Distributive drainage systems.

Jaime Ordóñez, Raquel Duque, Luis A. Camacho

Tuesday 30 june 2015

9:45 - 10:00h at Oceania (level 0)

Themes: (ST) River morphodynamics, (T) Sediment management and morphodynamics

Parallel session: 4B. Sediment - River

INTRODUCTION Natural Drainage on the earth Surface occurs along channels that transport the excess of runoff that cannot be infiltrated or evaporated by the subsoil or retained in Surface and subsurface basins. These channels transport also the volume of sediments that are produced by erosion in the mountain sides and 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 in the literature have a contributive nature, that is, 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 their channels. These systems occur mostly in high mountain areas, and in the wide alluvial valleys confined by mountains; the only exceptions occur in alluvial cones and piedmont bands formed by imbricating cones along the mountain chains. In the majority of septentrional and austral areas, with arid and semi-arid climates, alluvial cones 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 In tropical countries, alluvial cones and other zones of distributive drainage have Surface areas that are significantly larger and can easily be the location of large Engineering projects such as roads, oil ducts, airports, cities and so on. CASE STUDIES The extensive alluvial cone of the Pirai River in Bolivia, offers a typical example of the problematic situation of these distributive systems. Another typical example is the Arauca River in Colombia, and the difluence of the Bayonero branch and the Caño Limón branch of the later. CONCLUSION Distributive drainage systems usual in alluvial fans, continental and ocean deltas, present serious difficulties for the construction of large Engineering projects, particularly in large piedmont fans (Surface area over 1000 Km2). The study and proper characterization of these systems is indispensable for the success of many engineering works in America.