Seif Dunes Research Articles

Morphological characteristics and dynamic changes of seif dunes in the eastern margin of the Kumtagh Desert, China

The seif dune field over the gravel desert surface in the eastern margin of the Kumtagh Desert is a valuable experimental site for the observation of dune formation and dynamics. We used high-resolution remote sensing and station observation approaches, combined with wind and grain size data, to study the characteristics of the aeolian environment and the morphologies of and dynamic changes in seif dunes. We observed the ratio of the resultant drift potential (RDP) to the drift potential (DP), which was 0.37, associated with an obtuse bimodal wind regime. The drift potentials in the west-northwest (WNW) and east-northeast (ENE) directions were dominant, and the angle between the two primary DP directions was 135.00°. The dune orientations ranged from 168.75°–213.75°, which were parallel to the resultant drift direction (186.15°). The dune lengths ranged from 51.68 to 1932.11 m with a mean value of 344.91 m. The spacings of the dunes ranged from 32.34 to 319.77 m with a mean value of 93.39 m. The mean grain size of the sediments became finer, and the sorting became better from upwind tail to downwind tip, which indicated that the sediment of the seif dunes in the study region may be transported from northward to southward. The rate of increase in the length, the mean longitudinal migration rate of the dune tail, and the mean longitudinal extension rate of the dune tip (also called elongation rate) were 4.93, 4.63, and 9.55 m/a, respectively. The mean lateral migration vector of the seif dunes was approximately 0.11 m/a towards the west (–0.11 m/a), while the mean amplitude of lateral migration was 0.53 m/a, ignoring the direction of lateral migration. We found that the seif dune field formed first beside seasonal rivers, which can provide sediment, and then expanded downwind.

Carbonate sedimentation around Jebel Dhanna: models for parts of the buried Holocene sabkha sequences elsewhere along the Abu Dhabi coastline

The early Holocene mainland coast of Abu Dhabi Emirate consisted of a series of embayments between headlands of Miocene inliers and Pleistocene carbonate seif dunes. Subsequently, most of those embayments were back-filled to form coastal sabkhas. The facies and gross depositional characteristics of modern embayments adjacent to Jebel Dhanna are described and used as analogues for former embayments such as those landward of Abu Dhabi Island. Unfortunately, the sedimentology of both modern and earlier embayments have been severely or completely destroyed by industrial and urban developments but vintage aerial photography and satellite imagery have revealed the former topography.

A morphological insight into the Martian dune geomorphology

Aeolian process is the most active and extensive contemporary geomorphological process on the Mars, bearing rich information on surface processes, environment and evolution history. High-resolution remote sensing images covering the whole planet are interpreted and processed by GIS software to analyze the morphological characteristics of the Martian dunes on global basis, and discuss their aeolian geomorphological implications. A tentative dune classification scheme is proposed for the Mars. Dunes types on the Mars mainly include sand sheet, dome dune, barchan, barchanoid ridge, transverse ridge, linear ridges, seif dune, raked linear dune, nail dune, square dune network, ladder-like dune network, scaled dune network, honeycomb dune network, star dune, and obstacle dune. It is suggested that the Mars shares similar dune types with the Earth to a large extent, but is dominated by simple dune types. Complex and compound dunes are rarely seen on the Mars so that the cumulative probability curves of dune morphological parameters are characterized by a single section. Barchans, barchanoid ridges and transverse ridges are the most popular, but linear dunes that account for about 50% on the Earth only have 10% on the Mars. The simplicity of dune geomorphology on the Mars is also manifested by the small patches of the dune fields the Martian dune-fields. The Martian dunes are generally smaller on the Mars, in contradiction with theoretical inferences that the scale of dunes increases the ratio of particle/air density. The morphological parameters of the Martian dunes are well correlated, similar to the Earth dunes, but the correlationship is different, implying the similar formation mechanism but different development conditions. It is also suggested that the Martian analogues of terrestrial aeolian landforms, such as those found on the Tibetan Plateau, are good examples to study the Martian aeolian geomorphology. The simplicity of dune geomorphology on the Mars probably suggests that dunes on the Mars are developed with insufficient sediment availability and short development history. Inactive recent exogenic processes are probably responsible for insufficient sediment availability on the Mars. Insufficient sediment availability would be an interesting topic for future research, which may have extensive earth science significances.

