Improving navigation and preventing erosion near San Pablo, Bolivar

In this report a study is carried out on the Magdalena River near the village of San Pablo, Colombia. The Magdalena River near San Pablo experiences two major issues which are related to navigation and erosion. The major problem for inland navigation is the sharp bend of the Magdalena River at San Pablo, which has too small a radius, causing problems for navigation. Furthermore, there are some parts in the considered river reach where the water depth is insufficient according to the requirements for navigability. Regarding the erosion problem near San Pablo, the main driving force is assumed to be high flow velocities and large near-bank water depths due to spiral flow in the outer bend. The main flow passing by the river bank of San Pablo gradually takes soil particles away, resulting in a continuous erosion process. In addition, the developed steep slope and height of the river bank results in mass failure mechanisms, increasing the erosion problems even further in the future. Several possiblemeasures have been analysed to solve this navigational problem, such as river training works consisting of groynes, longitudinal dams and a spit, where the latter one consists of a land tongue and a longitudinal dam connected to it. Measures with an even bigger impact are also considered, such as a bend cut-off and a river bypass. There are several possible measures to solve the erosion problem, such as a revetment, groynes and geogrids. Both the navigability and erosion problem can be solved in an integral way by a combination of different measures. In this study five alternatives are proposed, which are groynes, a spit, longitudinal dams, a bend cut-off and a river bypass. With the help of a multi-criteria analysis the most promising alternatives have been selected, which showed the spit to be the best in terms of reducing erosion and increasing navigation possibilities. With the help of the Delft3D model the general up- and downstream consequences have been explored of the most promising alternatives. The large river training structures, such as longitudinal dams and a bend cut-off, do not result in the desired behaviour of the river reach. Therefore, it is not advised to completely restrict the river in its flow conditions since this in general leads to more difficulties as the restriction does not allow the river to get to its equilibrium within a short period of time. However, both the groynes and the spit pose fewer restrictions to the river flow and show large improvements of the current situation in terms of navigability and erosion. The groynes are especially successful in reducing erosion problems at the river bank of San Pablo. However, the spit is more effective in driving the main part of the flow through the current navigation channel, which results in a deeper and wider channel. Therefore, it is recommended to combine the spit and the groynes to obtain an integrated solution for both erosion and navigation problems. However, for both river training works, erosion problems are encountered downstream of San Pablo. This should be a major part of future research since problems should be solved instead of shifted up- or downstream.