Blowout Development Research Articles

Feedbacks between Biotic and Abiotic Processes Governing the Development of Foredune Blowouts: A Review

This paper reviews the initiation, development, and closure of foredune blowouts with focus on biotic-abiotic interactions. There is a rich body of literature describing field measurements and model simulations in and around foredune blowouts. Despite this abundance of data there is no conceptual framework available linking biotic and abiotic observations to pathways of blowout development (e.g., erosional blowout growth or vegetation induced blowout closure). This review identifies morphological and ecological processes facilitating the transition between blowout development stages and sets them in the context of existing conceptual frameworks describing biotic-abiotic systems. By doing so we are able to develop a new conceptual model linking blowout development to the dominance of its governing processes. More specifically we link blowout initiation to the dominance of abiotic (physical) processes, blowout development to the dominance of biotic-abiotic (bio-geomorphological) processes and blowout closure to the dominance of biotic (ecological) processes. Subsequently we identify further steps to test the proposed conceptual model against existing observations and show possibilities to include it in numerical models able to predict blowout development for various abiotic and biotic conditions.

Phases of blowout initiation and stabilization on Padre Island revealed through ground-penetrating radar and remotely sensed imagery

Blowouts cover a large section of North Padre Island, Texas, and have previously been associated with overgrazing and vehicles driving between the low water line and the toe of the foredune. In the current study, the evolution and migration of 18 blowouts within Padre Island National Seashore are tracked using aerial photographs and satellite images since 1969. Representative active and stabilized trough blowouts are also examined using ground-penetrating radar surveys and remotely sensed imagery to identify the phases of blowout initiation and stabilization between 1969 and 2010. At least five distinct phases can be identified in the radar surfaces and facies of the active blowout, including both the stabilization and reactivation of the blowout throat, whereas only four phases can be identified in the surfaces and facies of a blowout that completely stabilized in 2010. Results suggest that blowout evolution on Padre Island is episodic and is initiated by the opening and closing of the blowout throat during periods with elevated storm surge. The lack of reflectors at depth is evidence that the development of blowouts on Padre Island is a relatively new phenomenon initiated by overgrazing and maintained by compaction of the beach and backshore by vehicles.

Geomorphology and dynamics of a traveling cuspate foreland, Authie estuary, France

Cuspate forelands or salients occur all over the world in lakes, estuaries and on ocean shores, yet there have been few studies conducted on traveling cuspate forelands (or salients), that is, forelands that migrate or travel alongshore. This paper presents a study of a traveling foreland in the Authie estuary, France, termed the Bec du Perroquet. Historical shoreline changes may be traced from the 1200's AD and the region has experienced both marked intertidal-subtidal accretion extending from the south, and massive erosion in the north since this period. An analysis of aerial photographs from 1947 until the present shows that the original Bec foreland was established at the mouth of the Authie estuary, but gradually disappeared by the 1960's and a new foreland developed in the middle of the northern-central portion of the bay. This foreland was composed of a suite of foredune ridges which have been successively eroded on the northern margin and initiated on the southern margin as the foreland traveled or migrated southwards. As the foreland traveled south, from 1947 to 2009 the northern part of the bay retreated more than 350m, while mid-bay, the coastline retreated ~215m. As the foreland evolves and migrates, incipient foredunes can develop rapidly (e.g. 18 ridges formed in an 11week period), while at other times the ridges form slowly and may be eroded and disappear. Two or more foredune ridges may blend into a single ridge over time depending on the initial degree of vegetation cover on the ridge and swale set. Aeolian processes in dune swales are much more important in this system than in typical prograding foredune plain systems due to the sometimes marked lack of vegetation colonization in the swales following foredune ridge development, and aeolian deflation of the swales (along with blowout development) is important particularly when they become open conduits to the beach as erosion of the NW foreland proceeds. The ages of each of the surviving ridges on the foreland in 2009 have been determined, and the evolutionary path of the ridges ascertained. Formerly intact, relatively stable, continuous ridges evolve to erosional knobs, turrets and nebkha over time. Foredune ridges (and swales) can be extremely arcuate to semi-circular in form where the foreland and especially the spit extension are exposed to a wide range of wind directions and where the shoreline trends through an arc of at least 270°. This study illustrates a remarkable cycling of the formation, destruction and reformation (travel) of a cuspate foreland over a ~50+year period.

