Dune Erosion Research Articles

Modélisation de l’impact du changement climatique sur l’érosion des dunes. Application à la Camargue

Three climatic changes scenariis were investigated by an increase of energy and by the duration of an extreme storm. The wave caracteristics properties and the sea level time series of the referenced storm were increased of about 5, 10 and 20 % in order to define the dune erosion. We evidenced that the wave height displays more influence on the dune erosion than the sea level. Moreover, the storm duration, over 4 days, does not play an important role on the dune erosion. However, there is no proportional relationships between a weak storm energy increase and/or duration, and dune erosion. A small increase of storm will have a large impact on dune erosion in the future.

Rip currents, mega-cusps, and eroding dunes

Dune erosion is shown to occur at the embayment of beach mega-cusps O(200 m alongshore) that are associated with rip currents. The beach is the narrowest at the embayment of the mega-cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune, to undercut the dune and to cause dune erosion. Field measurements of dune, beach, and rip current morphology are acquired along an 18 km shoreline in southern Monterey Bay, California. This section of the bay consists of a sandy shoreline backed by extensive dunes, rising to heights exceeding 40 m. There is a large increase in wave height going from small wave heights in the shadow of a headland, to the center of the bay where convergence of waves owing to refraction over the Monterey Bay submarine canyon results in larger wave heights. The large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale. The strongly refracted waves and narrow bay aperture result in near normal wave incidence, resulting in well-developed, persistent rip currents along the entire shoreline. The alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95% confidence. The alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95% confidence. Therefore, it is concluded the mega-cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega-cusp.

A simple model for the spatially-variable coastal response to hurricanes

The vulnerability of a beach to extreme coastal change during a hurricane can be estimated by comparing the relative elevations of storm-induced water levels to those of the dune or berm. A simple model that defines the coastal response based on these elevations was used to hindcast the potential impact regime along a 50-km stretch of the North Carolina coast to the landfalls of Hurricane Bonnie on August 27, 1998, and Hurricane Floyd on September 16, 1999. Maximum total water levels at the shoreline were calculated as the sum of modeled storm surge, astronomical tide, and wave runup, estimated from offshore wave conditions and the local beach slope using an empirical parameterization. Storm surge and wave runup each accounted for ∼ 48% of the signal (the remaining 4% is attributed to astronomical tides), indicating that wave-driven process are a significant contributor to hurricane-induced water levels. Expected water levels and lidar-derived measures of pre-storm dune and berm elevation were used to predict the spatially-varying storm-impact regime: swash, collision, or overwash. Predictions were compared to the observed response quantified using a lidar topography survey collected following hurricane landfall. The storm-averaged mean accuracy of the model in predicting the observed impact regime was 55.4%, a significant improvement over the 33.3% accuracy associated with random chance. Model sensitivity varied between regimes and was highest within the overwash regime where the accuracies were 84.2% and 89.7% for Hurricanes Bonnie and Floyd, respectively. The model not only allows for prediction of the general coastal response to storms, but also provides a framework for examining the longshore-variable magnitudes of observed coastal change. For Hurricane Bonnie, shoreline and beach volume changes within locations that experienced overwash or dune erosion were two times greater than locations where wave runup was confined to the foreshore (swash regime). During Hurricane Floyd, this pattern became more pronounced as magnitudes of change were four times greater within the overwash regime than in the swash regime. Comparisons of pre-storm topography to a calm weather survey collected one year after Hurricane Floyd's landfall show long-term beach volume loss at overwash locations. Here, the volume of sand eroded from the beach was balanced by the volume of overwash deposits, indicating that the majority of the sand removed from the beach was transported landward across the island rather than being transported offshore. In overwash locations, sand was removed from the nearshore system and unavailable for later beach recovery, resulting in a more permanent response than observed within the other regimes. These results support the predictive capabilities of the storm scaling model and illustrate that the impact regimes provide a framework for explaining the longshore-variable coastal response to hurricanes.

Defining the storm erosion hazard for beaches

This paper presents a rational protocol for quantifying the dune erosion hazard resulting from coastal storms. The method is based on the application of calibrated and verified numerical models with the chosen level of risk based on long term wave data. A “design storm” is synthesised from long term wave data records of storm wave heights of various durations. The wave energy from such a storm is transformed from offshore to nearshore using a two-dimensional wave-refraction (and/or diffraction) modelling program. This provides appropriate nearshore wave conditions as input for a dune erosion model. The dune erosion model is calibrated and verified against ground survey data, usually obtained using photogrammetry. The storm erosion hazard risk assessment may then be quantified by the application of a chosen annual exceedance probability (AEP) wave height/duration event to the validated dune erosion model. This allows a “design” storm-bite to be determined at any location along a beach and it allows coastal managers to choose the level of coastal risk for existing and future foreshore development, based on an AEP approach.

