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Navigability at an Unstable Bifurcation

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Navigability at an Unstable Bifurcation Navigability at an unstable bifurcation The Montaño - Murindó bifurcation of the Atrato River in Colombia Master of Science Thesis report F.C.R. (Frank) Melman, August 2011 Graduation Committee: Prof. Dr. Ir. H.J. De Vriend Dr. Ir. E. Mosselman Dr. Ir. C.J. Sloff Dr. Ir. A.M. Talmon Delft University of Technology Faculty of Civil Engineering and Geosciences Section of Hydraulic Engineering Chair of River Engineering ‘Medir es Saber’ Kamerlingh Onnes Frank Melman Preface This document forms the final thesis report to accomplish the Master of Science degree in Hydraulic Engineering at Delft University of Technology, faculty of Civil Engineering and Geosciences. The thesis’ subject regards a navigability study of the Atrato River in Colombia, which zooms in on sedimentation problems that evolve around a bifurcation. This study is carried out at Deltares under the supervision of dr. ir. Mosselman and dr. ir. Sloff. From this point I would like to thank the people who made my graduation and overall studies possible. First of all, I would like to thank my daily supervisors dr. ir. E. Mosselman and dr. ir. C.J Sloff for without them this final work could not be done. In addition, I would like to thank the other members of my committee, prof. dr. ir. De Vriend and dr. ir. A. Talmon for their interest in my research. I would like to thank Deltares for providing me with a workspace and making expensive software available for me. I would like to thank my Colombian friends ir. J. Jimenez and ir. A. Montes for making my stay in Colombia safe, convenient, interesting and fun. Furthermore, their help in collecting data and information is appreciated. Our trip over the Atrato River and visit to La Piscina were very inspiring. Furthermore I would like to thank the port authorities of Quibdó for financing the trip to Colombia. I would like to thank my fellow graduate students at Deltares for making my stay there pleasant and worthwhile. The many talks and discussions regarding both academics and daily interests were both interesting and fun. In particular, I would like to thank Sander Post for putting up with me, not only the past year of finalising my Masters degree, but for all these years of studying. In addition to this last remark, I would like to thank my fellow students Dennis Joosten and Paul Witteveen for their tremendous help and moral support during these years at the faculty of Civil Engineering and Geosciences. Finally, I would like to thank my parents for giving me the opportunity to study, for the financial as well as the moral support. Frank Melman Delft, August 2011 Navigability at an unstable bifurcation - iii - Frank Melman Executive summary The starting point for this study is the increasing interest of the Colombian government in stimulating the economy of the Chocó region. In this department, sailing is the only mode of transportation so that the intersecting Atrato River is of paramount importance to the local people. The river flows parallel to the Pacific Ocean and into the Caribbean sea, which leads to the conclusion that this river has a unique geographical position. Creating an inter-oceanic connection with the existing river and a to-be-constructed road network will help improve infrastructure and should lead to improved living standards for the local people living in the Chocó department. The Atrato River should be navigable for bigger vessels when generating this inter-oceanic connection. A likely segment of the river that forms a critical point for navigation is the point of bifurcation where the river is divided in the Montaño and Murindó branch. Both branches should be reasonably navigable, but bifurcation morphodynamics possibly render the water depth in one branch smaller than the draught of the vessel. It is noted that the Montaño branch is considered the main corridor, as this branch is shorter and generally wider. The unknown morphological behaviour that may influence the navigability around the bifurcation is stated as the problem. The main objective is to model and understand the morphology that evolves around the river bifurcation and discuss possible measures for creating a sustainable navigable channel. River bifurcations are typical and complex features in river systems. The behaviour of every bifurcation is different, as the dynamics are largely determined by the specific characteristics of the bifurcation. The division of sediments is mainly determined by the local flow pattern and geometries. Phenomena like the Bulle effect, gravity pull and flow separation influence the local geometry and subsequently the division of flow discharge and sediments at the bifurcation. In order to create insights in these morphodynamics, a computational model has been set up. Using the software package Delft3D, a 2D morphodynamic model of the area around the bifurcation has been made. When modelling the real world using a computational model, a number of