Government Microblogs: Innovative Approaches for E-Government

As one of the typical social media categories, microblogs have become the hot vehicle for online communication and interaction since 2006. Not only individuals, but also organizations are all fascinated to open their own microblogs. Government microblogs launched by Chinese governmental agencies have showed the blowout development since 2011. These microblogs devote to release public information, guide public opinions, promote government and place images and fulfill administrative transparency. This paper will introduce the development of government microblogs in China and articulate suggestions for effective utilization of government microblogs as a vehicle for e-government.

Initiation and evolution of blowouts within Padre Island National Seashore, Texas

Blowouts once covered the entire northern section of North Padre Island as a result of overgrazing, but are now restricted to a 5.5 km length of beach where driving is permitted between the low water line and the toe of the foredune. A comparison between this driving section and a directly adjacent non-driving section of the beach reveals a significant difference in the ratio of storm run-up (Rhigh) to the elevation of the dune toe (Dlow), suggesting that the foredunes within the driving section are more susceptible to scarping and blowout development. Where driving is permitted, blowouts are located in areas where Rhigh/Dlow ratio is locally large, suggesting that the location of blowouts depends on the alongshore variation in storm surge. Results of a cluster analysis reveal two distinct blowout types, in which the relatively small blowouts of Cluster 1 are characterized by a small throat and limited expansion beyond the foredune. The large and more active blowouts of Cluster 2 tend to be found closer to the access road, where driving on the beach is more concentrated and the larger Rhigh/Dlow suggests a greater potential for scarping during relatively small storms. The development of these blowouts feeds back on the height of the foredune, making it susceptible to scarping and washover during tropical storms and hurricanes. It is argued that historical overgrazing and recent beach driving have significantly altered this landscape and represents a long-term threat to island resilience and have the potential to hasten island transgression with sea level rise.

Numerical Simulation for Gas-Liquid Two-Phase Flow along the Borehole after Air Cutting

When gas kick Occurs during drilling, because of pressure, temperature, coefficient of gas compressibility and other parameters changing continuously, gas will slip along the borehole and also accompany expansion some extent, and bottom hole differential pressure increases, resulting in the amount of invasion gas increasing continuously until blowout. The procedure of gas kick till blowout in the borehole is transient gas-liquid two-phase flow, studying on The development of gas-liquid two-phase flow parameters variation characteristics and bottom hole pressure variation characteristics plays an significant role to understand blowout occurrence and development characteristics. This paper using methane-mud as the circulating medium simulates the procedure of gas kick till blowout near the bottom under the condition which is almost the onsite drilling process, Analyzing the flow pattern, bottom hole pressure variation characteristics, and velocity distribution under the different stages of gas kick, different influx, and obtained an initial understanding.

Geomorphological and ecological features of blowouts in a western Mediterranean coastal dune complex: a case study of the Es Comú de Muro beach-dune system on the island of Mallorca, Spain

Many of the coastal dune systems along western Mediterranean shores are in an advanced state of fragmentation and show distinct signs of erosion, largely because of blowout development along the dune front. The Es Comu de Muro beach-dune system on the island of Mallorca (Spain) is a good example of this. In order to better understand and quantify the current situation, 58 blowouts along a ca. 1.5-km-long dune front were investigated. In each case, a number of morphometric and ecological variables were analyzed as a basis for comparison and classification, in particular blowout dimensions and orientation, inner morphometry and topography, morphological types, the role of vegetation in defining the state of the foremost dune line, and the link between vegetation and blowout typology. In comparison with a recent preliminary investigation, the results of the present study provide a more comprehensive picture of the advanced state of fragmentation along the dune front. The blowouts are not evenly distributed, highest densities occurring along the southernmost part of the beach, lowest densities along the northern part. The blowouts were subdivided into two categories on the basis of their shape and general structure, trough blowouts being the most prevalent, followed by mixed trough-saucer shapes. Distinctly saucer-shaped blowouts could not be distinguished. In addition, the blowouts were subdivided into two morphological categories, i.e. simple and branched. It was also possible to link the morphological state of the dune front to certain ecological parameters, in particular vegetation which, in the present case, comprised herbaceous and woody plants. Cluster analyses of species associations (Bray-Curtis similarity indices) were carried out on the basis of the presence/absence of each species. It is shown that, on account of presence counts and the degree of similarity of species associations, some species play a more important role in stabilizing the mobile dune sands than others, a foremost candidate being Ammophila arenaria. It is concluded that active support for such plants should form part of any management strategy aiming to reduce and reverse coastal dune fragmentation processes.