Origin and morphology of barchan and linear clay dunes in the Shuhongtu Basin, Alashan Plateau, China

Clay dunes are characterized by a relatively high content of silt and clay and are typically developed on the lee side margin of closed-basins in arid and semi-arid areas. Although they are widely distributed aeolian landforms on Earth, little is known about their development and distribution in Asia. Our field investigations have identified for the first time the occurrence of clay dunes in the Alashan Plateau, north China. The clay dunes occur in three isolated lowland areas of the Suhongtu (SHT) Basin and their morphologies (barchans, hooked barchans, asymmetrical barchans, and linear or seif dunes) differ substantially from those of the well-studied clay dunes in Australia, Africa and North America. These differences could be mainly attributed to the asymmetrical bimodal wind regime, prolonged drought and sediment physical properties, which indicate that dunes of any type can be formed in sand, silt or clay. Based on the known evolution of the SHT paleo-lake basin and regional climate change, we infer that the landscape of modern clay dunes in the desert of the Alashan Plateau does not predate MIS 5, and probably formed during the late Holocene. Analyses of grain-size distributions and the contents of trace and rare earth elements, combined with geomorphological characteristics, indicate that the source sediments of the clay dunes are mainly local Cretaceous-Tertiary and Quaternary red fluvial-lacustrine mudstone and sediments, while the ultimate source is the weathered and denuded products of granitoids of the East Altay Mountains. Comparison of the morphology and alignment of the clay dunes with the prevailing wind pattern tends to support Tsoar's (1984) model of the transition in dune shape from barchans to seif or linear dunes. During 2003–2013, the seif or linear dunes of the SHT Basin migrated laterally and elongated northeastward at rates of about 2–4 m/yr and 16–20 m/yr, respectively.

Bidirectional winds, barchan dune asymmetry and formation of seif dunes from barchans: a discussion

Barchan dunes—which have a crescent shape with two horns pointing downwind—may undergo a transition to a longitudinal (seif) dune under a bimodal wind regime. Understanding the barchan–seif dune transition is important for the research of dune field evolution and for the investigation of planetary climate and wind regimes. Two models have been proposed to explain the barchan–seif dune transition: Bagnold (The physics of blown sand and desert dunes. Methuen, London, 1941) and Tsoar (Z Geomorphol 28:99–103, 1984). The significance of both models has been investigated through much field and modeling works over the last few decades. However, the conditions for the barchan–seif dune transition as well as the models proposed to explain it are still poorly understood. To correct this situation, here we present and discuss some examples of asymmetric barchans and barchan–seif transitional dune morphologies occurring in nature and show how to characterize wind regimes and identify the relevance of different factors leading to the observed patterns (in addition to wind directionality). Bagnold’s and Tsoar’s models were conceived to explain the barchan–seif dune transition under asymmetric bimodal winds. They were not conceived to explain all types of barchan asymmetry. However, these models must be evaluated in the light of an insight that has been gained more recently, from field investigations, experiments and numerical simulations: The seif dune forms only if the divergence angle between the two main wind directions is ≥90°.

Morphodynamic mechanisms for the formation of asymmetric barchans: improvement of the Bagnold and Tsoar models

Asymmetric barchan dunes exhibit a marked asymmetry, this morphology develops from the combined effects of two dominant winds, with different directions and relative strengths, and can be described based on Bagnold’s and Tsoar’s models. In Bagnold’s model, the limb nearest to the strongest wind extends and is sustained and enhanced by a gentler wind nearly parallel to the barchan. Tsoar’s model expects that the limb farthest from the gentler wind extends in a manner similar to the evolution of seif dunes and that the limb closest to the gentler wind is eroded. To know which model will emerge with variations of the wind, we did some numerical simulations, and found that there exists a critical angle between orientations of the two winds, which decide the formation model of asymmetric barchans. When the angle is greater than this critical value, the effect of the bi-directional winds agrees with Tsoar’s model. When the angle is less than the critical value, the evolution of the two horns contradicts Tsoar’s model and agrees with Bagnold’s model. The critical value for this angle depends on the transport ratio of two winds.

