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Morphologic evolutionary systems
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
Denudation chronology is concerned with the reconstruction of the evolution of landforms for a given area through time by means of the absolute or relative dating of erosional and depositional events occurring under the influence of tectonic, eustatic, climatic, or other variations. The most simple reconstructions of denudation chronology rest on the assumption that landform assemblages are a palimpsest of superimposed parts of different erosional cycles each initiated by a change of baselevel of tectonic or eustatic origin. An obvious difficulty is that, as we have seen, the possible geomorphic results of negative movements of baselevel are more widespread and striking than those of positive movements, but because these results are predominantly erosional, they are more difficult to date. Unlike sedimentation, upon which most of the study of historical geology is based, erosional processes are historically defective in that they tend to destroy the signs of their own agency. It is for these reasons that large-scale cyclic studies of denudation chronology tend to identify the results of a supposed sequence of sporadic negative movements of baselevel, and they commonly apply to a long time period with few internal time constraints. The most ambiguous denudation chronologies employ widespread morphological evidence of accordances, flats and breaks of slope of supposed erosional origin to postulate a widespread series of complete or partial peneplains, or of planes of marine denudation formed close to pre-existing baselevels, and which have been relatively or absolutely uplifted by negative baselevel movements. By contrast, the most unambiguous denudation chronologies are those which are spatially limited to the immediate vicinity of rivers within which a datable sedimentary record may give a much clearer picture of erosional and depositional events over a short historical timespan.
Process-based approach on tidal inlet evolution – Part 1
Published in C. Marjolein Dohmen-Janssen, Suzanne J.M.H. Hulscher, River, Coastal and Estuarine Morphodynamics: RCEM 2007, 2019
D.M.P.K. Dissanayake, J.A. Roelvink
Natural rivers are self-formed features whose shapes are the result of interaction between erosion, deposition and transport of sediments. The study of their morphodynamics and the characterization of related sedimentary processes are of great interest not only to environmental engineers but also to hydrology and historical geology, contributing to the interpretation of stratigraphic records. In the present contribution we focus our attention on the long-term behaviour of meandering rivers, a very common pattern in nature, which belongs to a class of dynamical systems occurring at the spatial scale of the channel width and driven by the coexistence of complex linear and non-linear processes. On the short term time scale, the formation of meandering patterns can be suitably explained as an instability process, driven by bank erosion (bend instability). The planar development of the river is described by a non-linear integro-differential bend evolution equation, complemented with a suitable model for flow and bed topography in sinuous channels with cohesionless bed. On the long-term timescale, a further highly non-linear process must be accounted for, namely channel shortening via cutoff processes. Depending on the description adopted for the flow field, various mathematical models allowing the description of the temporal evolution of the channel axis can be developed. The problem then arise to compare the morphologic characteristics of the planimetric configurations obtained using the different flow field models as well as the differences/analogies between calculated patterns and those observed in the field. Usually, the comparison is pursued by considering a few typical variables (e.g., cartesian and intrinsic wavelengths, sinuosity, curvature) which, however, if separately investigated cannot provide an objective and discriminant description of the complex morphologic features exhibited by either calculated or observed planimetric configurations. In order to evaluate the differences/similarities of the patterns calculated considering different flow field models and to test the related long-term prediction capabilities, a significant sample of both computed planar configurations and of planimetric patterns extracted from Landsat mosaic images is analyzed through a more complete statistical method of characterization of channel axis configuration, based on a wide set of morphological variables.
Animal, Vegetable, Mineral? How Eighteenth-Century Science Disrupted the Natural Order
Published in Ambix, 2018
Gibson's book starts with a concise portrayal of natural history (historia naturalis) as it was developed in antiquity (Aristotle, Pliny the Elder), resumed in the Christian age (Albert the Great), and vigorously pursued since the Renaissance up to the eighteenth century (chapter 1). The first challenging natural being addressed in the second chapter is the sweet water polyp which is—as was discovered then—able to regenerate parts of its body that have been cut off. The capability of regenerating lost body parts was known of plants but not of animals. Was the polyp a zoophyte—a hybrid being between the kingdoms of plants and animals? The chapter further records efforts to capitalise on chemical analyses of plant and animal materials to achieve a reliable criterion for the discrimination of these two kingdoms. The third chapter deals with two related eighteenth-century controversies: the issue of whether plants reproduce sexually like animals (if so, an old clear criterion for discriminating plants from animals would prove to be invalid); and the debate between adherents of preformism and epigenesis, concerning the question of how ontogenesis ought to be understood. Chapter 4 shows that the borderline between minerals and living beings was at stake when naturalist tried to understand corals. It also pursues the impact that knowledge of fossils and their distribution in different geological strata had on the emergence of an historical geology, with which a history of nature itself became undeniable for the first time. The last chapter finally addresses opposing stances in plant physiology of the age: Are plants hydraulic machines or beings endowed with a vital force? It shows that in these stances no less than the question of ‘What is life?’ was renegotiated. In an epilogue, Gibson provides an outline of how these questions, which had vexed eighteenth-century naturalists, philosophers, and even a broader public, were dealt with in the life sciences of the nineteenth and twentieth centuries, based on theories such as that of the cell, of evolution, of genetics, and today of molecular biology—theories to which eighteenth-century naturalists could not resort. In this way Gibson reminds the reader to consider the theoretical, observational, and experimental conditions of the historical actors when judging their often curious theories.