The development of blowouts and foredunes in the Ilha Comprida barrier (Southeastern Brazil): the influence of Late Holocene climate changes on coastal sedimentation

Middle to Late Holocene barriers are conspicuous landforms in southeastern and southern Brazilian regions. The barriers in the coastal zones of northern Santa Catarina, Paraná and São Paulo states (27°19′–24°00′S) are formed mainly by beach ridge alignments and many barriers present foredune and blowout alignments in their seaward portion. The development of these eolian landforms appears to record a regional shift in coastal dynamics and barrier building. In this context, the Ilha Comprida barrier stands out for its well-developed and well-preserved foredunes and blowouts. Based on the presence or not and type of eolian landforms, the Ilha Comprida barrier can be divided seaward into inner, middle and outer units. The inner unit is formed entirely by beach ridges. The middle unit comprises a narrow belt of blowouts (up to 15 m high) aligned alongshore. Blowout lobes pointing NNW are indicative of their generation by southern winds. The outer unit is represented by low (≤1 m high) active or stabilized foredunes and a small transgressive dunefield (∼1 km 2). Twenty-seven luminescence ages (SAR protocol) obtained for the beach ridges, foredunes, and blowouts of these three units allow definition of a precise chronology of these landforms and calculation of rates of coastal progradation. The inner unit presents ages greater than 1004 ± 88 years. The blowouts of the middle unit show ages from 575 ± 47 to 172 ± 18 years. The ages of the outer unit are less than 108 ± 10 years. Rates of coastal progradation for the inner and outer units are 0.71–0.82 m/year and 0.86–2.23 m/year, respectively. The main phase of blowout development correlates well with the Little Ice Age (LIA) climatic event. These results indicate that southern winds in subtropical Brazil became increasingly more intense and/or frequent during the LIA. These conditions persist to the present and are responsible for the development of the eolian landforms in the outer unit. Thus, barrier geomorphology can record global climatic events. The sensitivity of barrier systems in subtropical Brazil to Late Holocene climate changes was favored by the relative sea level stillstand during this time. Luminescence dating makes it possible to analyze barrier geomorphology during Late Holocene climate changes operating on timescales of a hundred to thousand years. These results improve our knowledge of barrier building and will help in the evaluation of the impact of future climate changes on coastal settings.

