Flow within a Trough Blowout at Cape Cod (original) (raw)
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Spatial–temporal evolution of aeolian blowout dunes at Cape Cod
This paper explores historical evolution of blowouts at Cape Cod National Seashore (CCNS), USA — a site that hosts one of the world's highest densities of active and stabilized blowouts. The Spatial–Temporal Analysis of Moving Polygons (STAMP) method is applied to a multi-decadal dataset of aerial photography and LiDAR to extract patterns of two-dimensional movement andmorphometric changes in erosional deflation basins and depositional lobes. Blowout development in CCNS is characterized by several geometric (overlap) and movement (proximity) responses, including: i) generation and disappearance, ii) extension and contraction, iii) union or division, iv) clustering and v) divergence by stabilization. Other possible movement events include migration, amalgamation and proximal stabilization, but they were not observed in this study. Generation events were more frequent than disappearance events; the former were highest between 1985 and 1994, while the latter were highest between 2000 and 2005. High rates of areal change in erosional basins occurred between 1998 and 2000 (+3932 m2 a−1), the lowest rate (+333 m2 a−1) between 2005 and 2009, and the maximum rate (+4589 m2 a−1) between 2009 and 2011. Union events occurred mostly in recent years (2000–2012), while only one division was observed earlier (1985–1994). Net areal changes of lobes showed gradual growth from a period of contraction (−1119 m2 a−1) between 1998 and 2000 to rapid extension (+2030 m2 a−1) by 2010, which is roughly concurrent with rapid growth of erosional basins between 2005 and 2009. Blowouts extended radially in this multi-modal wind regime and, despite odd shapes initially, they became simpler in form (more circular) and larger over time. Net extension of erosional basins was toward ESE (109°) while depositional lobes extended SSE (147°). Lobes were aligned with the strongest (winter) sand drift vector although their magnitude of areal extension was only 33% that of the basins. These differences in extension responses likely result from more complex and evolving flow-form interactions inside erosional basins. Historical photographs and CCNS documents suggest that blowout evolution may be influenced by land-use changes, such as revegetation campaigns in 1985 that were followed by high blowout generation. High magnitude regional storm events (e.g., hurricanes) also play a role. The analytical framework presented provides a systematic means for twodimensional geomorphic change detection and pattern analysis that can be applied to other landscapes.
Aeolian Research
This paper examines the æolian dynamics of a deep bowl blowout within the foredune of the Greenwich Dunes, on the northeastern shore or Prince Edward Island, Canada. Masts of cup anemometers and sonic anemometers were utilized to measure flow velocities and directions during a strong regional ESE (offshore) wind event. The flow across the blowout immediately separated at the upwind rim crest, and within the blowout was strongly reversed. High, negative vertical flows occurred down the downwind (but seaward) vertical scarp which projected into the separation envelope and topographically forced flow back into the blowout. A pronounced, accelerated jet flow existed near the surface across the blowout basin, and the flow exhibited a complex, anti-clockwise structure with the near-surface flow following the contours around the blowout basin and lower slopes. Significant æolian sediment transport occurred across the whole bowl basin and sediment was delivered by saltation and suspension o...
Blowout evolution in a coastal dune: using GPR, aerial imagery and core records
The aim of this work is to present the results of GPR surveys from a large trough blowout and a saucer blowout associated to an energetic shoreline in a temperate- humid coastline (NW of Spain Traba sedimentary complex), cross-validated with core data and field exposures. Finally, to relate the sedimentological information obtained from the interpreted reflection profiles to the geomorphological development of the blowouts, as revealed by aerial photographs and other evidences. GPR can provide i8nsights into the internal stratigraphy of a blowout dune; this linking with aerial imagery provides a suitable tool. The pre-blowout dunes, in which the blowouts are developed, presented a similar radar facies and corresponded with sequences of the accretionary state of the dune complex. They overlie older deposits consisting of gray sands with organic-rich humic horizons, showing wetter conditions. The combination of different factors, such as previous topography, winds and wave erosion of foredunes and changes in vegetation cover, are the driving force to the initiation of blowouts. A detailed study of 2D/3D GPR surveys indicates that the internal sedimentary structure of the depositional lobe is more complicated than expected, and this pattern appears related mainly to changes in the vegetation cover. The results show how much the direction of blowouts migration is dependent on changes in near-surface wind flows due to the presence/absence of vegetation.
