Dune Face Research Articles

Modeling beach profile changes by typhoon impacts at Xiamen coast

For the Xiamen coast where typhoon frequently occurs, beaches are subject to severe erosion during typhoons. To investigate storm-induced beach profile changes at Xiamen coast, four inner XBeach models were applied using typhoon Dan as a case study. These numerical simulations utilized hydrodynamic and wave conditions determined from larger-scale outer and middle coupled Delft3D-FLOW and SWAN models. The models were validated against historic measurements of tidal level, storm tide, storm surge and beach profiles, thus showing the accuracy of outer and middle models to provide boundary conditions and the reliability of inner models to reflect beach profile changes during a typhoon process. The applicability of this modeling approach to Xiamen coast was verified. The results also demonstrated that an enormous amount of dune face erosion occurred at the selected beaches during the typhoon Dan process and the slopes in the vicinity of zero elevation for the chosen four beach profiles all turned out to be gentler after typhoon Dan. Nevertheless, these beaches suffered different impact degrees and processes during the typhoon influence period. Compared to swash and collision regimes, overwash and inundation regimes have the ability to alter beach profile rapidly in short time. Post-storm beach profile with and without vegetation indicated that vegetation is capable of protecting coastal beaches to some extent. By running the nested models, the simulated results can be employed in the management of the beach system and the design of beach nourishment projects at Xiamen coast.

Evaluation of XBeach performance for the erosion of a laboratory sand dune

A new set of laboratory data is used to investigate the bathymetry change of a steep sand dune exposed to waves and high water levels, and subsequently compared to the results of numerical simulations using XBeach. Bichromatic wave boundary conditions are used to simulate a combined short-wave and long-wave field for two water level elevations corresponding to collision with the dune face, and overwashing of the dune crest. In the collision regime case, episodic slumping due to the undercutting of the dune results in sudden erosional events followed by long periods of wave-driven reshaping at the dune toe. In the overwash regime case, morphological changes are faster and sediment transport rates are higher.The XBeach model was used to simulate wave-driven erosion of the dune at the two water levels observed in the laboratory. The model was not able to precisely recreate the cross-shore spatial variability of significant wave height observed in the experiments, however near-bed wave-orbital and mean current velocities were in good agreement with observations. Following rapid initial adjustment, the model results were in agreement with measured dune morphology at successive times. XBeach was sensitive to several parameters that control the rate of erosion including the critical avalanching slope under water, the threshold water depth and the sediment transport formulation, and performed well after careful selection of the best combination of these parameters. Overall, the model predictions were in better agreement with laboratory observations for dune erosion in the overwash regime case than the collision regime case.

Testing model parameters for wave‐induced dune erosion using observations from Hurricane Sandy

AbstractModels of dune erosion depend on a set of assumptions that dictate the predicted evolution of dunes throughout the duration of a storm. Lidar observations made before and after Hurricane Sandy at over 800 profiles with diverse dune elevations, widths, and volumes are used to quantify specific dune erosion model parameters including the dune face slope, which controls dune avalanching, and the trajectory of the dune toe, which controls dune migration. Wave‐impact models of dune erosion assume a vertical dune face and erosion of the dune toe along the foreshore beach slope. Observations presented here show that these assumptions are not always valid and require additional testing if these models are to be used to predict coastal vulnerability for decision‐making purposes. Observed dune face slopes steepened by 43% yet did not become vertical faces, and only 50% of the dunes evolved along a trajectory similar to the foreshore beach slope. Observations also indicate that dune crests were lowered during dune erosion. Moreover, analysis showed a correspondence between dune lowering and narrower beaches, smaller dune volumes, and/or longer wave impact.

Coastal Dunes and Plants: An Ecosystem-Based Alternative to Reduce Dune Face Erosion

ABSTRACT Martinez, M.L.; Silva, R.; Mendoza, E.; Oderiz, I., and Perez-Maqueo, O.M., 2016. Coastal dunes and plants: an ecosystem-based alternative to reduce dune face erosion.. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 303–307. Coconut Creek (Florida), ISSN 0749-0208. Future scenarios indicate that growing human encroachment on coasts, more frequent and stronger storms and sea level rise will result in worsening coastal squeeze. In consequence, human lives, property and infrastructure, as well as ecosystem services, will increasingly be threatened. It is therefore vital to find the means to maintain or increase the resilience and resistance of coastal zones. As an alternative to hard infrastructure, ecosystem-based coastal defense strategies have been recommended as better and more sustainable solutions. Thus, the goal of this study was to ...