Complex systems in aeolian geomorphology

Aeolian geomorphology provides a rich ground for investigating Earth surface processes and landforms as complex systems. Sand transport by wind is a classic dissipative process with non-linear dynamics, while dune field evolution is a prototypical self-organisation phenomenon. Both of these broad areas of aeolian geomorphology are discussed and analysed in the context of complexity and a systems approach. A feedback loop analysis of the aeolian boundary-layer-flow/sediment-transport/bedform interactions, based on contemporary physical models, reveals that the system is fundamentally unstable (or at most meta-stable) and likely to exhibit chaotic behaviour. Recent field-experimental research on aeolian streamers and spatio-temporal transport patterns, however, indicates that sand transport by wind may be wholly controlled by a self-similar turbulence cascade in the boundary layer flow, and that key aspects of transport event time-series can be fully reproduced from a combination of (self-organised) 1/ f forcing, motion threshold, and saltation inertia. The evolution of various types of bare-sand dunes and dune field patterns have been simulated successfully with self-organising cellular automata that incorporate only simplified physically-based interactions (rules). Because of their undefined physical scale, however, it not clear whether they in fact simulate ripples (bedforms) or dunes (landforms), raising fundamental cross-cutting questions regarding the difference between aeolian dunes, impact ripples, and subaqueous (current) ripples and dunes. An extended cellular automaton (CA) model, currently under development, incorporates the effects of vegetation in the aeolian environment and is capable of simulating the development of nebkhas, blow-outs, and parabolic coastal dunes. Preliminary results indicate the potential for establishing phase diagrams and attractor trajectories for vegetated aeolian dunescapes. Progress is limited, however, by a serious lack of appropriate concepts for quantifying meaningful state variables at the landscape scale. State variables currently used in the bare-sand models are far from capturing the rich 3D topography and patterns and are not sufficiently discriminative to distinguish different attractors. The vegetation component in the extended model, and indeed ecogeomorphic systems in general, pose even graver challenges to establishing appropriate state variables. A re-examination of older concepts, such as landscape entropy, perhaps complemented by recent developments in information theory, may be a potentially fruitful avenue for research, although the outlines of such an implementation are still rather vague.

The Impact of Exotic Dune Grass Species on Foredune Development in Australia and New Zealand: a case study of Ammophila arenaria and Thinopyrum junceiforme

Marram grass (Ammophila arenaria) and sea-wheat grass (Thinopyrum junceiforme) have been introduced to Australia and New Zealand. This study examines the morphology of incipient foredunes and established foredunes associated with these species at two sites, Mason Bay in southern New Zealand, and the Younghusband Peninsula in South Australia. Both species invaded the existing foredunes very rapidly. In both cases the antecedent topography comprised relatively sparsely vegetated, irregular foredunes. Invasion resulted in continuous, regular, evenly vegetated foredunes. At Mason Bay a massive foredune has formed since 1958, in conjunction with Ammophila. Thinopyrum has formed an incipient foredune, with a ramp or terrace morphology, along the Younghusband Peninsula, South Australia. In both cases gaps in the former foredune have been closed and the indigenous foredune vegetation has been displaced. Both species may decrease the frequency and severity of blowout development. They are likely to be resilient to aeolian processes of sedimentation compared with dunes formed by indigenous species. Ammophila survives burial, is tolerant of drought and is resistant to erosion associated with storm surge and high waves. Thinopyrum is very tolerant of salinity. These species may adversely affect the long-term development of coastal barriers by inhibiting transgressive dune development.

Morphodynamics and climate controls of two aeolian blowouts on the northern Great Plains, Canada

AbstractBlowouts are the most regionally pervasive active aeolian landform on the northern Great Plains of North America. This study reports a long‐term investigation into the morphological development of two adjacent blowouts in a continental dune field. The blowouts were monitored for a decade in the Bigstick Sand Hills of southwestern Saskatchewan, Canada. Topographic changes were determined from dense arrays of erosion pins in each blowout (1 per 4 m2, n = 171; and 1 per 16 m2, n = 150). Pin measurements were made 16 times between May 1994 and May 2004. Over the decade both blowouts expanded and more than doubled in volume. Differences in form–flow interactions have caused the larger of the two blowouts to deposit more than a metre of sediment within the deflation basin, and the smaller blowout to erode by more than a metre. A negative feedback effect was triggered when the larger blowout reached a critical size around 1994 (60 m × 36 m × 8·1 m, length × width × height) when sediment was no longer eroded from the deflation basin. A positive feedback in the smaller blowout continues to facilitate erosion from the deflation basin. Monthly observations since 2002 indicate that aspect plays an important role in the development of these blowouts by creating a spatial asymmetry in sediment availability. Sediment is more readily available throughout the year on south‐facing slopes, which receive greater insolation than north‐facing slopes and are often drier and more frequently thawed in this cold‐climate environment. Comparisons between climate data from a remote meteorological station 45 km to the southwest and sediment transport indices developed from the erosion pin data produced very few correlations significant at the 95 per cent confidence level. Nevertheless, the signs of the correlation coefficients indicate that sediment erosion and deposition in both blowouts respond similarly to the following climate variables recorded at the remote station: (i) the amount of precipitation, (ii) the transport capacity of the wind and (iii) transporting winds from a directional wedge between 180 and 330°. Taken altogether, the results from this study highlight the importance of climate and feedback effects in blowout development that may be extended to other blowouts in continental and coastal settings. Copyright © 2006 John Wiley & Sons, Ltd.