assumptions had to be made. The most important of them were appending a uniform grain size to the model, assuming pre-dominant bed load transport and making assumptions for the unknown water levels in both branches. Due to the lack of sufficient data available, a thorough morphodynamic calibration and verification is impossible. This implies that the model results should be interpreted with caution. It was shown that the model predicts a development to a stable, but often highly asymmetrical discharge division. The model attains this equilibrium after 50 years of morphological modelling. This suggests the bifurcation is unstable. The evolution to an asymmetrical discharge division is mainly governed by the imposed downstream boundary water levels because a head difference over the branches determines the division of flow over the branches. The calibrated model assumes no backwater effects, which means that the shorter Montaño branch (83 km) has a larger water level gradient than the Murindó branch (91.5 km). It was shown that the unknown head difference over the branches greatly influences the global development of the branches (i.e. aggradation /degradation). Because only one bathymetrical survey was executed, erosion and sedimentation rates are unknown, which further decreases the possibility of accurately modelling the area. An important result is that when imposing the same water level gradients in both branches, the model still predicts a development to a highly asymmetrical division of flow discharge (i.e. 75% through the Montaño branch). This means that local geometrical phenomena influence the division of flow and sediments. The sharp angle of offtake of the Murindó branch results in flow separation which in turn causes a sediment trap at the entrance of this branch. As a result, the cross-section reduces significantly, which means less discharge is directed through this branch. Another phenomenon is caused by the present river bend just upstream of the Navigability at an unstable bifurcation - iv - Frank Melman point of bifurcation. This leads to a gravitational pull along the transverse slope which initially leads to an increasing bed load transport through the Murindó branch. Another local phenomenon is the Bulle effect which leads more bed load through the Murindó branch due to the river bend formed by the upstream bend and the offtaking channel. The sharp bend direct after offtake of the Murindó branch induces a spiral flow which generates a decrease in cross- sectional area. With the current model, it is not possible to quantify the influence of these phenomena separately. The model also showed to be sensitive to several input parameters. Modelling when incorporating suspended sediment transport showed to predict a development towards a non- logical global evolution of both the longitudinal and transverse bed. When changing the uniform sediment size, the model does not show significant changes. The only striking difference is the time it takes the model to reach the asymmetrical discharge distribution. A smaller grain means a steeper transverse slope, which in turn leads to more bed load transport into the already aggrading Murindó branch. To quantify the navigable character of the river, a representative vessel has been chosen. The navigable character of the river is illustrated by appointing two scenarios, one in which the Montaño branch aggrades, and one in which the Murindó branch aggrades. It was shown that when modelling with a representative low discharge, the Montaño branch is always fairly navigable. The Murindó branch is poorly navigable when the Montaño branch has a slight water level gradient advantage. Discussing possible solutions is sometimes hypothetical as the global evolution is sensitive to the unknown downstream boundaries. When the river will be used more extensively in the future, channel buoying and traffic regulation seem to be simple and cost-effective measures. To erode the shallow inner bends due to spiral flow, bottom vanes can be placed to form a counteracting transverse flow. The possibility of dredging is a logical way to increase the navigable depth. It was shown by the model that the aggrading process starts quite rapidly which makes this possibility relatively expensive and time-consuming. In addition, the possibility of dredging is only a short-term solution. Different permanent river training structures like groynes and non-erodible layers are discussed but are expensive and hard to manage in such an environment. The possibility of two different bend cut-offs are discussed. It was shown that the angle of offtake greatly influences the division of flow discharge and sediments. Decreasing the offtake-angle of the Murindó branch from 90° to 30° means the Murindó branch gets dominant (60% flow discharge through this branch instead of 20%). As the internal connection between the branches can result in significant backwater effects, it can be a simple solution to cut off this connection. In conclusion, the present available data prove to be insufficient.
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