Morphology of Sand Dune with Vegetation Using a Numerical Simulation

Vegetated dune morphodynamics during recent stabilization of the Mu Us dune field, north-central China

Regional differences in the grain size characteristics of surface sediments from typical barchan dunes in arid zones

The influence of various factors, such as the natural properties of rock and in-situ stress conditions, results in uneven and rough fracture surfaces post-hydraulic fracturing. This significantly impacts the transport and placement of proppant within the fracture, thereby affecting the effectiveness of fracture stimulation. This study employs the rectangular wave method to characterize the roughness of fracture wall morphology, detailing the variation of roughness by altering the number and height of micro-protuberances, and constructs a three-dimensional model of rough fractures. The Euler–Euler model is utilized to simulate the placement and transport patterns of proppant within the fracture. Sand banks within the fracture profile are segmented based on proppant concentration, and the dimensionless area of each concentration interval is calculated to analyze the structure of sand banks and the suspension and settling effects of proppant. This research investigates the variation patterns of sand dune structures within fractures characterized by different levels of roughness and morphologies; it also examines the impact of injection velocity and fracturing fluid viscosity on the transport and placement of proppant within rough fractures. The findings indicate that the complex spatial structure of rough fractures modifies the edge shape of sand dunes. Moreover, it impedes proppant transport, leading to the formation of sand plugs near the wellbore. The maximum distance of sand placement for rough fractures is only 55.2% of that for fractures without considering roughness. The increase in the number and height of micro-protrusions enhances fracture roughness, leading to a stronger retarding effect. However, variations in these two types of roughness have distinct impacts on the morphology of sand dunes. Higher injection velocities facilitate the transport of proppant within rough fractures. The furthest distance of proppant placement at an injection velocity of 0.5 m3/min is only 68.4% of that at an injection velocity of 1.5 m3/min. The study’s findings contribute to a more intuitive understanding of the impact of rough fracture wall surfaces on the transport and placement patterns of proppant, providing a foundation for the optimization of fracturing design and operational parameters.

Types of internal structure and external morphology of submarine dunes under the influence of tide- and wind-driven processes (Dover Strait, northern France)

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.

Seasat and aircraft radar imagery of five areas of sand dunes in the southwestern United States and northwestern Mexico have been studied and compared to Landsat imagery and air photos. Radar imaging system parameters included two wavelengths (23.5 cm and 3.0 cm) and incidence angles ranging from 0° through 70°. Horizontal polarization was used on both transmit and receive for all radar imagery. Resolution ranged from 12 m for the best aircraft imagery to about 40 m for Seasat. Sand dunes studied behave as smooth surfaces for both 23.5 cm and 3.0 cm wavelength radars and the reflection of the radar energy from the dunes is specular. Measured surface irregularities of sand dunes are inadequate to cause either Bragg or incoherent scattering of the incident radar beam in the 3.0 to 23.5 cm wavelength region. This results in dunes that appear dark when imaged at incidence angles greater than the angle of repose because none of the dune faces are oriented properly (normal to the incident radar beam) to give a specular return to the antenna. A scattering model based on specular return from randomly oriented flat dune faces was used and experimentally verified using aircraft data. Imaging radar resolution must be adequate to define the changing distribution of dune faces that characterize the dune morphology. Linear dune features with interdune spacing of less than 3–4 resolution elements are not recognizable. Distributed dune types such as barchans or stars must be more than about 10 resolution elements in each direction to be defined on the imagery. Illumination direction of the radar beam is important; directional dune features must be oriented within about 60° of perpendicular to the radar illumination direction in order to be imaged. If such features have considerable topographic relief, the direction of radar illumination is less important. Because directional trends in unexplored regions cannot be predicted, availability of radar imagery from two directions greatly facilitates interpretation of dune morphology and hence conclusions about causative wind regimes.