Processes of Ammophila arenaria (Marram Grass) Invasion and Indigenous Species Displacement, Stewart Island, New Zealand

The present study (1) describes the rates and patterns of Ammophila arenaria (marram grass) invasion in a large transgressive dune system (Mason Bay) and on a prograding foredune-ridge barrier (Doughboy Bay), Stewart Island, New Zealand; (2) examines the impact of Ammophila on dune morphology and indigenous dune biota; and (3) assesses the significance of geomorphic processes in accounting for the patterns observed. Processes of Ammophila invasion are interpreted from evidence of landform development and vegetation change; field observations and survey of dune landforms and dune vegetation; the aerial photographic record and historic accounts of the local botany. The area dominated by Ammophila in the Mason Bay study area has increased from 1.4 ha in 1958, to 17.8 ha in 1978, to 74.9 ha in 1998; a 5,204 percent increase. Ammophila invasion of active dune systems in the study areas is clearly associated with dune forming processes—shadow dune development; migration of long-walled parabolic dunes; stoss face blowout development; and barrier progradation. The primary mechanism of native species displacement appears to be burial rather than competition for nutrients. Ammophila traps sand and builds dunes at rates that may exceed the threshold of tolerance of local native species. Desmoschoenus spiralis, the dominant indigenous foredune species, cannot co-exist with Ammophila in the active dune systems investigated.

Geomorphic response to Late Holocene climate variation and anthropogenic pressure, northeastern Prince Edward Island, Canada

Geomorphic response provides an index to climate change and variability. Differing time scales of both weather/climate events and landform response complicate interpretation. Anthropogenic modification, particularly in coastal and aeolian dune environments, represents a further geomorphic influence. Northeastern Prince Edward Island (PEI), Canada, is a marine-dominated environment subjected to rising sea level throughout the late Holocene. The combination of rising sea levels, increased anthropogenic utilization of the coast for residential, and tourism purposes, and limited offshore winter ice conditions have resulted in accelerated erosion and degradation of the dunes and coastline. The aeolian succession indicates that temporal and spatial transitions from isolated shield dunes to parabolic dunes to linked coastal transverse dune complexes are characteristic of the coastal dune fields. Sand supply is the most significant control on dune morphology, development, and extent. Superimposed dome dunes and shield dunes are prevalent in areas of anthropogenic disturbance. Dune migration along the coastal zone has varied considerably, with rates locally in excess of 10m/a over short times. Winters marked by relatively limited sea ice and scarce snow cover, resulting from more temperate winter conditions, allow increased erosion by strong storms. Deflation of dune crests and development of blowouts is notably accentuated along human trails. The climate variations recorded in Atlantic Canada over the past decade fall within the range of those recorded previously within the twentieth century, and thus do not indicate conclusive evidence of a pronounced trend of climate change in this region. However, analysis of northeastern PEI demonstrates that dune coastlines can evolve rapidly in response to short-term and intermediate-term climate variation, particularly when coupled with anthropogenic pressure. Similar geomorphic effects in combination with climate variation and storm activity should be anticipated along other dune coastlines that have been subjected to intense anthropogenic utilization during earlier periods of the Quaternary.

