Transporte de sedimentos del Arroyo Napostá Grande, Bahía Blanca, provincia de Buenos Aires

• Autores: Perez Enrri, Daniel Roberto; Perillo, Gerardo Miguel E.
• Año de publicación: 2002
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: Transporte de sedimento; Esfuerzo de corte de fondo; Carga de fondo; Arroyo Naposta Grande
• Institución: Consejo Nacional de Investigaciones Científicas y Técnicas

Se presentan resultados hidráulicos y sedimentológicos de una campaña al arroyo Naposta Grande. A pesar que existe un gran número de obras hidráulicas sobre el curso inferior del arroyo, no existen aún datos publicados acerca del caudal sólido transportados por el mismo. Se ubicó una sección de aforo adecuada, sobre la cual se determinó la batimetrÌa del área. Se midieron las velocidades en verticales separadas 0.5 m en toda la sección y a partir de estos valores se determinaron las curvas de igual velocidad en la sección con las que se calculó el caudal y la distribución de los esfuerzos de corte del fondo. Se obtuvieron muestras del sedimento transportado como carga de fondo por medio de trampas, verificándose transporte diferentes en cada faja. Paralelamente se calculó el transporte como carga de fondo por medio de cuatro diferentes ecuaciones y se comparó con los valores medidos, determinándose cual de las ecuaciones predijo mejor el valor del transporte. De las muestras de sedimento en suspensión se realizaron las curvas granulométricas y se calculó la velocidad de caída ponderada de esos sedimentos. Se estimó la concentración y el transporte de sedimento en suspensión por el método de difusión, determinándose además la distribución de concentraciones en la sección de aforo. Las características particulares del transporte en el curso inferior de arroyo Naposta llevan necesaria mente a determinar el transporte como carga de fondo en distintas fajas ya que la influencia de las orillas provoca una considerable diferencia en el transporte entre la zona de mayor profundidad y las zonas cercanas a las orillas, haciendo riesgoso considerar una sola posición como representativa de la sección en su totalidad. Las comparaciones entre las fórmulas de predicción de transporte como carga de fondo sugieren como la más adecuada a la de Einstein aunque predice en defecto. Le siguen las formulas de Yalin y de Bagnold. La fórmula suiza arrojó resultados muy por encima de los reales, debiéndose esto a que la arena del arroyo es muy fina y dicha ecuación es valida para partículas relativamente grandes (D>2 mm).
Hydraulic and sedimentological results of a field survey to the Napostá Grande Creek are presented. The lower, highly meandering course flows through Bahía Blanca City. This part of the creek is greatly affected by hydraulic man-made features such as railways, car and walking bridges, canalisations, a structure that separates the extreme flood water, urban rainwater collection, cleaning, etc. In addition, sand is being extracted from the channel for construction. Despite the remarkable influence that the stream has on the city, there are no quantitative records of the sediment transport within the stream. The objective of this work is to determine the liquid an solid flux over a section, evaluate the dynamic and turbulent characteristics of the flux, find a proper methodology suitable for this particular situation and define a formulation that predicts most accurately the bed load transport in the Napostá Creek. A proper cross-section after a recognition walk along several kilometres along the course of water has been done. As the stream shows several meanders it was difficult to locate an ideal cross section. The one chosen had some problems from the hydraulic point of view (partially symmetrical area, presence of a sand bar located in a rather short straight length), but on the other hand, the accessibility was excellent with available electric supply. Bathymetry of the surrounding area was carried out and a map was produced with isobaths with 0.5 m equidistance. Velocities at vertical profiles separated 0.5 m were obtained with a propeller along the cross-section and, based on these data, equal- velocity curves were drawn over the transversal section. These allowed the determination of the water flux and the bottom shear stress distribution along the cross section and wet perimeter. Within the section, bedload sediment transport was sampled using Helley-Smith kind bedload traps, at various locations, confirming different transport rates at each point. Traps gathered the sediments over 13 minute at each strip in five points, which led to a distribution of the sediment dragged close to the bottom. Higher transport rates were verified on the section thalweg diminishing shorewards. Granulometric curves were determined for the bed load samples and bottom ones. The coarser sediment was trapped only near the thalweg where there is an important velocity concentration. Close to the shores, only the finer fraction of the bedload was captured by the traps meaning that the energy available there does not involve all particle sizes in the movement. Calculation of the predicted bedload sediment transport was done, considering four well known equations (Meyer- Peter-Muller, Einstein, Bagnold and Yalin).The D50 criteria was adopted for the calculation of the bedload. The Swiss formula gave a 100% error in comparison with the measured values, whereas the rest of them showed great accuracy for the methodology employed. Einstein's formula gave the closest value to the measured one but underestimated it. On the other hand Bagnold's equation resulted in 17 % in excess. Yalin's formula produced a 15% understimation, about the same order than Einstein's formulation. Suspended sediment samples were obtained and their granulometry determined through the Oden's Curve technique. Based on them, weighted settling velocities were calculated for each sample (Perez et al., 1997). Suspended sediment concentration profiles were determined by the diffusion method. Then the concentration distribution was integrated over the area so that total flux and flux distribution were obtained. Also a grid was calculated and curves of equal flux are presented. Net suspended sediment transport across the area was found as the addition of the two kind of transports.
Fil: Perez Enrri, Daniel Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur; Argentina
Fil: Perillo, Gerardo Miguel E.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur; Argentina