The 2005 la conchita landslide, california: part 2 - modeling

Abstract

This is Part II of a two-part causation analysis of the January 10, 2005 La Conchita landslide. This paper describes the development and application of a two-dimensional finite difference model (FLO-2D) model to simulate the debris flow over a fixed substrate. The model domain 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. An inflow hydrograph, representing the volume of displaced material, was specified as a line input at the base of the headscarp area. The sediment concentration in the input hydrograph was varied (from zero to 0.7) over the 14-second duration of the hydrograph. Samples of debris collected from test pits in the debris flow provided a saturated density of 1,762 kg m-3. An initial estimate of the yield stress (stress) of the debris of 5,257 Pa was calculated using parameters derived from pre- and post-event topographic maps of the area. Because no independent means of calculation was available, the dynamic viscosity of the debris was adopted from values contained in the FLO-2D Users Manual. The Manning’s bottom roughness coefficient for each grid cell was based on estimates of the surface and vegetation characteristics of the area. A series of simulations were performed to evaluate travel path variations for three differing wall and slope con gurations present in the area between 1995 and 2005. Sensitivity analyses performed for each of the simulations by varying the yield stress (strength) and dynamic viscosity by ± 20% had a minor effect on the extent of debris flow runout. Sensitivity analyses were also performed by varying the debris volume by one-half and 1.5 times the original volume. Model results show a significant difference in debris runout as a result of these inputs. Two lobes of debris formed during the transport phase. The spatial distribution of the main lobe of the debris flow was similar to actual conditions in all of the simulations performed. The spatial distribution of the minor lobe, however, generally differed from that predicted by the analysis.