Modelling and Projection of The Morphological Evolution of The Coca River After The Collapse of The San Rafael Waterfall

Between February 2nd and 11th, 2020, the natural collapse of the San Rafael waterfall located in the course of the Coca River in Ecuador occurred. The waterfall was located approximately 19 km downstream from the intake and 48 km upstream from the outflow of the Coca Codo Sinclair hydroelectric plant (CCS), respectively. The 150 m high waterfall was made up of a lava flow that stem from the Reventador volcano. The petrified lava wall acted as a natural dam that retained fluvial sediments and material produced by volcanic activity. As a result of its collapse, the discontinuity of the river profile was totally exposed to the constant action of the water. This discontinuity triggered a process of adaptation towards a new morphological equilibrium along a stretch of more than 70 km. The search for a new equilibrium is reflected in the gradual reduction of the discontinuity in the river profile, and in the new slope that it assumes. This implies the regressive erosion and headcut migration upstream, and the sedimentation of the eroded material downstream of the site of the former waterfall. The rate of the headcut migration upstream, as well as the rate of sedimentation downstream, are rapid. Consequently, the potential occurrence of two failure modes of the CCS plant is foreseen within the considered period of its operational life. On the one hand, the advance of the headcut implies the degradation of the bed around the intake facilities, and on the other hand, the progressive sedimentation threatens to prevent the discharge of the turbinated flow. So far the morphological changes involve the erosion of 250 hm3 of sediment, and therefore, the need for mitigation and erosion control actions are urgent. The detailed projection of the evolution of the erosion process is crucial for the conception and definition of potential interventions. In that regard, the projection of the morphological evolution is based on the implementation of a global hydromorphological model of the entire stretch of the river that is subject to the processes of regressive erosion and progressive sedimentation. This global model is conceived from a holistic perspective of the problem, for which the fundamental equations that describe the morphodynamic cycle are used, as well as the record of a sequence of topographic and geologic surveys of the river section under analysis. The idea is to project the temporal evolution of the longitudinal river profile that can be affected by the processes of regressive erosion and progressive sedimentation. This paper summarizes the implementation of the model and describes the scenarios and projections analyzed with the aim to estimate the magnitude of the implications related to the river bed degradation around the intake, and the bed aggradation around the outflow facilities of the CCS hydropower plant.