Geomorphology, 2009
This study reports the responses of three-dimensional near-surface airflow over a vegetated foredune to variations in the conditions of incident flow during an 8-h experiment. Two parallel measurement transects were established on morphologically different dune profiles: i) a taller, concave-convex West foredune transect with 0.5-m high, densely vegetated (45%), seaward incipient foredune, and ii) a shorter, concavestraight East foredune transect with lower, sparsely vegetated (14%) seaward incipient foredune. Five stations on each transect from the incipient dune to the crest were equipped with ultrasonic anemometers at 0.6 and 1.65 m height and logged at 1 Hz. Incident conditions were recorded from a 4-m tower over a flat beach. Winds increased from 6 m s − 1 to N 20 m s − 1 and were generally obliquely onshore (ENE, 73°). Three subevents and the population of 10-minute averages of key properties of flow (U, W, S, CV U ) from all sample locations on the East transect (n = 235) are examined to identify location-and profile-specific responses over 52°of the incident direction of flow (from 11 to 63°onshore). Topographic steering and forcing cause major deviations in the properties and vectors of near-surface flow from the regional wind. Topographic forcing on the concave-straight dune profile increases wind speed and steadiness toward the crest, with speed-up values to 65% in the backshore. Wind speed and steadiness of flow are least responsive to changes in incident angle in the backshore because of stagnation of flow and are most responsive at the lower stoss under pronounced streamline compression. On the steeper concaveconvex profile, speed and steadiness decrease toward the crest because of stagnation of flow at the toe and flow expansion at the slope inflection point on the lower stoss. Net downward vertical velocity occurs over both profiles, increases toward the crest, and reflects enhanced turbulent momentum conveyance toward the surface. All of these flow responses are enhanced with faster speeds of incident flow and/or more onshore winds. Significant onshore steering of near-surface vectors of flow (to 37°) occurs and is greatest closer to the surface and during highly oblique winds (~15°onshore). Therefore, even subtle effects of streamline compression and amplification of flow under alongshore conditions effectively steer flow and sand transport toward the dune. As topographic forcing and steering cause significant, three-dimensional deviations in near-surface properties of flow, most regional-scale and/or two-dimensional models of dune process-response dynamics are insufficient for characterizing coastal and desert dune sediment budgets and morphodynamics. In particular, deflection of sand transport vectors with greater fetch distances than those derived from regional winds may occur. Coincident flow, transport and morphological response data are required to better quantitatively model these processes.
Earth Surface Processes and Landforms, 2014
Blowouts are depressions that occur on coastal dunes, deserts and grasslands. The absence of vegetation in blowouts permits high speed winds to entrain and remove sediment. Whereas much research has examined patterns of wind flow and sediment transport on the stoss slopes and lee of sand dunes, no study has yet investigated the connections between secondary air-flow structures and sediment transport in a blowout where zones of streamline compression, expansion and steering are less clearly delineated. In this study we investigated the variability of sediment flux and its relation to near-surface wind speed and turbulence within a trough blowout during wind flow that was oblique to the axis of the blowout. Wind flow was measured using six, three-dimensional (3D) ultrasonic anemometers while sediment flux by eight sand traps, all operating at 25 Hz.