Topographic changes associated with coastal dune blowouts at island beach state park, New Jersey

AbstractTopographic changes in two blowouts located in Island Beach State Park, New Jersey, USA were monitored over the winter of 1981‐1982. Elevation changes were measured with erosion pins, and sediment traps placed at comparable locations in each blowout monitored the amount of sand moved by the wind. Discrete wind events were identified from regional data, and morphological data for the intervals with the highest onshore and offshore wind speeds are examined in detail.Vegetation is the primary influence on the development of the two blowouts. Blowout A is characterized by eroding sidewalls, a stable base, and an accreting blowout rim. High rates of sediment transport occur through the blowout throat which results in accretion on the vegetated rim. This blowout is an active sediment transfer system. Vegetation causes a large amount of deposition in the throat of blowout B. As vegetation was buried over the winter, the area of deposition migrated inland. Sidewall erosion also occurred in blowout B. Little change was recorded on the blowout rim. Blowout B is a recovering system where sediment is delivered to the blowout floor from the beach by onshore winds and from the blowout rim by offshore winds where it is stabilized by vegetation.The development of foredune blowouts is governed largely by vegetation cover on the dune crest and by sidewall erosion during offshore and onshore winds. Blowout recovery depends on vegetation growth and sediment deposition in the throat, and on the role of the sidewalls as sources of sediment which is deposited elsewhere within the system. Foredune blowouts are dynamic systems in which positive feedbacks in sediment availability and vegetation growth lead to a cycle of development and closure.

The effects of changing wind regimes on the development of blowouts in the coastal dunes of The Netherlands

Blowouts are the main features of aeolian activity in many dune areas. To assess the impact of future climatic change on the geomorphological processes prevailing in a dune landscape it is essential to understand blowout formation and identify the meteorological parameters which are important.The development, that is, local erosion and accumulation, of six blowouts in a dune terrain along the Dutch coast has been related to wind velocity and wind direction, as measured at a nearby standard meteorological station. Blowout changes correlate best with wind velocities between 6.25 and 12.5 m/s (measured at 10 m height) which are the critical wind velocities for moving particles in the 0.15 to 0.42 mm range. These winds mostly blow from the southwest. Consequently, the blowouts are elongated in the same direction.Extreme aeolian events such as northwestern storms have little effect on blowout development compared to events which have a lower magnitude but occur with a higher frequency. An eventual shift towards higher effective wind velocities would probably result not in larger blowouts but in a break-down of the whole system, especially if this shift were accompanied by a change in wind direction. The accumulation of sand in the blowouts during storms should be seen as a first step of adaptation to a higher energy level.

The development of dune blowouts, as measured with erosion pins and sequential air photos

The development of blowouts in dune terrains where no stabilization measures are taken requires constant watch to detect unwanted developments at an early stage. Two methods for investigating blowout development are in use. Since 1983, blowouts in a 40 ha section of Meijendel, a coastal dune area near The Hague have been twice yearly measured with erosion pins. Most of the 32 to 35 blowouts grew in length against the prevailing SW wind. The total blowout deflation area increased from 2.8 to 3.3% of the terrain in 4 years time. Air photos of 1958, 1967, 1977, 1984 and 1986 were used to study blowouts in a nearby area, De Blink north of Noordwijk, one of the most severely eroded dune terrains along the Dutch mainland coast. Total deflation area here increased from 0.8% in 1958 to 3.7% in 1986. There is a definite modal length class but the size is site-specific: 25 to 30 m in Meijendel, 15 to 20 m in De Blink. Many blowouts disappear shortly after they are formed. From the results it appears that the processes involved in the formation of blowouts can best be investigated with erosion pins whereas aerial photographs are preferable when changes in the spatial and temporal pattern of blowouts and the contributing factors are of interest.

Landscape development in Dutch coastal dunes: the breakdown and restoration of geomorphological and geohydrological processes

