Matrix granulometry of catastrophic debris flows (December 1999) in central coastal Venezuela

Abstract

After 2 days of intense precipitation, devastating flooding and debris flows affected the central coast of Venezuela on December 15–16, 1999. These events killed nearly 30,000 people and destroyed or damaged 126,000 homes. I visited the disaster area in March 2000; coastal valleys affected by debris flows were extensively covered by an interstitial matrix that included large amounts of often impressive boulders, up to 35–40 tons in weight. Granulometric analyses of four matrix samples from the Naiguatá valley showed they were all similarly coarse; their main constituent was sand, and grains <0.063 mm (silt and clay) made up only 1.3–14.2% by weight of the matrix. Median ( D 50) matrix grain-size was used to calculate—by interpolation on published diagrams (Water Resources Research, 21 (1985) 1511)—the minimum sediment concentration needed for debris flows to reach the yield-strength threshold required to mobilize large blocks. The data imply that debris flows needed at least 53–62% sediment by volume to transport matrix-supported boulders. Given the possibility that some of the fines may have been lost in suspension during flow or by subsequent flooding, I recalculated median matrix grain-sizes assuming that either 50% or 75% of the silt and clay had not been preserved in the deposits sampled. The new D 50 values did not alter results substantially, as a 50–57% sediment concentration would have been still needed for such rough matrix to support boulders. Alternatively, the textures discussed here may approximately describe the actual particle-size distribution of the matrix involved in transporting boulders during the debris flows. This is suggested by the presence, 6 months after the flooding events, of a remnant sandy matrix still preserved beneath and between large boulders (Wieczorek, personal communication, 2000 and 2001. United States Department of the Interior, U.S. Geological Survey, Reston, VA 20192, USA). The above implies that the Venezuelan debris flows were of the granular, noncohesive type (cf. Scott, K.M., Vallance, J.W., Pringle, P.T., 1995. Sedimentology, behavior, and hazards of debris flows at Mount Rainier, Washington. U.S. Geological Survey Professional Paper 1547, 56 pp.). Data suggest the following events must have taken place in coastal Venezuela. Torrential rains quickly caused flooding along creeks and rivers, which must have rapidly attained hyperconcentrated streamflow. Bulking of stream alluvium in flood surges may also have taken place. However, the storm triggered widespread landsliding along the steep, unstable flanks of the Coastal Cordillera; many slides started in upper-basin areas, underlain by gneiss bedrock (Peña de Mora). Such slides released vast amounts of coarse, sandy debris, which mixed with turbulent floodwaters to produce numerous granular, noncohesive debris flows. These flows, despite their coarse matrix, exhibited intense particle collisions and interactions, thus were able to effectively mobilize vast masses of large boulders.