Titan’s climate includes dynamic processes at multiple scales, ranging from local turbulence to global super-rotation. Several mesoscale models are currently being developed which will permit simulations of surface winds at kilometric resolutions. However, mesoscale modeling requires high-resolution topography as an input, as it can be one of the primary controls on surface wind directions and amplitudes. The Cassini-Huygens mission, which spent 13 years in orbit around Saturn, mapped Titan’s surface at resolutions up to about 300 m/pixels with its RADAR instrument, which operated at a wavelength of 2.17 cm (13.78 GHz frequency, Ku-band). Titan’s best topography maps (Corlies et al., 2017) consist in a combination of radar altimetry, the SARTopo dataset (Stiles et al., 2009), and SAR stereoradargrammetry, interpolated where data is unavailable. The available topographic information thus has low spatial resolution, unequal coverage, and very variable accuracy (e.g., Fig. 1), rendering it unusable as an input for mesoscale modeling. In the absence of better topography data for the foreseeable future, we create fully parametrized synthetic topography, based on the surface geomorphology and the available topography. A similar method is used to create synthetic maps of the surface albedo, aerodynamic roughness length, and thermal inertia, whose influence on surface winds can thus be tested.In order to study (1) the surface atmospheric and sedimentary environment at the Dragonfly landing site and (2) the role of surface liquids in mesoscale atmospheric circulation, we focus on two regions: a 1000-km-wide region centered on the Dragonfly landing site near Selk crater, and a 250-km-wide area in the northern small lakes region. We first create a geomorphological map based on Cassini Synthetic Aperture Radar (SAR) images (Fig. 1), simpler than previous geomorphological maps of these regions (Malaska et al., 2016; Birch et al., 2017) in order to minimize the number of parameters. Each geomorphological unit (dune fields, mountains, plains, craters, lakes, valleys…) is given a fully parametrized shape, and a random surface topographic roughness is added everywhere (except on the smooth lakes). Reasonable values of the parameters are chosen from existing topography data, resulting in maps such as the one in Fig. 2. The same method is applied to create synthetic maps of the thermal inertia, surface albedo, and aerodynamic roughness z0: each unit is assigned a value chosen from the literature.The resulting topography, albedo, thermal inertia, and roughness maps are simplified and assume that all features of a unit are identical: these maps therefore look reasonable, but are non-unique and cannot match well with the observations. However, they have the great advantage of being parametrized, which allows us to vary each parameter and test its influence on mesoscale model outputs such as surface winds. Some key questions that we are seeking to answer include: At what height do crater rims and mountains on Titan start to significantly influence surface winds? How do the winds in a realistic lakes region compare to those over an idealized circular lake? Can topography cause wind patterns responsible for the dune morphologies observed on Titan, including cross-hatched dunes at the Dragonfly landing site?In the equatorial region, which is a dry, desert environment, we use the mesoscale model developed by Lefèvre et al. (abstract in this session), which couples the Weather-Research Forecast (WRF) nonhydrostatic dynamical core (Skamarock et al., 2008) with the LMD Titan PCM physics package (Lebonnois et al., 2012). Meanwhile, in the North polar small lakes region, we use the mesoscale Titan WRF, which reproduces the methane cycle, and can therefore model the evaporation and transport of methane (Chatain et al., abstract in this session).While other presentations at EPSC (Chatain et al., Lefèvre et al.) will focus on the mesoscale modeling efforts, including the effect of topography on the model, we will detail the synthetic topography model and present preliminary results on the relationship between surface winds and dune morphology for a “best guess” topography. In particular, we use the surface winds output by the mesoscale model in the equatorial regions as inputs to the IPGP dune morphodynamics model (Narteau et al., 2009) to attempt to reproduce the reticulated dunes observed around Selk crater (Malaska et al., 2016). Figure 1: Mapping the Selk crater region from Cassini SAR and topographyFigure 2: A synthetic topography map of the Selk crater region. Altitude is given in meters. 

Object-based Dune Analysis: Automated dune mapping and pattern characterization for Ganges Chasma and Gale crater, Mars

The degraded linear dunefield of north west Ngamiland, Botswana occurs in a seismically active area, lying to the northwest of the Okavango graben, widely considered to be the tail end of the East African Rift system. To assess the effects of neotectonism on the dunes, an area was selected for examination close to the Gumare fault, which bounds the graben on the northwest side. Digital SRTM data were converted to light shaded representation of the topography. It was found that dune forms only occur along the margin of the rift and on both sides of incising valleys, graded to the graben floor. A selection of long profiles showed dune crests standing some 25 m above the straats at the edge of the rift. The relative relief pinches out away from the fault, towards interfluves that do not depict dune morphology. Landsat satellite imagery shows linear features continuous across the flat interfluves, inviting the incorrect inference from the vegetation that the linear dune forms occur there. A model is proposed that an original linear dune field formed under arid conditions, was entirely flattened under wetter conditions, crest material being washed into the straats, thus obliterating the original dune morphology. Upon rifting and stream incision, these forms are being replicated, an example of equifinality. Replication is suggested to be by the action of infiltrating water, controlled by the groundwater gradients. These observations suggest that the active dunefield significantly predates the tectonic processes. With the additional time required for degradation and replication it would suggest that the dunes are of considerable antiquity. It has already been recognised that the dunefield has a complex history of construction and destruction, to which must now be added a process of base level-controlled replication.