Numerical modelling of airflow over an idealised transverse dune
Environmental Modelling and Software, 2004
57 The general flow structure over transverse aeolian dunes is now well documented through both field studies and wind tunnel 58 experiments. Research on windward (stoss) slopes of dunes is extensive and has recently been complemented by research on the 59 lee-side flow structure. However, a number of technical deficiencies in wind tunnel instrumentation and a lack of detailed resolution 60 in and appropriate turbulence instrumentation for field research have resulted in an incomplete quantified characterisation of the 61 flow structure over aeolian dunes. This study applies a two-dimensional numerical model with an RNG-modified -⑀ turbulence 62 model to simulate the time-averaged flow field over an idealized aeolian dune. The model is successfully validated with wind tunnel 63 experimental data. Results indicate that the model accurately predicts the flow patterns over the dune, producing regions of flow 64 stagnation at the toe, acceleration up the stoss slope and a region of flow separation and reversal in the lee. Further development 65 and application of this model will allow examination of flow-form interaction, the testing of more complex isolated dune morpho-66 logies, and characterisation of flow over multiple dunes.
Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry
Geomorphology, 2004
A Computational Fluid Dynamics (CFD) model (PHOENICSk 3.5) previously validated for wind tunnel measurements is used to simulate the streamwise and vertical velocity flow fields over idealized transverse dunes of varying height (h) and stoss slope basal length (L). The model accurately reproduced patterns of: flow deceleration at the dune toe; stoss flow acceleration; vertical lift in the crest region; lee-side flow separation, re-attachment and reversal; and flow recovery distance. Results indicate that the flow field over transverse dunes is particularly sensitive to changes in dune height, with an increase in height resulting in flow deceleration at the toe, streamwise acceleration and vertical lift at the crest, and an increase in the extent of, and strength of reversed flows within, the lee-side separation cell. In general, the length of the separation zone varied from 3 to 15 h from the crest and increased over taller, steeper dunes. Similarly, the flow recovery distance ranged from 45 to >75 h and was more sensitive to changes in dune height. For the range of dune shapes investigated in this study, the differing effects of height and stoss slope length raise questions regarding the applicability of dune aspect ratio as a parameter for explaining airflow over transverse dunes. Evidence is also provided to support existing research on: streamline curvature and the maintenance of sand transport in the toe region; vertical lift in the crest region and its effect on grainfall delivery; relations between the turbulent shear layer and downward forcing of flow re-attachment; and extended flow recovery distances beyond the separation cell. Field validation is required to test these findings in natural settings. Future applications of the model will characterize turbulence and shear stress fields, examine the effects of more complex isolated dune forms and investigate flow over multiple dunes. D
2016
33 Measurements of lee-side airflow response from an extensive array of meteorological instruments 34 combined with smoke and flow streamer visualization is used to examine the development and 35 morphodynamic significance of the lee-side separation vortex over closely spaced transverse dune 36 ridges. A differential deflection mechanism is presented that explains the three-dimensional pattern 37 of lee-side airflow structure for a variety of incident flow angles. These flow patterns produce 38 reversed, along-dune and deflected surface sand transport in the lee that result in a net ‘lateral 39 diversion’ of sand mass transport over one dune wavelength for incident angles as small as 10° 40 from crest-transverse (i.e., 80° from the crest line). This lateral displacement in fluid mass transport 41 increases markedly with incident flow angle, when expressed as the absolute value of the total 42 deflection in degrees. Reversed flow and multidirectional sand transport occur for incident...
Lee Angle Effects in Near Bed Turbulence: An Experimental Study on Low and Sharp Angle Dunes
International Journal of Hydraulic Engineering, 2012
This research presents recent advances on morphodynamic modeling over gravel dunes. Boundary-layer separation over gravel fixed dunes is investigated by Acoustic Doppler Velocimetry (ADV). Using the measurements of flow over dunes at laboratory scale, examined the influence of dune lee sides on the separation of flow. Experiments were conducted in a horizontal flu me. A train of 2D fixed dunes were installed on the bottom of the flu me starting just downstream of the entrance on the flu me. Experiments were carried out using 2 types of dunes with different lee slopes, (38 and 8 degree). The results of quadrant analyses base on the stress fraction SHf versus the hole size at t wo elevations near the bed and near water surface for two angles of lee side, ,at the central axis of the flu me also investigated. Both experimental observations and quadrant results were in agreement about the influence of dune characteristics on the flow separation in different dune lee angles.