Fluvial Geomorphology - Essay Example

13 Mar. 2008 FLUVIAL GEOMORPHOLOGY INTRODUCTION Geomorphology is the study of landforms and the processes that shape them. Fluvial Geomorphology is the study of role of rivers in forming the morphology of the earth. In short it is the study of landform evolution related to rivers. Rivers are significant geo morphological agents, they show an amazing diversity of form and behavior and transfer water and sediment from the land surface to the oceans. Highly dynamic in nature, river channels adjust and evolve over timescales that range from hours to tens of thousands of years or more and are found in a wide range of environments. An understanding of fluvial geomorphology has proved its use in channel preservation, environmentally sensitive design, and the restoration of degraded river channels. The basic concepts in fluvial geomorphology are Equilibrium, Regime Theory and Channel Geometry, Geomorphic Thresholds and Scale. Equilibrium state is one in which the input of mass and energy to a specific system equals the outputs from the same system. In fluvial geomorphology it is this equilibrium state that the stream channels tend to achieve Regime theory is grounded on the propensity of a stream system to obtain an equilibrium state under constant environmental conditions. The Regime Theory has a set of empirical equations relating channel shape to discharge, bank resistance and sediment load. It laid the foundation for a large body of work in Fluvial Geomorphology poring on the geometric properties of equilibrium alluvial channels and their adjustments to discharge and sediment transport regimes. HISTORY & PHILOSOPHY OF STUDY OF FLUVIAL GEOMORPHOLOGY Many of the concepts in fluvial geomorphology can be traced to European origins; however, "Classical" American geomorphology as expressed by W.M. Davis has its roots in the Surveys of the Western United States conducted by the U.S. Geological and Geographical Survey following the Civil War. The leading figures in this period of exploration were John Wesley Powell, Gore Karl Gilbert, and Clarence E. Dutton. Others of note during this time frame were Ferdinand V. Hayden, Lt. George N. Wheeler, and Archibald R. Marvine. As the west was being explored and the landforms analyzed, these individuals formulated several key ideas about geomorphology. Clarence Dutton made contributions by creating an awareness of isostatic adjustments and descriptions of landforms. lie also discussed the "Great Denudation," a period of extensive erosion which he felt created the Colorado Plateau. His writings also contained several references to the idea of parallel retreat of slopes. This concept is based upon a belief that hillsides maintain their angle of slope and form as erosion occurs. The first fluvial geomorphic model was the fluvial geographical cycle or the cycle of erosion, developed by William Morris Davis between 1884 and 1899. The cycle was inspired by theories of evolution, and was depicted as a sequence by which a river would cut a valley more and more deeply, but then erosion of side valleys would eventually flatten out the terrain again, now at a lower elevation. The cycle could be started over by uplift of the terrain. The model is today considered too much of a simplification to be especially useful in practice. The Geographical Cycle, as envisioned by Davis, starts with the rapid uplifting of a plain and the beginning of fluvial erosion. Erosion of this initial stage soon produces the second stage, youth. This stage is characterized by low relief and poor drainage with road flat water divides. As the erosion process continues, relief increases until the mature stage is reached. At this time, narrow ridges form water divides and very little flat terrain remains. Additional erosion leads to the old age stage in which relief in slight and low flat plains art dominant. The "almost featureless" plain resulting from the Geographical Cycle was termed a peneplain by Davis. Among suggested examples of peneplains are the Rocky Mountain Peneplain in the Colorado Front Range and the Sherman Peneplain which forms the Gangplank area of Wyoming. The views expressed by Davis were adopted by many geographers, geologists, and geo morphologists because they were logical and easy to teach. The Geographical Cycle as presented by Davis has become known as the Geomorphic Cycle, Geologic Cycle, Cycle of Land-mass Denudation, and Cycle of Erosion in various geomorphology texts used in the United States. Although Davis' thoughts were established in American geomorphology study, there were several oppositions to that model. Penck considered the process of landscape formation to be the same in all climates with the type of crustal unit and crustal movement being responsible for differences in landscapes. Thus, he added crustal movement to Davis' structure, process, and time. Davis believed that the overall objective of geomorphic research was the determination of where a landscape fit into the Geographical Cycle. Penck thought that such research should serve to determine the history of a landscape. Penck also stated that the Davisian idea of rapid uplift followed by a period of slower uplift was not normal and proposed his viewr that uplift began slowly then accelerated. Many Americans claimed that Penck had expressed the concept that valley walls undergo parallel retreat with erosion rather than that a gradual lowering of hilltops occurs. Parallel retreat of slopes does not fit into the Geo-graphical Cycle because the resulting landscape would be a low plain with steep sharp ridges rather than the smooth peneplain. One of the most fundamental changes that have occurred in fluvial geomorphology over the past several decades is the dramatic increase in the number of studies with an applied dimension. The original purpose of fluvial geomorphology is to provide knowledge of the evolutionary history of landscapes through water bodies, has been supplemented by the goal of explaining and predicting landscape dynamics for societal benefit. This supplemental focus implies that geo morphological studies conducted on human time scales no longer need to be justified solely on the basis of their contribution to the goal of understanding geologic-scale landscape evolution. A recent work in fluvial geomorphology has identified