SynopsisIn Dutch coastal dunes, geomorphological and geohydrological processes have been adversely affected by intensive utilisation of the dunes. In the past, wide-scale stabilisation, notably for reasons of coastal defence, stopped nearly all aeolian activity. Rejuvenation did not take place and succession to mature-type ecosystems increased. Artificial infiltration with eutrophic river water and extraction of groundwater for drinking water catchment modified the geohydrology of the wet-dune environment.Today, actions are being taken to restore geomorphological and geohydrological processes. Managing authorities join forces with universities and policy makers to strike the right balance between conflicting interests. Applied research helps to develop management strategies.Two types of research are discussed: (a) geomorphological work on the development of blow-outs; (b) geohydrological work on the development of dune slacks. Questions considered are: do blow-outs grow to an unlimited extent or do they have optimum size and shape corresponding to aerodynamic conditions? Are there natural stabilisation mechanisms for blow-outs? What is the ecological impact of drinking water catchment? Which aspects should be considered to restore the dune slack environment in catchment areas?Our findings are that blow-outs in The Netherlands have an optimum length:width ratio of about 2 with a blow-out length varying between 20 and 30 m. After 10–15 years without artificial stabilisation measures, the areas studied present an average of about one blow-out per hectare. Natural stabilisation mechanisms include algae which colonise deflation sites inside the blow-out and low secondary dunes which develop at the immediate leeward side of the blow-out. Blow-outs situated at or near downhill slopes of 6–12° may easily extend into larger deflation surfaces. Water erosion, probably caused by the water-repellency of humus-rich top soil, often precedes wind erosion on south- and south-east oriented slopes.Drinking water production caused dessiccation of dune slacks and the disappearance of associated vegetation. In areas where infiltration with river water from outside the dunes is also applied, inudation and eutrophication drastically changed the dune environment. Nitrophilous plants appeared, especialy along banks of infiltration ponds, but also in seepage ponds. The plant cover depends on nutrient concentration of the (infiltrated) groundwater but especially on the flux of the groundwater. A high rate of flux leads to high nutrient loads, encouraging the growth and eventually dominance of nitrophilous plants. Restoration of dune slacks depends on the quality and the flux of the groundwater, on the streamlines of the groundwater flow in the dune body in relation to the orientation of the seepage ponds, on the presence of peaty or clayey adsorptive layers in the subsoil and possibly also on the presence of a rainwater lens occurring on top of the infiltration water.

A simulation model of blowout development

AbstractBlowouts are the main feature of wind erosion in the dunes along the Dutch coast. A number of characteristics, such as their growth against the prevailing wind and the spontaneous stabilization when certain dimensions are reached, are believed to be caused by a prominent quality of the southwest wind, viz. its gustiness. This hypothesis is tested in a two‐dimensional computer model which simulates the effect of wind gusts on a sandy surface along the main axis of the blowout. A number of constraints based on field observations are built into the model. The results closely resemble the longitudinal section of the blowout.

The development of blowouts in ‘de blink’, a coastal dune area near Noordwijkerhout, The Netherlands

AbstractSurface changes in six blowouts in the coastal dunes near Noordwijkerhout between s‐Gravenhage and Zandvoort were measured weekly over a period of two years at 80 erosion pin sites. Readings were correlated with wind variables derived from nearby Valkenburg airbase for the same weekly periods. The values of the wind variables is determined by the number of hours that specific average wind speeds and directions were recorded.The first part of this paper deals with the process of wind erosion. Special attention is given to the relationship between surface changes and wind speed, wind direction, season and rain. Surface changes are associated with wind speeds as low as 1.25 m/s, but net lowering, i.e. erosion, is best correlated with wind speeds between 8.75 and 10 m/s. At higher wind speed there is a tendency for the blowouts to be filled in. Winds from an easterly direction generally correlate with accumulation, whereas winds from the west correspond with accumulation at low speeds and with erosion at high speeds. Springtime and autumn are the most important seasons for blowout development. The effect of rain seems to be limited: the correlations are not improved by excluding rainy days, perhaps because wet sand will be dried rapidly by strong winds.In the next part of the paper, the response of blowouts to the erosion process is discussed. Wind erosion is dominant at the windward side because contrarily to what is often assumed the erosivity decreases as soon as the air is loaded with sand. As a result, blowouts grow southwestwards against the prevailing wind. Nevertheless, much sand is also shifted by winds from other directions. The forms of the blowouts determine in their turn the efficacy of the wind. The errosivity of the wind is most pronounced in circular blowouts, it decreases as the blowout grows in length.Finally, a model of blowout development is proposed which is controlled by width, depth and length ratios.