CONTROLING BED SCOUR IN A TORRENT 20 YEARS AFTER AMUDFLOW

摘要

On November 13, 1985, a phreatic eruption of the Ruiz Volcano in the central cordillera of the Colombian Andes melted part of its snow cap generating devastating mudflows along four rivers with dramatic consequences. On the Lagunilla River alone, Armero, a city with a population of 50,000, was totally destroyed killing 50% of the inhabitants. Along the Guali River, two larger cities, Mariquita and Honda, were mostly spared, but the long term effects of the mudflow can still be felt today, as the river bed continues to scour over a reach in excess of 10 kilometers from its mouth in the Magdalena River. In the aftermath of the mudflow, several bank control schemes were designed and built with no success, leading to the more expensive design of a system of drop structure bed stabilizers, along the last 7 km reach of the river, in 1988, which could not be built for different reasons until today. The Guali River is a mountain torrent with an average discharge of 40 cms, which maintains a high slope, in excess of 10%, through all its course down to the Magdalena River; maximum recorded hydrologic floods reached 1000 cms, while the mudflow was estimated at 6000 cms; average sediment load of the river has been estimated at one million tons/year, while the mudflow sediment load was estimated at 15 million tons during the 4 hours that it lasted in Honda. The paper analyzes the consequences of the mudflow through the last 23 years, and the final design of a reduced system of only 8 drop structures to protect the last 3 Km of the river course, through the city of Honda, near the junction of the Guali and the Magdalena rivers.