anatomizing and wandering gravel-bed rivers as distinct types that differ fundamentally from meandering, braided, or straight rivers Given below are four different types of research that has gone into fluvial geomorphology Another of the major areas of discussion in fluvial geomorphology concerning the Geographical Cycle is the idea of peneplanation. Parallel ret-eat of slopes, which original" had been inferred by Dutton and also attributed to Walter Penck, results in a different landscape than the peneplain of the Davisian cycle. L.C. King has been one of the leading proponents of this concept. His work in the semi-arid portions of Africa also led to his conclusions that the peneplain as described by Davis did not exist. In addition to the study of landfo'ms, interest in stream characteristics and morphology was renewed following World War II. Gilbert had written a paper in 1914 in which he concluded that stream capacity was dependent upon discharge, straeam gradient, and the nature of the stream load. This served as a background for the 1956 study of ephemeral streams by Leopold and Miller. Later, fluvial geomorphologists reexamined the ideas expressed in .His works in an attempt to determine cause and effect relationships. Schumm, while examining the type sediments along stream banks, was able to correlate the sediments to the nature of the stream cross section. This correlation indicated that a cause and effect relationship does exist between two factors. The relationship of this and similar research to other fields of geomorphology is quite strong. Morisawa expressed the view that, "To understand fully and interpret landforms a knowledge of the physical principles by which a river operates is necessary". Research, along the lines mentioned, indicate a shift in the overall objective of geomorphology. Geomorphology was no longer used as a means to deduce earth history but rather earth processes became the focus of research. RECENT DEVELOPMENTS One significant aspect of the general system and specifically of the open systems approach to geomorphology has been the idea that a landscape may have several origins. This approach is directly opposed to the Davisian concept of a single origin. Hack discussed the problem of "relict landforms" and felt that, "If... sudden diastrophic movements occur, relict landforms may be present in the topography until a new steady state is achieved. This has become one area of present day discussion and analysis. A relict landform is defined as being, "situated within an environment where it apparently could not have formed.' Relict forms include the Sand Hills of Nebraska, a similar form in northern Some recent papers in geomorphology have reflected combination of geographical approaches in fluvial geomorphology with remote sensing, historical perspective, and analysis of longitudinal or inter-system complexity. Increasing interactions with biology have led to use of new term to characterize this branch of discipline: bio-geo-morphology. Investigation of rivers require a multidisciplinary approach which makes fluvial geo morphology interact with engineering, environmental sciences and management. Nigeria, and deranged drainage patterns. Existence of these forms indicates an "initial stage of disequilibrium." Today fluvial geomorphology encompasses three- main views: The old "classical" geomorphology of Davis, the dynamic equilibrium of Hack and Strahler, and the peneplanation cycle of King. Because of its cyclic nature, the peneplanation theory has encountered much of the same opposition as the Geographical Cycle. Classical and dynamic equilibrium have in effect been combined. The great majority of geomorphology courses taught in 1971 used texts which could be considered "classical" in approach with additional instruction in quantitative aspects. This is in keeping with Strahler's point that both types of instruction are important in geography and that the "explanatory-descriptive" forms of instruction are best at the introductory level. He also states that quantitative methodology is of greater significance at the advanced level of research and study. CONCLUSION As we can see from the ongoing discussion, fluvial geo morphology has a history that has led the way to new developments in the study of soil erosion, control and restoration activities. River engineering has coming under increasing public scrutiny given failures to prevent flood hazards and economic and environmental concerns. It is important to assess the contribution that fluvial geomorphology can make in the future to river engineering. In particular, the need for fluvial geomorphology to be an integral part in engineering projects, that is, to be integral to the planning, implementation, and post-project appraisal stages of engineering projects is highlighted. Areas in which geo morphologists will increasingly be able to complement engineers in river management include risk and environmental impact assessment, floodplain planning, river audits, determination of in stream flow needs, river restoration, and design of ecologically acceptable channels and structures. For the future fluvial geomorphology faces the following challenges: gaining full acceptance by the engineering profession; widespread utilization of new technologies including GPS, GIS, image analysis of satellite and airborne remote sensing data, computer-based hydraulic modeling and geophysical techniques; dovetailing engineering approaches to the study of river channels which emphasize reach-scale flow resistance, shear stresses, and material strength with catchments scale geomorphic approaches, empirical predictions, bed and bank processes, landform evolution, and magnitude-frequency concepts; producing accepted river channel typologies; fundamental research aimed at producing more reliable deterministic equations for prediction of bed and bank stability and bed load transport; and collaboration with aquatic biologists to determine the role and importance of geo morphologically and hydraulically defined habitats. Fluvial Geomorphology. 11 Feb. 2008. 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Sciencedirect. 09 Mar. 2008. http://www.sciencedirect.com/science_ob=ArticleURL&_udi=B6V93-3YSY1BF-W&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=3002e6af68c95a709db7a6d8fd402820 Read More