Numerical Study of Dune Erosion Induced by Tsunami

Coastal dunes are natural bed forms caused by wind and wave actions. They run parallel to the shoreline close to the beach and thus have an important role to protect land against external force due to tsunami. To evaluate such protective effects of sand dunes, their erodible property must be considered since some part of them might be washed out during tsunami flooding. In this paper, a numericalm odelf or dune deformationi nducedb y tsunami is presented. The model is based on the Boussinesq-type equation coupled with the sediment advection equation and existing empirical formulae of sediment transport rate. The model performanc is confirmed through comparison with available laboratory data. Finally, the dune erosion induced by tsunami is discussed on the basis of the model application.

Hurricanes 2004: An overview of their characteristics and coastal change

Four hurricanes battered the state of Florida during 2004, the most affecting any state since Texas endured four in 1884. Each of the storms changed the coast differently. Average shoreline change within the right front quadrant of hurricane force winds varied from 1 m of shoreline advance to 20 m of retreat, whereas average sand volume change varied from 11 to 66 m3 m−1 of net loss (erosion). These changes did not scale simply with hurricane intensity as described by the Saffir-Simpson Hurricane Scale. The strongest storm of the season, category 4 Hurricane Charley, had the least shoreline retreat. This was likely because of other factors like the storm's rapid forward speed and small size that generated a lower storm surge than expected. Two of the storms, Hurricanes Frances and Jeanne, affected nearly the same area on the Florida east coast just 3 wk apart. The first storm, Frances, although weaker than the second, caused greater shoreline retreat and sand volume erosion. As a consequence, Hurricane Frances may have stripped away protective beach and exposed dunes to direct wave attack during Jeanne, although there was significant dune erosion during both storms. The maximum shoreline change for all four hurricanes occurred during Ivan on the coasts of eastern Alabama and the Florida Panhandle. The net volume change across a barrier island within the Ivan impact zone approached zero because of massive overwash that approximately balanced erosion of the beach. These data from the 2004 hurricane season will prove useful in developing new ways to scale and predict coastal-change effects during hurricanes.

Dynamics of intertidal gravel dunes

This paper examines the dynamics of intertidal gravel dunes subjected to high‐energy unsteady and reversing tidal currents in the Severn Estuary, UK. The dunes were composed of shale particles with median grain size, D50, around 4 mm and had mean heights and wavelengths of 60 cm and 7 m, respectively. Acoustic instruments were deployed above a dune to measure the streamwise profile and flow turbulence. Radar was used to map a section of the dune field on two occasions. Measurements during the peak spring ebb tides showed that when the measured bed shear stress exceeded the threshold for entrainment of the coarse surface armor, sediments were eroded from the upper surface of the dune. The dune prograded in the direction of the ebb flow a distance O(1 m), and some eroded sediments were deposited on the stoss slope downstream. The dune was then reworked during the following flood tide to reestablish approximately the same pre‐ebb profile shape and location. Radar data showed also that the exposed dunes at low water occupied approximately the same location over a period of 4 days during peak spring tides. The relationship between the depth‐mean current and the net volume of dune sediments moved was found to conform to a power law. The predicted critical bed shear stress, τcrit, for D50 = 4 mm was approximately 1 N/m2. However, owing to a coarse armor layer on the surface of the dune, a measured τcrit value O(4 N/m2) was required to initiate dune erosion. Although the Van Rijn formula was found to overestimate both the height and wavelength of dunes at this site, the dunes appear to be equilibrium bed forms controlled by local hydraulics and sediment supply. Sediment transport predicted by bed load formula were within a factor of two of the observed mass transport of dune sediments for ebb tides.

