THE JANUARY 10, 2005 LA CONCHITA LANDSLIDE

This presentation reports the findings of a geologic/geomorphologic investigation and a FLO-2D modeling analysis of the January 10, 2005 La Conchita landslide. The 2005 La Conchita landslide occurred about 130 km northwest of Los Angeles, California, following two weeks of heavy rainfall. The landslide mobilized over 30,000 m³ of wet debris, forming two distinct lobes. The main lobe, comprising 90% of the deposit, moved into place as a large-scale debris flow. This lobe entered a residential community at the foot of the slope, killing 10 persons and damaging or destroying 36 residences. The source area exhibited numerous unfavorable geologic characteristics. Movement was triggered by the addition of water from several sources. The main lobe eroded and entrained over 4,000 m³ of material along its path. Video and eyewitness accounts indicate it was moving at around 6 to 10 m/s in the mid slope area, dropping to 5 m/s or less in the community. The main lobe exhibits textures characteristic of debris flows elsewhere, including raised lateral levees and a surface pattern of ridges and troughs. The minor lobe, comprising about 10% of the total landslide volume, instead exhibits a hummocky, irregular surface texture. Runout of the landslide was modeled using FLO-2D, a commercial software package used to model the movement of water or sediment-water slurries over a fixed substrate. The model space for the analysis consisted of 25,614 square grid cells, each measuring 1.52 m on a side, and was developed using digitized pre- and post-event topographic maps. Model inputs included an inflow hydrograph, sediment concentration, saturated density, yield strength, dynamic viscosity and Manning’s bottom roughness. A series of simulations were performed to evaluate travel path variations for three differing wall and slope configurations present in the area between 1995 and 2005. Sensitivity analyses were performed for each of the simulations by varying the yield strength, dynamic viscosity and debris volume. The FLO-2D program was very successful in predicting the runout of the main lobe of the debris flow. The spatial distribution of the minor lobe, however, generally differed from that predicted by the analysis due to travel through heavy brush, the deposition of the debris in pulses, and other complicating factors.