Niveo-aeolian sand deposition in subarctic dunes,eastern coast of Hudson Bay,Québec,Canada

Aeolian sand transport during winter and the snow-free season was assessed quantitatively by direct year-round field measurements along transects on the lee side of parabolic dunes in subarctic Québec. In 1987–1988, niveo-aeolian deposition was more important than aeolian sedimentation in three of the four study sites, and contributed > 75% of the total annual accumulation in exposed sites and < 25% in protected forest sites. The maximum depth of interstratified snow and sand deposits (3.5 m) was recorded in March. Semi-permanent snow lenses may persist longer than 2 years in the aeolian sediments. After dissipation of snow, 22 cm of sand (as a maximum) accumulated on the slipface of the most active dunes, whereas only minor sand accumulation occurred in distant areas from active sand erosion. Wind-driven sand was dispersed over 7.4 km² in the Whapmagoostui-Kuujjuarapik area. The acumulation of snow and sand during the snow season, together with spring thaw and collapse of the niveo-aeolian deposit, caused different types of injuries to trees, especially in 1985 and 1987 when a maximum of torn branches was recorded over the last 10 year period.     

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.     

Integrated coastal dune management: checklists

The main objective was to assess dune vulnerability—a reduced ability to adapt to change, which is of serious concern not only in Western Europe but on a world scale. A main root checklist together with two daughter ones were devised in order to achieve this objective. The main root checklist comprised six categories (site and dune morphology, beach condition, surface character of the seaward 200 m of dune, vegetation, anthropogenic impact and protection measures in place) and dunes were classified according to their vulnerability compared to protection measures. The Resilience daughter checklist encompassed dune erosion, sand input, sand retention, degradation by use and dune management. This was compared with risk factors such as obstacles to dune transgression and recreation. The geomorphology, aeolian, vegetation, anthropogenic, marine influences daughter checklist looked at 30 dune systems in the Gulf of Cadiz and assessed dune condition by a dune vulnerability index based on the above conditioning influences. Values for all checklists were expressed as percentages. Over 200 dune sites in Spain, France, Portugal and the UK were analysed by means of these checklists. Results confirmed that regional trends concerned with dune vulnerability could be assessed, although successful differentiation at the end of the dune spectrum (large sand sheets, e.g. Pyla, France and meadow formation, e.g. many of the UK systems) and at the local scale were muted. An innovative aerial digital photographic system was developed and photographs were successfully used to assess dune condition, anthropogenic impact on dune systems, etc., i.e. the checklist parameters and was used to modify checklist procedures in assessing regional dune degradation. Ca 70% of the checklist parameters could be obtained by this means. This also provided a photographic record of the current dune condition. Principal component analysis provided a strong justification for the majority of checklist questions and the approach and relevance of the technique. The enumerated techniques provide valuable tools for dune managers. Rapid assessment of large areas of coastline can be obtained at a low cost and the resulting remote sensed images can provide a permanent record of the condition of the photographed dunes.     

Lateral migration of linear dunes in the Strzelecki desert,Australia

Linear dunes in the Strzelecki Desert trend roughly south-north. Sand transport, which is toward the NNE, has caused the dunes to migrate eastward while they extend or migrate northward. Eastward lateral migration is evidenced by: (1) asymmetrical shape of the dunes; east-facing slopes are several times as steep as west-facing slopes; (2) asymmetrical accumulation of loose recently transported sand (relatively abundant on east-facing slopes); (3) asymmetrical outcropping of older semiconsolidated aeolian sand on the dune surface (more abundant on west-facing slopes); and (4) east-dipping foreset beds that underly the west-facing flanks of some dunes. Dunes in the Strzelecki Desert are still active in the sense that sand is transported along and across many dune crests. However, the dunes are composed primarily of Pleistocene strata, indicating that the trend of the dunes was established before the Holocene. The obliquity of the dunes to the transport direction is not merely an aberration of the wind regime of the last few decades. Preferential accumulation of sand on east-facing flanks indicates that the dunes migrated eastward several metres during the Holocene. Moreover, the west-facing flanks of some dunes have experienced a minimum of tens of metres of erosion. This asymmetric erosion and deposition were caused by dune obliquity and lateral migration that may have begun as early as the Pleistocene. Dunes in the Strzelecki Desert and in the adjacent Simpson Desert display a variety of grossly different internal structures. Computer graphics experiments demonstrate that many of these differences in structure can be explained by different angles of climb of the dunes.     

Bedload transport and sediment yield in the Onyx River,Antarctica

Bedload transport measurements were made in a braided reach of the Onyx River, Wright Valley, Antarctica, during summer 1984/85. Transport was predominantly of sand in the form of dunes, which moved in a band down the centre of the channels, the perimeters of which were composed of a gravel pavement created during short duration high flows in earlier years. Transport rates at-a-point and past-a-cross-section were highly variable in space and time, even under conditions of constant discharge, and it was inferred that many factors other than hydraulic conditions—particularly sediment supply—control transport rates. An empirical power function relationship between sediment discharge and water discharge was used to predict an average annual total sediment discharge of 3400 t y^?1 past the study reach. This gives a specific sediment yield of 5.9 t km^?2 · y^?1, which is two orders of magnitude less than values for Arctic and Alpine proglacial rivers, and confirms earlier conclusions that sedimentation rates on Antarctic sandur are much lower than in the arctic.     

