By Ross Stein and David Jacobson, Temblor
Click here for Spanish version
Late last week, heavy rainfall in Colombia resulted in landslides and mudflows devastating the city of Mocoa. As of this afternoon, 254 people are confirmed to have died, with many more missing. The worst natural disaster to strike the country in years occurred as rivers breached their banks, carrying rocks and trees through the city of 40,000. Because it happened at night, many people were in their homes when the deluge smashed through the town, destroying houses and carrying away cars.
In the World Resources Institute’s Global Flood Analyzer, unlike the larger Rio Caqueta to the east, Mocoa is not shown as having a flood risk. Because of the impact this flood had on the country, and perhaps its unexpected nature, we took a closer look at why Mocoa is susceptible to flooding.
Why was Mocoa flooded?
Mocoa straddles the Mocoa River, which drains the Eastern Cordillera of the Colombian Andes. Here, the ranges have been uplifted by the Mocoa Thrust Fault. In successive earthquakes over perhaps a million years, the fault has repeatedly jacked up the range front, and downdropped the basin to the south (where Puerto Guzman lies I the map below). This tends to back-tilt the stream channel, blocking its outflow from the range front to the plains. Given enough time and water, the stream would erode and deepen its valley until it forms a continuous concave-up topographic riverbed profile, erasing the effect of the faulting.
A competition between the fault and rain
At every range front fault traversed by a stream, there is a competition between the ability of the thrust fault to uplift the range, and the ability of the stream to erode its channel. In arid, active tectonic settings, the fault usually wins (think Nevada, Utah, or Baja California); in humid areas or where the faults have very low slip rates, the stream wins.
Despite a verdant setting, near Mocoa, the fault has prevailed, with the range front uplifted along the fault, tilted downward-to-the-north. The result is a ‘necking’ of the river valley, evident from the river terraces, which get closer to the stream as one approaches the fault. There has not been enough water coursing through the stream to fully erode the valley. At the town of Mocoa, there is a local low point where the valley widens before it necks to the south. When the river floods, Mocoa’s wide, flat expanse is inundated.
A braided river channel
Examining the Mocoa River in Google Earth, it is apparent that upstream of the neck, where the topography is flatter, the river is braided. Think of turning the water in a garden hose to the max and letting the hose drop to the ground; it snakes back and forth. In slow motion, this makes for braided channels.
It is also evident to us that the city is built on abandoned (former) river meanders that are not much higher than the current channel. This makes the city much more susceptible to flooding. When a river reaches flood stage, it is more likely to follow paths of least resistance, which in this case was the eastern portion of the city.
So, what caused the flood?
In the near-term: Water, and lots of it, falling in the Mocoa river catchment basin. Second, the Mocoa thrust fault, that has created a locally flat area and disturbed the river channel depth. And, finally, building a city atop old river meanders, sites of the river in the not so distant past.
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