Geomorphosites and Geotourism in the Kavir National Park (Central Iran)

Geotourism and geomorphotourism are considered the most important sub-categories of tourism industry in recent years, and their systematic development based on the principles of proper planning and creating required infrastructure ­ can have a direct impact on sustainable development of every country. With regard to the climate and tectonic conditions, Iran has the most versatile landforms and ­exquisite geological phenomena that can attract many geoscientists as well as adventurous tourists. The purpose of this article is introducing Kavir national park (Maranjab Desert) geomorphosites, and exploring ways to develop geotourism in this area. Geomorphotourism is tourism with emphasis on visiting geomorphologic phenomena and landscapes. At the same time, the integration of historic and cultural heritage, as well as ecological attractions, highlight its importance and further increases its impact on development. The results of the studies show that Kavir national park area is among the most capable regions for sustainable tourism development, because it is one of the most accessible desert areas of Iran, it also has a moderate temperature, geological and tourism attraction diversity, diversity of wilderness and vegetation species, outstanding desert landscape, along with cultural and historic attractions. The region takes advantage of landscapes such as saif (seif) dunes, barchan dunes, ripple marks, ­desert pavements, sand dunes, nebkhas, salt lakes, salt polygons, inflated and puffy salt surfaces, Sargardan Island (Wandering Island) and so on. Kavir national park has a lot of potential for carrying out further studies and the impact on economic development and operation at national and international levels. This article seeks to introduce mentioned potentials in Kavir national park region.

Origins of barchan dune asymmetry: Insights from numerical simulations

Barchan dunes — crescent-shaped dunes that form in areas of unidirectional winds and low sand avail- ability — commonly display an asymmetric shape, with one limb extended downwind. Several factors have been identified as potential causes for barchan dune asymmetry on Earth and Mars: asymmetric bimodal wind regime, topography, influx asymmetry and dune collision. However, the dynamics and potential range of barchan morphologies emerging under each specific scenario that leads to dune asym- metry are far from being understood. In the present work, we use dune modeling in order to investigate the formation and evolution of asymmetric barchans. We find that a bimodal wind regime causes limb extension when the divergence angle between primary and secondary winds is larger than 90, whereas the extended limb evolves into a seif dune if the ratio between secondary and primary transport rates is larger than 25%. Calculations of dune formation on an inclined surface under constant wind direction also lead to barchan asymmetry, however no seif dune is obtained from surface tilting alone. Asymmetric barchans migrating along a tilted surface move laterally, with transverse migration velocity proportional to the slope of the terrain. Limb elongation induced by topography can occur when a barchan crosses a topographic rise. Furthermore, transient asymmetric barchan shapes with extended limb also emerge during collisions between dunes or due to an asymmetric influx. Our findings can be useful for making quantitative inference on local wind regimes or spatial heterogeneities in transport conditions of plane- tary dune fields hosting asymmetric barchans.

Particle-size fractionation of eolian sand along the Sinai-Negev erg of Egypt and Israel

Eolian sand fractions along the west-east transport system of the northern Sinai Peninsula–northwestern Negev erg of Egypt and Israel were analyzed in this study with regard to source, dune geomorphology, eolian transport, and paleoclimate. The studied erg is composed of active linear (seif) dunes in northern Sinai (western part), and stabilized vegetated linear dunes in the NW Negev dune field (eastern part). Linear seif dunes differ from vegetated linear dunes in their vegetation cover, linearity, internal structure, and dynamics. Sand samples were analyzed for sand-grain morphology and particle-size distribution. Although both dune types are continuous landforms with similar orientations and sand-grain roundness values, the linear dunes of Sinai are coarser grained than the Negev vegetated linear dunes. The vegetated linear dunes have a variable but higher proportion of very fine sand (50–125 μm) content and a varying but lower sand fining ratio (defined as the ratio of fine sand percentage to very fine sand percentage). From these observations, we infer that fractionation of sand occurred along the studied eolian transport path during periods of enhanced windiness. Very fine sands are suggested to have been transported by saltation and low suspension from source deposits and sand sheets. We suggest that a significant proportion of the very fine sand fraction of Nile Delta sands has been transported downwind through northern Sinai during the late Pleistocene, especially when linear dunes reached the NW Negev due to last-glacial period windiness and probably larger sediment supply. Generally decreasing wind velocities and increasing precipitation along the west-east dune transport path enhanced vegetative cover in the northern Negev and enabled deposition of the very fine sand component within the dunes and probably further downwind. Our results suggest that particle-size distribution can elucidate much about erg history over time scales of a glacial-interglacial cycle, especially in cases where the sand provenance is of a single dominant source.

