ecological field datasets and process-based numerical modeling, post-storm dune recovery is assessed following Hurricane Matthew (2016).

Raising New Orleans: Historical Analogs and Future Environmental Risks

Raising New Orleans: Historical Analogs and Future Environmental Risks

Numerical modeling of the morphodynamic response of a low-lying barrier island beach and foredune system inundated during Hurricane Ike using XBeach and CSHORE

Follet's Island (FI) is a sediment-starved barrier island located on the Upper Texas Coast; a stretch of coastline along the Gulf of Mexico that experiences on average four hurricanes and four tropical storms per decade. During Hurricane Ike, water levels and wave heights at FI exceeded the 100-year and 40-year return values, respectively, leading to significant overtopping and morphology changes of this low-lying barrier island. The physical processes governing the real-time morphodynamic response of the beach and dune system during 96h of hurricane impact were modeled using XBeach (2D) and CSHORE (1D). Hydrodynamic boundary conditions were obtained from ADCIRC/SWAN model runs validated with measured buoy and wave gauge data while LiDAR surveys provided pre- and post-storm measured topography.XBeach displayed a decent model skill of 0.34 and provided numerical outputs of the entire 2D domain such as topography, suspended sediment load and bed load which was very useful in visualizing erosion and deposition patterns. CSHORE also displayed a decent model skill of 0.33 and was able to accurately predict the post-storm beach slope and shoreline, but was less effective at simulating the foredune morphology. Modeling results show that the complete morphodynamic response of FI to Hurricane Ike was governed by a sequence of impact regimes, including swash, collision, overwash, inundation, and storm surge ebb.

Numerical simulation of a low-lying barrier island's morphological response to Hurricane Katrina

Tropical cyclones that enter or form in the Gulf of Mexico generate storm surge and large waves that impact low-lying coastlines along the Gulf Coast. The Chandeleur Islands, located 161 km east of New Orleans, Louisiana, have endured numerous hurricanes that have passed nearby. Hurricane Katrina (landfall near Waveland MS, 29 Aug 2005) caused dramatic changes to the island elevation and shape. In this paper the predictability of hurricane-induced barrier island erosion and accretion is evaluated using a coupled hydrodynamic and morphodynamic model known as XBeach. Pre- and post-storm island topography was surveyed with an airborne lidar system. Numerical simulations utilized realistic surge and wave conditions determined from larger-scale hydrodynamic models. Simulations included model sensitivity tests with varying grid size and temporal resolutions. Model-predicted bathymetry/topography and post-storm survey data both showed similar patterns of island erosion, such as increased dissection by channels. However, the model under predicted the magnitude of erosion. Potential causes for under prediction include (1) errors in the initial conditions (the initial bathymetry/topography was measured three years prior to Katrina), (2) errors in the forcing conditions (a result of our omission of storms prior to Katrina and/or errors in Katrina storm conditions), and/or (3) physical processes that were omitted from the model (e.g., inclusion of sediment variations and bio-physical processes).

Effect of Hurricane Ivan on Coastal Dunes of Santa Rosa Barrier Island, Florida: Characterized on the Basis of Pre- and Poststorm LIDAR Surveys

Abstract Santa Rosa Island, situated along the northwestern Florida coast facing the Gulf of Mexico, is an 85-km-long wave-dominated low-lying barrier island with well-developed incipient and established dunes. In this paper, we examine the regional-scale effect on coastal dunes by a strong category 3 hurricane, Ivan, through comparison of pre- and poststorm airborne LIDAR (light detecting and ranging) surveys. On the basis of pre-Ivan LIDAR survey data, the elevation of the berm and back beach is typically 2.0 m above MSL (mean sea level). Incipient dunes range from 2.5 to 10 m above MSL, or 0.5 to 8.0 m above the surrounding beach. The hummocky dunes that developed over relic washover platforms are typically less than 4.0 m above MSL. The densely vegetated, established dune fields are composed of dunes less than 7.0 m high and intradune wetlands lying at less than 1.0 m above MSL. The entire island was severely affected by Ivan, which made landfall about 45 km to the west in September 2004. The landscap...