Sand mining impacts on long-term dune erosion in southern Monterey Bay

Southern Monterey Bay was the most intensively mined shoreline (with sand removed directly from the surf zone) in the U.S. during the period from 1906 until 1990, when the mines were closed following hypotheses that the mining caused coastal erosion. It is estimated that the yearly averaged amount of mined sand between 1940 and 1984 was 128,000 m 3/yr, which is approximately 50% of the yearly average dune volume loss during this period. To assess the impact of sand mining, erosion rates along an 18 km range of shoreline during the times of intensive sand mining (1940–1990) are compared with the rates after sand mining ceased (1990–2004). Most of the shoreline is composed of unconsolidated sand with extensive sand dunes rising up to a height of 46 m, vulnerable to the erosive forces of storm waves. Erosion is defined here as a recession of the top edge of the dune. Recession was determined using stereo-photogrammetry, and LIDAR and GPS surveys. Long-term erosion rates vary from about 0.5 m/yr at Monterey to 1.5 m/yr in the middle of the range, and then decrease northward. Erosion events are episodic and occur when storm waves and high tides coincide, allowing swash to undercut the dune and resulting in permanent recession. Erosion appears to be correlated with the occurrence of El Niños. The calculated volume loss of the dune in southern Monterey Bay during the 1997–98 El Niño winter was 1,820,000 m 3, which is almost seven times the historical annual mean dune erosion of 270,000 m 3/yr. The alongshore variation in recession rates appears to be a function of the alongshore gradient in mean wave energy and depletions by sand mining. After cessation of sand mining in 1990, the erosion rates decreased at locations in the southern end of the bay but have not significantly changed at other locations.

Flood-risk mapping for storm-surge events and sea-level rise using lidar for southeast New Brunswick

Coastal flooding from storm-surge events and sea-level rise is a major issue in Atlantic Canada. Airborne light detection and ranging (lidar) has the spatial density and vertical precision required to map coastal areas at risk of flooding from water levels typically 1–2 m higher than predicted tides during storm surges. In this study, a large section of the New Brunswick coast along Northumberland Strait was surveyed in 2003 and 2004 using two lidar systems. Water levels from a major storm-surge event in January 2000 were surveyed using a global positioning system (GPS) and used as a benchmark for flood-risk maps. Maps of flood depth were also generated for all water levels and used for socioeconomic and ecosystem impact assessment. Flood-risk maps were constructed using standard geographical information system (GIS) processing routines to determine the spatial extent of inundation for a given water level. The high resolution of the lidar digital elevation model (DEM) captured embankments such as raised roadbeds that could prevent flooding inland. Where connectivity was present due to culverts or bridges, the DEM was notched across the roadbed to simulate the connection between the ocean and upstream low-lying areas in the GIS. An automated routine was then used to generate maps of flood extent for water levels at 10 cm increments from 0 to 4 m above mean sea level. Validation of the flood-risk and flood-depth maps for the January 2000 storm-surge water level by field visits indicates that the simulations are generally accurate to within 10–20 cm. The lidar data were also used to evaluate the potential for overtopping and dune erosion on a large coastal spit, La Dune de Bouctouche. This showed a high vulnerability to storm damage for critical habitats on the spit. The lidar-derived maps produced in this study are now available to coastal communities and regional planners for use in the planning process and to assist in development of long-term adaptation strategies.

Reconciling Foreshore Development and Dune Erosion on Three Queensland Beaches: An Historical Perspective

Abstract Yeppoon Main beach, Kinka beach and Keppel Sands beach are nestled around Keppel Bay in Queensland, Australia. Before white settlements were developed adjacent to these beaches, they were comprised of extensive sand dune systems operating in dynamic equilibrium. Over the past century, there has been a popular misunderstanding about the way beach systems work, evident in all kinds of foreshore development along these beaches. This has led to a delayed but cascading effect of worsening dune and beach erosion. Once sea walls and other kinds of foreshore construction begin, historically it becomes unlikely that the authorities will stop and reverse such development. Rather, more expensive construction becomes the remedy for the earlier mistakes as the local government attempts to reconcile coastal development and dune erosion. There is also the issue of how sand was regarded and whether that was a motivating factor for its removal. Was sand perceived as a 'nuisance' and 'readily renewable', and therefore dispensable? Of this trio of beaches, Kinka beach is fortunate to have experienced the earliest implementation of so called soft protection that initiated a natural renourishment resulting in the sea wall mostly disappearing under a build up of sand. This paper examines the historical development, as well as the motivation behind this development, of these three beaches and the subsequent implications for the beaches during the inevitable big seas and cyclones. This coastal environmental history informs local coastal communities about the importance of foresight in protecting dune systems in their natural state.