The Wabar impact craters, Saudi Arabia, revisited

AbstractThe very young Wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933. We describe field observations made during a visit to the Wabar impact site, provide analytical data on the material collected, and combine these data with poorly known information discovered during the recovery of the largest meteorites. During our visit in March 2008, only two craters (Philby‐B and 11 m) were visible; Philby‐A was completely covered by sand. Mapping of the ejecta field showed that the outcrops are strongly changing over time. Combining information from different visitors with our own and satellite images, we estimate that the large seif dunes over the impact site migrate by approximately 1.0–2.0 m yr−1 southward. Shock lithification took place even at the smallest, 11 m crater, but planar fractures (PFs) and undecorated planar deformation features (PDFs), as well as coesite and stishovite, have only been found in shock‐lithified material from the two larger craters. Shock‐lithified dune sand material shows perfectly preserved sedimentary structures including cross‐bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding, slickensides, and radiating striation resembling shatter cones. The composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite. We observed a partial decoupling of Fe and Ni in the black impact glass, probably due to partitioning of Ni into unoxidized metal droplets. The absence of a Ca‐enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet, which has an estimated thickness of 20–30 m.

Observations of wind ripple migration on an Egyptian seif dune using an inexpensive digital timelapse camera

Abstract I report experiments on the observation of sand ripples on the Quattani dunes in Egypt using an inexpensive ($150) digital timelapse camera. Simple ripples in the fine sand had a wavelength of 10 cm and propagated downwind (windspeed ∼10 m/s) at a speed of ∼3 cm/min, and Y-junction defects moved one wavelength forward in the propagating ripple pattern over ∼6 min. This defect propagation speed of ∼0.5 times the ripple speed is rather slower than suggested in idealized models. Compound ripples at a nearby site were also observed – the megaripples moved at ∼0.2 cm/min while smaller superposed normal ripples moved ∼3–4× faster: the small ripples did not propagate through the larger ones. Experimental technique and some aspects of the design of a windsock/gnomon for use in further studies are discussed. A similar experimental setup should be able to observe fast-moving barchans.

Dune formation under bimodal winds

The study of dune morphology represents a valuable tool in the investigation of planetary wind systems--the primary factor controlling the dune shape is the wind directionality. However, our understanding of dune formation is still limited to the simplest situation of unidirectional winds: There is no model that solves the equations of sand transport under the most common situation of seasonally varying wind directions. Here we present the calculation of sand transport under bimodal winds using a dune model that is extended to account for more than one wind direction. Our calculations show that dunes align longitudinally to the resultant wind trend if the angle(w) between the wind directions is larger than 90 degrees. Under high sand availability, linear seif dunes are obtained, the intriguing meandering shape of which is found to be controlled by the dune height and by the time the wind lasts at each one of the two wind directions. Unusual dune shapes including the "wedge dunes" observed on Mars appear within a wide spectrum of bimodal dune morphologies under low sand availability.

Gravel bodies in the Kumtagh Desert and their geomorphological implications

Although developed in predominantly arid environments, desert geomorphology sometimes bears information about past environmental change. China’s Kumtagh Desert has only recently been studied by researchers. These investigations revealed the existence of several unique features compared with other Chinese deserts, including gravel bodies distributed in the northern part of the desert. This paper summarizes the results of previous research in the region, as well as information obtained from remote sensing and field investigations, then discusses the geological background, underlying Quaternary geomorphology, and the types, development stages, and geomorphological implications of the desert’s gravel bodies. Four main types of gravel bodies and four developmental stages were identified. It is suggested that the great elevation difference between the Altyn Tagh Mountains and the Kumtagh Desert caused by uplift of the Qinghai-Tibetan Plateau constitutes the overall background for the development of gravel bodies, but subsequent climatic events were the direct causes of their evolution. The gravel bodies resulted from alluvial deposits on loose fine sediments mainly of aeolian origin that were subsequently subjected to wind erosion. Erosion cycles then progressively scattered these gravels to form gravel sheets. The gravel bodies have three main geomorphological implications: they record anomalous ancient climate events after the dune field formed and have little influence on the development of nearby seif dunes in the northeast, demonstrate that the Altyn Tagh Mountains are the main sediment source for the Kumtagh Desert, and suggest that the evolution of the environment and geomorphology of the Kumtagh Desert have been influenced by neotectonic movements.

