The Recurrent Debris Flows in Mocoa - Colombia

On March 31 of 2017, between 10:00 pm and 1:00 am on April 1, 2017, 5 inches of rain fell in the watersheds of two small creeks in the city of Mocoa, capital of the state of Putumayo, in Colombia, triggering major debris flows, which destroyed more than 500 residences in 17 barrios, killing well over 500 people.

This type of event that is occurring with greater frequency in the last 10 years has claimed over 5,000 deaths in Colombia in the last five. The rainfall in the Mocoa event having a recurrence interval of about 100 years, falling in the degraded, steep, upper watershed slopes with characteristic slopes between 40% and 45% and silty clayey soils caused the destructive events. The debris flows concentrated in the Taruca and Taruquita creeks, affluents of the major drainage systems of the Mulato and Sangoyaco rivers, themselves tributaries of the torrential Mocoa River, which borders the distal front of the alluvial fan of Mocoa.

The flows of water, mud, stones and trees also devastated roads, bridges and government buildings affecting even the correctional facility in town, ad perturbing the lives of its more than 45,000 inhabitants. A history of similar events is recorded in several lobules of the alluvial fan of Mocoa, formed by an extensive distributive drainage system as old as 5 million years. A similar event in the Taruca creek in 1961, an avalanche of the Mulato River in 1989 and similar events as recently as 2014 have been recorded. Since the general areas in risk were known, this mayor loss of life was unprecedented, and show major lack of attentiveness from the local and state authorities regarding the gravity of the events.

This article presents the characteristic morphodynamics and flow behavior of two different debris floods in the alluvial fans of Mocoa. Two particular events in 2017 and 2018 are reviewed. Special attention is given to the composition of debris materials involved, their origin and their behavior under flow: The liquid fraction composed of water, fine silts, clays, and sands; the thicker solid fraction, with blocks, palisades, sand, and mud, and the fine final fraction of runoff.

KEY WORDS

Debris flow, Mocoa, Sangoyaco and Mulato rivers, Torrential Drainage Systems