Phytolith assemblages as indicators of coastal environmental changes and hurricane overwash deposition

We demonstrate that phytolith assemblages are a useful proxy for reconstructing coastal environmental changes and for validating the overwash origin of sand layers in palaeotempestology studies. Phytolith analysis was conducted on 50 topsoil or surface sediment samples collected from a variety of coastal plant communities or depositional environments in the southeastern USA. The data suggest that different coastal subenvironments can be distinguished by their modern phytolith assemblages. For example, coastal marsh samples contain a diverse phytolith assemblage dominated by smooth-elongate and square morpho-types and maritime forest samples are dominated by phytoliths from palms and broadleaf dicotyledonous plants. Remarkably, the phytolith assemblages from sand dunes are characterized by high percentages of two-horned-tower, flat-tower, spool/horned-tower and short-saddle types. Phytolith analysis of three prehistoric sand layers in a sediment core from Western Lake, northwestern Florida, shows that they contain a phytolith assemblage similar to those characteristic of sand dunes and interdune meadows. These observations are confirmed by the results of principal components analysis and discriminant analysis on the modern and fossil phytolith data sets. Our study results support the interpretation that the sand layers in Western Lake were indeed formed by the erosion of sand dunes during overwash processes caused by landfalling catastrophic hurricanes.

Vulnerability of Poland's Coast to Sea-Level Rise

Over the last decades, the Polish coast, about 500 km long and predominantly featured by sandy, low-lying beaches, has been exposed to various threats resulting from intensified climate change and Accelerated Sea-Level Rise (ASLR). This has manifested itself in the growing intensity of shoreline, dune and cliff erosion and by the increasing necessity of their protection.The current study is both a summary and an extension of the existing Polish studies and analyses on the present and predicted influence of ASLR on the coast. First, the Polish coast was divided into three basic area types (AREA I, II, III) according to their geographic and socio-economic background. Then, two different scenarios of accelerated sea-level rise (ASLR1 — 30 cm/100 yrs and ASLR2 — 100 cm/100 yrs) were assumed. After that threats of land loss and the risk of its temporary or partial inundation was analyzed in connection with the assessment of the material and social costs and losses. These analyses were made within the framework of two adaptation scenarios, i.e. retreat (do nothing) and full protection.The performed analyses have shown that the greatest threat of partial or full land loss and the associated material and social costs are expected to occur in two regions of the Polish coast. One of them is situated in the eastern sector of the Polish coast and covers the agglomeration of Gdańsk and the Żulawy polders. The other is located in the west and comprises low-lying areas around the Szczecin Lagoon and the vicinity of the Odra river mouth. These areas both require intensive care and protection efforts.

Determining Seawall Crest Levels Using RiskAnalysis Methodology

PARASODE-BALI (Probabilistic Assessment of Risks Associated with Seawall Overtopping, Dune Erosion and Breakwater Armour Layer Instability) is a software package which employs the Level II First Order Reliability Method (FORM) for the design and safety assessment of sea defences. This paper provides an example of the use of PARASODE-BALI in designing a seawall subject to a combination of swell and locally-generated wave conditions. It illustrates the different results obtained from two overtopping models which it can employ to predict the minimum crest level necessary to limit seawall overtopping to some specified permissible value. Comparisons between the models show that, for large discharges associated with structural safety, they give similar results. However, the differences are greater for the small discharges commonly required to ensure the safety of people and property in urban areas. The paper also lists many of the wide range of consequences and potential effects of wave overtopping in urban areas.

Le formazioni boschive litoranee in provincia di Matera: funzioni e problematiche gestionali

The origin, current conditions and the environmental role of the costal forest growing on sand dunes along the Ionian coast (province of Matera, Basilicata, Italy) are discussed. Mostly planted in the years 1950-1970, these stands (mainly made up by Pinus halepensis) assure an important windbreak function, protecting the inland cultivations from dangerous marine winds, represent a typical landscape marker and currently host important turistic resorts. The main management issues concern: erosion of coastal sand dunes; recurrent disturbances by fires and grazing; inadequate silvicultural treatments (lack of thinnings), often causing excessive stand density and tree mortality.

Use of Remote Sensing Techniques to Determine the Effects of Grazing on Vegetation Cover and Dune Elevation at Assateague Island National Seashore: Impact of Horses

The effects of grazing by feral horses on vegetation and dune topography at Assateague Island National Seashore were investigated using color-infrared imagery, lidar surveys, and field measurements. Five pairs of fenced and unfenced plots (300 m2) established in 1993 on sand flats and small dunes with similar elevation, topography, and vegetation cover were used for this study. Color-infrared imagery from 1998 and field measurements from 2001 indicated that there was a significant difference in vegetation cover between the fenced and unfenced plot-pairs over the study period. Fenced plots contained a higher percentage of vegetation cover that was dominated by American beachgrass (Ammophila breviligulata). Lidar surveys from 1997, 1999, and 2000 showed that there were significant differences in elevation and topography between fenced and unfenced plot-pairs. Fenced plots were, on average, 0.63 m higher than unfenced plots, whereas unfenced plots had generally decreased in elevation after establishment in 1993. Results demonstrate that feral horse grazing has had a significant impact on dune formation and has contributed to the erosion of dunes at Assateague Island. The findings suggest that unless the size of the feral horse population is reduced, grazing will continue to foster unnaturally high rates of dune erosion into the future. In order to maintain the natural processes that historically occurred on barrier islands, much larger fenced exclosures would be required to prevent horse grazing.