Pseudo-feathery dunes in the Kumtagh Desert

Abstract The Kumtagh Desert is the last explored desert in China. The unique patterns on aerial photographs and satellite images have led previous researchers to the conclusion that the Kumtagh Desert is the only place over the world where typical feathery dunes are developed, and that some unique wind regimes are at work. Recent field investigation reveals that the feathery patterns in fact reflect the albedo contrasts between the seif dunes, bright dune-like drifts and the dark inter-dune lag sediments. The wind regime in the area containing the so-called feathery dunes is typical of the regime that produces seif dunes that were considered to be shafts of the feathery dunes, and the vanes of the feathery dunes are in fact bright dune-like drifts with indistinct height from the surroundings rather than true transverse dunes. Further analysis indicates that the sediments of seif dunes, dune-like drifts and dark inter-dune flats differ in grain size distribution, mineralogy, reflectance spectrum and particle micro-morphology. Sediments that constitute seif dunes and dune-like drifts are finer and poorly rounded, contain relatively more quartz, and hence have higher albedo, while the sediments that constitute the dark inter-dune flats are coarser and better rounded, contain far less quartz but relatively more andesite and dacite, and hence have lower albedo. As a result, what was originally perceived as unique feathery dunes are in fact only pseudo-feathery dunes composed primarily of seif dunes. This discovery also demonstrates that blowing sand drifts can form dune-like patterns that remain visible for long periods of time. It is important to distinguish between the patterns of true dunes and those of dune-like drifts (pseudo-dunes) that cause albedo contrasts in interpreting remote sensing images.

A second look at western Sinai seif dunes and their lateral migration

Tsoar et al. [Tsoar, H., Blumberg, D.G., Stoler, Y., 2004. Elongation and migration of sand dunes. Geomorphology 57, 293–302.] reported that seif dunes in the western Sinai Desert did not migrate laterally between 1973 and 1999. If the planform sinuosities of the dunes are removed by filtering, spatial averaging, or linear regression, however, it is evident that the dunes did, in fact, migrate laterally roughly 13 m during this 26-year period. The measured migration distance is 1–2 orders of magnitude greater than the rms co-registration error Tsoar et al. determined for the first and last air photos that were used to map the dunes. The western Sinai dunes provide another example demonstrating that linear dunes can migrate laterally, and they illustrate some of the difficulties in documenting systematic lateral motion. Lateral migration of a dune can be important geologically or geomorphologically, even where migration is too slow to detect from repeated topographic surveys. This article explains the wind conditions for the lateral migration of seif dunes in western Sinai and the possible wind occurrences that would not lead to such a migration.

Elongation and migration of sand dunes

Two distinct processes are known to act on dynamic dunes, the process of migration by erosion on the windward side and deposition on the lee side, typical for transverse dunes, and the process of elongation typical for linear dunes. These two processes are determined by wind direction relative to the dune alignment. This article reviews the assertion that linear dunes experience lateral displacement in addition to elongation. Fieldwork on vegetated linear dunes (VLDs) and GIS work on seif dunes indicates no lateral migration for these dunes. Linear dunes can shift laterally only when a slip face, formed on the lee side, reaches the plinth of the dune. The winds from both sides of the seif dune are never symmetric; usually winds from one direction are more dominant and effective. The outcome is the formation of peaks and saddles along the dune. The strongest winds create a slip face on the lee side of the peak segments of the dune, oblique to the dune alignment, which reaches the base of the dune and displaces the peak downwind along the dune alignment. The internal structure of the seif dune is formed mostly by this dominant wind direction and gives the impression that the dune has shifted laterally. On the other hand, there are cases in which the wind directions relative to dune alignment fall between those of transverse and seif dunes. In such cases, both processes act on the dune, which subsequently experiences migration as well as elongation.

A Simple Model for the Various Pattern Dynamics of Dunes

A simple computational model is proposed that reproduces various aspects of the complex dynamics of dunes such as the barchan dunes formation process, the evolution process from a barchan dune to a seif dune, the network-dunes formation under time-dependent directional winds, etc. Although this model may be oversimplified in several respects, there is a hope that it helps us to sift relevant factors out the vast sea of numerous factors influencing the rich dynamics of desert dunes.