Response of vegetated dune–beach systems to storm conditions

Human encroachment on the coasts is extensive and expected to increase over the coming decades. This proximity to the sea is coupled with potentially more frequent strong tropical cyclones and eustatic sea level rise, and thus human life, property and infrastructure are threatened. In this scenario, it is vital to find the means to maintain or increase the resilience and resistance of coastal zones. As an alternative to engineering solutions, ecosystem-based coastal defence strategies have been recommended as better and more sustainable solutions, although it has not been clearly demonstrated that they really work. In 24 wave flume experiments, we studied the effects of four densities of vegetation cover (none, low, medium, high) on the movement of sediment along two beach–dune profiles (with and without a berm), under three storm conditions (mild, moderate and intense). Erosion regimes of collision and overwash were observed in the dune profiles with a berm, whereas swash and overwash regimes were observed when no berm was present. In the profiles with a collision regime, vegetation decreased the amount of erosion. For both profiles, in the tests with the largest wave conditions, vegetation prevented overwash and thereby erosion of the landward side of the dune. Erosion of the dune face was reduced by vegetation, particularly with a strong storm, when Iribarren numbers were greater. In summary, vegetation reduced net erosion on the dune face, regardless of the wave conditions, the morphology of the beach–dune profile, or the mode of erosion.

Observations and modeling of alongshore variability in dune erosion at Egmond aan Zee, the Netherlands

Dunes can erode within a few hours when exposed to high storm surge levels and large waves. If the dunes are the primary defense, such as in the Netherlands, this could result in flooding of the hinterland when dunes breach. Models are often used to analyze how dunes will respond to extreme conditions, but they can also be applied to study milder events, and with that gain more insight in the processes of dune erosion. There is, however, growing demand for validating existing models under field conditions. Here, we calibrate the XBeach dune erosion model with pre- and post-storm topography measurements of the dune-erosion event in January 2012 at Egmond aan Zee, The Netherlands. Furthermore, hydrodynamical data of the intertidal zone along a cross-shore transect in the same area collected 3months prior to the storm event are used to hydrodynamically calibrate and validate XBeach. During the January 2012 storm the dune erosion was variable alongshore, from the erosion of embryo dunes, the forming of a dune scarp to considerable slumping of the entire dune face. This caused the erosion volume V to range from 5 to 40m3/m. The calibrated model reproduces the alongshore variability in V with reasonable accuracy. Additional simulations show that the alongshore variation in V is due to variability on the initial dune topography (primarily steepness), and that variability in beach and nearshore bathymetry was of secondary importance.

Dune Height Estimation on Titan Exploiting Pairs of Synthetic Aperture Radar Images With Different Observation Angles

Widespread longitudinal dunes have been identified on Titan thanks to the 2.2-cm wavelength Cassini Synthetic Aperture Radar (SAR) instrument. Understanding the properties of these surface features, such as material composition and dune height, is very important for giving new clues about the Titan geology and climate. One of the major difficulties in the estimation of dune heights using SAR occurs when the material composition of the dunes is heterogeneous. In this paper, we propose a novel method for dune height estimation, which takes into account material heterogeneity, and in particular, the case in which the interdune exhibits different dielectric properties with respect to the remaining part of the dune. Paired data acquisitions with orthogonal observations are considered for separating the dielectric from the geometric effect on the backscattering coefficients in order to retrieve the slope and thus the height of the dunes. The results for a test area located in the Fensal region indicate that the slopes of the dune faces are generally lower than 5° and the heights range between 40 and 110 m.

Spatial and temporal variation of community composition and species cover following dune restoration in the Devesa de Albufera (Valencia, Spain).