An analytical model to predict dune erosion due to wave impact

An analytical model is developed to calculate recession distance and eroded volume for coastal dunes during severe storms. The transport relationship used in the model is based on wave impact theory, where individual swash waves hitting the dune face induce the erosion. Combining this relationship with the sediment volume conservation equation describes the response of the dune to high waves and water levels. Four different data sets on dune erosion, originating from the laboratory and the field, were employed to validate the model and to determine the value of an empirical transport coefficient appearing in the analytical solutions. The time evolution of dune recession observed in the different data sets was well described by the model, but the empirical coefficient showed some variation between cases, especially for the field data. In practical applications of the model, it is recommended to use a range of coefficient values to include an uncertainty estimate of calculated quantities, such as recession distance and eroded volume.

Modifications morphodynamiques et phénomènes rétroactifs en baie de Saint-Michel-en-Grève (Côtes-d'Armor) (Morphodynamic changes and feedback processes in the Saint-Michel-en-Grève bay, Cotes-d'Armor)

Two technical operations concerning the mechanical opening of the channel rivers of the Yar and the Roscoat in the Saint-Michel-en-Grève bay were carried out between 1998 and 1999. These operations were undertaken in order to eliminate the confluence of the two rivers on the shore and to allow the fill up of a large depression which was located on the upper part of the beach, just behind the confluence. The fill up of the depression brought important morphodynamic changes inducing feedback phenomenon such as dune erosion located on each side of the rivers. A monitoring based on topographic measurements allowed to analyse and quantify these processes.

Vulnerability Indicators for Coastal Dunes

This paper describes the development of a new parameter to characterize dune vulnerability to storm-induced erosion. Existing indicators of dune erosion vulnerability are examined, including expected cross-sectional erosion values calculated using storm characteristics. We extract a series of 110 pre- and poststorm profiles at cross-shore transects, spaced at approximately 300 m alongshore on digital terrain models of a North Carolina barrier island. Dune failure and survival are designated based on a percentage of the cross-sectional area eroded—50% or greater erosion indicates failure, less than 50% erosion, survival. Crest height does not prove to be an effective predictor of dune vulnerability. Existing cross-sectional area based parameters show some success in predicting erosion vulnerability. We improve the dune failure and survival prediction success rate using a new parameter, a surrogate moment of inertia, the erosion resistance.

The response of Ammophila breviligulata and Spartina patens (Poaceae) to grazing by feral horses on a dynamic mid‐Atlantic barrier island

Ammophila breviligulata, American beachgrass, and Spartina patens, salt meadow hay, have been grazed by feral horses on the dunes of Assateague Island for hundreds of years; however, because of a significant increase in the horse population since the 1960s, overgrazing and dune erosion have become problems. Grazing was assessed on foredunes of four different morphologies along a 21-km stretch of the Maryland portion of the island using 17 exclosure plot pairs. In addition to decreased cover and biomass of the two species, plant structure was significantly affected by grazing. Leaf length and width, stem diameter, and stem density of A. breviligulata and stem diameter of S. patens were reduced in the grazed plots. Especially sensitive to grazing were reproductive characteristics. Percentage of plants in flower, height of flowering stems, and inflorescence length were all significantly reduced by grazing (nongrazed individuals measured). Species composition was not affected by horse accessibility.

Application of prototype flume tests for beach nourishment assessment

The development of underwater profiles and sand losses from the dune respectively the beach by various water level and wave conditions has been studied worldwide in large wave flumes since 1955. Virtually all major results collected over the last decades are documented and described in this paper. Moreover, to this overview results from the MAST III-SAFE experiments, carried out in the Grosser Wellenkanal (GWK) in Hannover, are summarized and discussed in more detail. The tests were focused on beach and dune stability under given conditions with normal and raised water levels. The results are concentrated on surf zone profiles, beach slope and erosion. Furthermore, dune erosion with and without barriers, built into the dune face, and perched surf zone by means of an underwater sill were investigated.