Buried pipe is usually used for laying submarine pipeline. However, in the process of submarine trenching and pipe laying, the pipe laying route is not a straight line, some are perpendicular to the direction of sand dune ridge, and some are along the direction of sand dune ridge. In order to study the scouring effect of the two paths, the numerical scouring simulation of the two methods was carried out. Firstly, the Sediment Scour Model in FLOW-3D was used to simulate the vertical jet scouring of single nozzle. The numerical simulation results were in good agreement with the experimental results in the references, which verified the reliability of FLOW-3D Sediment Scour Model to simulate the real jet scouring. Then, the numerical model of biaxial flow pump passing through sand dunes was established, and the scouring characteristics in the direction of the vertical dune ridge line and along the direction of the dune ridge line were compared. The results show that the ditch depth in these two ways is similar, but the ditch width along sand dune ridge line is wider, which can provide a certain reference for practical construction.

The kinematics and morphodynamics of low‐amplitude, small‐scale sand waves developed over migrating dunes are examined using data drawn from laboratory experiments. We refer to the superimposed features as “sand sheets,” a general descriptive term for low‐amplitude bed waves that are not easily classified as ripples, dunes, or bars. Within the experiments, the sheets formed downstream of the reattachment point at a distance that was invariant with dune size. Some sheets lacked slip faces composed of sand grains avalanching down a slope near the angle of repose. Over equilibrium dunes, three to four sand sheets were observed per 100 s. Sheet thickness was 10% of the height of the dune upon which they were superimposed; they migrated at 8 to 10 times the dune rate; they had nearly constant lengths over the full range of dune lengths and flow conditions; and they had aspect ratios of ∼0.025. Dunes and sand sheets represent distinct scales of sediment transport with different migration rates. However, sediment transport rates, calculated from the sand sheet and dune morphologies, are nearly identical. For transport equivalence to occur, sand sheets migrating at 10 times the dune rate must be 0.1 times the size, which is consistent with the morphological observations. Superimposed bed waves on dunes are often considered simply as additional roughness elements, but these results indicate that such bed waves are the agency by which the dune bed form itself moves downstream.

The Sixteen Mile Beach Complex is one of numerous active dune accumulations along the West Coast of southern Africa. The beach complex is composed of a sandy beach, coast-parallel dunes, and a dune cordon. Grain size analyses, calcium carbonate content, sand texture and composition, and radiocarbon ages were determined to understand the Holocene evolution of the Sixteen Mile Beach Complex. Changes in sand grain size and dune morphology allow the complex to be divided into three parts. The beach and the dune cordon at the southern end are composed of fine sand. The central beach has a rapid decrease in the fine sand fraction that coincides with the transition from the dune cordon to a single large coast-parallel dune ridge. The northern beach is composed of medium sand and consists of a series of prograded, vegetated coast-parallel dune ridges. The formation of these distinct regimes is a reflection of the different amounts of wave energy received by the complex from the predominant southwest swel...

We present a systematic assessment of the complex aeolian bedforms across the western rim of Hellas Planitia, the largest impact basin on Mars. Repeat orbital imaging shows crescentic, transverse, and star dunes across the Hellespontus Montes region migrate, converge, and occasionally reverse course. Dune morphology and sand ripple migration identify three effective wind regimes that vary spatially and temporally, correlating with atmospheric modeling. Peak sand transport occurs during southern spring and summer to the west and southwest, driven by daytime anabatic slope winds originating from Hellas. Migration rates decline with waning temperatures, when eastward drainage winds into Hellas dominate nights annually and during the colder seasons. However, these diurnally and seasonally cyclic slope winds can be highly localized, as nearby barchan fields swiftly migrate exclusively west where eastward winds are mitigated by topography. These novel findings highlight the complexity of surface‐atmosphere interactions in shaping aeolian systems on Mars.

Shows that the morphology of the sand dunes in the Beni-Abbes region, Algeria, is due not only to the force and direction of the wind (both present-day and Quaternary), but also to the granulometric composition of component sands.

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.