Factors controlling the survival of coastal dunes during multiple hurricane impacts in 2004 and 2005: Santa Rosa barrier island, Florida

Santa Rosa Island is an 85 km-long, wave-dominated low-lying barrier island situated along the northwestern Florida coast, facing the Gulf of Mexico. The entire island was severely impacted by Ivan, a strong category 3 hurricane that made landfall about 45 km to the west in September of 2004. Ten months later in July of 2005, Dennis, another category 3 hurricane, made landfall about 30 km east of the western tip of the island. Santa Rosa Island is characterized by well-developed but relatively low dunefields, described in this paper as incipient and established dunes, based on the presence of grassy and woody types of vegetation, respectively. The dunes were severely eroded by the two hurricanes. This paper investigates the factors controlling the regional-scale destruction and survival of the dunefields. Dune survival is controlled by: 1) hurricane characteristics, including intensity, duration, and frequency, and 2) morphological parameters including width of the barrier island, height and width of the dunefields, vegetation type, distance of the dunes to the ocean, and continuity of the dunefields. Three processes of dune destruction are described including, from most to least severe, inundation, overwash, and scarping. The interaction of all the above factors determines the different dune responses to the storm impacts. In general, the extensive and densely woody vegetated dunefields near the bay-side shoreline survived the storms, while the discontinuous dunes with grassy vegetation near the Gulf shoreline were almost completely destroyed.

Abstract: The Importance of Cyclogenesis on the Short-Term Evolution of Gulf Coast Barriers

ABSTRACT Over the past several years, research has been conducted to determine the importance of cyclones on the morphodynamics and morphological maintenance of barrier islands along the northern Gulf of Mexico. Data are presented indicating the complexity and highly variable response of a 12 km stretch of barrier along the Florida panhandle to cold front passage across the northern Gulf, in addition to the impacts of tropical storms, hurricanes and winter storms in the Gulf of Mexico. Two temporal scales are used to evaluate short-term response and adjustment to these events, i.e., months and a near five year record of sediment volume change, the combination of which have resulted in a time series of change for the subaerial and nearshore environments. These data incorporate the morphological signature of six tropical storms/hurricanes and approximately 100 frontal passages. Three important conclusions have been reached based on these data: (1) low-lying barrier islands, which are vulnerable to overwash, can conserve mass during catastrophic hurricanes (e.g., Hurricane Opal, a strong category 4 hurricane near landfall); (2) less severe hurricanes can promote rapid dune and berm aggradation and contribute sediment to the entire barrier system; and (3) cold fronts play a critical role in the post-storm adjustment of the barrier by deflating the subaerial portion of the overwash terrace and eroding its marginal lobe. The data presented also indicate that barrier systems along the northern Gulf do not necessarily enter an immediate post-storm recovery phase after catastrophic hurricanes, even though they are nested in sediment-rich nearshore environments. It is anticipated that these findings will have important implications for better understanding the longer-term evolution of barrier systems in mid-latitude, micro-tidal settings elsewhere around the globe.

SEDIMENTATION PATTERNS IN A TIDAL INLET SYSTEM MORICHES INLET, NEW YORK

A detailed study of the historical development of Moriches Inlet, Long Island (New York), was completed to determine the morphodynamic interaction of tidally influenced processes and the effects of man-made alterations on the inlet channel and affiliated flood and ebbtidal deltas. The south shore of Long Island in the vicinity of Moriches Inlet is a microtidal, wave-dominated (mixed-energy) environment. Characteristic of this setting, the inlet bisects a low-lying barrier island backed by an open bay and has a prominent flood-tidal delta. In this study, bathymetric charts of the inlet, bay, and barrier nearshore zones were contoured for analysis. Using a polar planimeter, the areas between isobaths from mean high water to the bottom surface were measured. This information was used to determine the volumetric distribution of sediment and water in the system. In addition, harmonic analysis was applied on digitized bay-tide records to ascertain the relationships of the semidiurnal overtide constituents. From the results, mean rise/fall duration differences, related to the conservation of mass, were calculated. The results of the quantitative spatial analysis indicate that Moriches Inlet was flood-dominant from breach in 1931 until closure in 1951. After reopening in 1953, the inlet became ebb-dominant as a result of inlet stabilization and extensive dredge-and-spoil operations in the inlet and bay. Good correlations exist between net sedimentation in the inlet with (1) bay-surface area change, (2) water storage capabilities of the flood-tidal delta, and (3) the cross-sectional area of the inlet. The relationship among these variables suggests that there is a system-wide response to the change in the hydrodynamics caused by man-made alterations, resulting in ebb dominance.

Numerical Model for Storm Surges in Galveston Bay

A numerical scheme for computation of two-dimensional, nonlinear storm surges and tides is applied to Galveston Bay, Tex. Allowance is made for the overflow of low-lying barrier islands and for the flooding of and recession from low-lying land areas adjacent to the bay. The initial surge or tide, or both, at the seaward end of the bay outside the barrier islands is prescribed. The system has been calibrated for spring tide conditions and for the surge produced by hurricane Carla (September, 1961). Verification of the system is carried out for hurricane Cindy (September, 1963).