Plant populations were reintroduced to the coastal dune bar of the Devesa de Albufera from 1988 to 2004; different coastline sections received different species composition and cover. With the aim to detect spatial and temporal variation of floristic diversity, we compared current species composition and cover across the length of the Devesa and across the dune bar with those imposed at the time of restoration. Non-metric multidimensional scaling (NMDS) detected significant differences both across the dune faces and across the coast sections. Differences across the dune faces reflect the sea-inland ecological gradient and resulted from a spatial rearrangement of plant populations: Calystegia soldanella, Achillea maritima and Polygonum maritimum prefer the windward face; Malcolmia littorea and Lagurus ovatus the leeward. Differences across coast sections are related to those at restoration time, with a slow trend towards the homogenization of plant communities. At the current level of anthropic pressure, the plant cover is likely to evolve following the trends pointed out in this research.

Vegetation Dynamics of Coastal Dunes with Juniperus spp. in Crete, Gavdos and Chrysi Islands (Greece)

We aimed to determine the composition, structure and ecological processes of the vegetation of the coastal dunes with Juniperus spp. in Crete, Gavdos and Chrysi in the South Aegean, Greece, in order to apply sound habitat management and restoration. Vegetation composition, structure and zonation were investigated with plots and transects. Data from seven study sites were classified using TWINSPAN. The major patterns of the vegetation data and their relation to environmental variables were explored by DCA and CCA ordination techniques. Ellenberg indicators for moisture, nutrients, and salt were used to characterise the community types identified. The habitat’s floristic composition includes 142 plant species. Five principal community types were identified. Vegetation distribution was related to geomorphology and disturbance gradients. The analysis of transect data identified 20 vegetation units on incipient dune, foredune, interdune and hind dune. Vegetation and geomorphological data were used to construct sand dune profiles for each site, while a set of 36 keystone and 76 indicator species were identified. The sites examined have varied levels of dune development and face different threats. Habitat management should address grazing and trampling at the local level but also land use changes at the catchment level.

ANALYSIS AND MODELLING OF EXTREME LOCALISED DUNE EROSION EVENTS ALONG DENMARK'S NORTH SEA HIGH WATER DUNE BARRIER

This paper presents the results of an investigation into cases of extreme localised dune erosion along Denmark’s west coast. Cases of extreme dune erosion of over 20m are identified between 1977 and 2012. These are compared to the closest storm events. However, no correlation is found between the occurrence of these extreme erosion events and storm intensity or return period. A general increasing prevalence of sand bars is identified as well as an absence of extreme dune erosion in several of the more recent intense storms. Results of XBeach numerical modelling on a local example of extreme dune erosion showed that the offshore bathymetry and sand bar size has a significant effect on the amount of erosion that occurs at the dune face during a storm.

DUNE EROSION PHYSICAL, ANALYTICAL AND NUMERICAL MODELLING

The present paper provides an overview of the large-scale physical model experiments performed at the Canal d'Investigaciò i Esperimentaciò Marìtima (CIEM), Laboratori d'Enginyeria Marìtima (LIM), Universitat Politècnica de Catalunya (UPC), Barcelona, within the EU-Hydralab III Integrated Infrastructure Initiative. The model tests have been carried out in a flume with a sandy dune exposed to a combination of water levels and wave conditions. Different regimes of wave attacks on the sandy/beach dune system have been investigated; in particular, the study provides a unique set of large-scale physical data concerning the storm waves induced dune overwash (Tomasicchio et al. 2011a; Tomasicchio et al.2011b). The effects of various "load parameters† on the dune erosion process generation, including dune recession rates in terms of the retreat of the dune face, Î"x, and the eroded volume, Î"V, have been investigated and discussed. The laboratory data sets have been adopted to calibrate and verify the analytical model proposed by Larson et al. (2004) in order to calculate the values of Î"V at specific time intervals. Furthermore, the profile measurements have been used to calibrate and verify the numerical model C-SHORE (Kobayashi et al. 2007) predicting the beach-dune profile modifications over the near-shore region (Tomasicchio et al. 2011a; Tomasicchio et al.2011b).

Modelling storm impacts on beaches, dunes and barrier islands

A new nearshore numerical model approach to assess the natural coastal response during time-varying storm and hurricane conditions, including dune erosion, overwash and breaching, is validated with a series of analytical, laboratory and field test cases. Innovations include a non-stationary wave driver with directional spreading to account for wave-group generated surf and swash motions and an avalanching mechanism providing a smooth and robust solution for slumping of sand during dune erosion. The model performs well in different situations including dune erosion, overwash and breaching with specific emphasis on swash dynamics, avalanching and 2DH effects; these situations are all modelled using a standard set of parameter settings. The results show the importance of infragravity waves in extending the reach of the resolved processes to the dune front. The simple approach to account for slumping of the dune face by avalanching makes the model easily applicable in two dimensions and applying the same settings good results are obtained both for dune erosion and breaching.

