Debris flows in southern Puerto Rico

Of the many types of landslides common to Puerto Rico, debris flows--the mobilization and flow of rock and soil down steep slopes--are the most abundant in many areas. On October 5-8, 1985, a tropical storm produced extreme rainfalls and consequent widespread debris flows along the south-central coast of Puerto Rico. Locally, 24-hr rainfall exceeded 560 mm, 4-day rainfall exceeded 750 mm, and intensities reached 70 mm/hr. Most of the flows occurred in an area where Tertiary limestone and detrital sediments form 20 degrees to 40 degrees slopes covered by less than 2 m of colluvium or residuum. Many areas received more rainfall and had steeper slopes but contained far fewer flows. The distribution of flows probably was caused by a small cell of very intense rainfall or by localized engineering properties of the colluvium that rendered it more susceptible to failure in the area of concentration. Most of the flows resulted from storm-induced buildup of pore pressure at the colluvium/bedrock contact. The debris generally failed in disk-shaped slabs as much as 15 m across and 0.5 to 1.5 m thick. All the colluvium in the source areas mobilized and exposed bedrock surfaces parallel to the original ground surface. The flowing debris scoured surficial soil down to bedrock and destabilized preexisting gully walls, which triggered additional thin debris slides into the channel. Material contributed by channel scouring and side-slope debris sliding commonly made up 90 to 95 percent of the debris deposit. Debris flows are recurrent phenomena in southern Puerto Rico. Several flows incised channels into older debris-flow deposits, and in some areas, as many as three successively older debris deposits were exposed. Preliminary dating of debris-flow deposits suggests that recurrence intervals for flows at a given site could range from several decades to several hundred years.