Major fluvial erosion and a 500-Mt sediment pulse triggered by lava-dam failure, Rio Coca, Ecuador

Authors: BARRERA CRESPO, PEDRO - Electric Corp Of Ecuador; ESPINOZA GIRON, PABLO - Electric Corp Of Ecuador; BEDOYA, RENAN - Electric Corp Of Ecuador; GIBSON, STANFORD - Us Army Corp Of Engineers (USACE); EAST, AMY - Us Geological Survey (USGS); Langendoen, Eddy; BOYD, PAUL - Us Army Corp Of Engineers (USACE)

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2023
Publication Date: 3/15/2024
Citation: Barrera Crespo, P., Espinoza Giron, P., Bedoya, R., Gibson, S., East, A., Langendoen, E.J., Boyd, P. 2024. Major fluvial erosion and a 500-Mt sediment pulse triggered by lava-dam failure, Rio Coca, Ecuador. Earth Surface Processes and Landforms. 49(3): 1058-1080. https://onlinelibrary.wiley.com/journal/10969837.

Interpretive Summary: In February 2020 a 150 meter tall waterfall collapsed on the Rio Coca, Ecuador, which has resulted in rapid erosion of the river bed that is propagating upstream towards the Coca Coda Sinclair hydroelectric project. Geomorphic adjustment of the Rio Coca represents a highly unusual natural disaster threatening life, property, water quality, the regional economy, major infrastructure, and energy security. ARS researchers in Oxford, Mississippi, cooperated with experts from the US Army Corps of Engineers, US Geological Survey, and the Electric Corporation of Ecuador to characterize the geomorphic adjustment of the Rio Coca since the waterfall collapse and the geotechnical properties of the bed and bank materials. The river's boundary materials comprise four erodible geologic units consisting of volcaniclastic and lacustrine deposits that mostly fall within the medium erodibility class. In the three years after the collapse of the waterfall, the erosion front migrated almost 13 km upstream through these geologic units, and is within 7 km of the dam. About 500 Mega tonnes of sediment have been removed and transported downstream. The erodibility information of the geologic units is used by the Government of Ecuador to perform hazard assessments and develop appropriately dimensioned mitigation measures.

Technical Abstract: The collapse of a 144-m-high lava-dam waterfall on the Río Coca, Ecuador, in February 2020 initiated a catastrophic watershed—regressive erosion upstream and a massive sediment pulse downstream—as the river evolves toward a new equilibrium grade. Evolution of this river corridor after a sudden base-level fall embodies the “complex response” concepts long understood through laboratory experiments, numerical modeling, and smaller-scale field studies, but that have not been observed in the field before on this scale. This paper presents geomorphic and geotechnical data to characterize the evolution of the Río Coca since 2020. In the three years after the collapse of the lava dam, the erosion front migrated almost 13 km upstream along the mainstem river and triggered secondary headcuts that are migrating up tributaries. Erosion of the mainstem and tributary valleys generated a sediment pulse estimated to be 277 million cubic meters and ~500 Mt over three years, depositing sediment up to tens of meters thick over tens of kilometers downstream from the former waterfall site. This sediment pulse is one of the largest in modern times, comparable to the annual sediment load of a major continent-draining river but with orders-of-magnitude greater sediment yield. Geomorphic adjustment of the Río Coca represents a highly unusual natural disaster threatening life, property, water quality, the regional economy, major infrastructure, and energy security. However, this event also provides a rare opportunity to learn how a large autogenic watershed disturbance and recovery evolve, with important lessons for interpreting the sedimentary record of volcanic landscapes.