Prediction of dune erosion due to storms

This paper presents results of experimental and mathematical modelling of beach and dune erosion under storm events. Re-analysis of the experimental results on dune erosion in small-scale and large-scale flumes shows that the dune erosion for extreme conditions is somewhat smaller than that based on earlier analysis results. Dune erosion caused by wave impact has been modelled by a cross-shore profile model (CROSMOR-model), which is based on a ‘wave by wave’ modelling approach solving the wave energy equation for each individual wave. The model has been applied to the recent Deltaflume experiments on dune erosion. The three main processes affecting dune erosion have been taken into account: the generation of low-frequency effects, the production of extra turbulence due to wave breaking and wave collision and the sliding of the dune face due to wave impact. The calibrated model can very well simulate the observed dune erosion above the storm surge level during storm events in small-scale facilities, large-scale facilities and in the protoype (1953 storm in The Netherlands) using the same model settings. The mathematical model results have been used to develop a new simplified dune erosion rule.

Large-scale dune erosion tests to study the influence of wave periods

Large-scale physical model tests were performed to quantify the effects of the wave period on dune erosion. Attention was focussed on 2D cross-shore effects in a situation with sandy dunes and extreme water levels and wave conditions. Besides profile measurements, detailed measurements in time and space of water pressure, flow velocities and sediment concentrations were performed in the near near-shore area. It was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face. Tests with double-peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the T m − 1,0 spectral mean wave period, better than the peak wave period, T p . The effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the Dutch coast. More details of the measurements and additional analyses of physical processes are described in an accompanying paper by Van Thiel de Vries et al. [Van Thiel de Vries, J.S.M., van Gent, M.R.A., Reniers, A.J.H.M. and Walstra, D.J.R., submitted for publication. Analysis of dune erosion processes in large scale flume experiments, In this volume of Coastal Engineering.].

Analysis of dune erosion processes in large-scale flume experiments

Large-scale physical model tests were conducted with different wave periods to examine the physical processes driving dune erosion. The model tests have been carried out in a flume (2DV) with a sandy dune exposed to extreme surge and wave conditions [Van Gent, M.R.A., Van Thiel de Vries, J.S.M., Coeveld, E.M., De Vroeg, J.H. and Van de Graaff, J., 2008. Large-scale dune erosion tests to study the effect of wave periods. Coastal Engineering. doi:10.1016/j.coastaleng.2008.04.003.]. Detailed measurements in time and space of water pressure, flow velocities and sediment concentrations were performed in the near shore area. The data revealed that both short- and long waves are important to inner surf hydrodynamics. Depth averaged flows are directed offshore and increase towards the shore line. The corresponding mean sediment concentrations rise sharply towards the dune face (up to 50 g/l near the bed). The strong increase in the mean sediment concentration towards the dune face correlates well with the maximum wave surface slope which in turn is coupled to both the pressure gradient and the near-bed wave-breaking induced turbulence. Analysis shows that the pressure gradient is only partially coupled to the flow acceleration suggesting that the latter cannot always be used as a proxy for the first. Weak correlation is obtained with the near-bed flows related to the bed shear stress. Tests with a larger wave period resulted in a larger dune erosion volume. During these tests more wave energy (combined incident and infragravity waves) reached the dune face, but more importantly, this wave energy is dissipated by fewer waves resulting in more intense wave breakers and steeper wave fronts. It is therefore expected that the wave-breaking induced near-bed turbulence increases resulting in significantly higher (O(100%)) mean sediment concentrations. In addition the mean flow velocities are comparable, yielding a substantially larger offshore directed sediment transport capacity. This increase in offshore directed transport is only partially compensated by a concurrent increase in the wave related onshore transport capacity associated with intrawave processes, resulting in a net increase in the dune erosion rate.

