Active Region Formation through the Negative Effective Magnetic Pressure Instability

The negative effective magnetic-pressure instability operates on scales encompassing many turbulent eddies, which correspond to convection cells in the Sun. This instability is discussed here in connection with the formation of active regions near the surface layers of the Sun. This instability is related to the negative contribution of turbulence to the mean magnetic pressure that causes the formation of large-scale magnetic structures. For an isothermal layer, direct numerical simulations and mean-field simulations of this phenomenon are shown to agree in many details, for example the onset of the instability occurs at the same depth. This depth increases with increasing field strength, such that the growth rate of this instability is independent of the field strength, provided the magnetic structures are fully contained within the domain. A linear stability analysis is shown to support this finding. The instability also leads to a redistribution of turbulent intensity and gas pressure that could provide direct observational signatures.     

Flow separation at the inner (convex) and outer (concave) banks of constant‐width and widening open‐channel bends

There is a paucity of data and insight in the mechanisms of, and controls on flow separation and recirculation at natural sharply‐curved river bends. Herein we report on successful laboratory experiments that elucidate flow structure in one constant‐width bend and a second bend with an outer‐bank widening. The experiments were performed with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport. In the constant‐width bend with immobile bed, a zone of mainly horizontal flow separation (vertical rotational axis) formed at the inner bank that did not contain detectable flow recirculation, and an outer‐bank cell of secondary flow with streamwise oriented rotational axis. Surprisingly, the bend with widening at the outer bank and immobile bed did not lead to a transverse expansion of the flow. Rather, flow in the outer‐bank widening weakly recirculated around a vertical axis and hardly interacted with the inner part of the bend, which behaved as a constant‐width bend. In the mobile bed experiment, downstream of the bend apex a pronounced depositional bar developed at the inside of the bend and pronounced scour occurred at the outside. Moreover the deformed bed promoted flow separation over the bar, including return currents. In the constant‐width bend, the topographic steering impeded the generation of an outer‐bank cell of secondary flow. In the bend with outer‐bank widening, the topographic steering induced an outward expansion of the flow, whereby the major part of the discharge was conveyed in the central part of the widening section. Flow in the outer‐bank widening was highly three dimensional and included return currents near the bottom. In conclusion, the experiments elucidated three distinct processes of flow separation common in sharp bends: flow separation at the inner bank, an outer‐bank cell of secondary flow, and flow separation and recirculation in an outer‐bank widening. Copyright © 2012 John Wiley & Sons, Ltd.     

The life cycle of crevasse splays as a key mechanism in the aggradation of alluvial ridges and river avulsion

Accommodation space in the unconfined distal part of low‐gradient fluvial fans facilitates abundant floodplain deposition. Here, the development of crevasse splays plays a key role in the aggradation of alluvial ridges and subsequent river avulsion. This study presents an analysis of different stages in the evolution of crevasse splays based on observations made in the modern‐day Río Colorado dryland fluvial fan fringing the endorheic Altiplano Basin in Bolivia. A generic life cycle is proposed in which crevasse‐splay channels adjust towards a graded equilibrium profile with their lower‐lying distal termini acting as a local base level. Initial development is dominantly controlled by the outflow of floodwater, promoting erosion near the crevasse apex and deposition towards the splay fringes. When proximal incision advances to below the maximum level of floodplain inundation, return flow occurs during the waning stage of flooding. This floodwater reflux leads to a temporary repositioning of the local base level to the deeper trunk‐channel thalweg at the apex of the crevasse‐splay channels. The resultant decrease in the floodplainward gradient of these channels ultimately leads to backfilling and abandonment of the crevasse splay, leaving a subtle local elevation of the floodplain. Consecutive splays form an alluvial ridge through lateral amalgamation and subsequent vertical stacking, which is mirrored by the aggradation of their parent channel floor. As this alluvial ridge becomes increasingly perched above the surrounding floodplain, splay equilibration may cause incision of the levee crevasse down to or below its trunk channel thalweg, leading to an avulsion. The mechanisms proposed in this study are relevant to fluvial settings promoting progradational avulsions. The relatively rapid accumulation rate and high preservation potential of crevasse splays in this setting makes them an important constituent of the resultant fluvial stratigraphy, amongst which are hydrocarbon‐bearing successions. Copyright © 2018 John Wiley & Sons, Ltd.     

Accessibility and land-use patterns at the forest fringe in the northeastern part of the Philippines

Accessibility is considered to be one of the most important determinants of use and land-cover change. In rural land-use change studies, the accessibility situation is often described by simple measures of the distance to a location of interest. In this paper, different measures of access are tested for a rural area at the forest fringe in the northeastern Philippines. The accessibility measures addressed range from simple distance measures to land-use type specific transport costs and a population potential measure. The different measures are tested based on their capacity to explain the spatial pattern of different land-use types. A comparison of the findings based on a spatial analysis and an analysis of household level data is made. It is concluded that the relation between land use and accessibility is dependent on the specific characteristics of the different land-use types. The (dis-)advantages of the use of the different accessibility measures are discussed.