Author(s): Jaime I. Ordoñez; Raquel Duque
Linked Author(s): Raquel Duque
Keywords: Distributive drainage; Alluvial Fans; Deltaic zones
Abstract: Natural Drainage on the earth Surface occurs along channels that transport the excess of runoff that cannot be retained in surface and subsurface basins, infiltrated in the subsoil or evaporated to the atmosphere. These channels transport also the volume of sediments that are produced by erosion in the mountain sides; the interaction between these two elements in the presence of vegetation and geologic structure, define the size and shape of the drainage systems and the watersheds themselves. Despite the variety of patterns that such interaction is likely to produce, given the variety of geological structure, soil and vegetation types, climatic and erosive conditions, most natural drainage systems considered in the hydrologic and geomorphologic literature have a contributive nature, so that, small watersheds within a bigger one always tend to coalesce in systems where the liquid and solid discharges increase in the downstream direction due to the confluence of the various channels. These systems occur mostly in high mountain areas, and in the wide alluvial valleys confined by mountains; some exceptions however occur in the piedmont regions, along alluvial cones and inclined depositional bands formed by imbricating fans along the mountain chains. In the majority of septentrional and austral areas, with arid and semi-arid climates, alluvial fans are a very small part of watersheds, and tend to be over sighted, due to the low human occupancy and the minor importance they have on large engineering projects. These systems occur also in the lowlands along deltaic systems reaching to the sea, and in less known continental deltas in particular regions inland, where rivers empty into large depositional basins from dried out lakes or inland seas. In tropical countries, alluvial fans and other zones of distributive drainage have surface areas that are significantly larger, (well over 1000 Km2) ,and because of their moderate climate and water abundancy are usually the location of large cities, generating also major engineering projects such as roads, oil ducts, airports, cities and so on. The present article discuss the nature and properties of some of these distributive drainage systems particularly those related to large alluvial fans in tropical countries, where the special environmental situations they bring about, present serious difficulties for the construction of engineering projects. The study and proper characterization of these systems is indispensable for the success of many such projects in the tropical zone.
Year: 2015