Downstream Impacts of The Rio Coca Headcut

The February 2020 collapse of the 150-m tall San Rafael Waterfall, a lava dam formerly providing natural grade control on the Rio Coca, Ecuador, initiated a headcut that has progressed 14 km upstream and eroded 200 million tons of sediment. This headcut is threatening the Coca Coda Sinclair diversion structure, which is another 6km upstream, and causing the loss of residential areas and infrastructure through valley backwasting; these time-sensitive threats upstream of the former waterfall have received most of the attention and analysis.

However, 200 million tons of eroded sediment must go somewhere. One of the largest sediment pulses in the Anthropocene era is advancing downstream, where bed aggradation and excess sediment transport also threaten Ecuador’s most important hydroelectric project. The outlet structure for this project is 45 kilometers downstream of the collapsed waterfall. This structure will become inoperable if the bed aggrades more than 6m and will have to shut down operations periodically between 2 and 6 meters of deposition. The sediment pulse has reached this location and has deposited about 2 meters to date as of summer 2022.

This talk presents the downstream impacts and processes of this massive sediment pulse. It includes data and modeling results developed for La Corporación Eléctrica del Ecuador (CELEC), the Ecuadorian agency responding to this event, and the assessments from a multi-agency team of US Government subject matter experts in sediment-pulse dynamics.