Post-Chippewa oxygen isotope record from Cowles Bog, southern Lake Michigan basin

Previous studies of sediments and molluscs recovered from vibracores at Cowles Bog, a fen located in the Indiana Dunes National Lakeshore, along the south shore of Lake Michigan, reveal long and short term water level fluctuations during the last 6000 years. Low water events are indicated by zones of organic detritus, in which occasionally, iron oxide and calcium carbonate nodules, as well as selenite crystals have been precipitated. Oxygen isotope data from aragonitic shells of the gastropod Amnicola limosa (Say) collected from a sediment core provide a record of Middle to Late Holocene environmental changes for the fen. These data are in good agreement with previous interpretations of water level fluctuations based on changes in lithology and molluscan faunal abundance and composition. Below 366 cm the molluscan record is either absent or represented by shell fragments. The condition of shells in this interval suggests that the molluscs may have been exposed to subaerial weathering and reworking of older Holocene lake sediments, possibly during the low water Chippewa phase in the Lake Michigan basin (10000 YBP to 6000 YBP). Above 366 cm the core is characterized by a well preserved molluscan fauna. Relatively light isotopic values for the interval between 366 cm to 300 cm correlate with the transition from non-fossiliferous sands, peat and diamict to silty marl and calcareous sand, with a molluscan fauna dominated by taxa associated with permanent water bodies. The event producing these alterations, the Nipissing Transgression, marks a change from subaerial to permanent lacustrine conditions that were not characterized by high net evaporation. Evidence for another series of environmental changes occurs between 284 cm and 198 cm. This evidence includes the: (1) appearance of aquatic molluscs at 280 cm that are associated with water bodies subject to significant seasonal water level changes; (2) intermittent accumulations of iron oxide nodules, calcium carbonate nodules, and organic layers interbedded with crudely horizontal layers of fine, calcareous, sand, suggesting periodic water level oscillations; (3) onset of major excursions in the oxygen isotopic values between 260 cm to 198 cm. Relatively high ¹⁸O (PDB) values, possibly indicating evaporative enrichment of the water, correlate with a prominent shell debris layer at a depth of about 235 cm. Taken together, this evidence suggests that the core site was in the process of becoming isolated from Lake Michigan. This isolation occurred during a series of low water events during the later part of the Nipissing Transgression.     

Turbulence characteristics of flow region over a series of 2-D dune shaped structures

This paper addresses the development of a flow region associated with turbulence and stress characteristics over a series of 2-D asymmetric dunes placed successively at the flume surface. Experiments were conducted over twelve asymmetric dunes of mean length 32 cm, crest height 3 cm and the dune width almost as wide as width of the flume, using 3-D Micro-ADV at the Indian Statistical Institute, Calcutta. The variations of turbulence statistics along the flow affected by the wavy bottom roughness have been studied. Quadrant decomposition of the instantaneous Reynolds shear stress has been adopted to calculate the contribution of ejection and sweeping events in shear stress generation. The relative dominance of two events are found to contribute in a cyclic manner (spatially) in the near bed region, whereas such phenomenon seems to be disappeared towards the main flow.     

Assessing performance of a “nature-based” foredune restoration project,Oceano Dunes,California, USA

The Oceano Dunes near Pismo Beach, California is part of a large transgressive dune system that extends up to 5 km inland and hosts a state park that has been managed for off-highway vehicle recreation since 1982, although vehicle activity has existed in the dunes for almost 90 years. As a result, foredunes have been largely obliterated and sand surfaces in vehicle use areas are highly emissive of dust-sized particles, causing frequent exceedances of state air quality standards. To reduce dust emissions from the dunes, a nature-based foredune restoration strategy using five different treatments over a 20-ha site was implemented in February 2020. The research hypothesis is that treatments will differ in their ability to promote deposition and dune development and that more intensive planting-based treatments will outperform simpler treatments. We test this using biannual high-resolution uncrewed aerial system (UAS) surveys to quantify sediment budgets, sand exchanges between beach, foredune, and backdune components, changes in plant cover, and related dune development over a 2-year period (October 2019–2021). After two full wind and plant-growth seasons, results show that all treatments are maintaining a positive sediment budget (net accumulation), most are developing sizable nebkha dunes (an important stage in foredune development in this region), and some are increasing plant cover and species richness. There is no clear winner, yet two treatments (broadcast native plant and sterile grass seeds, and a high-density straw planting node with native seedlings) are performing well toward developing an incipient foredune. These results will inform an adaptive management process that could entail further modifications to enhance foredune development. Based on this experience, and with reference to other types of restoration projects, we refine existing criteria used to assess the performance of “dynamic” dune restoration efforts to include settings that involve revegetation (vs. devegetation) as a means for foredune re-establishment.     

Late Pleistocene dune construction in the Central Sand Plain of Wisconsin, USA

Wisconsin's Central Sand Plain east of the Wisconsin River is composed of eolian sand forming high-relief dunes surrounded by sand sheets and scattered low-relief dunes. To establish a maximum age for dune formation, three samples for optical dating were taken from glacial Lake Wisconsin lacustrine sediment that underlies eolian sand. These age estimates range from 19.3 to 13.6ka. Age estimates taken from within or at the base of the dunes range from 14.0 to 10.6ka. Samples taken from < 2m of the ground surface were slightly younger, indicating dunes were stabilized between 11.8 and 5.5ka. The younger ages near the surface of some dunes were most likely the result of pedoturbation or localized problems with applying the optical dating method. The majority of the optical age estimates from dunes (18 of 21) indicated that most of the dunes were active between 14 and 10ka and that most dune activity ended by 10ka. These ages suggest that localized activity on dune crests may have occurred in the Holocene but would have been limited to < 1m of sand accumulation. The timing of dune activity and the lack of any significant Holocene reactivation suggest that dune activation in this setting cannot be attributed solely to changes in aridity. Instead, we attribute dune formation to changes in sediment availability from either sand inputs from the Wisconsin River or the melting of permafrost.     

Detailed bed topography and sediment load measurements for two stepdown flows in a laboratory flume

Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bedforms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bedforms respond to rapid changes in flow rate,...