IIUNAM Modifications to the Tula Riverbed to Prevent Flooding When

Modifications to the Tula riverbed to prevent flooding when receiving the stream flow from the Eastern Wastewater Tunnel (Túnel Emisor Oriente) Óscar A. Fuentes Mariles, Ph.D., and Faustino de Luna Cruz, M.Sc. To reduce the risk of flooding in Mexico City and its suburbs, a hydraulic project named Wastewater System Drainage Exit [Portal de Salida del Sistema de Drenaje] is being adapted to lead the waters from the Eastern Wastewater Tunnel [Túnel Emisor Oriente – TEO], combined with those from the Central Wastewater Tunnel [Túnel Emisor Central – TEC] and El Salto river that receives the waters from the Western Wastewater Tunnel (Túnel Emisor Poniente - TEP). The TEO will enable removal of a portion of the water from the Remedios River, the Grand Canal and the General Drain [Dren General], which will significantly ease the drainage system in the Valley of Mexico. Much of the output stream flow discharged by the portal project will be carried by the Tula river to the Endhó dam. The magnitude of these stream flows, with a yearly occurrence probability of 0.02 (return period of 50 years), is approximately 400 m3 / s (150 m3 / s of the TEC, 170 m3 / s of the TEO and 80 m3 / s of the El Salto river). Due to the increase of drainage water to be discharged at the Tula river, it is necessary to perform enlargements to its channel to prevent overflows, especially in the urban areas of the cities Cooperativa Cruz Azul and Tula. Further, along the course of the Tula river, the output stream flows of the Requena dam (100 m3 / s) and the Tlautla (213 m3 / s) and Rosas (225 m3 / s) rivers are added at different points, in addition to those of other natural flows of lesser magnitude. A hydraulic performance study was carried out at the Institute of Engineering of the National Autonomous University of Mexico (Universidad Nacional Autónoma de México - IIUNAM), based on a simulation of the Tula River water flow and its inflows from the Tlautla and Rosas rivers (Figure 1). The study was at the request of the National Water Commission (–CONAGUA, by its initials in Spanish). The physical characteristics of the riverbed were taken into account in the study, including the shapes of the channel along its entire length, the bottom topographic elevation, the height of the levees, and the sizes of the sediments, vegetation, obstacles, and bridge piers, among others. Hydrologic analyses were also conducted by the staff of the Federal Commission of Electricity on the Rosas and IIUNAM Tlautla rivers, the discharges of the Requena dam, the El Salto river and the discharge stream flows based on the work of the Output Portal of the Wastewater System. The dimensions of the irregular shapes of the riverbed were specified at distances less than 200 m and throughout 23 km to calculate how the straps and water velocities varied along its course. Additionally, the magnitude of the stream flow that overflowed at different times was calculated, and the most unfavorable conditions that occurred were identified. Various hydrological events that are likely to occur within a given year with values between 0.01 and 0.5 were also considered. The calculations were performed using computer programs with numerical methods using water movement models that change in time and using others in which the fluid displacement is the same at any time (made in IIUNAM). Special attention was given to overflows in urban areas, to elevations and velocities reached by the water under the 10 vehicle bridges crossing the Tula riverbed, as well as in the confluences of the Tlautla and Rosas rivers. The obtained results demonstrate that, to prevent the water from flooding the river, it is necessary to extend or modify the final 18 km of the Tula river. Taking the hydrological conditions as design criteria with a 0.02 probability of occurrence, three proposals were selected for modifying this riverbed: the first is to form a regular section with a concrete coating so that the channel has a trapezoidal section of concrete with four different slopes, preventing the excavations of the river’s bed from becoming excessive. The second proposal is to increase the height of the levees by placing stone-filled gabions in the form of 1 m3 cubes on IIUNAM top of the existing levees. The third is a combination of the two, which includes adding a trapezoidal channel in some stretches of the river and increasing the elevation of the levees in others. The proposals considered that the trapezoidal shape of the riverbed should be built with a certain possible width in the upper part because in the urban areas through which the river passes, there are homes on both sides that must not be affected by physical changes to the riverbed. Again, computer programs were applied to simulate the water flow in the three proposed modifications, and it was found that in the first proposal, the existing bridges were not overtaken. In the second and third proposals, only two bridges would not operate properly because the water would pass over them, which would cause the failure. Additionally, it was noted that each bridge requires its own study due to the strong water currents that pass under the bridges. Finally, the approximate costs obtained for the three proposals verified that the first option is the most expensive, while the second is the most economical. It is desirable in the future that the potential benefits of the changes in the riverbed be analyzed for other possible hydrologic conditions and the operation of the Requena dam. IIUNAM .

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