A numerical model of flow over sand waves in water of finite depth

Summary. A model of turbulent flow above sand waves in water of finite depth is described. Closure assumptions are based on an eddy viscosity proportional to the square root of the local value of turbulent kinetic energy and mixing length dependent upon distance from the lower boundary. Results are presented for some idealized cases to investigate the effects of wave slope, water depth, Froude number and wave shape. The implications of the model for the transport of sediment are discussed and the development of the wave investigated. It is found that the crest of the wave will become sharper for lower flow rates, as has been observed in the sea. Comparisons are made with recent measurements made over sand waves in the Columbia River.     

Doppler tomography of accretion regions in algols

The technique of Doppler Tomography has been used to image the accretion regions in five short-period Algols. There is clear evidence of gas flows along the predicted free-fall path of the gas stream as well as asymmetric disk-like structures around the mass gainer. Another source of H emission is associated with the cool magnetically active star.     

Discrete-element, individual-based and agent-based models: Tools for interdisciplinary enquiry in geography?

Discrete-element, individual-based and agent-based modelling techniques represent the interactions of individuals or entities with one another and their environments by sets of computational rules. These methods are now being used in geophysics, ecology, zoology, and increasingly in the social sciences. A characteristic is that the “agents” are embedded in an environment with which they interact, so that joint evolution of physical, ecological and socio-economic systems can be studied; this makes them of interest in integrated geographical investigation. Of particular interest is the fact that they allow the study of macro-scale emergent behaviour generated by multiple individual actions. Discrete techniques have the potential to create integrated models that cross disciplines. Similar computational methods can be employed to control the spatial search process, deal with irregular or changing boundaries, and to track the evolution of systems where preservation of heterogeneity across space and time is important. The complex series of feedbacks between the different types of system can thus be modelled within a single computational framework. Humans modelled in this way as “agents” can allow the collective effect of many interacting individuals to generate emergent structures at the community scale. The structural characteristics of their social and natural environments will influence their behaviour, while at the same time constituting emergent consequences of their actions. The principal challenge of the latter is to find those sets of rules that represent the beliefs and desires of real human agents, accurately reflecting the cultural context, while allowing us to explore the social and economic limitations of agents to take action. This paper reviews and illustrates discrete methods applicable to a range of geographical enquiries, and with particular reference to individual- and agent-based methods, assesses the rich diversity of approaches that must be combined to take full advantage of their potential to explore interactions of social and environmental processes.     

Evolution of Mesozoic fluvial systems along the SE flank of the West Siberian Basin, Russia

The Mesozoic stratigraphy in the subsurface of the West Siberian Basin contains prolific hydrocarbon accumulations, and thus the depositional environments of marine and marginal marine Jurassic and Cretaceous age sediments are well-established. However, no information is currently available on strata of equivalent age that crop out along the SE basin margin in the Mariinsk–Krasnoyarsk region, despite the potential of these exposures to supply important information on the sediment supply routes into the main basin. Detailed sedimentological analysis of Jurassic–Cretaceous clastic sediments, in conjunction with palaeo-botanical data, reveals five facies associations that reflect deposition in a range of continental environments. These include sediments that were deposited in braided river systems, which were best developed in the Early Jurassic. These early river systems infilled the relics of a topography that was possibly inherited from earlier Triassic rifting. More mature fluvial land systems evolved in the Mid to Late Jurassic. By the Mid Jurassic, well-defined overbank areas had become established, channel abandonment was commonplace, and mudrocks were deposited on floodplains. Coal deposition occurred in mires, which were subject to periodic incursions by crevasse splay processes. Cretaceous sedimentation saw a renewed influx of sand-grade sediment into the region. It is proposed that landscape evolution throughout the Jurassic was driven simply by peneplanation rather than tectonic processes. By contrast, the influx of sandstones in the Cretaceous is tentatively linked to hinterland rejuvenation/ tectonic uplift, possibly coeval with the growth of large deltaic clinoform complexes of the Neocomian in the basin subsurface.     

Ecological aspects of plant colonisation of the Krakatau Islands

Species assemblage data from the Krakatau Islands, Indonesia, are presented for the period 1883 to 1989 (including previously unpublished data from the 1989 survey). Since 1934, 16 additional families of higher plants have colonised. Recent arrivals at the family level are mostly of zoochorous species of forest tree, indicating (subject to the effects of disturbance) a continuing increase in potential niche space within the island interiors. The data for Rakata (an uninterrupted prisere) conform to a successional explanation in which identifiable ecological groups of plants exhibit differing colonization and turnover patterns. Animal-dispersed canopy tree species and species which are widespread within the group, exhibit a very low probability of extinction once they have colonized successfully. There are, however, several constraints on the rapid spread of species within the group, in particular those connected to local dispersal (eg lack of large terrestrial mammals). In respect of dispersal to the group, partial survey data for the island of Sebesi from 1921 (revised) and 1989 provides the basis for comment as to the changing biogeographical circumstances of the Sunda Straits and the role of Sebesi as a stepping stone island.The varied data discussed in the paper indicates that with the exception of the strand-line, no component of the Krakatau flora or vegetation has yet approached a stable composition. Both floral and faunal diversification are argued to be proximally controlled not only by dispersal opportunities but also by the dynamics of the dominant life-forms of the system, ie, the forest trees. Such hierarchical links, across trophic boundaries, should receive greater recognition in the construction of island biogeographical theory.