Pleistocene Carbonate Seif Dunes and their Role in the Development of Complex Past and Present Coastlines of the U.A.E.

ABSTRACTIsolated outcrops of Pleistocene carbonate aeolianites and their cappings of shallow water Pleistocene marine limestones are described largely from Al Dabb’iya on the eastern end of the Great Pearl Bank, Abu Dhabi. Their sedimentological characteristics and depositional settings are discussed in relation to mainland Pleistocene aeolianite trends observed from satellite images. A comparison of the internal dune cross-bedding with dune morphologies suggests changing wind regimes that probably resulted from meteorological changes induced by the onset of a Pleistocene glacial period. The complex palaeo-geomorphology imposed by the dunes strongly influenced later Pleistocene coastal marine sedimentation that was concentrated in a series of shallow embayments separated by low-relief seif dunes forming peninsulas. Similar marine settings extended into the Holocene along parts of the United Arab Emirates coastline and still persist locally.

A Quaternary proximal foreland ramp and its continental fringe, Arabian Gulf, UAE

Abstract During Quaternary time, the southern flank of the Arabian Gulf can be considered as a classic, foreland basin ramp setting in which the sedimentological processes operating changed drastically along strike. The combined effects of sea-level fluctuations and wind, which have dominated the Quaternary coastal morphology and coastal evolution, are observed at a resolution rarely achieved in the stratigraphic record of palaeoramp sequences. The facies distributions and diagenetic characteristics of the relatively thin Pleistocene-Holocene strata provide valuable insights into the complex stratigraphy that can develop within an inner ramp and its continental fringe. In the UAE, Pleistocene sediments comprising mainly aeolian dunes overlie a deeply eroded topography of Upper Miocene strata. These sediments become increasingly carbonate rich and calcite cemented northwards towards the coast. Here they experienced severe aeolian deflation due to the later marine flooding of the aeolian sources on the formerly exposed areas lower down the ramp profile, and wind erosion along the coastal area because of a lack of sediment supply. The antecedent topography of Miocene and Pleistocene escarpments, mesas, islands, cemented seif dunes and coastal deflationary ‘Stokes surfaces’ provided a highly varied Holocene ramp margin undergoing coastal progradation (with offlap) during the 1–2 m relative sea-level fall of late Holocene time. Since then, marine carbonate deposition and evaporite diagenesis have dominated the Holocene sequence, but it is aeolian processes that largely control their ultimate distribution within the constraints of the contemporaneous sea-levels. Longshore currents, lateral and leeward accretion, infilling of stranded lagoons and deflation on a massive scale testify to the crucial role of wind in the development of this proximal ramp. The Holocene aeolian sediments, though extensive, are very different in character from the Pleistocene sediments. Two generations of Holocene supratidal anhydrites are recognized. The earlier one was partly transgressive, but the later one is regressive. In addition to the present-day oolith factories on the ebb tidal deltas at the seaward ends of tidal channels between the barrier islands of central Abu Dhabi, oolith factories also exist on exposed beaches and tidal embayments of western Abu Dhabi. They also occurred within Pleistocene and Holocene embayments. Flat-topped and sometimes shingled hardgrounds representing isochronous events extend over very large areas. So also do commonly observed iron oxide bands formed beneath the sabkha surfaces.

Coastal dunes: sensitive or not?

This paper introduces a classification of coastal dune systems into two main morphodynamic classes, namely retentive and transgressive dune systems. Retentive systems include coastal dune types where sand accumulation within vegetation is dominant over other processes. In this category we include such morphological types as hummock dunes, foredunes, and retention ridges, including sub-environments such as precipitation ridges that form the landward boundary of transgressive dunefields. Transgressive systems include those coastal dune types where sand transport is dominant over other processes, and the dunes are unvegetated and mobile. In this category we include such morphological types as parabolic dunes, reversing transverse dunes, barchans, seif dunes, transgressive sheet dunefields and headland-bypass dunefields. Components of the two main morphodynamic types of coastal dune systems range in sensitivity from sensitive to robust. Retentive dune systems, being vegetated, are sensitive and fragile. Mobile dunes that form the major component of transgressive dune systems are robust and resilient. This morphodynamic classification is particularly useful because the two types of dune systems have diametrically opposed sensitivities and hence different management requirements.