Hydraulic Evaluation of an Auxiliary Spillway of Gatun Lake System

The expansion of the Panama Canal through the construction of a third set of locks for larger ships requires increasing the reservoir levels of Gatun Lake, which results in a decrease in its capacity to attenuate floods and the consequent need to have a higher discharge capacity to the existing one. In effect, the original hydraulic spillway was designed for a flow rate close to 4,500 m³/s, verifying in a recent study carried out by the National Institute of Water (INA) from Argentina that this spillway can discharge 6,070 m³/s for increased levels of the lake. This increase in the discharge capacity is not enough to reach the maximum instantaneous flow of 13,000 m³/s for the 10,000-year recurrence that results from the hydrological and operational conditions of the system analyzed within the context of the expansion project of the Panama Canal. Two alternatives of auxiliary spillways were defined in two consecutive phases, with the purpose to reach the maximum flow discharge between one of these alternatives and the existing spillway. Then, INA carried out the evaluation of two alternatives of auxiliary spillways, by means of the construction and the exploitation of two physical models with length scale 1:40. The first alternative consisted in a project of a hydraulic spillway destined to discharge a total flow of approximately 6,800 m³/s, whose location was defined by the Panama Canal Authority in the left abutment of the dam and its discharge at the same Chagres River. During the development of the hydraulic studies, and even though their results were satisfactory, this alternative was rejected due to the presence of a geological fault in the chosen site. For decision of the Panama Canal Authority, additional studies were carried out to define the location and characteristics of the second alternative of auxiliary spillway, which would be located between the first pair of locks and the third set of locks. Geological aspects and the limitations of the location of this structure have imposed some singular conditions to its design and its flow discharge, which is less than the first option of auxiliary spillway. As a consequence of these factors, the maximum discharge flow of the complex would be reached by means of the simultaneous operation of principal and auxiliary spillways and the opening of the gates of located in the culverts. In the present article, the most important aspects of each of the studies are highlighted, which define the hydraulic behavior of both alternatives of auxiliary spillway, with special emphasis on the second of the alternatives that was finally selected for the definitive project.