MODELING DISTRIBUTION OF COLLUVIUM AND SHALLOW GROUNDWATER ON LANDSLIDE-SUSCEPTIBLE SLOPES FOR PARTS OF SEATTLE, WASHINGTON

Damaging and deadly landslides have occurred along the coastal bluffs above Puget Sound during recent wet winters. Most of these landslides have been shallow failures of saturated colluvium that mobilized into fast-moving, long-runout debris flows. Colluvium in the region primarily mantles parts of the coastal bluffs from which it is derived, but also occurs on isolated, steep inland slopes. Additional landslide losses will likely be incurred in the future as development in these areas continues. As part of efforts to predict landslide occurrence in the region, methods were developed to estimate the distribution of colluvium and shallow groundwater perched within. The methods were used to generate GIS-based colluvium and groundwater models for parts of Seattle, Washington. The models were developed using regional geologic maps and topographic data along with colluvium thickness and groundwater depth observations obtained from 84 subsurface exploration logs. Hillslope gradient, elevation, distance to primary colluvial source area, and gradient and relief of the source area were used to predict colluvium thickness. Time of year, modeled distribution of regional groundwater, hydraulic conductivity and gradient of the base of the colluvium were used to predict groundwater depth. Hydraulic conductivity was assumed to decrease with distance from the primary colluvial source area because source material becomes finer grained down section. Relations for estimating both colluvium thickness and groundwater depth were varied for several different colluvial environments identified during the study. Predicted depths of colluvium and groundwater in the models were within 25% of the known values at over 83% of the locations at which subsurface data were available. The colluvium thickness model indicates that thickness is inversely proportional to source-area gradient and relief, consistent with the idea that high, steep primary source areas are due to recent, large-scale bluff retreat accompanied by loss of colluvium to Puget Sound. Results of the shallow groundwater model support the assumption that hydraulic conductivity of colluvium decreases downslope. Potential relationships between colluvium thickness, shallow groundwater depth, and historical landslides are currently being evaluated.