Beach–dune interactions on the dry–temperate Danube delta coast

Beach–dune seasonal elevation changes, aeolian sand transport measurements, bathymetric surveys and shoreline evolution assessments were used to investigate annual and seasonal patterns of dune development on Sfântu Gheorghe beach, the Danube delta coast, from 1997 to 2004. Dune volume increased consistently (1.96 m 3 m − 1 y − 1 to 5.1 m 3 m − 1 y − 1 ) over this 7-year period with higher rates in the southward (downdrift) direction. Dune aggradation is periodically limited by storms, each of which marks a new evolutionary phase of the beach–dune system. As a consequence of the variable beach morphology and vegetation density during a year, foredune growth occurs during the April–December interval while between December and April a slightly erosive tendency is present. The pattern of erosion and deposition shown by the topographical surveys is in good agreement with the sand transport measurements and demonstrates the presence of a vigorous sand flux over the foredunes which is 20–50% smaller than on the beach. This high sand flux, due to low precipitation and sparse vegetation cover, creates an aerodynamically efficient morphology on the seaward dune slope. The seaward dune face accretes during low to medium onshore winds (5.5–12 m s − 1 ) and erodes during high winds (> 12 m s − 1 ).

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.

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.

Lava–sediment interaction in desert settings; are all peperite-like textures the result of magma–water interaction?

This study reports on lava–sediment interaction in dry depositional settings focusing on the Early Cretaceous volcano-sedimentary sequence in the Huab Basin, NW Namibia, as a detailed example. Here an active aeolian sand sea (erg) system was progressively engulfed by lavas of the Etendeka Flood Basalt Province ∼133 Ma BP. This volcanic flooding resulted in: (1) the unprecedented preservation of large parts of the active dune system; (2) the development of a variety of sediment interlayers; and (3) a record of excellent examples of the dynamic interaction between lava and aeolian sand. The lava–sediment interaction happens on a variety of scales from simple sediment interbeds within the lavas at a large scale down to complex breccia horizons and bulbous lava–sediment contacts at small scales. ‘Peperite’ like textures are found where hot lava has dynamically interacted with unconsolidated aeolian sands. Such interaction could involve lava cascading down the front of dune faces, or active ‘aa’ style flow fronts ‘bulldozing’ into and oversteepening dune faces causing rapid influx of sand into the active flow front. Resulting breccia horizons have been found up to 5 m thick. Locally, the aeolian sand is baked to a quartzite by the hot lava. In contrast, on other surfaces, the lava leaves striations in the sand as it flows across it indicating local flow directions and less pronounced interaction. In places, wind ripples and topset beds are preserved on the stoss side of the dune indicating the passive nature of emplacement for some of the lava flows. Peperites are often thought to form due to the presence of water within the unconsolidated sediments that the juvenile magma comes into contact with. However, examples from arid environments demonstrate that peperite-forming processes are diverse and may not always require water. Therefore, it is suggested that the term peperite should be used to describe deposits consisting of sediments mixed with juvenile magmatic components where it is clear that there has been a dynamic interaction between the two. This leaves the presence, or lack, of water as an issue for interpreting the processes by which the peperite formed.

Dunes and Microdunes on Venus: Why Were So Few Found in the Magellan Data?

A search through cycle 1, 2, and 3 Magellan radar data covering 98% of the surface of Venus revealed very few dunes. Only two possible dune fields and several areas that may contain microdunes smaller than the resolution of the images (75 m) were identified. The Aglaonice dune field was identified in the cycle 1 images by the specular returns characteristic of dune faces oriented perpendicular to the radar illumination. Cycle 1 and 2 data of the Fortuna-Meshkenet dune field indicate that there has been no noticeable movement of the dunes over an 8-month period. The dunes, which are oriented both parallel and perpendicular to the radar illumination, appear to be dark features on a brighter substrate. Bright and dark patches that were visible in either cycle 1 or 2 data, but not both, allowed identification of several regions in the southern part of Venus that may contain microdunes. The microdunes are associated with several parabolic crater deposits in the region and are probably similar to those formed in wind tunnel experiments under Venus-like conditions. Bragg scattering and/or subpixel reflections from the near-normal face on asymmetric microdunes may account for these bright and dark patches. Look-angle effects and the lack of sufficient sand-size particles seem to be the most likely reasons so few dunes were identified in Magellan data. Insufficient wind speeds, thinness of sand cover, and difficulty in identifying isolated dunes may also be contributors to the scarcity of dunes.